rocksdb/db/merge_helper.cc
Mayank Agarwal f1bf169484 Counter for merge failure
Summary:
With Merge returning bool, it can keep failing silently(eg. While faling to fetch timestamp in TTL). We need to detect this through a rocksdb counter which can get bumped whenever Merge returns false. This will also be super-useful for the mcrocksdb-counter service where Merge may fail.
Added a counter NUMBER_MERGE_FAILURES and appropriately updated db/merge_helper.cc

I felt that it would be better to directly add counter-bumping in Merge as a default function of MergeOperator class but user should not be aware of this, so this approach seems better to me.

Test Plan: make all check

Reviewers: dnicholas, haobo, dhruba, vamsi

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12129
2013-08-13 14:25:42 -07:00

193 lines
7.8 KiB
C++

#include "merge_helper.h"
#include "db/dbformat.h"
#include "leveldb/comparator.h"
#include "leveldb/db.h"
#include "leveldb/merge_operator.h"
#include <string>
#include <stdio.h>
namespace leveldb {
// PRE: iter points to the first merge type entry
// POST: iter points to the first entry beyond the merge process (or the end)
// keys_, operands_ are updated to reflect the merge result.
// keys_ stores the list of keys encountered while merging.
// operands_ stores the list of merge operands encountered while merging.
// keys_[i] corresponds to operands_[i] for each i.
void MergeHelper::MergeUntil(Iterator* iter, SequenceNumber stop_before,
bool at_bottom, shared_ptr<Statistics> stats) {
// Get a copy of the internal key, before it's invalidated by iter->Next()
// Also maintain the list of merge operands seen.
keys_.clear();
operands_.clear();
keys_.push_front(iter->key().ToString());
operands_.push_front(iter->value().ToString());
success_ = false; // Will become true if we hit Put/Delete or bottom
// We need to parse the internal key again as the parsed key is
// backed by the internal key!
// Assume no internal key corruption as it has been successfully parsed
// by the caller.
// Invariant: keys_.back() will not change. Hence, orig_ikey is always valid.
ParsedInternalKey orig_ikey;
ParseInternalKey(keys_.back(), &orig_ikey);
bool hit_the_next_user_key = false;
ParsedInternalKey ikey;
std::string merge_result; // Temporary value for merge results
for (iter->Next(); iter->Valid(); iter->Next()) {
assert(operands_.size() >= 1); // Should be invariants!
assert(keys_.size() == operands_.size());
if (!ParseInternalKey(iter->key(), &ikey)) {
// stop at corrupted key
if (assert_valid_internal_key_) {
assert(!"corrupted internal key is not expected");
}
break;
}
if (user_comparator_->Compare(ikey.user_key, orig_ikey.user_key) != 0) {
// hit a different user key, stop right here
hit_the_next_user_key = true;
break;
}
if (stop_before && ikey.sequence <= stop_before) {
// hit an entry that's visible by the previous snapshot, can't touch that
break;
}
// At this point we are guaranteed that we need to process this key.
if (kTypeDeletion == ikey.type) {
// hit a delete
// => merge nullptr with operands_
// => store result in operands_.back() (and update keys_.back())
// => change the entry type to kTypeValue for keys_.back()
// We are done! Return a success if the merge passes.
success_ = user_merge_operator_->Merge(ikey.user_key, nullptr,
operands_, &merge_result,
logger_);
// We store the result in keys_.back() and operands_.back()
// if nothing went wrong (i.e.: no operand corruption on disk)
if (success_) {
std::string& key = keys_.back(); // The original key encountered
orig_ikey.type = kTypeValue;
UpdateInternalKey(&key[0], key.size(),
orig_ikey.sequence, orig_ikey.type);
swap(operands_.back(), merge_result);
} else {
RecordTick(stats, NUMBER_MERGE_FAILURES);
}
// move iter to the next entry (before doing anything else)
iter->Next();
return;
}
if (kTypeValue == ikey.type) {
// hit a put
// => merge the put value with operands_
// => store result in operands_.back() (and update keys_.back())
// => change the entry type to kTypeValue for keys_.back()
// We are done! Success!
const Slice value = iter->value();
success_ = user_merge_operator_->Merge(ikey.user_key, &value,
operands_, &merge_result,
logger_);
// We store the result in keys_.back() and operands_.back()
// if nothing went wrong (i.e.: no operand corruption on disk)
if (success_) {
std::string& key = keys_.back(); // The original key encountered
orig_ikey.type = kTypeValue;
UpdateInternalKey(&key[0], key.size(),
orig_ikey.sequence, orig_ikey.type);
swap(operands_.back(), merge_result);
} else {
RecordTick(stats, NUMBER_MERGE_FAILURES);
}
// move iter to the next entry
iter->Next();
return;
}
if (kTypeMerge == ikey.type) {
// hit a merge
// => merge the operand into the front of the operands_ list
// => use the user's associative merge function to determine how.
// => then continue because we haven't yet seen a Put/Delete.
assert(!operands_.empty()); // Should have at least one element in it
keys_.push_front(iter->key().ToString());
operands_.push_front(iter->value().ToString());
while (operands_.size() >= 2) {
// Returns false when the merge_operator can no longer process it
if (user_merge_operator_->PartialMerge(ikey.user_key,
Slice(operands_[0]),
Slice(operands_[1]),
&merge_result,
logger_)) {
// Merging of operands (associative merge) was successful.
// Replace these frontmost two operands with the merge result
keys_.pop_front();
operands_.pop_front();
swap(operands_.front(), merge_result);
} else {
// Merging of operands (associative merge) returned false.
// The user merge_operator does not know how to merge these operands.
// So we just stack them up until we find a Put/Delete or end of key.
break;
}
}
continue;
}
}
// We are sure we have seen this key's entire history if we are at the
// last level and exhausted all internal keys of this user key.
// NOTE: !iter->Valid() does not necessarily mean we hit the
// beginning of a user key, as versions of a user key might be
// split into multiple files (even files on the same level)
// and some files might not be included in the compaction/merge.
//
// There are also cases where we have seen the root of history of this
// key without being sure of it. Then, we simply miss the opportunity
// to combine the keys. Since VersionSet::SetupOtherInputs() always makes
// sure that all merge-operands on the same level get compacted together,
// this will simply lead to these merge operands moving to the next level.
//
// So, we only perform the following logic (to merge all operands together
// without a Put/Delete) if we are certain that we have seen the end of key.
bool surely_seen_the_beginning = hit_the_next_user_key && at_bottom;
if (surely_seen_the_beginning) {
// do a final merge with nullptr as the existing value and say
// bye to the merge type (it's now converted to a Put)
assert(kTypeMerge == orig_ikey.type);
assert(operands_.size() >= 1);
assert(operands_.size() == keys_.size());
success_ = user_merge_operator_->Merge(ikey.user_key, nullptr,
operands_, &merge_result,
logger_);
if (success_) {
std::string& key = keys_.back(); // The original key encountered
orig_ikey.type = kTypeValue;
UpdateInternalKey(&key[0], key.size(),
orig_ikey.sequence, orig_ikey.type);
// The final value() is always stored in operands_.back()
swap(operands_.back(),merge_result);
} else {
RecordTick(stats, NUMBER_MERGE_FAILURES);
// Do nothing if not success_. Leave keys() and operands() as they are.
}
}
}
} // namespace leveldb