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rocksdb/table/index_builder.h

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// Copyright (c) 2011-present, 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.
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#pragma once
#include <assert.h>
#include <inttypes.h>
#include <string>
#include <unordered_map>
#include "rocksdb/comparator.h"
#include "table/block_based_table_factory.h"
#include "table/block_builder.h"
#include "table/format.h"
namespace rocksdb {
// The interface for building index.
// Instruction for adding a new concrete IndexBuilder:
// 1. Create a subclass instantiated from IndexBuilder.
// 2. Add a new entry associated with that subclass in TableOptions::IndexType.
// 3. Add a create function for the new subclass in CreateIndexBuilder.
// Note: we can devise more advanced design to simplify the process for adding
// new subclass, which will, on the other hand, increase the code complexity and
// catch unwanted attention from readers. Given that we won't add/change
// indexes frequently, it makes sense to just embrace a more straightforward
// design that just works.
class IndexBuilder {
public:
static IndexBuilder* CreateIndexBuilder(
BlockBasedTableOptions::IndexType index_type,
const InternalKeyComparator* comparator,
const SliceTransform* prefix_extractor, int index_block_restart_interval,
uint64_t index_per_partition);
// Index builder will construct a set of blocks which contain:
// 1. One primary index block.
// 2. (Optional) a set of metablocks that contains the metadata of the
// primary index.
struct IndexBlocks {
Slice index_block_contents;
std::unordered_map<std::string, Slice> meta_blocks;
};
explicit IndexBuilder(const InternalKeyComparator* comparator)
: comparator_(comparator) {}
virtual ~IndexBuilder() {}
// Add a new index entry to index block.
// To allow further optimization, we provide `last_key_in_current_block` and
// `first_key_in_next_block`, based on which the specific implementation can
// determine the best index key to be used for the index block.
// @last_key_in_current_block: this parameter maybe overridden with the value
// "substitute key".
// @first_key_in_next_block: it will be nullptr if the entry being added is
// the last one in the table
//
// REQUIRES: Finish() has not yet been called.
virtual void AddIndexEntry(std::string* last_key_in_current_block,
const Slice* first_key_in_next_block,
const BlockHandle& block_handle) = 0;
// This method will be called whenever a key is added. The subclasses may
// override OnKeyAdded() if they need to collect additional information.
virtual void OnKeyAdded(const Slice& key) {}
// Inform the index builder that all entries has been written. Block builder
// may therefore perform any operation required for block finalization.
//
// REQUIRES: Finish() has not yet been called.
inline Status Finish(IndexBlocks* index_blocks) {
// Throw away the changes to last_partition_block_handle. It has no effect
// on the first call to Finish anyway.
BlockHandle last_partition_block_handle;
return Finish(index_blocks, last_partition_block_handle);
}
// This override of Finish can be utilized to build the 2nd level index in
// PartitionIndexBuilder.
//
// index_blocks will be filled with the resulting index data. If the return
// value is Status::InComplete() then it means that the index is partitioned
// and the callee should keep calling Finish until Status::OK() is returned.
// In that case, last_partition_block_handle is pointer to the block written
// with the result of the last call to Finish. This can be utilized to build
// the second level index pointing to each block of partitioned indexes. The
// last call to Finish() that returns Status::OK() populates index_blocks with
// the 2nd level index content.
virtual Status Finish(IndexBlocks* index_blocks,
const BlockHandle& last_partition_block_handle) = 0;
// Get the estimated size for index block.
virtual size_t EstimatedSize() const = 0;
protected:
const InternalKeyComparator* comparator_;
};
// This index builder builds space-efficient index block.
//
// Optimizations:
// 1. Made block's `block_restart_interval` to be 1, which will avoid linear
// search when doing index lookup (can be disabled by setting
// index_block_restart_interval).
// 2. Shorten the key length for index block. Other than honestly using the
// last key in the data block as the index key, we instead find a shortest
// substitute key that serves the same function.
class ShortenedIndexBuilder : public IndexBuilder {
public:
explicit ShortenedIndexBuilder(const InternalKeyComparator* comparator,
int index_block_restart_interval)
: IndexBuilder(comparator),
index_block_builder_(index_block_restart_interval) {}
virtual void AddIndexEntry(std::string* last_key_in_current_block,
const Slice* first_key_in_next_block,
const BlockHandle& block_handle) override {
if (first_key_in_next_block != nullptr) {
comparator_->FindShortestSeparator(last_key_in_current_block,
*first_key_in_next_block);
} else {
comparator_->FindShortSuccessor(last_key_in_current_block);
}
std::string handle_encoding;
block_handle.EncodeTo(&handle_encoding);
index_block_builder_.Add(*last_key_in_current_block, handle_encoding);
}
virtual Status Finish(
IndexBlocks* index_blocks,
const BlockHandle& last_partition_block_handle) override {
index_blocks->index_block_contents = index_block_builder_.Finish();
return Status::OK();
}
virtual size_t EstimatedSize() const override {
return index_block_builder_.CurrentSizeEstimate();
}
private:
BlockBuilder index_block_builder_;
};
// HashIndexBuilder contains a binary-searchable primary index and the
// metadata for secondary hash index construction.
// The metadata for hash index consists two parts:
// - a metablock that compactly contains a sequence of prefixes. All prefixes
// are stored consectively without any metadata (like, prefix sizes) being
// stored, which is kept in the other metablock.
// - a metablock contains the metadata of the prefixes, including prefix size,
// restart index and number of block it spans. The format looks like:
//
// +-----------------+---------------------------+---------------------+
// <=prefix 1
// | length: 4 bytes | restart interval: 4 bytes | num-blocks: 4 bytes |
// +-----------------+---------------------------+---------------------+
// <=prefix 2
// | length: 4 bytes | restart interval: 4 bytes | num-blocks: 4 bytes |
// +-----------------+---------------------------+---------------------+
// | |
// | .... |
// | |
// +-----------------+---------------------------+---------------------+
// <=prefix n
// | length: 4 bytes | restart interval: 4 bytes | num-blocks: 4 bytes |
// +-----------------+---------------------------+---------------------+
//
// The reason of separating these two metablocks is to enable the efficiently
// reuse the first metablock during hash index construction without unnecessary
// data copy or small heap allocations for prefixes.
class HashIndexBuilder : public IndexBuilder {
public:
explicit HashIndexBuilder(const InternalKeyComparator* comparator,
const SliceTransform* hash_key_extractor,
int index_block_restart_interval)
: IndexBuilder(comparator),
primary_index_builder_(comparator, index_block_restart_interval),
hash_key_extractor_(hash_key_extractor) {}
virtual void AddIndexEntry(std::string* last_key_in_current_block,
const Slice* first_key_in_next_block,
const BlockHandle& block_handle) override {
++current_restart_index_;
primary_index_builder_.AddIndexEntry(last_key_in_current_block,
first_key_in_next_block, block_handle);
}
virtual void OnKeyAdded(const Slice& key) override {
auto key_prefix = hash_key_extractor_->Transform(key);
bool is_first_entry = pending_block_num_ == 0;
// Keys may share the prefix
if (is_first_entry || pending_entry_prefix_ != key_prefix) {
if (!is_first_entry) {
FlushPendingPrefix();
}
// need a hard copy otherwise the underlying data changes all the time.
// TODO(kailiu) ToString() is expensive. We may speed up can avoid data
// copy.
pending_entry_prefix_ = key_prefix.ToString();
pending_block_num_ = 1;
pending_entry_index_ = static_cast<uint32_t>(current_restart_index_);
} else {
// entry number increments when keys share the prefix reside in
// different data blocks.
auto last_restart_index = pending_entry_index_ + pending_block_num_ - 1;
assert(last_restart_index <= current_restart_index_);
if (last_restart_index != current_restart_index_) {
++pending_block_num_;
}
}
}
virtual Status Finish(
IndexBlocks* index_blocks,
const BlockHandle& last_partition_block_handle) override {
FlushPendingPrefix();
primary_index_builder_.Finish(index_blocks, last_partition_block_handle);
index_blocks->meta_blocks.insert(
{kHashIndexPrefixesBlock.c_str(), prefix_block_});
index_blocks->meta_blocks.insert(
{kHashIndexPrefixesMetadataBlock.c_str(), prefix_meta_block_});
return Status::OK();
}
virtual size_t EstimatedSize() const override {
return primary_index_builder_.EstimatedSize() + prefix_block_.size() +
prefix_meta_block_.size();
}
private:
void FlushPendingPrefix() {
prefix_block_.append(pending_entry_prefix_.data(),
pending_entry_prefix_.size());
PutVarint32Varint32Varint32(
&prefix_meta_block_,
static_cast<uint32_t>(pending_entry_prefix_.size()),
pending_entry_index_, pending_block_num_);
}
ShortenedIndexBuilder primary_index_builder_;
const SliceTransform* hash_key_extractor_;
// stores a sequence of prefixes
std::string prefix_block_;
// stores the metadata of prefixes
std::string prefix_meta_block_;
// The following 3 variables keeps unflushed prefix and its metadata.
// The details of block_num and entry_index can be found in
// "block_hash_index.{h,cc}"
uint32_t pending_block_num_ = 0;
uint32_t pending_entry_index_ = 0;
std::string pending_entry_prefix_;
uint64_t current_restart_index_ = 0;
};
} // namespace rocksdb