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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).
// An example code demonstrating how to use CompactFiles, EventListener,
// and GetColumnFamilyMetaData APIs to implement custom compaction algorithm.
#include <mutex>
#include <string>
#include "rocksdb/db.h"
#include "rocksdb/env.h"
#include "rocksdb/options.h"
using namespace rocksdb;
std::string kDBPath = "/tmp/rocksdb_compact_files_example";
struct CompactionTask;
// This is an example interface of external-compaction algorithm.
// Compaction algorithm can be implemented outside the core-RocksDB
// code by using the pluggable compaction APIs that RocksDb provides.
class Compactor : public EventListener {
// Picks and returns a compaction task given the specified DB
// and column family. It is the caller's responsibility to
// destroy the returned CompactionTask. Returns "nullptr"
// if it cannot find a proper compaction task.
virtual CompactionTask* PickCompaction(
DB* db, const std::string& cf_name) = 0;
// Schedule and run the specified compaction task in background.
virtual void ScheduleCompaction(CompactionTask *task) = 0;
// Example structure that describes a compaction task.
struct CompactionTask {
DB* _db, Compactor* _compactor,
const std::string& _column_family_name,
const std::vector<std::string>& _input_file_names,
const int _output_level,
const CompactionOptions& _compact_options,
bool _retry_on_fail)
: db(_db),
retry_on_fail(_retry_on_fail) {}
DB* db;
Compactor* compactor;
const std::string& column_family_name;
std::vector<std::string> input_file_names;
int output_level;
CompactionOptions compact_options;
bool retry_on_fail;
// A simple compaction algorithm that always compacts everything
// to the highest level whenever possible.
class FullCompactor : public Compactor {
explicit FullCompactor(const Options options) : options_(options) {
compact_options_.compression = options_.compression;
compact_options_.output_file_size_limit =
// When flush happens, it determines whether to trigger compaction. If
// triggered_writes_stop is true, it will also set the retry flag of
// compaction-task to true.
void OnFlushCompleted(
DB* db, const FlushJobInfo& info) override {
CompactionTask* task = PickCompaction(db, info.cf_name);
if (task != nullptr) {
if (info.triggered_writes_stop) {
task->retry_on_fail = true;
// Schedule compaction in a different thread.
// Always pick a compaction which includes all files whenever possible.
CompactionTask* PickCompaction(
DB* db, const std::string& cf_name) override {
ColumnFamilyMetaData cf_meta;
std::vector<std::string> input_file_names;
for (auto level : cf_meta.levels) {
for (auto file : level.files) {
if (file.being_compacted) {
return nullptr;
return new CompactionTask(
db, this, cf_name, input_file_names,
options_.num_levels - 1, compact_options_, false);
// Schedule the specified compaction task in background.
void ScheduleCompaction(CompactionTask* task) override {
options_.env->Schedule(&FullCompactor::CompactFiles, task);
static void CompactFiles(void* arg) {
std::unique_ptr<CompactionTask> task(
Status s = task->db->CompactFiles(
printf("CompactFiles() finished with status %s\n", s.ToString().c_str());
if (!s.ok() && !s.IsIOError() && task->retry_on_fail) {
// If a compaction task with its retry_on_fail=true failed,
// try to schedule another compaction in case the reason
// is not an IO error.
CompactionTask* new_task = task->compactor->PickCompaction(
task->db, task->column_family_name);
Options options_;
CompactionOptions compact_options_;
int main() {
Options options;
options.create_if_missing = true;
// Disable RocksDB background compaction.
options.compaction_style = kCompactionStyleNone;
// Small slowdown and stop trigger for experimental purpose.
options.level0_slowdown_writes_trigger = 3;
options.level0_stop_writes_trigger = 5;
options.listeners.emplace_back(new FullCompactor(options));
DB* db = nullptr;
DestroyDB(kDBPath, options);
Status s = DB::Open(options, kDBPath, &db);
// if background compaction is not working, write will stall
// because of options.level0_stop_writes_trigger
for (int i = 1000; i < 99999; ++i) {
db->Put(WriteOptions(), std::to_string(i),
std::string(500, 'a' + (i % 26)));
// verify the values are still there
std::string value;
for (int i = 1000; i < 99999; ++i) {
db->Get(ReadOptions(), std::to_string(i),
assert(value == std::string(500, 'a' + (i % 26)));
// close the db.
delete db;
return 0;