rocksdb/db_stress_tool/db_stress_driver.cc
Yanqin Jin 1777e5f7e9 Snapshots with user-specified timestamps (#9879)
Summary:
In RocksDB, keys are associated with (internal) sequence numbers which denote when the keys are written
to the database. Sequence numbers in different RocksDB instances are unrelated, thus not comparable.

It is nice if we can associate sequence numbers with their corresponding actual timestamps. One thing we can
do is to support user-defined timestamp, which allows the applications to specify the format of custom timestamps
and encode a timestamp with each key. More details can be found at https://github.com/facebook/rocksdb/wiki/User-defined-Timestamp-%28Experimental%29.

This PR provides a different but complementary approach. We can associate rocksdb snapshots (defined in
https://github.com/facebook/rocksdb/blob/7.2.fb/include/rocksdb/snapshot.h#L20) with **user-specified** timestamps.
Since a snapshot is essentially an object representing a sequence number, this PR establishes a bi-directional mapping between sequence numbers and timestamps.

In the past, snapshots are usually taken by readers. The current super-version is grabbed, and a `rocksdb::Snapshot`
object is created with the last published sequence number of the super-version. You can see that the reader actually
has no good idea of what timestamp to assign to this snapshot, because by the time the `GetSnapshot()` is called,
an arbitrarily long period of time may have already elapsed since the last write, which is when the last published
sequence number is written.

This observation motivates the creation of "timestamped" snapshots on the write path. Currently, this functionality is
exposed only to the layer of `TransactionDB`. Application can tell RocksDB to create a snapshot when a transaction
commits, effectively associating the last sequence number with a timestamp. It is also assumed that application will
ensure any two snapshots with timestamps should satisfy the following:
```
snapshot1.seq < snapshot2.seq iff. snapshot1.ts < snapshot2.ts
```

If the application can guarantee that when a reader takes a timestamped snapshot, there is no active writes going on
in the database, then we also allow the user to use a new API `TransactionDB::CreateTimestampedSnapshot()` to create
a snapshot with associated timestamp.

Code example
```cpp
// Create a timestamped snapshot when committing transaction.
txn->SetCommitTimestamp(100);
txn->SetSnapshotOnNextOperation();
txn->Commit();

// A wrapper API for convenience
Status Transaction::CommitAndTryCreateSnapshot(
    std::shared_ptr<TransactionNotifier> notifier,
    TxnTimestamp ts,
    std::shared_ptr<const Snapshot>* ret);

// Create a timestamped snapshot if caller guarantees no concurrent writes
std::pair<Status, std::shared_ptr<const Snapshot>> snapshot = txn_db->CreateTimestampedSnapshot(100);
```

The snapshots created in this way will be managed by RocksDB with ref-counting and potentially shared with
other readers. We provide the following APIs for readers to retrieve a snapshot given a timestamp.
```cpp
// Return the timestamped snapshot correponding to given timestamp. If ts is
// kMaxTxnTimestamp, then we return the latest timestamped snapshot if present.
// Othersise, we return the snapshot whose timestamp is equal to `ts`. If no
// such snapshot exists, then we return null.
std::shared_ptr<const Snapshot> TransactionDB::GetTimestampedSnapshot(TxnTimestamp ts) const;
// Return the latest timestamped snapshot if present.
std::shared_ptr<const Snapshot> TransactionDB::GetLatestTimestampedSnapshot() const;
```

We also provide two additional APIs for stats collection and reporting purposes.

```cpp
Status TransactionDB::GetAllTimestampedSnapshots(
    std::vector<std::shared_ptr<const Snapshot>>& snapshots) const;
// Return timestamped snapshots whose timestamps fall in [ts_lb, ts_ub) and store them in `snapshots`.
Status TransactionDB::GetTimestampedSnapshots(
    TxnTimestamp ts_lb,
    TxnTimestamp ts_ub,
    std::vector<std::shared_ptr<const Snapshot>>& snapshots) const;
```

To prevent the number of timestamped snapshots from growing infinitely, we provide the following API to release
timestamped snapshots whose timestamps are older than or equal to a given threshold.
```cpp
void TransactionDB::ReleaseTimestampedSnapshotsOlderThan(TxnTimestamp ts);
```

Before shutdown, RocksDB will release all timestamped snapshots.

Comparison with user-defined timestamp and how they can be combined:
User-defined timestamp persists every key with a timestamp, while timestamped snapshots maintain a volatile
mapping between snapshots (sequence numbers) and timestamps.
Different internal keys with the same user key but different timestamps will be treated as different by compaction,
thus a newer version will not hide older versions (with smaller timestamps) unless they are eligible for garbage collection.
In contrast, taking a timestamped snapshot at a certain sequence number and timestamp prevents all the keys visible in
this snapshot from been dropped by compaction. Here, visible means (seq < snapshot and most recent).
The timestamped snapshot supports the semantics of reading at an exact point in time.

Timestamped snapshots can also be used with user-defined timestamp.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9879

Test Plan:
```
make check
TEST_TMPDIR=/dev/shm make crash_test_with_txn
```

Reviewed By: siying

Differential Revision: D35783919

Pulled By: riversand963

fbshipit-source-id: 586ad905e169189e19d3bfc0cb0177a7239d1bd4
2022-06-10 16:07:03 -07:00

198 lines
5.8 KiB
C++

// 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).
//
// 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.
//
#ifdef GFLAGS
#include "db_stress_tool/db_stress_common.h"
#include "utilities/fault_injection_fs.h"
namespace ROCKSDB_NAMESPACE {
void ThreadBody(void* v) {
ThreadState* thread = reinterpret_cast<ThreadState*>(v);
SharedState* shared = thread->shared;
if (!FLAGS_skip_verifydb && shared->ShouldVerifyAtBeginning()) {
thread->shared->GetStressTest()->VerifyDb(thread);
}
{
MutexLock l(shared->GetMutex());
shared->IncInitialized();
if (shared->AllInitialized()) {
shared->GetCondVar()->SignalAll();
}
while (!shared->Started()) {
shared->GetCondVar()->Wait();
}
}
thread->shared->GetStressTest()->OperateDb(thread);
{
MutexLock l(shared->GetMutex());
shared->IncOperated();
if (shared->AllOperated()) {
shared->GetCondVar()->SignalAll();
}
while (!shared->VerifyStarted()) {
shared->GetCondVar()->Wait();
}
}
if (!FLAGS_skip_verifydb) {
thread->shared->GetStressTest()->VerifyDb(thread);
}
{
MutexLock l(shared->GetMutex());
shared->IncDone();
if (shared->AllDone()) {
shared->GetCondVar()->SignalAll();
}
}
}
bool RunStressTest(StressTest* stress) {
SystemClock* clock = db_stress_env->GetSystemClock().get();
SharedState shared(db_stress_env, stress);
stress->InitDb(&shared);
stress->FinishInitDb(&shared);
if (FLAGS_sync_fault_injection) {
fault_fs_guard->SetFilesystemDirectWritable(false);
}
if (FLAGS_write_fault_one_in) {
fault_fs_guard->EnableWriteErrorInjection();
}
uint32_t n = FLAGS_threads;
uint64_t now = clock->NowMicros();
fprintf(stdout, "%s Initializing worker threads\n",
clock->TimeToString(now / 1000000).c_str());
shared.SetThreads(n);
if (FLAGS_compaction_thread_pool_adjust_interval > 0) {
shared.IncBgThreads();
}
if (FLAGS_continuous_verification_interval > 0) {
shared.IncBgThreads();
}
if (FLAGS_create_timestamped_snapshot_one_in > 0) {
shared.IncBgThreads();
}
std::vector<ThreadState*> threads(n);
for (uint32_t i = 0; i < n; i++) {
threads[i] = new ThreadState(i, &shared);
db_stress_env->StartThread(ThreadBody, threads[i]);
}
ThreadState bg_thread(0, &shared);
if (FLAGS_compaction_thread_pool_adjust_interval > 0) {
db_stress_env->StartThread(PoolSizeChangeThread, &bg_thread);
}
ThreadState continuous_verification_thread(0, &shared);
if (FLAGS_continuous_verification_interval > 0) {
db_stress_env->StartThread(DbVerificationThread,
&continuous_verification_thread);
}
ThreadState snapshots_gc_thread(0, &shared);
if (FLAGS_create_timestamped_snapshot_one_in > 0) {
db_stress_env->StartThread(SnapshotGcThread, &snapshots_gc_thread);
}
// Each thread goes through the following states:
// initializing -> wait for others to init -> read/populate/depopulate
// wait for others to operate -> verify -> done
{
MutexLock l(shared.GetMutex());
while (!shared.AllInitialized()) {
shared.GetCondVar()->Wait();
}
if (shared.ShouldVerifyAtBeginning()) {
if (shared.HasVerificationFailedYet()) {
fprintf(stderr, "Crash-recovery verification failed :(\n");
} else {
fprintf(stdout, "Crash-recovery verification passed :)\n");
}
}
// This is after the verification step to avoid making all those `Get()`s
// and `MultiGet()`s contend on the DB-wide trace mutex.
stress->TrackExpectedState(&shared);
now = clock->NowMicros();
fprintf(stdout, "%s Starting database operations\n",
clock->TimeToString(now / 1000000).c_str());
shared.SetStart();
shared.GetCondVar()->SignalAll();
while (!shared.AllOperated()) {
shared.GetCondVar()->Wait();
}
now = clock->NowMicros();
if (FLAGS_test_batches_snapshots) {
fprintf(stdout, "%s Limited verification already done during gets\n",
clock->TimeToString((uint64_t)now / 1000000).c_str());
} else if (FLAGS_skip_verifydb) {
fprintf(stdout, "%s Verification skipped\n",
clock->TimeToString((uint64_t)now / 1000000).c_str());
} else {
fprintf(stdout, "%s Starting verification\n",
clock->TimeToString((uint64_t)now / 1000000).c_str());
}
shared.SetStartVerify();
shared.GetCondVar()->SignalAll();
while (!shared.AllDone()) {
shared.GetCondVar()->Wait();
}
}
for (unsigned int i = 1; i < n; i++) {
threads[0]->stats.Merge(threads[i]->stats);
}
threads[0]->stats.Report("Stress Test");
for (unsigned int i = 0; i < n; i++) {
delete threads[i];
threads[i] = nullptr;
}
now = clock->NowMicros();
if (!FLAGS_skip_verifydb && !FLAGS_test_batches_snapshots &&
!shared.HasVerificationFailedYet()) {
fprintf(stdout, "%s Verification successful\n",
clock->TimeToString(now / 1000000).c_str());
}
stress->PrintStatistics();
if (FLAGS_compaction_thread_pool_adjust_interval > 0 ||
FLAGS_continuous_verification_interval > 0 ||
FLAGS_create_timestamped_snapshot_one_in > 0) {
MutexLock l(shared.GetMutex());
shared.SetShouldStopBgThread();
while (!shared.BgThreadsFinished()) {
shared.GetCondVar()->Wait();
}
}
if (shared.HasVerificationFailedYet()) {
fprintf(stderr, "Verification failed :(\n");
return false;
}
return true;
}
} // namespace ROCKSDB_NAMESPACE
#endif // GFLAGS