merge_helper.h
// 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 MERGE_HELPER_H
#define MERGE_HELPER_H
#include <deque>
#include <string>
#include <vector>
#include "db/dbformat.h"
#include "db/merge_context.h"
#include "db/range_del_aggregator.h"
#include "rocksdb/compaction_filter.h"
#include "rocksdb/env.h"
#include "rocksdb/slice.h"
#include "util/stop_watch.h"
namespace rocksdb {
class Comparator;
class Iterator;
class Logger;
class MergeOperator;
class Statistics;
class InternalIterator;
class MergeHelper {
public:
MergeHelper(Env* env, const Comparator* user_comparator,
const MergeOperator* user_merge_operator,
const CompactionFilter* compaction_filter, Logger* logger,
bool assert_valid_internal_key, SequenceNumber latest_snapshot,
int level = 0, Statistics* stats = nullptr,
const std::atomic<bool>* shutting_down = nullptr);
// Wrapper around MergeOperator::FullMergeV2() that records perf statistics.
// Result of merge will be written to result if status returned is OK.
// If operands is empty, the value will simply be copied to result.
// Set `update_num_ops_stats` to true if it is from a user read, so that
// the latency is sensitive.
// Returns one of the following statuses:
// - OK: Entries were successfully merged.
// - Corruption: Merge operator reported unsuccessful merge.
static Status TimedFullMerge(const MergeOperator* merge_operator,
const Slice& key, const Slice* value,
const std::vector<Slice>& operands,
std::string* result, Logger* logger,
Statistics* statistics, Env* env,
Slice* result_operand = nullptr,
bool update_num_ops_stats = false);
// Merge entries until we hit
// - a corrupted key
// - a Put/Delete,
// - a different user key,
// - a specific sequence number (snapshot boundary),
// - REMOVE_AND_SKIP_UNTIL returned from compaction filter,
// or - the end of iteration
// iter: (IN) points to the first merge type entry
// (OUT) points to the first entry not included in the merge process
// range_del_agg: (IN) filters merge operands covered by range tombstones.
// stop_before: (IN) a sequence number that merge should not cross.
// 0 means no restriction
// at_bottom: (IN) true if the iterator covers the bottem level, which means
// we could reach the start of the history of this user key.
//
// Returns one of the following statuses:
// - OK: Entries were successfully merged.
// - MergeInProgress: Put/Delete not encountered, and didn't reach the start
// of key's history. Output consists of merge operands only.
// - Corruption: Merge operator reported unsuccessful merge or a corrupted
// key has been encountered and not expected (applies only when compiling
// with asserts removed).
// - ShutdownInProgress: interrupted by shutdown (*shutting_down == true).
//
// REQUIRED: The first key in the input is not corrupted.
Status MergeUntil(InternalIterator* iter,
RangeDelAggregator* range_del_agg = nullptr,
const SequenceNumber stop_before = 0,
const bool at_bottom = false);
// Filters a merge operand using the compaction filter specified
// in the constructor. Returns the decision that the filter made.
// Uses compaction_filter_value_ and compaction_filter_skip_until_ for the
// optional outputs of compaction filter.
CompactionFilter::Decision FilterMerge(const Slice& user_key,
const Slice& value_slice);
// Query the merge result
// These are valid until the next MergeUntil call
// If the merging was successful:
// - keys() contains a single element with the latest sequence number of
// the merges. The type will be Put or Merge. See IMPORTANT 1 note, below.
// - values() contains a single element with the result of merging all the
// operands together
//
// IMPORTANT 1: the key type could change after the MergeUntil call.
// Put/Delete + Merge + ... + Merge => Put
// Merge + ... + Merge => Merge
//
// If the merge operator is not associative, and if a Put/Delete is not found
// then the merging will be unsuccessful. In this case:
// - keys() contains the list of internal keys seen in order of iteration.
// - values() contains the list of values (merges) seen in the same order.
// values() is parallel to keys() so that the first entry in
// keys() is the key associated with the first entry in values()
// and so on. These lists will be the same length.
// All of these pairs will be merges over the same user key.
// See IMPORTANT 2 note below.
//
// IMPORTANT 2: The entries were traversed in order from BACK to FRONT.
// So keys().back() was the first key seen by iterator.
// TODO: Re-style this comment to be like the first one
const std::deque<std::string>& keys() const { return keys_; }
const std::vector<Slice>& values() const {
return merge_context_.GetOperands();
}
uint64_t TotalFilterTime() const { return total_filter_time_; }
bool HasOperator() const { return user_merge_operator_ != nullptr; }
// If compaction filter returned REMOVE_AND_SKIP_UNTIL, this method will
// return true and fill *until with the key to which we should skip.
// If true, keys() and values() are empty.
bool FilteredUntil(Slice* skip_until) const {
if (!has_compaction_filter_skip_until_) {
return false;
}
assert(compaction_filter_ != nullptr);
assert(skip_until != nullptr);
assert(compaction_filter_skip_until_.Valid());
*skip_until = compaction_filter_skip_until_.Encode();
return true;
}
private:
Env* env_;
const Comparator* user_comparator_;
const MergeOperator* user_merge_operator_;
const CompactionFilter* compaction_filter_;
const std::atomic<bool>* shutting_down_;
Logger* logger_;
bool assert_valid_internal_key_; // enforce no internal key corruption?
bool allow_single_operand_;
SequenceNumber latest_snapshot_;
int level_;
// the scratch area that holds the result of MergeUntil
// valid up to the next MergeUntil call
// Keeps track of the sequence of keys seen
std::deque<std::string> keys_;
// Parallel with keys_; stores the operands
mutable MergeContext merge_context_;
StopWatchNano filter_timer_;
uint64_t total_filter_time_;
Statistics* stats_;
bool has_compaction_filter_skip_until_ = false;
std::string compaction_filter_value_;
InternalKey compaction_filter_skip_until_;
bool IsShuttingDown() {
// This is a best-effort facility, so memory_order_relaxed is sufficient.
return shutting_down_ && shutting_down_->load(std::memory_order_relaxed);
}
};
// MergeOutputIterator can be used to iterate over the result of a merge.
class MergeOutputIterator {
public:
// The MergeOutputIterator is bound to a MergeHelper instance.
explicit MergeOutputIterator(const MergeHelper* merge_helper);
// Seeks to the first record in the output.
void SeekToFirst();
// Advances to the next record in the output.
void Next();
Slice key() { return Slice(*it_keys_); }
Slice value() { return Slice(*it_values_); }
bool Valid() { return it_keys_ != merge_helper_->keys().rend(); }
private:
const MergeHelper* merge_helper_;
std::deque<std::string>::const_reverse_iterator it_keys_;
std::vector<Slice>::const_reverse_iterator it_values_;
};
} // namespace rocksdb
#endif
