rocksdb/table/cuckoo_table_builder.cc

//  Copyright (c) 2014, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the root directory of this source tree. An additional grant
//  of patent rights can be found in the PATENTS file in the same directory.

#ifndef ROCKSDB_LITE
#include "table/cuckoo_table_builder.h"

#include <assert.h>
#include <algorithm>
#include <limits>
#include <string>
#include <vector>

#include "db/dbformat.h"
#include "rocksdb/env.h"
#include "rocksdb/table.h"
#include "table/block_builder.h"
#include "table/format.h"
#include "table/meta_blocks.h"
#include "util/autovector.h"
#include "util/random.h"

namespace rocksdb {
const std::string CuckooTablePropertyNames::kEmptyKey =
      "rocksdb.cuckoo.bucket.empty.key";
const std::string CuckooTablePropertyNames::kNumHashTable =
      "rocksdb.cuckoo.hash.num";
const std::string CuckooTablePropertyNames::kMaxNumBuckets =
      "rocksdb.cuckoo.bucket.maxnum";
const std::string CuckooTablePropertyNames::kValueLength =
      "rocksdb.cuckoo.value.length";
const std::string CuckooTablePropertyNames::kIsLastLevel =
      "rocksdb.cuckoo.file.islastlevel";

// Obtained by running echo rocksdb.table.cuckoo | sha1sum
extern const uint64_t kCuckooTableMagicNumber = 0x926789d0c5f17873ull;

CuckooTableBuilder::CuckooTableBuilder(
    WritableFile* file, double hash_table_ratio,
    uint32_t max_num_hash_table, uint32_t max_search_depth,
    const Comparator* user_comparator,
    uint64_t (*get_slice_hash)(const Slice&, uint32_t, uint64_t))
    : num_hash_table_(2),
      file_(file),
      hash_table_ratio_(hash_table_ratio),
      max_num_hash_table_(max_num_hash_table),
      max_search_depth_(max_search_depth),
      is_last_level_file_(false),
      has_seen_first_key_(false),
      ucomp_(user_comparator),
      get_slice_hash_(get_slice_hash),
      closed_(false) {
  properties_.num_entries = 0;
  // Data is in a huge block.
  properties_.num_data_blocks = 1;
  properties_.index_size = 0;
  properties_.filter_size = 0;
}

void CuckooTableBuilder::Add(const Slice& key, const Slice& value) {
  if (properties_.num_entries >= kMaxVectorIdx - 1) {
    status_ = Status::NotSupported("Number of keys in a file must be < 2^32-1");
    return;
  }
  ParsedInternalKey ikey;
  if (!ParseInternalKey(key, &ikey)) {
    status_ = Status::Corruption("Unable to parse key into inernal key.");
    return;
  }
  // Determine if we can ignore the sequence number and value type from
  // internal keys by looking at sequence number from first key. We assume
  // that if first key has a zero sequence number, then all the remaining
  // keys will have zero seq. no.
  if (!has_seen_first_key_) {
    is_last_level_file_ = ikey.sequence == 0;
    has_seen_first_key_ = true;
    smallest_user_key_.assign(ikey.user_key.data(), ikey.user_key.size());
    largest_user_key_.assign(ikey.user_key.data(), ikey.user_key.size());
  }
  // Even if one sequence number is non-zero, then it is not last level.
  assert(!is_last_level_file_ || ikey.sequence == 0);
  if (is_last_level_file_) {
    kvs_.emplace_back(std::make_pair(
          ikey.user_key.ToString(), value.ToString()));
  } else {
    kvs_.emplace_back(std::make_pair(key.ToString(), value.ToString()));
  }
  properties_.num_entries++;

  // In order to fill the empty buckets in the hash table, we identify a
  // key which is not used so far (unused_user_key). We determine this by
  // maintaining smallest and largest keys inserted so far in bytewise order
  // and use them to find a key outside this range in Finish() operation.
  // Note that this strategy is independent of user comparator used here.
  if (ikey.user_key.compare(smallest_user_key_) < 0) {
    smallest_user_key_.assign(ikey.user_key.data(), ikey.user_key.size());
  } else if (ikey.user_key.compare(largest_user_key_) > 0) {
    largest_user_key_.assign(ikey.user_key.data(), ikey.user_key.size());
  }
}

Status CuckooTableBuilder::MakeHashTable(std::vector<CuckooBucket>* buckets) {
  uint64_t num_buckets = kvs_.size() / hash_table_ratio_;
  buckets->resize(num_buckets);
  uint64_t make_space_for_key_call_id = 0;
  for (uint32_t vector_idx = 0; vector_idx < kvs_.size(); vector_idx++) {
    uint64_t bucket_id;
    bool bucket_found = false;
    autovector<uint64_t> hash_vals;
    Slice user_key = is_last_level_file_ ? kvs_[vector_idx].first :
      ExtractUserKey(kvs_[vector_idx].first);
    for (uint32_t hash_cnt = 0; hash_cnt < num_hash_table_; ++hash_cnt) {
      uint64_t hash_val = get_slice_hash_(user_key, hash_cnt, num_buckets);
      if ((*buckets)[hash_val].vector_idx == kMaxVectorIdx) {
        bucket_id = hash_val;
        bucket_found = true;
        break;
      } else {
        if (ucomp_->Compare(user_key, is_last_level_file_
              ? Slice(kvs_[(*buckets)[hash_val].vector_idx].first)
              : ExtractUserKey(
                kvs_[(*buckets)[hash_val].vector_idx].first)) == 0) {
          return Status::NotSupported("Same key is being inserted again.");
        }
        hash_vals.push_back(hash_val);
      }
    }
    while (!bucket_found && !MakeSpaceForKey(hash_vals,
          ++make_space_for_key_call_id, buckets, &bucket_id)) {
      // Rehash by increashing number of hash tables.
      if (num_hash_table_ >= max_num_hash_table_) {
        return Status::NotSupported("Too many collissions. Unable to hash.");
      }
      // We don't really need to rehash the entire table because old hashes are
      // still valid and we only increased the number of hash functions.
      uint64_t hash_val = get_slice_hash_(user_key,
          num_hash_table_, num_buckets);
      ++num_hash_table_;
      if ((*buckets)[hash_val].vector_idx == kMaxVectorIdx) {
        bucket_found = true;
        bucket_id = hash_val;
        break;
      } else {
        hash_vals.push_back(hash_val);
      }
    }
    (*buckets)[bucket_id].vector_idx = vector_idx;
  }
  return Status::OK();
}

Status CuckooTableBuilder::Finish() {
  assert(!closed_);
  closed_ = true;
  std::vector<CuckooBucket> buckets;
  Status s = MakeHashTable(&buckets);
  if (!s.ok()) {
    return s;
  }
  // Determine unused_user_key to fill empty buckets.
  std::string unused_bucket;
  if (!kvs_.empty()) {
    std::string unused_user_key = smallest_user_key_;
    int curr_pos = unused_user_key.size() - 1;
    while (curr_pos >= 0) {
      --unused_user_key[curr_pos];
      if (Slice(unused_user_key).compare(smallest_user_key_) < 0) {
        break;
      }
      --curr_pos;
    }
    if (curr_pos < 0) {
      // Try using the largest key to identify an unused key.
      unused_user_key = largest_user_key_;
      curr_pos = unused_user_key.size() - 1;
      while (curr_pos >= 0) {
        ++unused_user_key[curr_pos];
        if (Slice(unused_user_key).compare(largest_user_key_) > 0) {
          break;
        }
        --curr_pos;
      }
    }
    if (curr_pos < 0) {
      return Status::Corruption("Unable to find unused key");
    }
    if (is_last_level_file_) {
      unused_bucket = unused_user_key;
    } else {
      ParsedInternalKey ikey(unused_user_key, 0, kTypeValue);
      AppendInternalKey(&unused_bucket, ikey);
    }
  }
  properties_.fixed_key_len = unused_bucket.size();
  uint32_t value_length = kvs_.empty() ? 0 : kvs_[0].second.size();
  uint32_t bucket_size = value_length + properties_.fixed_key_len;
  properties_.user_collected_properties[
        CuckooTablePropertyNames::kValueLength].assign(
        reinterpret_cast<const char*>(&value_length), sizeof(value_length));

  unused_bucket.resize(bucket_size, 'a');
  // Write the table.
  uint32_t num_added = 0;
  for (auto& bucket : buckets) {
    if (bucket.vector_idx == kMaxVectorIdx) {
      s = file_->Append(Slice(unused_bucket));
    } else {
      ++num_added;
      s = file_->Append(kvs_[bucket.vector_idx].first);
      if (s.ok()) {
        s = file_->Append(kvs_[bucket.vector_idx].second);
      }
    }
    if (!s.ok()) {
      return s;
    }
  }
  assert(num_added == NumEntries());
  properties_.raw_key_size = num_added * properties_.fixed_key_len;
  properties_.raw_value_size = num_added * value_length;

  uint64_t offset = buckets.size() * bucket_size;
  properties_.data_size = offset;
  unused_bucket.resize(properties_.fixed_key_len);
  properties_.user_collected_properties[
    CuckooTablePropertyNames::kEmptyKey] = unused_bucket;
  properties_.user_collected_properties[
    CuckooTablePropertyNames::kNumHashTable].assign(
        reinterpret_cast<char*>(&num_hash_table_), sizeof(num_hash_table_));
  uint64_t num_buckets = buckets.size();
  properties_.user_collected_properties[
    CuckooTablePropertyNames::kMaxNumBuckets].assign(
        reinterpret_cast<const char*>(&num_buckets), sizeof(num_buckets));
  properties_.user_collected_properties[
    CuckooTablePropertyNames::kIsLastLevel].assign(
        reinterpret_cast<const char*>(&is_last_level_file_),
        sizeof(is_last_level_file_));

  // Write meta blocks.
  MetaIndexBuilder meta_index_builder;
  PropertyBlockBuilder property_block_builder;

  property_block_builder.AddTableProperty(properties_);
  property_block_builder.Add(properties_.user_collected_properties);
  Slice property_block = property_block_builder.Finish();
  BlockHandle property_block_handle;
  property_block_handle.set_offset(offset);
  property_block_handle.set_size(property_block.size());
  s = file_->Append(property_block);
  offset += property_block.size();
  if (!s.ok()) {
    return s;
  }

  meta_index_builder.Add(kPropertiesBlock, property_block_handle);
  Slice meta_index_block = meta_index_builder.Finish();

  BlockHandle meta_index_block_handle;
  meta_index_block_handle.set_offset(offset);
  meta_index_block_handle.set_size(meta_index_block.size());
  s = file_->Append(meta_index_block);
  if (!s.ok()) {
    return s;
  }

  Footer footer(kCuckooTableMagicNumber);
  footer.set_metaindex_handle(meta_index_block_handle);
  footer.set_index_handle(BlockHandle::NullBlockHandle());
  std::string footer_encoding;
  footer.EncodeTo(&footer_encoding);
  s = file_->Append(footer_encoding);
  return s;
}

void CuckooTableBuilder::Abandon() {
  assert(!closed_);
  closed_ = true;
}

uint64_t CuckooTableBuilder::NumEntries() const {
  return properties_.num_entries;
}

uint64_t CuckooTableBuilder::FileSize() const {
  if (closed_) {
    return file_->GetFileSize();
  } else if (properties_.num_entries == 0) {
    return 0;
  }
  // This is not the actual size of the file as we need to account for
  // hash table ratio. This returns the size of filled buckets in the table
  // scaled up by a factor of 1/hash_table_ratio.
  return ((kvs_[0].first.size() + kvs_[0].second.size()) *
      properties_.num_entries) / hash_table_ratio_;
}

// This method is invoked when there is no place to insert the target key.
// It searches for a set of elements that can be moved to accommodate target
// key. The search is a BFS graph traversal with first level (hash_vals)
// being all the buckets target key could go to.
// Then, from each node (curr_node), we find all the buckets that curr_node
// could go to. They form the children of curr_node in the tree.
// We continue the traversal until we find an empty bucket, in which case, we
// move all elements along the path from first level to this empty bucket, to
// make space for target key which is inserted at first level (*bucket_id).
// If tree depth exceedes max depth, we return false indicating failure.
bool CuckooTableBuilder::MakeSpaceForKey(
    const autovector<uint64_t>& hash_vals,
    const uint64_t make_space_for_key_call_id,
    std::vector<CuckooBucket>* buckets,
    uint64_t* bucket_id) {
  struct CuckooNode {
    uint64_t bucket_id;
    uint32_t depth;
    uint32_t parent_pos;
    CuckooNode(uint64_t bucket_id, uint32_t depth, int parent_pos)
      : bucket_id(bucket_id), depth(depth), parent_pos(parent_pos) {}
  };
  // This is BFS search tree that is stored simply as a vector.
  // Each node stores the index of parent node in the vector.
  std::vector<CuckooNode> tree;
  // We want to identify already visited buckets in the current method call so
  // that we don't add same buckets again for exploration in the tree.
  // We do this by maintaining a count of current method call, which acts as a
  // unique id for this invocation of the method. We store this number into
  // the nodes that we explore in current method call.
  // It is unlikely for the increment operation to overflow because the maximum
  // no. of times this will be called is <= max_num_hash_table_ + kvs_.size().
  for (uint32_t hash_cnt = 0; hash_cnt < num_hash_table_; ++hash_cnt) {
    uint64_t bucket_id = hash_vals[hash_cnt];
    (*buckets)[bucket_id].make_space_for_key_call_id =
      make_space_for_key_call_id;
    tree.push_back(CuckooNode(bucket_id, 0, 0));
  }
  bool null_found = false;
  uint32_t curr_pos = 0;
  while (!null_found && curr_pos < tree.size()) {
    CuckooNode& curr_node = tree[curr_pos];
    uint32_t curr_depth = curr_node.depth;
    if (curr_depth >= max_search_depth_) {
      break;
    }
    CuckooBucket& curr_bucket = (*buckets)[curr_node.bucket_id];
    for (uint32_t hash_cnt = 0; hash_cnt < num_hash_table_; ++hash_cnt) {
      uint64_t child_bucket_id = get_slice_hash_(
          is_last_level_file_ ? kvs_[curr_bucket.vector_idx].first
          : ExtractUserKey(Slice(kvs_[curr_bucket.vector_idx].first)),
          hash_cnt, buckets->size());
      if ((*buckets)[child_bucket_id].make_space_for_key_call_id ==
          make_space_for_key_call_id) {
        continue;
      }
      (*buckets)[child_bucket_id].make_space_for_key_call_id =
        make_space_for_key_call_id;
      tree.push_back(CuckooNode(child_bucket_id, curr_depth + 1,
            curr_pos));
      if ((*buckets)[child_bucket_id].vector_idx == kMaxVectorIdx) {
        null_found = true;
        break;
      }
    }
    ++curr_pos;
  }

  if (null_found) {
    // There is an empty node in tree.back(). Now, traverse the path from this
    // empty node to top of the tree and at every node in the path, replace
    // child with the parent. Stop when first level is reached in the tree
    // (happens when 0 <= bucket_to_replace_pos < num_hash_table_) and return
    // this location in first level for target key to be inserted.
    uint32_t bucket_to_replace_pos = tree.size()-1;
    while (bucket_to_replace_pos >= num_hash_table_) {
      CuckooNode& curr_node = tree[bucket_to_replace_pos];
      (*buckets)[curr_node.bucket_id] =
        (*buckets)[tree[curr_node.parent_pos].bucket_id];
      bucket_to_replace_pos = curr_node.parent_pos;
    }
    *bucket_id = tree[bucket_to_replace_pos].bucket_id;
  }
  return null_found;
}

}  // namespace rocksdb
#endif  // ROCKSDB_LITE