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WritePrepared Txn: Move DB class to its own file

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Summary:
Move  WritePreparedTxnDB from pessimistic_transaction_db.h to its own header, write_prepared_txn_db.h
Closes https://github.com/facebook/rocksdb/pull/3114

Differential Revision: D6220987

Pulled By: maysamyabandeh

fbshipit-source-id: 18893fb4fdc6b809fe117dabb544080f9b4a301b
This commit is contained in:
Maysam Yabandeh 2017-11-02 11:05:55 -07:00 committed by Facebook Github Bot
parent 6778690b51
commit 60d83df23d
10 changed files with 888 additions and 823 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
CMakeLists.txt
View File

@ -554,6 +554,7 @@ set(SOURCES
utilities/transactions/transaction_lock_mgr.cc
utilities/transactions/transaction_util.cc
utilities/transactions/write_prepared_txn.cc
utilities/transactions/write_prepared_txn_db.cc
utilities/ttl/db_ttl_impl.cc
utilities/write_batch_with_index/write_batch_with_index.cc
utilities/write_batch_with_index/write_batch_with_index_internal.cc
@ -576,12 +577,12 @@ if(WIN32)
port/win/win_logger.cc
port/win/win_thread.cc
port/win/xpress_win.cc)
if(WITH_JEMALLOC)
list(APPEND SOURCES
port/win/win_jemalloc.cc)
endif()
else()
list(APPEND SOURCES
port/port_posix.cc

1
TARGETS
View File

@ -256,6 +256,7 @@ cpp_library(
"utilities/transactions/transaction_lock_mgr.cc",
"utilities/transactions/transaction_util.cc",
"utilities/transactions/write_prepared_txn.cc",
"utilities/transactions/write_prepared_txn_db.cc",
"utilities/ttl/db_ttl_impl.cc",
"utilities/write_batch_with_index/write_batch_with_index.cc",
"utilities/write_batch_with_index/write_batch_with_index_internal.cc",

1
src.mk
View File

@ -202,6 +202,7 @@ LIB_SOURCES = \
utilities/transactions/transaction_lock_mgr.cc \
utilities/transactions/transaction_util.cc \
utilities/transactions/write_prepared_txn.cc \
utilities/transactions/write_prepared_txn_db.cc \
utilities/ttl/db_ttl_impl.cc \
utilities/write_batch_with_index/write_batch_with_index.cc \
utilities/write_batch_with_index/write_batch_with_index_internal.cc \

486
utilities/transactions/pessimistic_transaction_db.cc
View File

@ -11,7 +11,6 @@
#include "utilities/transactions/pessimistic_transaction_db.h"
#include <inttypes.h>
#include <string>
#include <unordered_set>
#include <vector>
@ -25,6 +24,7 @@
#include "util/sync_point.h"
#include "utilities/transactions/pessimistic_transaction.h"
#include "utilities/transactions/transaction_db_mutex_impl.h"
#include "utilities/transactions/write_prepared_txn_db.h"
namespace rocksdb {
@ -135,26 +135,6 @@ Status PessimisticTransactionDB::Initialize(
return s;
}
Status WritePreparedTxnDB::Initialize(
const std::vector<size_t>& compaction_enabled_cf_indices,
const std::vector<ColumnFamilyHandle*>& handles) {
auto dbimpl = reinterpret_cast<DBImpl*>(GetRootDB());
assert(dbimpl != nullptr);
auto rtxns = dbimpl->recovered_transactions();
for (auto rtxn : rtxns) {
AddPrepared(rtxn.second->seq_);
}
SequenceNumber prev_max = max_evicted_seq_;
SequenceNumber last_seq = db_impl_->GetLatestSequenceNumber();
AdvanceMaxEvictedSeq(prev_max, last_seq);
db_impl_->SetSnapshotChecker(new WritePreparedSnapshotChecker(this));
auto s = PessimisticTransactionDB::Initialize(compaction_enabled_cf_indices,
handles);
return s;
}
Transaction* WriteCommittedTxnDB::BeginTransaction(
const WriteOptions& write_options, const TransactionOptions& txn_options,
Transaction* old_txn) {
@ -166,17 +146,6 @@ Transaction* WriteCommittedTxnDB::BeginTransaction(
}
}
Transaction* WritePreparedTxnDB::BeginTransaction(
const WriteOptions& write_options, const TransactionOptions& txn_options,
Transaction* old_txn) {
if (old_txn != nullptr) {
ReinitializeTransaction(old_txn, write_options, txn_options);
return old_txn;
} else {
return new WritePreparedTxn(this, write_options, txn_options);
}
}
TransactionDBOptions PessimisticTransactionDB::ValidateTxnDBOptions(
const TransactionDBOptions& txn_db_options) {
TransactionDBOptions validated = txn_db_options;
@ -571,458 +540,5 @@ void PessimisticTransactionDB::UnregisterTransaction(Transaction* txn) {
transactions_.erase(it);
}
Status WritePreparedTxnDB::Get(const ReadOptions& options,
ColumnFamilyHandle* column_family,
const Slice& key, PinnableSlice* value) {
// We are fine with the latest committed value. This could be done by
// specifying the snapshot as kMaxSequenceNumber.
SequenceNumber seq = kMaxSequenceNumber;
if (options.snapshot != nullptr) {
seq = options.snapshot->GetSequenceNumber();
}
WritePreparedTxnReadCallback callback(this, seq);
bool* dont_care = nullptr;
// Note: no need to specify a snapshot for read options as no specific
// snapshot is requested by the user.
return db_impl_->GetImpl(options, column_family, key, value, dont_care,
&callback);
}
// Struct to hold ownership of snapshot and read callback for iterator cleanup.
struct WritePreparedTxnDB::IteratorState {
IteratorState(WritePreparedTxnDB* txn_db, SequenceNumber sequence,
std::shared_ptr<ManagedSnapshot> s)
: callback(txn_db, sequence), snapshot(s) {}
WritePreparedTxnReadCallback callback;
std::shared_ptr<ManagedSnapshot> snapshot;
};
namespace {
static void CleanupWritePreparedTxnDBIterator(void* arg1, void* arg2) {
delete reinterpret_cast<WritePreparedTxnDB::IteratorState*>(arg1);
}
} // anonymous namespace
Iterator* WritePreparedTxnDB::NewIterator(const ReadOptions& options,
ColumnFamilyHandle* column_family) {
std::shared_ptr<ManagedSnapshot> own_snapshot = nullptr;
SequenceNumber snapshot_seq = kMaxSequenceNumber;
if (options.snapshot != nullptr) {
snapshot_seq = options.snapshot->GetSequenceNumber();
} else {
auto* snapshot = db_impl_->GetSnapshot();
snapshot_seq = snapshot->GetSequenceNumber();
own_snapshot = std::make_shared<ManagedSnapshot>(db_impl_, snapshot);
}
assert(snapshot_seq != kMaxSequenceNumber);
auto* cfd = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family)->cfd();
auto* state = new IteratorState(this, snapshot_seq, own_snapshot);
auto* db_iter =
db_impl_->NewIteratorImpl(options, cfd, snapshot_seq, &state->callback);
db_iter->RegisterCleanup(CleanupWritePreparedTxnDBIterator, state, nullptr);
return db_iter;
}
Status WritePreparedTxnDB::NewIterators(
const ReadOptions& options,
const std::vector<ColumnFamilyHandle*>& column_families,
std::vector<Iterator*>* iterators) {
std::shared_ptr<ManagedSnapshot> own_snapshot = nullptr;
SequenceNumber snapshot_seq = kMaxSequenceNumber;
if (options.snapshot != nullptr) {
snapshot_seq = options.snapshot->GetSequenceNumber();
} else {
auto* snapshot = db_impl_->GetSnapshot();
snapshot_seq = snapshot->GetSequenceNumber();
own_snapshot = std::make_shared<ManagedSnapshot>(db_impl_, snapshot);
}
iterators->clear();
iterators->reserve(column_families.size());
for (auto* column_family : column_families) {
auto* cfd = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family)->cfd();
auto* state = new IteratorState(this, snapshot_seq, own_snapshot);
auto* db_iter =
db_impl_->NewIteratorImpl(options, cfd, snapshot_seq, &state->callback);
db_iter->RegisterCleanup(CleanupWritePreparedTxnDBIterator, state, nullptr);
iterators->push_back(db_iter);
}
return Status::OK();
}
void WritePreparedTxnDB::Init(const TransactionDBOptions& /* unused */) {
// Adcance max_evicted_seq_ no more than 100 times before the cache wraps
// around.
INC_STEP_FOR_MAX_EVICTED =
std::max(SNAPSHOT_CACHE_SIZE / 100, static_cast<size_t>(1));
snapshot_cache_ = unique_ptr<std::atomic<SequenceNumber>[]>(
new std::atomic<SequenceNumber>[SNAPSHOT_CACHE_SIZE] {});
commit_cache_ = unique_ptr<std::atomic<CommitEntry64b>[]>(
new std::atomic<CommitEntry64b>[COMMIT_CACHE_SIZE] {});
}
// Returns true if commit_seq <= snapshot_seq
bool WritePreparedTxnDB::IsInSnapshot(uint64_t prep_seq,
uint64_t snapshot_seq) const {
// Here we try to infer the return value without looking into prepare list.
// This would help avoiding synchronization over a shared map.
// TODO(myabandeh): read your own writes
// TODO(myabandeh): optimize this. This sequence of checks must be correct but
// not necessary efficient
if (prep_seq == 0) {
// Compaction will output keys to bottom-level with sequence number 0 if
// it is visible to the earliest snapshot.
return true;
}
if (snapshot_seq < prep_seq) {
// snapshot_seq < prep_seq <= commit_seq => snapshot_seq < commit_seq
return false;
}
if (!delayed_prepared_empty_.load(std::memory_order_acquire)) {
// We should not normally reach here
ReadLock rl(&prepared_mutex_);
if (delayed_prepared_.find(prep_seq) != delayed_prepared_.end()) {
// Then it is not committed yet
return false;
}
}
auto indexed_seq = prep_seq % COMMIT_CACHE_SIZE;
CommitEntry64b dont_care;
CommitEntry cached;
bool exist = GetCommitEntry(indexed_seq, &dont_care, &cached);
if (exist && prep_seq == cached.prep_seq) {
// It is committed and also not evicted from commit cache
return cached.commit_seq <= snapshot_seq;
}
// else it could be committed but not inserted in the map which could happen
// after recovery, or it could be committed and evicted by another commit, or
// never committed.
// At this point we dont know if it was committed or it is still prepared
auto max_evicted_seq = max_evicted_seq_.load(std::memory_order_acquire);
if (max_evicted_seq < prep_seq) {
// Not evicted from cache and also not present, so must be still prepared
return false;
}
// When advancing max_evicted_seq_, we move older entires from prepared to
// delayed_prepared_. Also we move evicted entries from commit cache to
// old_commit_map_ if it overlaps with any snapshot. Since prep_seq <=
// max_evicted_seq_, we have three cases: i) in delayed_prepared_, ii) in
// old_commit_map_, iii) committed with no conflict with any snapshot (i)
// delayed_prepared_ is checked above
if (max_evicted_seq < snapshot_seq) { // then (ii) cannot be the case
// only (iii) is the case: committed
// commit_seq <= max_evicted_seq_ < snapshot_seq => commit_seq <
// snapshot_seq
return true;
}
// else (ii) might be the case: check the commit data saved for this snapshot.
// If there was no overlapping commit entry, then it is committed with a
// commit_seq lower than any live snapshot, including snapshot_seq.
if (old_commit_map_empty_.load(std::memory_order_acquire)) {
return true;
}
{
// We should not normally reach here
ReadLock rl(&old_commit_map_mutex_);
auto old_commit_entry = old_commit_map_.find(prep_seq);
if (old_commit_entry == old_commit_map_.end() ||
old_commit_entry->second <= snapshot_seq) {
return true;
}
}
// (ii) it the case: it is committed but after the snapshot_seq
return false;
}
void WritePreparedTxnDB::AddPrepared(uint64_t seq) {
ROCKS_LOG_DEBUG(info_log_, "Txn %" PRIu64 " Prepareing", seq);
// TODO(myabandeh): Add a runtime check to ensure the following assert.
assert(seq > max_evicted_seq_);
WriteLock wl(&prepared_mutex_);
prepared_txns_.push(seq);
}
void WritePreparedTxnDB::RollbackPrepared(uint64_t prep_seq,
uint64_t rollback_seq) {
ROCKS_LOG_DEBUG(
info_log_, "Txn %" PRIu64 " rolling back with rollback seq of " PRIu64 "",
prep_seq, rollback_seq);
std::vector<SequenceNumber> snapshots =
GetSnapshotListFromDB(kMaxSequenceNumber);
// TODO(myabandeh): currently we are assuming that there is no snapshot taken
// when a transaciton is rolled back. This is the case the way MySQL does
// rollback which is after recovery. We should extend it to be able to
// rollback txns that overlap with exsiting snapshots.
assert(snapshots.size() == 0);
if (snapshots.size()) {
throw std::runtime_error(
"Rollback reqeust while there are live snapshots.");
}
WriteLock wl(&prepared_mutex_);
prepared_txns_.erase(prep_seq);
bool was_empty = delayed_prepared_.empty();
if (!was_empty) {
delayed_prepared_.erase(prep_seq);
bool is_empty = delayed_prepared_.empty();
if (was_empty != is_empty) {
delayed_prepared_empty_.store(is_empty, std::memory_order_release);
}
}
}
void WritePreparedTxnDB::AddCommitted(uint64_t prepare_seq,
uint64_t commit_seq) {
ROCKS_LOG_DEBUG(info_log_, "Txn %" PRIu64 " Committing with %" PRIu64,
prepare_seq, commit_seq);
auto indexed_seq = prepare_seq % COMMIT_CACHE_SIZE;
CommitEntry64b evicted_64b;
CommitEntry evicted;
bool to_be_evicted = GetCommitEntry(indexed_seq, &evicted_64b, &evicted);
if (to_be_evicted) {
auto prev_max = max_evicted_seq_.load(std::memory_order_acquire);
if (prev_max < evicted.commit_seq) {
// Inc max in larger steps to avoid frequent updates
auto max_evicted_seq = evicted.commit_seq + INC_STEP_FOR_MAX_EVICTED;
AdvanceMaxEvictedSeq(prev_max, max_evicted_seq);
}
// After each eviction from commit cache, check if the commit entry should
// be kept around because it overlaps with a live snapshot.
CheckAgainstSnapshots(evicted);
}
bool succ =
ExchangeCommitEntry(indexed_seq, evicted_64b, {prepare_seq, commit_seq});
if (!succ) {
// A very rare event, in which the commit entry is updated before we do.
// Here we apply a very simple solution of retrying.
// TODO(myabandeh): do precautions to detect bugs that cause infinite loops
AddCommitted(prepare_seq, commit_seq);
return;
}
{
WriteLock wl(&prepared_mutex_);
prepared_txns_.erase(prepare_seq);
bool was_empty = delayed_prepared_.empty();
if (!was_empty) {
delayed_prepared_.erase(prepare_seq);
bool is_empty = delayed_prepared_.empty();
if (was_empty != is_empty) {
delayed_prepared_empty_.store(is_empty, std::memory_order_release);
}
}
}
}
bool WritePreparedTxnDB::GetCommitEntry(const uint64_t indexed_seq,
CommitEntry64b* entry_64b,
CommitEntry* entry) const {
*entry_64b = commit_cache_[indexed_seq].load(std::memory_order_acquire);
bool valid = entry_64b->Parse(indexed_seq, entry, FORMAT);
return valid;
}
bool WritePreparedTxnDB::AddCommitEntry(const uint64_t indexed_seq,
const CommitEntry& new_entry,
CommitEntry* evicted_entry) {
CommitEntry64b new_entry_64b(new_entry, FORMAT);
CommitEntry64b evicted_entry_64b = commit_cache_[indexed_seq].exchange(
new_entry_64b, std::memory_order_acq_rel);
bool valid = evicted_entry_64b.Parse(indexed_seq, evicted_entry, FORMAT);
return valid;
}
bool WritePreparedTxnDB::ExchangeCommitEntry(const uint64_t indexed_seq,
CommitEntry64b& expected_entry_64b,
const CommitEntry& new_entry) {
auto& atomic_entry = commit_cache_[indexed_seq];
CommitEntry64b new_entry_64b(new_entry, FORMAT);
bool succ = atomic_entry.compare_exchange_strong(
expected_entry_64b, new_entry_64b, std::memory_order_acq_rel,
std::memory_order_acquire);
return succ;
}
void WritePreparedTxnDB::AdvanceMaxEvictedSeq(SequenceNumber& prev_max,
SequenceNumber& new_max) {
// When max_evicted_seq_ advances, move older entries from prepared_txns_
// to delayed_prepared_. This guarantees that if a seq is lower than max,
// then it is not in prepared_txns_ ans save an expensive, synchronized
// lookup from a shared set. delayed_prepared_ is expected to be empty in
// normal cases.
{
WriteLock wl(&prepared_mutex_);
while (!prepared_txns_.empty() && prepared_txns_.top() <= new_max) {
auto to_be_popped = prepared_txns_.top();
delayed_prepared_.insert(to_be_popped);
prepared_txns_.pop();
delayed_prepared_empty_.store(false, std::memory_order_release);
}
}
// With each change to max_evicted_seq_ fetch the live snapshots behind it.
// We use max as the version of snapshots to identify how fresh are the
// snapshot list. This works because the snapshots are between 0 and
// max, so the larger the max, the more complete they are.
SequenceNumber new_snapshots_version = new_max;
std::vector<SequenceNumber> snapshots;
bool update_snapshots = false;
if (new_snapshots_version > snapshots_version_) {
// This is to avoid updating the snapshots_ if it already updated
// with a more recent vesion by a concrrent thread
update_snapshots = true;
// We only care about snapshots lower then max
snapshots = GetSnapshotListFromDB(new_max);
}
if (update_snapshots) {
UpdateSnapshots(snapshots, new_snapshots_version);
}
while (prev_max < new_max && !max_evicted_seq_.compare_exchange_weak(
prev_max, new_max, std::memory_order_acq_rel,
std::memory_order_relaxed)) {
};
}
const std::vector<SequenceNumber> WritePreparedTxnDB::GetSnapshotListFromDB(
SequenceNumber max) {
InstrumentedMutex(db_impl_->mutex());
return db_impl_->snapshots().GetAll(nullptr, max);
}
void WritePreparedTxnDB::UpdateSnapshots(
const std::vector<SequenceNumber>& snapshots,
const SequenceNumber& version) {
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:start");
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:start");
#ifndef NDEBUG
size_t sync_i = 0;
#endif
WriteLock wl(&snapshots_mutex_);
snapshots_version_ = version;
// We update the list concurrently with the readers.
// Both new and old lists are sorted and the new list is subset of the
// previous list plus some new items. Thus if a snapshot repeats in
// both new and old lists, it will appear upper in the new list. So if
// we simply insert the new snapshots in order, if an overwritten item
// is still valid in the new list is either written to the same place in
// the array or it is written in a higher palce before it gets
// overwritten by another item. This guarantess a reader that reads the
// list bottom-up will eventaully see a snapshot that repeats in the
// update, either before it gets overwritten by the writer or
// afterwards.
size_t i = 0;
auto it = snapshots.begin();
for (; it != snapshots.end() && i < SNAPSHOT_CACHE_SIZE; it++, i++) {
snapshot_cache_[i].store(*it, std::memory_order_release);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:", ++sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:", sync_i);
}
#ifndef NDEBUG
// Release the remaining sync points since they are useless given that the
// reader would also use lock to access snapshots
for (++sync_i; sync_i <= 10; ++sync_i) {
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:", sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:", sync_i);
}
#endif
snapshots_.clear();
for (; it != snapshots.end(); it++) {
// Insert them to a vector that is less efficient to access
// concurrently
snapshots_.push_back(*it);
}
// Update the size at the end. Otherwise a parallel reader might read
// items that are not set yet.
snapshots_total_.store(snapshots.size(), std::memory_order_release);
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:end");
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:end");
}
void WritePreparedTxnDB::CheckAgainstSnapshots(const CommitEntry& evicted) {
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:start");
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:start");
#ifndef NDEBUG
size_t sync_i = 0;
#endif
// First check the snapshot cache that is efficient for concurrent access
auto cnt = snapshots_total_.load(std::memory_order_acquire);
// The list might get updated concurrently as we are reading from it. The
// reader should be able to read all the snapshots that are still valid
// after the update. Since the survived snapshots are written in a higher
// place before gets overwritten the reader that reads bottom-up will
// eventully see it.
const bool next_is_larger = true;
SequenceNumber snapshot_seq = kMaxSequenceNumber;
size_t ip1 = std::min(cnt, SNAPSHOT_CACHE_SIZE);
for (; 0 < ip1; ip1--) {
snapshot_seq = snapshot_cache_[ip1 - 1].load(std::memory_order_acquire);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:",
++sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:", sync_i);
if (!MaybeUpdateOldCommitMap(evicted.prep_seq, evicted.commit_seq,
snapshot_seq, !next_is_larger)) {
break;
}
}
#ifndef NDEBUG
// Release the remaining sync points before accquiring the lock
for (++sync_i; sync_i <= 10; ++sync_i) {
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:", sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:", sync_i);
}
#endif
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:end");
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:end");
if (UNLIKELY(SNAPSHOT_CACHE_SIZE < cnt && ip1 == SNAPSHOT_CACHE_SIZE &&
snapshot_seq < evicted.prep_seq)) {
// Then access the less efficient list of snapshots_
ReadLock rl(&snapshots_mutex_);
// Items could have moved from the snapshots_ to snapshot_cache_ before
// accquiring the lock. To make sure that we do not miss a valid snapshot,
// read snapshot_cache_ again while holding the lock.
for (size_t i = 0; i < SNAPSHOT_CACHE_SIZE; i++) {
snapshot_seq = snapshot_cache_[i].load(std::memory_order_acquire);
if (!MaybeUpdateOldCommitMap(evicted.prep_seq, evicted.commit_seq,
snapshot_seq, next_is_larger)) {
break;
}
}
for (auto snapshot_seq_2 : snapshots_) {
if (!MaybeUpdateOldCommitMap(evicted.prep_seq, evicted.commit_seq,
snapshot_seq_2, next_is_larger)) {
break;
}
}
}
}
bool WritePreparedTxnDB::MaybeUpdateOldCommitMap(
const uint64_t& prep_seq, const uint64_t& commit_seq,
const uint64_t& snapshot_seq, const bool next_is_larger = true) {
// If we do not store an entry in old_commit_map we assume it is committed in
// all snapshots. if commit_seq <= snapshot_seq, it is considered already in
// the snapshot so we need not to keep the entry around for this snapshot.
if (commit_seq <= snapshot_seq) {
// continue the search if the next snapshot could be smaller than commit_seq
return !next_is_larger;
}
// then snapshot_seq < commit_seq
if (prep_seq <= snapshot_seq) { // overlapping range
WriteLock wl(&old_commit_map_mutex_);
old_commit_map_empty_.store(false, std::memory_order_release);
old_commit_map_[prep_seq] = commit_seq;
// Storing once is enough. No need to check it for other snapshots.
return false;
}
// continue the search if the next snapshot could be larger than prep_seq
return next_is_larger;
}
WritePreparedTxnDB::~WritePreparedTxnDB() {
// At this point there could be running compaction/flush holding a
// SnapshotChecker, which holds a pointer back to WritePreparedTxnDB.
// Make sure those jobs finished before destructing WritePreparedTxnDB.
db_impl_->CancelAllBackgroundWork(true/*wait*/);
}
} // namespace rocksdb
#endif // ROCKSDB_LITE

334
utilities/transactions/pessimistic_transaction_db.h
View File

@ -162,339 +162,5 @@ class WriteCommittedTxnDB : public PessimisticTransactionDB {
Transaction* old_txn) override;
};
// A PessimisticTransactionDB that writes data to DB after prepare phase of 2PC.
// In this way some data in the DB might not be committed. The DB provides
// mechanisms to tell such data apart from committed data.
class WritePreparedTxnDB : public PessimisticTransactionDB {
public:
explicit WritePreparedTxnDB(
DB* db, const TransactionDBOptions& txn_db_options,
size_t snapshot_cache_bits = DEF_SNAPSHOT_CACHE_BITS,
size_t commit_cache_bits = DEF_COMMIT_CACHE_BITS)
: PessimisticTransactionDB(db, txn_db_options),
SNAPSHOT_CACHE_BITS(snapshot_cache_bits),
SNAPSHOT_CACHE_SIZE(static_cast<size_t>(1ull << SNAPSHOT_CACHE_BITS)),
COMMIT_CACHE_BITS(commit_cache_bits),
COMMIT_CACHE_SIZE(static_cast<size_t>(1ull << COMMIT_CACHE_BITS)),
FORMAT(COMMIT_CACHE_BITS) {
Init(txn_db_options);
}
explicit WritePreparedTxnDB(
StackableDB* db, const TransactionDBOptions& txn_db_options,
size_t snapshot_cache_bits = DEF_SNAPSHOT_CACHE_BITS,
size_t commit_cache_bits = DEF_COMMIT_CACHE_BITS)
: PessimisticTransactionDB(db, txn_db_options),
SNAPSHOT_CACHE_BITS(snapshot_cache_bits),
SNAPSHOT_CACHE_SIZE(static_cast<size_t>(1ull << SNAPSHOT_CACHE_BITS)),
COMMIT_CACHE_BITS(commit_cache_bits),
COMMIT_CACHE_SIZE(static_cast<size_t>(1ull << COMMIT_CACHE_BITS)),
FORMAT(COMMIT_CACHE_BITS) {
Init(txn_db_options);
}
virtual ~WritePreparedTxnDB();
virtual Status Initialize(
const std::vector<size_t>& compaction_enabled_cf_indices,
const std::vector<ColumnFamilyHandle*>& handles) override;
Transaction* BeginTransaction(const WriteOptions& write_options,
const TransactionOptions& txn_options,
Transaction* old_txn) override;
using DB::Get;
virtual Status Get(const ReadOptions& options,
ColumnFamilyHandle* column_family, const Slice& key,
PinnableSlice* value) override;
using DB::NewIterator;
virtual Iterator* NewIterator(const ReadOptions& options,
ColumnFamilyHandle* column_family) override;
using DB::NewIterators;
virtual Status NewIterators(
const ReadOptions& options,
const std::vector<ColumnFamilyHandle*>& column_families,
std::vector<Iterator*>* iterators) override;
// Check whether the transaction that wrote the value with seqeunce number seq
// is visible to the snapshot with sequence number snapshot_seq
bool IsInSnapshot(uint64_t seq, uint64_t snapshot_seq) const;
// Add the trasnaction with prepare sequence seq to the prepared list
void AddPrepared(uint64_t seq);
// Rollback a prepared txn identified with prep_seq. rollback_seq is the seq
// with which the additional data is written to cancel the txn effect. It can
// be used to idenitfy the snapshots that overlap with the rolled back txn.
void RollbackPrepared(uint64_t prep_seq, uint64_t rollback_seq);
// Add the transaction with prepare sequence prepare_seq and commit sequence
// commit_seq to the commit map
void AddCommitted(uint64_t prepare_seq, uint64_t commit_seq);
struct CommitEntry {
uint64_t prep_seq;
uint64_t commit_seq;
CommitEntry() : prep_seq(0), commit_seq(0) {}
CommitEntry(uint64_t ps, uint64_t cs) : prep_seq(ps), commit_seq(cs) {}
bool operator==(const CommitEntry& rhs) const {
return prep_seq == rhs.prep_seq && commit_seq == rhs.commit_seq;
}
};
struct CommitEntry64bFormat {
explicit CommitEntry64bFormat(size_t index_bits)
: INDEX_BITS(index_bits),
PREP_BITS(static_cast<size_t>(64 - PAD_BITS - INDEX_BITS)),
COMMIT_BITS(static_cast<size_t>(64 - PREP_BITS)),
COMMIT_FILTER(static_cast<uint64_t>((1ull << COMMIT_BITS) - 1)) {}
// Number of higher bits of a sequence number that is not used. They are
// used to encode the value type, ...
const size_t PAD_BITS = static_cast<size_t>(8);
// Number of lower bits from prepare seq that can be skipped as they are
// implied by the index of the entry in the array
const size_t INDEX_BITS;
// Number of bits we use to encode the prepare seq
const size_t PREP_BITS;
// Number of bits we use to encode the commit seq.
const size_t COMMIT_BITS;
// Filter to encode/decode commit seq
const uint64_t COMMIT_FILTER;
};
// Prepare Seq (64 bits) = PAD ... PAD PREP PREP ... PREP INDEX INDEX ...
// INDEX Detal Seq (64 bits) = 0 0 0 0 0 0 0 0 0 0 0 0 DELTA DELTA ...
// DELTA DELTA Encoded Value = PREP PREP .... PREP PREP DELTA DELTA
// ... DELTA DELTA PAD: first bits of a seq that is reserved for tagging and
// hence ignored PREP/INDEX: the used bits in a prepare seq number INDEX: the
// bits that do not have to be encoded (will be provided externally) DELTA:
// prep seq - commit seq + 1 Number of DELTA bits should be equal to number of
// index bits + PADs
struct CommitEntry64b {
constexpr CommitEntry64b() noexcept : rep_(0) {}
CommitEntry64b(const CommitEntry& entry, const CommitEntry64bFormat& format)
: CommitEntry64b(entry.prep_seq, entry.commit_seq, format) {}
CommitEntry64b(const uint64_t ps, const uint64_t cs,
const CommitEntry64bFormat& format) {
assert(ps < static_cast<uint64_t>(
(1ull << (format.PREP_BITS + format.INDEX_BITS))));
assert(ps <= cs);
uint64_t delta = cs - ps + 1; // make initialized delta always >= 1
// zero is reserved for uninitialized entries
assert(0 < delta);
assert(delta < static_cast<uint64_t>((1ull << format.COMMIT_BITS)));
rep_ = (ps << format.PAD_BITS) & ~format.COMMIT_FILTER;
rep_ = rep_ | delta;
}
// Return false if the entry is empty
bool Parse(const uint64_t indexed_seq, CommitEntry* entry,
const CommitEntry64bFormat& format) {
uint64_t delta = rep_ & format.COMMIT_FILTER;
// zero is reserved for uninitialized entries
assert(delta < static_cast<uint64_t>((1ull << format.COMMIT_BITS)));
if (delta == 0) {
return false; // initialized entry would have non-zero delta
}
assert(indexed_seq < static_cast<uint64_t>((1ull << format.INDEX_BITS)));
uint64_t prep_up = rep_ & ~format.COMMIT_FILTER;
prep_up >>= format.PAD_BITS;
const uint64_t& prep_low = indexed_seq;
entry->prep_seq = prep_up | prep_low;
entry->commit_seq = entry->prep_seq + delta - 1;
return true;
}
private:
uint64_t rep_;
};
// Struct to hold ownership of snapshot and read callback for cleanup.
struct IteratorState;
private:
friend class WritePreparedTransactionTest_IsInSnapshotTest_Test;
friend class WritePreparedTransactionTest_CheckAgainstSnapshotsTest_Test;
friend class WritePreparedTransactionTest_CommitMapTest_Test;
friend class WritePreparedTransactionTest_SnapshotConcurrentAccessTest_Test;
friend class WritePreparedTransactionTest;
friend class PreparedHeap_BasicsTest_Test;
friend class WritePreparedTxnDBMock;
friend class WritePreparedTransactionTest_AdvanceMaxEvictedSeqBasicTest_Test;
friend class WritePreparedTransactionTest_BasicRecoveryTest_Test;
friend class WritePreparedTransactionTest_IsInSnapshotEmptyMapTest_Test;
friend class WritePreparedTransactionTest_RollbackTest_Test;
void Init(const TransactionDBOptions& /* unused */);
// A heap with the amortized O(1) complexity for erase. It uses one extra heap
// to keep track of erased entries that are not yet on top of the main heap.
class PreparedHeap {
std::priority_queue<uint64_t, std::vector<uint64_t>, std::greater<uint64_t>>
heap_;
std::priority_queue<uint64_t, std::vector<uint64_t>, std::greater<uint64_t>>
erased_heap_;
public:
bool empty() { return heap_.empty(); }
uint64_t top() { return heap_.top(); }
void push(uint64_t v) { heap_.push(v); }
void pop() {
heap_.pop();
while (!heap_.empty() && !erased_heap_.empty() &&
heap_.top() == erased_heap_.top()) {
heap_.pop();
erased_heap_.pop();
}
while (heap_.empty() && !erased_heap_.empty()) {
erased_heap_.pop();
}
}
void erase(uint64_t seq) {
if (!heap_.empty()) {
if (seq < heap_.top()) {
// Already popped, ignore it.
} else if (heap_.top() == seq) {
pop();
} else { // (heap_.top() > seq)
// Down the heap, remember to pop it later
erased_heap_.push(seq);
}
}
}
};
// Get the commit entry with index indexed_seq from the commit table. It
// returns true if such entry exists.
bool GetCommitEntry(const uint64_t indexed_seq, CommitEntry64b* entry_64b,
CommitEntry* entry) const;
// Rewrite the entry with the index indexed_seq in the commit table with the
// commit entry <prep_seq, commit_seq>. If the rewrite results into eviction,
// sets the evicted_entry and returns true.
bool AddCommitEntry(const uint64_t indexed_seq, const CommitEntry& new_entry,
CommitEntry* evicted_entry);
// Rewrite the entry with the index indexed_seq in the commit table with the
// commit entry new_entry only if the existing entry matches the
// expected_entry. Returns false otherwise.
bool ExchangeCommitEntry(const uint64_t indexed_seq,
CommitEntry64b& expected_entry,
const CommitEntry& new_entry);
// Increase max_evicted_seq_ from the previous value prev_max to the new
// value. This also involves taking care of prepared txns that are not
// committed before new_max, as well as updating the list of live snapshots at
// the time of updating the max. Thread-safety: this function can be called
// concurrently. The concurrent invocations of this function is equivalent to
// a serial invocation in which the last invocation is the one with the
// largetst new_max value.
void AdvanceMaxEvictedSeq(SequenceNumber& prev_max, SequenceNumber& new_max);
virtual const std::vector<SequenceNumber> GetSnapshotListFromDB(
SequenceNumber max);
// Update the list of snapshots corresponding to the soon-to-be-updated
// max_eviceted_seq_. Thread-safety: this function can be called concurrently.
// The concurrent invocations of this function is equivalent to a serial
// invocation in which the last invocation is the one with the largetst
// version value.
void UpdateSnapshots(const std::vector<SequenceNumber>& snapshots,
const SequenceNumber& version);
// Check an evicted entry against live snapshots to see if it should be kept
// around or it can be safely discarded (and hence assume committed for all
// snapshots). Thread-safety: this function can be called concurrently. If it
// is called concurrently with multiple UpdateSnapshots, the result is the
// same as checking the intersection of the snapshot list before updates with
// the snapshot list of all the concurrent updates.
void CheckAgainstSnapshots(const CommitEntry& evicted);
// Add a new entry to old_commit_map_ if prep_seq <= snapshot_seq <
// commit_seq. Return false if checking the next snapshot(s) is not needed.
// This is the case if the entry already added to old_commit_map_ or none of
// the next snapshots could satisfy the condition. next_is_larger: the next
// snapshot will be a larger value
bool MaybeUpdateOldCommitMap(const uint64_t& prep_seq,
const uint64_t& commit_seq,
const uint64_t& snapshot_seq,
const bool next_is_larger);
// The list of live snapshots at the last time that max_evicted_seq_ advanced.
// The list stored into two data structures: in snapshot_cache_ that is
// efficient for concurrent reads, and in snapshots_ if the data does not fit
// into snapshot_cache_. The total number of snapshots in the two lists
std::atomic<size_t> snapshots_total_ = {};
// The list sorted in ascending order. Thread-safety for writes is provided
// with snapshots_mutex_ and concurrent reads are safe due to std::atomic for
// each entry. In x86_64 architecture such reads are compiled to simple read
// instructions. 128 entries
static const size_t DEF_SNAPSHOT_CACHE_BITS = static_cast<size_t>(7);
const size_t SNAPSHOT_CACHE_BITS;
const size_t SNAPSHOT_CACHE_SIZE;
unique_ptr<std::atomic<SequenceNumber>[]> snapshot_cache_;
// 2nd list for storing snapshots. The list sorted in ascending order.
// Thread-safety is provided with snapshots_mutex_.
std::vector<SequenceNumber> snapshots_;
// The version of the latest list of snapshots. This can be used to avoid
// rewrittiing a list that is concurrently updated with a more recent version.
SequenceNumber snapshots_version_ = 0;
// A heap of prepared transactions. Thread-safety is provided with
// prepared_mutex_.
PreparedHeap prepared_txns_;
// 10m entry, 80MB size
static const size_t DEF_COMMIT_CACHE_BITS = static_cast<size_t>(21);
const size_t COMMIT_CACHE_BITS;
const size_t COMMIT_CACHE_SIZE;
const CommitEntry64bFormat FORMAT;
// commit_cache_ must be initialized to zero to tell apart an empty index from
// a filled one. Thread-safety is provided with commit_cache_mutex_.
unique_ptr<std::atomic<CommitEntry64b>[]> commit_cache_;
// The largest evicted *commit* sequence number from the commit_cache_
std::atomic<uint64_t> max_evicted_seq_ = {};
// Advance max_evicted_seq_ by this value each time it needs an update. The
// larger the value, the less frequent advances we would have. We do not want
// it to be too large either as it would cause stalls by doing too much
// maintenance work under the lock.
size_t INC_STEP_FOR_MAX_EVICTED = 1;
// A map of the evicted entries from commit_cache_ that has to be kept around
// to service the old snapshots. This is expected to be empty normally.
// Thread-safety is provided with old_commit_map_mutex_.
std::map<uint64_t, uint64_t> old_commit_map_;
// A set of long-running prepared transactions that are not finished by the
// time max_evicted_seq_ advances their sequence number. This is expected to
// be empty normally. Thread-safety is provided with prepared_mutex_.
std::set<uint64_t> delayed_prepared_;
// Update when delayed_prepared_.empty() changes. Expected to be true
// normally.
std::atomic<bool> delayed_prepared_empty_ = {true};
// Update when old_commit_map_.empty() changes. Expected to be true normally.
std::atomic<bool> old_commit_map_empty_ = {true};
mutable port::RWMutex prepared_mutex_;
mutable port::RWMutex old_commit_map_mutex_;
mutable port::RWMutex commit_cache_mutex_;
mutable port::RWMutex snapshots_mutex_;
};
class WritePreparedTxnReadCallback : public ReadCallback {
public:
WritePreparedTxnReadCallback(WritePreparedTxnDB* db, SequenceNumber snapshot)
: db_(db), snapshot_(snapshot) {}
// Will be called to see if the seq number accepted; if not it moves on to the
// next seq number.
virtual bool IsCommitted(SequenceNumber seq) override {
return db_->IsInSnapshot(seq, snapshot_);
}
private:
WritePreparedTxnDB* db_;
SequenceNumber snapshot_;
};
} // namespace rocksdb
#endif // ROCKSDB_LITE

2
utilities/transactions/snapshot_checker.cc
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@ -9,7 +9,7 @@
#include <assert.h>
#endif // ROCKSDB_LITE
#include "utilities/transactions/pessimistic_transaction_db.h"
#include "utilities/transactions/write_prepared_txn_db.h"
namespace rocksdb {

1
utilities/transactions/write_prepared_transaction_test.cc
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@ -36,6 +36,7 @@
#include "utilities/merge_operators.h"
#include "utilities/merge_operators/string_append/stringappend.h"
#include "utilities/transactions/pessimistic_transaction_db.h"
#include "utilities/transactions/write_prepared_txn_db.h"
#include "port/port.h"

2
utilities/transactions/write_prepared_txn.cc
View File

@ -15,7 +15,7 @@
#include "rocksdb/status.h"
#include "rocksdb/utilities/transaction_db.h"
#include "utilities/transactions/pessimistic_transaction.h"
#include "utilities/transactions/pessimistic_transaction_db.h"
#include "utilities/transactions/write_prepared_txn_db.h"
namespace rocksdb {

515
utilities/transactions/write_prepared_txn_db.cc Normal file
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@ -0,0 +1,515 @@
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#ifndef ROCKSDB_LITE
#ifndef __STDC_FORMAT_MACROS
#define __STDC_FORMAT_MACROS
#endif
#include "utilities/transactions/write_prepared_txn_db.h"
#include <inttypes.h>
#include <string>
#include <unordered_set>
#include <vector>
#include "db/db_impl.h"
#include "rocksdb/db.h"
#include "rocksdb/options.h"
#include "rocksdb/utilities/transaction_db.h"
#include "util/mutexlock.h"
#include "util/sync_point.h"
#include "utilities/transactions/pessimistic_transaction.h"
#include "utilities/transactions/transaction_db_mutex_impl.h"
namespace rocksdb {
Status WritePreparedTxnDB::Initialize(
const std::vector<size_t>& compaction_enabled_cf_indices,
const std::vector<ColumnFamilyHandle*>& handles) {
auto dbimpl = reinterpret_cast<DBImpl*>(GetRootDB());
assert(dbimpl != nullptr);
auto rtxns = dbimpl->recovered_transactions();
for (auto rtxn : rtxns) {
AddPrepared(rtxn.second->seq_);
}
SequenceNumber prev_max = max_evicted_seq_;
SequenceNumber last_seq = db_impl_->GetLatestSequenceNumber();
AdvanceMaxEvictedSeq(prev_max, last_seq);
db_impl_->SetSnapshotChecker(new WritePreparedSnapshotChecker(this));
auto s = PessimisticTransactionDB::Initialize(compaction_enabled_cf_indices,
handles);
return s;
}
Transaction* WritePreparedTxnDB::BeginTransaction(
const WriteOptions& write_options, const TransactionOptions& txn_options,
Transaction* old_txn) {
if (old_txn != nullptr) {
ReinitializeTransaction(old_txn, write_options, txn_options);
return old_txn;
} else {
return new WritePreparedTxn(this, write_options, txn_options);
}
}
Status WritePreparedTxnDB::Get(const ReadOptions& options,
ColumnFamilyHandle* column_family,
const Slice& key, PinnableSlice* value) {
// We are fine with the latest committed value. This could be done by
// specifying the snapshot as kMaxSequenceNumber.
SequenceNumber seq = kMaxSequenceNumber;
if (options.snapshot != nullptr) {
seq = options.snapshot->GetSequenceNumber();
}
WritePreparedTxnReadCallback callback(this, seq);
bool* dont_care = nullptr;
// Note: no need to specify a snapshot for read options as no specific
// snapshot is requested by the user.
return db_impl_->GetImpl(options, column_family, key, value, dont_care,
&callback);
}
// Struct to hold ownership of snapshot and read callback for iterator cleanup.
struct WritePreparedTxnDB::IteratorState {
IteratorState(WritePreparedTxnDB* txn_db, SequenceNumber sequence,
std::shared_ptr<ManagedSnapshot> s)
: callback(txn_db, sequence), snapshot(s) {}
WritePreparedTxnReadCallback callback;
std::shared_ptr<ManagedSnapshot> snapshot;
};
namespace {
static void CleanupWritePreparedTxnDBIterator(void* arg1, void* arg2) {
delete reinterpret_cast<WritePreparedTxnDB::IteratorState*>(arg1);
}
} // anonymous namespace
Iterator* WritePreparedTxnDB::NewIterator(const ReadOptions& options,
ColumnFamilyHandle* column_family) {
std::shared_ptr<ManagedSnapshot> own_snapshot = nullptr;
SequenceNumber snapshot_seq = kMaxSequenceNumber;
if (options.snapshot != nullptr) {
snapshot_seq = options.snapshot->GetSequenceNumber();
} else {
auto* snapshot = db_impl_->GetSnapshot();
snapshot_seq = snapshot->GetSequenceNumber();
own_snapshot = std::make_shared<ManagedSnapshot>(db_impl_, snapshot);
}
assert(snapshot_seq != kMaxSequenceNumber);
auto* cfd = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family)->cfd();
auto* state = new IteratorState(this, snapshot_seq, own_snapshot);
auto* db_iter =
db_impl_->NewIteratorImpl(options, cfd, snapshot_seq, &state->callback);
db_iter->RegisterCleanup(CleanupWritePreparedTxnDBIterator, state, nullptr);
return db_iter;
}
Status WritePreparedTxnDB::NewIterators(
const ReadOptions& options,
const std::vector<ColumnFamilyHandle*>& column_families,
std::vector<Iterator*>* iterators) {
std::shared_ptr<ManagedSnapshot> own_snapshot = nullptr;
SequenceNumber snapshot_seq = kMaxSequenceNumber;
if (options.snapshot != nullptr) {
snapshot_seq = options.snapshot->GetSequenceNumber();
} else {
auto* snapshot = db_impl_->GetSnapshot();
snapshot_seq = snapshot->GetSequenceNumber();
own_snapshot = std::make_shared<ManagedSnapshot>(db_impl_, snapshot);
}
iterators->clear();
iterators->reserve(column_families.size());
for (auto* column_family : column_families) {
auto* cfd = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family)->cfd();
auto* state = new IteratorState(this, snapshot_seq, own_snapshot);
auto* db_iter =
db_impl_->NewIteratorImpl(options, cfd, snapshot_seq, &state->callback);
db_iter->RegisterCleanup(CleanupWritePreparedTxnDBIterator, state, nullptr);
iterators->push_back(db_iter);
}
return Status::OK();
}
void WritePreparedTxnDB::Init(const TransactionDBOptions& /* unused */) {
// Adcance max_evicted_seq_ no more than 100 times before the cache wraps
// around.
INC_STEP_FOR_MAX_EVICTED =
std::max(SNAPSHOT_CACHE_SIZE / 100, static_cast<size_t>(1));
snapshot_cache_ = unique_ptr<std::atomic<SequenceNumber>[]>(
new std::atomic<SequenceNumber>[SNAPSHOT_CACHE_SIZE] {});
commit_cache_ = unique_ptr<std::atomic<CommitEntry64b>[]>(
new std::atomic<CommitEntry64b>[COMMIT_CACHE_SIZE] {});
}
// Returns true if commit_seq <= snapshot_seq
bool WritePreparedTxnDB::IsInSnapshot(uint64_t prep_seq,
uint64_t snapshot_seq) const {
// Here we try to infer the return value without looking into prepare list.
// This would help avoiding synchronization over a shared map.
// TODO(myabandeh): read your own writes
// TODO(myabandeh): optimize this. This sequence of checks must be correct but
// not necessary efficient
if (prep_seq == 0) {
// Compaction will output keys to bottom-level with sequence number 0 if
// it is visible to the earliest snapshot.
return true;
}
if (snapshot_seq < prep_seq) {
// snapshot_seq < prep_seq <= commit_seq => snapshot_seq < commit_seq
return false;
}
if (!delayed_prepared_empty_.load(std::memory_order_acquire)) {
// We should not normally reach here
ReadLock rl(&prepared_mutex_);
if (delayed_prepared_.find(prep_seq) != delayed_prepared_.end()) {
// Then it is not committed yet
return false;
}
}
auto indexed_seq = prep_seq % COMMIT_CACHE_SIZE;
CommitEntry64b dont_care;
CommitEntry cached;
bool exist = GetCommitEntry(indexed_seq, &dont_care, &cached);
if (exist && prep_seq == cached.prep_seq) {
// It is committed and also not evicted from commit cache
return cached.commit_seq <= snapshot_seq;
}
// else it could be committed but not inserted in the map which could happen
// after recovery, or it could be committed and evicted by another commit, or
// never committed.
// At this point we dont know if it was committed or it is still prepared
auto max_evicted_seq = max_evicted_seq_.load(std::memory_order_acquire);
if (max_evicted_seq < prep_seq) {
// Not evicted from cache and also not present, so must be still prepared
return false;
}
// When advancing max_evicted_seq_, we move older entires from prepared to
// delayed_prepared_. Also we move evicted entries from commit cache to
// old_commit_map_ if it overlaps with any snapshot. Since prep_seq <=
// max_evicted_seq_, we have three cases: i) in delayed_prepared_, ii) in
// old_commit_map_, iii) committed with no conflict with any snapshot (i)
// delayed_prepared_ is checked above
if (max_evicted_seq < snapshot_seq) { // then (ii) cannot be the case
// only (iii) is the case: committed
// commit_seq <= max_evicted_seq_ < snapshot_seq => commit_seq <
// snapshot_seq
return true;
}
// else (ii) might be the case: check the commit data saved for this snapshot.
// If there was no overlapping commit entry, then it is committed with a
// commit_seq lower than any live snapshot, including snapshot_seq.
if (old_commit_map_empty_.load(std::memory_order_acquire)) {
return true;
}
{
// We should not normally reach here
ReadLock rl(&old_commit_map_mutex_);
auto old_commit_entry = old_commit_map_.find(prep_seq);
if (old_commit_entry == old_commit_map_.end() ||
old_commit_entry->second <= snapshot_seq) {
return true;
}
}
// (ii) it the case: it is committed but after the snapshot_seq
return false;
}
void WritePreparedTxnDB::AddPrepared(uint64_t seq) {
ROCKS_LOG_DEBUG(info_log_, "Txn %" PRIu64 " Prepareing", seq);
// TODO(myabandeh): Add a runtime check to ensure the following assert.
assert(seq > max_evicted_seq_);
WriteLock wl(&prepared_mutex_);
prepared_txns_.push(seq);
}
void WritePreparedTxnDB::RollbackPrepared(uint64_t prep_seq,
uint64_t rollback_seq) {
ROCKS_LOG_DEBUG(
info_log_, "Txn %" PRIu64 " rolling back with rollback seq of " PRIu64 "",
prep_seq, rollback_seq);
std::vector<SequenceNumber> snapshots =
GetSnapshotListFromDB(kMaxSequenceNumber);
// TODO(myabandeh): currently we are assuming that there is no snapshot taken
// when a transaciton is rolled back. This is the case the way MySQL does
// rollback which is after recovery. We should extend it to be able to
// rollback txns that overlap with exsiting snapshots.
assert(snapshots.size() == 0);
if (snapshots.size()) {
throw std::runtime_error(
"Rollback reqeust while there are live snapshots.");
}
WriteLock wl(&prepared_mutex_);
prepared_txns_.erase(prep_seq);
bool was_empty = delayed_prepared_.empty();
if (!was_empty) {
delayed_prepared_.erase(prep_seq);
bool is_empty = delayed_prepared_.empty();
if (was_empty != is_empty) {
delayed_prepared_empty_.store(is_empty, std::memory_order_release);
}
}
}
void WritePreparedTxnDB::AddCommitted(uint64_t prepare_seq,
uint64_t commit_seq) {
ROCKS_LOG_DEBUG(info_log_, "Txn %" PRIu64 " Committing with %" PRIu64,
prepare_seq, commit_seq);
auto indexed_seq = prepare_seq % COMMIT_CACHE_SIZE;
CommitEntry64b evicted_64b;
CommitEntry evicted;
bool to_be_evicted = GetCommitEntry(indexed_seq, &evicted_64b, &evicted);
if (to_be_evicted) {
auto prev_max = max_evicted_seq_.load(std::memory_order_acquire);
if (prev_max < evicted.commit_seq) {
// Inc max in larger steps to avoid frequent updates
auto max_evicted_seq = evicted.commit_seq + INC_STEP_FOR_MAX_EVICTED;
AdvanceMaxEvictedSeq(prev_max, max_evicted_seq);
}
// After each eviction from commit cache, check if the commit entry should
// be kept around because it overlaps with a live snapshot.
CheckAgainstSnapshots(evicted);
}
bool succ =
ExchangeCommitEntry(indexed_seq, evicted_64b, {prepare_seq, commit_seq});
if (!succ) {
// A very rare event, in which the commit entry is updated before we do.
// Here we apply a very simple solution of retrying.
// TODO(myabandeh): do precautions to detect bugs that cause infinite loops
AddCommitted(prepare_seq, commit_seq);
return;
}
{
WriteLock wl(&prepared_mutex_);
prepared_txns_.erase(prepare_seq);
bool was_empty = delayed_prepared_.empty();
if (!was_empty) {
delayed_prepared_.erase(prepare_seq);
bool is_empty = delayed_prepared_.empty();
if (was_empty != is_empty) {
delayed_prepared_empty_.store(is_empty, std::memory_order_release);
}
}
}
}
bool WritePreparedTxnDB::GetCommitEntry(const uint64_t indexed_seq,
CommitEntry64b* entry_64b,
CommitEntry* entry) const {
*entry_64b = commit_cache_[indexed_seq].load(std::memory_order_acquire);
bool valid = entry_64b->Parse(indexed_seq, entry, FORMAT);
return valid;
}
bool WritePreparedTxnDB::AddCommitEntry(const uint64_t indexed_seq,
const CommitEntry& new_entry,
CommitEntry* evicted_entry) {
CommitEntry64b new_entry_64b(new_entry, FORMAT);
CommitEntry64b evicted_entry_64b = commit_cache_[indexed_seq].exchange(
new_entry_64b, std::memory_order_acq_rel);
bool valid = evicted_entry_64b.Parse(indexed_seq, evicted_entry, FORMAT);
return valid;
}
bool WritePreparedTxnDB::ExchangeCommitEntry(const uint64_t indexed_seq,
CommitEntry64b& expected_entry_64b,
const CommitEntry& new_entry) {
auto& atomic_entry = commit_cache_[indexed_seq];
CommitEntry64b new_entry_64b(new_entry, FORMAT);
bool succ = atomic_entry.compare_exchange_strong(
expected_entry_64b, new_entry_64b, std::memory_order_acq_rel,
std::memory_order_acquire);
return succ;
}
void WritePreparedTxnDB::AdvanceMaxEvictedSeq(SequenceNumber& prev_max,
SequenceNumber& new_max) {
// When max_evicted_seq_ advances, move older entries from prepared_txns_
// to delayed_prepared_. This guarantees that if a seq is lower than max,
// then it is not in prepared_txns_ ans save an expensive, synchronized
// lookup from a shared set. delayed_prepared_ is expected to be empty in
// normal cases.
{
WriteLock wl(&prepared_mutex_);
while (!prepared_txns_.empty() && prepared_txns_.top() <= new_max) {
auto to_be_popped = prepared_txns_.top();
delayed_prepared_.insert(to_be_popped);
prepared_txns_.pop();
delayed_prepared_empty_.store(false, std::memory_order_release);
}
}
// With each change to max_evicted_seq_ fetch the live snapshots behind it.
// We use max as the version of snapshots to identify how fresh are the
// snapshot list. This works because the snapshots are between 0 and
// max, so the larger the max, the more complete they are.
SequenceNumber new_snapshots_version = new_max;
std::vector<SequenceNumber> snapshots;
bool update_snapshots = false;
if (new_snapshots_version > snapshots_version_) {
// This is to avoid updating the snapshots_ if it already updated
// with a more recent vesion by a concrrent thread
update_snapshots = true;
// We only care about snapshots lower then max
snapshots = GetSnapshotListFromDB(new_max);
}
if (update_snapshots) {
UpdateSnapshots(snapshots, new_snapshots_version);
}
while (prev_max < new_max && !max_evicted_seq_.compare_exchange_weak(
prev_max, new_max, std::memory_order_acq_rel,
std::memory_order_relaxed)) {
};
}
const std::vector<SequenceNumber> WritePreparedTxnDB::GetSnapshotListFromDB(
SequenceNumber max) {
InstrumentedMutex(db_impl_->mutex());
return db_impl_->snapshots().GetAll(nullptr, max);
}
void WritePreparedTxnDB::UpdateSnapshots(
const std::vector<SequenceNumber>& snapshots,
const SequenceNumber& version) {
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:start");
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:start");
#ifndef NDEBUG
size_t sync_i = 0;
#endif
WriteLock wl(&snapshots_mutex_);
snapshots_version_ = version;
// We update the list concurrently with the readers.
// Both new and old lists are sorted and the new list is subset of the
// previous list plus some new items. Thus if a snapshot repeats in
// both new and old lists, it will appear upper in the new list. So if
// we simply insert the new snapshots in order, if an overwritten item
// is still valid in the new list is either written to the same place in
// the array or it is written in a higher palce before it gets
// overwritten by another item. This guarantess a reader that reads the
// list bottom-up will eventaully see a snapshot that repeats in the
// update, either before it gets overwritten by the writer or
// afterwards.
size_t i = 0;
auto it = snapshots.begin();
for (; it != snapshots.end() && i < SNAPSHOT_CACHE_SIZE; it++, i++) {
snapshot_cache_[i].store(*it, std::memory_order_release);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:", ++sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:", sync_i);
}
#ifndef NDEBUG
// Release the remaining sync points since they are useless given that the
// reader would also use lock to access snapshots
for (++sync_i; sync_i <= 10; ++sync_i) {
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:", sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:", sync_i);
}
#endif
snapshots_.clear();
for (; it != snapshots.end(); it++) {
// Insert them to a vector that is less efficient to access
// concurrently
snapshots_.push_back(*it);
}
// Update the size at the end. Otherwise a parallel reader might read
// items that are not set yet.
snapshots_total_.store(snapshots.size(), std::memory_order_release);
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:end");
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:end");
}
void WritePreparedTxnDB::CheckAgainstSnapshots(const CommitEntry& evicted) {
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:start");
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:start");
#ifndef NDEBUG
size_t sync_i = 0;
#endif
// First check the snapshot cache that is efficient for concurrent access
auto cnt = snapshots_total_.load(std::memory_order_acquire);
// The list might get updated concurrently as we are reading from it. The
// reader should be able to read all the snapshots that are still valid
// after the update. Since the survived snapshots are written in a higher
// place before gets overwritten the reader that reads bottom-up will
// eventully see it.
const bool next_is_larger = true;
SequenceNumber snapshot_seq = kMaxSequenceNumber;
size_t ip1 = std::min(cnt, SNAPSHOT_CACHE_SIZE);
for (; 0 < ip1; ip1--) {
snapshot_seq = snapshot_cache_[ip1 - 1].load(std::memory_order_acquire);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:",
++sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:", sync_i);
if (!MaybeUpdateOldCommitMap(evicted.prep_seq, evicted.commit_seq,
snapshot_seq, !next_is_larger)) {
break;
}
}
#ifndef NDEBUG
// Release the remaining sync points before accquiring the lock
for (++sync_i; sync_i <= 10; ++sync_i) {
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:", sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:", sync_i);
}
#endif
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:end");
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:end");
if (UNLIKELY(SNAPSHOT_CACHE_SIZE < cnt && ip1 == SNAPSHOT_CACHE_SIZE &&
snapshot_seq < evicted.prep_seq)) {
// Then access the less efficient list of snapshots_
ReadLock rl(&snapshots_mutex_);
// Items could have moved from the snapshots_ to snapshot_cache_ before
// accquiring the lock. To make sure that we do not miss a valid snapshot,
// read snapshot_cache_ again while holding the lock.
for (size_t i = 0; i < SNAPSHOT_CACHE_SIZE; i++) {
snapshot_seq = snapshot_cache_[i].load(std::memory_order_acquire);
if (!MaybeUpdateOldCommitMap(evicted.prep_seq, evicted.commit_seq,
snapshot_seq, next_is_larger)) {
break;
}
}
for (auto snapshot_seq_2 : snapshots_) {
if (!MaybeUpdateOldCommitMap(evicted.prep_seq, evicted.commit_seq,
snapshot_seq_2, next_is_larger)) {
break;
}
}
}
}
bool WritePreparedTxnDB::MaybeUpdateOldCommitMap(
const uint64_t& prep_seq, const uint64_t& commit_seq,
const uint64_t& snapshot_seq, const bool next_is_larger = true) {
// If we do not store an entry in old_commit_map we assume it is committed in
// all snapshots. if commit_seq <= snapshot_seq, it is considered already in
// the snapshot so we need not to keep the entry around for this snapshot.
if (commit_seq <= snapshot_seq) {
// continue the search if the next snapshot could be smaller than commit_seq
return !next_is_larger;
}
// then snapshot_seq < commit_seq
if (prep_seq <= snapshot_seq) { // overlapping range
WriteLock wl(&old_commit_map_mutex_);
old_commit_map_empty_.store(false, std::memory_order_release);
old_commit_map_[prep_seq] = commit_seq;
// Storing once is enough. No need to check it for other snapshots.
return false;
}
// continue the search if the next snapshot could be larger than prep_seq
return next_is_larger;
}
WritePreparedTxnDB::~WritePreparedTxnDB() {
// At this point there could be running compaction/flush holding a
// SnapshotChecker, which holds a pointer back to WritePreparedTxnDB.
// Make sure those jobs finished before destructing WritePreparedTxnDB.
db_impl_->CancelAllBackgroundWork(true /*wait*/);
}
} // namespace rocksdb
#endif // ROCKSDB_LITE

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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#pragma once
#ifndef ROCKSDB_LITE
#include <mutex>
#include <queue>
#include <set>
#include <string>
#include <unordered_map>
#include <vector>
#include "db/db_iter.h"
#include "db/read_callback.h"
#include "db/snapshot_checker.h"
#include "rocksdb/db.h"
#include "rocksdb/options.h"
#include "rocksdb/utilities/transaction_db.h"
#include "utilities/transactions/pessimistic_transaction.h"
#include "utilities/transactions/pessimistic_transaction_db.h"
#include "utilities/transactions/transaction_lock_mgr.h"
#include "utilities/transactions/write_prepared_txn.h"
namespace rocksdb {
// A PessimisticTransactionDB that writes data to DB after prepare phase of 2PC.
// In this way some data in the DB might not be committed. The DB provides
// mechanisms to tell such data apart from committed data.
class WritePreparedTxnDB : public PessimisticTransactionDB {
public:
explicit WritePreparedTxnDB(
DB* db, const TransactionDBOptions& txn_db_options,
size_t snapshot_cache_bits = DEF_SNAPSHOT_CACHE_BITS,
size_t commit_cache_bits = DEF_COMMIT_CACHE_BITS)
: PessimisticTransactionDB(db, txn_db_options),
SNAPSHOT_CACHE_BITS(snapshot_cache_bits),
SNAPSHOT_CACHE_SIZE(static_cast<size_t>(1ull << SNAPSHOT_CACHE_BITS)),
COMMIT_CACHE_BITS(commit_cache_bits),
COMMIT_CACHE_SIZE(static_cast<size_t>(1ull << COMMIT_CACHE_BITS)),
FORMAT(COMMIT_CACHE_BITS) {
Init(txn_db_options);
}
explicit WritePreparedTxnDB(
StackableDB* db, const TransactionDBOptions& txn_db_options,
size_t snapshot_cache_bits = DEF_SNAPSHOT_CACHE_BITS,
size_t commit_cache_bits = DEF_COMMIT_CACHE_BITS)
: PessimisticTransactionDB(db, txn_db_options),
SNAPSHOT_CACHE_BITS(snapshot_cache_bits),
SNAPSHOT_CACHE_SIZE(static_cast<size_t>(1ull << SNAPSHOT_CACHE_BITS)),
COMMIT_CACHE_BITS(commit_cache_bits),
COMMIT_CACHE_SIZE(static_cast<size_t>(1ull << COMMIT_CACHE_BITS)),
FORMAT(COMMIT_CACHE_BITS) {
Init(txn_db_options);
}
virtual ~WritePreparedTxnDB();
virtual Status Initialize(
const std::vector<size_t>& compaction_enabled_cf_indices,
const std::vector<ColumnFamilyHandle*>& handles) override;
Transaction* BeginTransaction(const WriteOptions& write_options,
const TransactionOptions& txn_options,
Transaction* old_txn) override;
using DB::Get;
virtual Status Get(const ReadOptions& options,
ColumnFamilyHandle* column_family, const Slice& key,
PinnableSlice* value) override;
using DB::NewIterator;
virtual Iterator* NewIterator(const ReadOptions& options,
ColumnFamilyHandle* column_family) override;
using DB::NewIterators;
virtual Status NewIterators(
const ReadOptions& options,
const std::vector<ColumnFamilyHandle*>& column_families,
std::vector<Iterator*>* iterators) override;
// Check whether the transaction that wrote the value with seqeunce number seq
// is visible to the snapshot with sequence number snapshot_seq
bool IsInSnapshot(uint64_t seq, uint64_t snapshot_seq) const;
// Add the trasnaction with prepare sequence seq to the prepared list
void AddPrepared(uint64_t seq);
// Rollback a prepared txn identified with prep_seq. rollback_seq is the seq
// with which the additional data is written to cancel the txn effect. It can
// be used to idenitfy the snapshots that overlap with the rolled back txn.
void RollbackPrepared(uint64_t prep_seq, uint64_t rollback_seq);
// Add the transaction with prepare sequence prepare_seq and commit sequence
// commit_seq to the commit map
void AddCommitted(uint64_t prepare_seq, uint64_t commit_seq);
struct CommitEntry {
uint64_t prep_seq;
uint64_t commit_seq;
CommitEntry() : prep_seq(0), commit_seq(0) {}
CommitEntry(uint64_t ps, uint64_t cs) : prep_seq(ps), commit_seq(cs) {}
bool operator==(const CommitEntry& rhs) const {
return prep_seq == rhs.prep_seq && commit_seq == rhs.commit_seq;
}
};
struct CommitEntry64bFormat {
explicit CommitEntry64bFormat(size_t index_bits)
: INDEX_BITS(index_bits),
PREP_BITS(static_cast<size_t>(64 - PAD_BITS - INDEX_BITS)),
COMMIT_BITS(static_cast<size_t>(64 - PREP_BITS)),
COMMIT_FILTER(static_cast<uint64_t>((1ull << COMMIT_BITS) - 1)) {}
// Number of higher bits of a sequence number that is not used. They are
// used to encode the value type, ...
const size_t PAD_BITS = static_cast<size_t>(8);
// Number of lower bits from prepare seq that can be skipped as they are
// implied by the index of the entry in the array
const size_t INDEX_BITS;
// Number of bits we use to encode the prepare seq
const size_t PREP_BITS;
// Number of bits we use to encode the commit seq.
const size_t COMMIT_BITS;
// Filter to encode/decode commit seq
const uint64_t COMMIT_FILTER;
};
// Prepare Seq (64 bits) = PAD ... PAD PREP PREP ... PREP INDEX INDEX ...
// INDEX Detal Seq (64 bits) = 0 0 0 0 0 0 0 0 0 0 0 0 DELTA DELTA ...
// DELTA DELTA Encoded Value = PREP PREP .... PREP PREP DELTA DELTA
// ... DELTA DELTA PAD: first bits of a seq that is reserved for tagging and
// hence ignored PREP/INDEX: the used bits in a prepare seq number INDEX: the
// bits that do not have to be encoded (will be provided externally) DELTA:
// prep seq - commit seq + 1 Number of DELTA bits should be equal to number of
// index bits + PADs
struct CommitEntry64b {
constexpr CommitEntry64b() noexcept : rep_(0) {}
CommitEntry64b(const CommitEntry& entry, const CommitEntry64bFormat& format)
: CommitEntry64b(entry.prep_seq, entry.commit_seq, format) {}
CommitEntry64b(const uint64_t ps, const uint64_t cs,
const CommitEntry64bFormat& format) {
assert(ps < static_cast<uint64_t>(
(1ull << (format.PREP_BITS + format.INDEX_BITS))));
assert(ps <= cs);
uint64_t delta = cs - ps + 1; // make initialized delta always >= 1
// zero is reserved for uninitialized entries
assert(0 < delta);
assert(delta < static_cast<uint64_t>((1ull << format.COMMIT_BITS)));
rep_ = (ps << format.PAD_BITS) & ~format.COMMIT_FILTER;
rep_ = rep_ | delta;
}
// Return false if the entry is empty
bool Parse(const uint64_t indexed_seq, CommitEntry* entry,
const CommitEntry64bFormat& format) {
uint64_t delta = rep_ & format.COMMIT_FILTER;
// zero is reserved for uninitialized entries
assert(delta < static_cast<uint64_t>((1ull << format.COMMIT_BITS)));
if (delta == 0) {
return false; // initialized entry would have non-zero delta
}
assert(indexed_seq < static_cast<uint64_t>((1ull << format.INDEX_BITS)));
uint64_t prep_up = rep_ & ~format.COMMIT_FILTER;
prep_up >>= format.PAD_BITS;
const uint64_t& prep_low = indexed_seq;
entry->prep_seq = prep_up | prep_low;
entry->commit_seq = entry->prep_seq + delta - 1;
return true;
}
private:
uint64_t rep_;
};
// Struct to hold ownership of snapshot and read callback for cleanup.
struct IteratorState;
private:
friend class WritePreparedTransactionTest_IsInSnapshotTest_Test;
friend class WritePreparedTransactionTest_CheckAgainstSnapshotsTest_Test;
friend class WritePreparedTransactionTest_CommitMapTest_Test;
friend class WritePreparedTransactionTest_SnapshotConcurrentAccessTest_Test;
friend class WritePreparedTransactionTest;
friend class PreparedHeap_BasicsTest_Test;
friend class WritePreparedTxnDBMock;
friend class WritePreparedTransactionTest_AdvanceMaxEvictedSeqBasicTest_Test;
friend class WritePreparedTransactionTest_BasicRecoveryTest_Test;
friend class WritePreparedTransactionTest_IsInSnapshotEmptyMapTest_Test;
friend class WritePreparedTransactionTest_RollbackTest_Test;
void Init(const TransactionDBOptions& /* unused */);
// A heap with the amortized O(1) complexity for erase. It uses one extra heap
// to keep track of erased entries that are not yet on top of the main heap.
class PreparedHeap {
std::priority_queue<uint64_t, std::vector<uint64_t>, std::greater<uint64_t>>
heap_;
std::priority_queue<uint64_t, std::vector<uint64_t>, std::greater<uint64_t>>
erased_heap_;
public:
bool empty() { return heap_.empty(); }
uint64_t top() { return heap_.top(); }
void push(uint64_t v) { heap_.push(v); }
void pop() {
heap_.pop();
while (!heap_.empty() && !erased_heap_.empty() &&
heap_.top() == erased_heap_.top()) {
heap_.pop();
erased_heap_.pop();
}
while (heap_.empty() && !erased_heap_.empty()) {
erased_heap_.pop();
}
}
void erase(uint64_t seq) {
if (!heap_.empty()) {
if (seq < heap_.top()) {
// Already popped, ignore it.
} else if (heap_.top() == seq) {
pop();
} else { // (heap_.top() > seq)
// Down the heap, remember to pop it later
erased_heap_.push(seq);
}
}
}
};
// Get the commit entry with index indexed_seq from the commit table. It
// returns true if such entry exists.
bool GetCommitEntry(const uint64_t indexed_seq, CommitEntry64b* entry_64b,
CommitEntry* entry) const;
// Rewrite the entry with the index indexed_seq in the commit table with the
// commit entry <prep_seq, commit_seq>. If the rewrite results into eviction,
// sets the evicted_entry and returns true.
bool AddCommitEntry(const uint64_t indexed_seq, const CommitEntry& new_entry,
CommitEntry* evicted_entry);
// Rewrite the entry with the index indexed_seq in the commit table with the
// commit entry new_entry only if the existing entry matches the
// expected_entry. Returns false otherwise.
bool ExchangeCommitEntry(const uint64_t indexed_seq,
CommitEntry64b& expected_entry,
const CommitEntry& new_entry);
// Increase max_evicted_seq_ from the previous value prev_max to the new
// value. This also involves taking care of prepared txns that are not
// committed before new_max, as well as updating the list of live snapshots at
// the time of updating the max. Thread-safety: this function can be called
// concurrently. The concurrent invocations of this function is equivalent to
// a serial invocation in which the last invocation is the one with the
// largetst new_max value.
void AdvanceMaxEvictedSeq(SequenceNumber& prev_max, SequenceNumber& new_max);
virtual const std::vector<SequenceNumber> GetSnapshotListFromDB(
SequenceNumber max);
// Update the list of snapshots corresponding to the soon-to-be-updated
// max_eviceted_seq_. Thread-safety: this function can be called concurrently.
// The concurrent invocations of this function is equivalent to a serial
// invocation in which the last invocation is the one with the largetst
// version value.
void UpdateSnapshots(const std::vector<SequenceNumber>& snapshots,
const SequenceNumber& version);
// Check an evicted entry against live snapshots to see if it should be kept
// around or it can be safely discarded (and hence assume committed for all
// snapshots). Thread-safety: this function can be called concurrently. If it
// is called concurrently with multiple UpdateSnapshots, the result is the
// same as checking the intersection of the snapshot list before updates with
// the snapshot list of all the concurrent updates.
void CheckAgainstSnapshots(const CommitEntry& evicted);
// Add a new entry to old_commit_map_ if prep_seq <= snapshot_seq <
// commit_seq. Return false if checking the next snapshot(s) is not needed.
// This is the case if the entry already added to old_commit_map_ or none of
// the next snapshots could satisfy the condition. next_is_larger: the next
// snapshot will be a larger value
bool MaybeUpdateOldCommitMap(const uint64_t& prep_seq,
const uint64_t& commit_seq,
const uint64_t& snapshot_seq,
const bool next_is_larger);
// The list of live snapshots at the last time that max_evicted_seq_ advanced.
// The list stored into two data structures: in snapshot_cache_ that is
// efficient for concurrent reads, and in snapshots_ if the data does not fit
// into snapshot_cache_. The total number of snapshots in the two lists
std::atomic<size_t> snapshots_total_ = {};
// The list sorted in ascending order. Thread-safety for writes is provided
// with snapshots_mutex_ and concurrent reads are safe due to std::atomic for
// each entry. In x86_64 architecture such reads are compiled to simple read
// instructions. 128 entries
static const size_t DEF_SNAPSHOT_CACHE_BITS = static_cast<size_t>(7);
const size_t SNAPSHOT_CACHE_BITS;
const size_t SNAPSHOT_CACHE_SIZE;
unique_ptr<std::atomic<SequenceNumber>[]> snapshot_cache_;
// 2nd list for storing snapshots. The list sorted in ascending order.
// Thread-safety is provided with snapshots_mutex_.
std::vector<SequenceNumber> snapshots_;
// The version of the latest list of snapshots. This can be used to avoid
// rewrittiing a list that is concurrently updated with a more recent version.
SequenceNumber snapshots_version_ = 0;
// A heap of prepared transactions. Thread-safety is provided with
// prepared_mutex_.
PreparedHeap prepared_txns_;
// 10m entry, 80MB size
static const size_t DEF_COMMIT_CACHE_BITS = static_cast<size_t>(21);
const size_t COMMIT_CACHE_BITS;
const size_t COMMIT_CACHE_SIZE;
const CommitEntry64bFormat FORMAT;
// commit_cache_ must be initialized to zero to tell apart an empty index from
// a filled one. Thread-safety is provided with commit_cache_mutex_.
unique_ptr<std::atomic<CommitEntry64b>[]> commit_cache_;
// The largest evicted *commit* sequence number from the commit_cache_
std::atomic<uint64_t> max_evicted_seq_ = {};
// Advance max_evicted_seq_ by this value each time it needs an update. The
// larger the value, the less frequent advances we would have. We do not want
// it to be too large either as it would cause stalls by doing too much
// maintenance work under the lock.
size_t INC_STEP_FOR_MAX_EVICTED = 1;
// A map of the evicted entries from commit_cache_ that has to be kept around
// to service the old snapshots. This is expected to be empty normally.
// Thread-safety is provided with old_commit_map_mutex_.
std::map<uint64_t, uint64_t> old_commit_map_;
// A set of long-running prepared transactions that are not finished by the
// time max_evicted_seq_ advances their sequence number. This is expected to
// be empty normally. Thread-safety is provided with prepared_mutex_.
std::set<uint64_t> delayed_prepared_;
// Update when delayed_prepared_.empty() changes. Expected to be true
// normally.
std::atomic<bool> delayed_prepared_empty_ = {true};
// Update when old_commit_map_.empty() changes. Expected to be true normally.
std::atomic<bool> old_commit_map_empty_ = {true};
mutable port::RWMutex prepared_mutex_;
mutable port::RWMutex old_commit_map_mutex_;
mutable port::RWMutex commit_cache_mutex_;
mutable port::RWMutex snapshots_mutex_;
};
class WritePreparedTxnReadCallback : public ReadCallback {
public:
WritePreparedTxnReadCallback(WritePreparedTxnDB* db, SequenceNumber snapshot)
: db_(db), snapshot_(snapshot) {}
// Will be called to see if the seq number accepted; if not it moves on to the
// next seq number.
virtual bool IsCommitted(SequenceNumber seq) override {
return db_->IsInSnapshot(seq, snapshot_);
}
private:
WritePreparedTxnDB* db_;
SequenceNumber snapshot_;
};
} // namespace rocksdb
#endif // ROCKSDB_LITE