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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).
import org.rocksdb.*;
import static java.nio.charset.StandardCharsets.UTF_8;
/**
* Demonstrates using Transactions on an OptimisticTransactionDB with
* varying isolation guarantees
*/
public class OptimisticTransactionSample {
private static final String dbPath = "/tmp/rocksdb_optimistic_transaction_example";
public static final void main(final String args[]) throws RocksDBException {
try(final Options options = new Options()
.setCreateIfMissing(true);
final OptimisticTransactionDB txnDb =
OptimisticTransactionDB.open(options, dbPath)) {
try (final WriteOptions writeOptions = new WriteOptions();
final ReadOptions readOptions = new ReadOptions()) {
////////////////////////////////////////////////////////
//
// Simple OptimisticTransaction Example ("Read Committed")
//
////////////////////////////////////////////////////////
readCommitted(txnDb, writeOptions, readOptions);
////////////////////////////////////////////////////////
//
// "Repeatable Read" (Snapshot Isolation) Example
// -- Using a single Snapshot
//
////////////////////////////////////////////////////////
repeatableRead(txnDb, writeOptions, readOptions);
////////////////////////////////////////////////////////
//
// "Read Committed" (Monotonic Atomic Views) Example
// --Using multiple Snapshots
//
////////////////////////////////////////////////////////
readCommitted_monotonicAtomicViews(txnDb, writeOptions, readOptions);
}
}
}
/**
* Demonstrates "Read Committed" isolation
*/
private static void readCommitted(final OptimisticTransactionDB txnDb,
final WriteOptions writeOptions, final ReadOptions readOptions)
throws RocksDBException {
final byte key1[] = "abc".getBytes(UTF_8);
final byte value1[] = "def".getBytes(UTF_8);
final byte key2[] = "xyz".getBytes(UTF_8);
final byte value2[] = "zzz".getBytes(UTF_8);
// Start a transaction
try(final Transaction txn = txnDb.beginTransaction(writeOptions)) {
// Read a key in this transaction
byte[] value = txn.get(readOptions, key1);
assert(value == null);
// Write a key in this transaction
txn.put(key1, value1);
// Read a key OUTSIDE this transaction. Does not affect txn.
value = txnDb.get(readOptions, key1);
assert(value == null);
// Write a key OUTSIDE of this transaction.
// Does not affect txn since this is an unrelated key.
// If we wrote key 'abc' here, the transaction would fail to commit.
txnDb.put(writeOptions, key2, value2);
// Commit transaction
txn.commit();
}
}
/**
* Demonstrates "Repeatable Read" (Snapshot Isolation) isolation
*/
private static void repeatableRead(final OptimisticTransactionDB txnDb,
final WriteOptions writeOptions, final ReadOptions readOptions)
throws RocksDBException {
final byte key1[] = "ghi".getBytes(UTF_8);
final byte value1[] = "jkl".getBytes(UTF_8);
// Set a snapshot at start of transaction by setting setSnapshot(true)
try(final OptimisticTransactionOptions txnOptions =
new OptimisticTransactionOptions().setSetSnapshot(true);
final Transaction txn =
txnDb.beginTransaction(writeOptions, txnOptions)) {
final Snapshot snapshot = txn.getSnapshot();
// Write a key OUTSIDE of transaction
txnDb.put(writeOptions, key1, value1);
// Read a key using the snapshot.
readOptions.setSnapshot(snapshot);
final byte[] value = txn.getForUpdate(readOptions, key1, true);
assert (value == null);
try {
// Attempt to commit transaction
txn.commit();
throw new IllegalStateException();
} catch(final RocksDBException e) {
// Transaction could not commit since the write outside of the txn
// conflicted with the read!
assert(e.getStatus().getCode() == Status.Code.Busy);
}
txn.rollback();
} finally {
// Clear snapshot from read options since it is no longer valid
readOptions.setSnapshot(null);
}
}
/**
* Demonstrates "Read Committed" (Monotonic Atomic Views) isolation
*
* In this example, we set the snapshot multiple times. This is probably
* only necessary if you have very strict isolation requirements to
* implement.
*/
private static void readCommitted_monotonicAtomicViews(
final OptimisticTransactionDB txnDb, final WriteOptions writeOptions,
final ReadOptions readOptions) throws RocksDBException {
final byte keyX[] = "x".getBytes(UTF_8);
final byte valueX[] = "x".getBytes(UTF_8);
final byte keyY[] = "y".getBytes(UTF_8);
final byte valueY[] = "y".getBytes(UTF_8);
try (final OptimisticTransactionOptions txnOptions =
new OptimisticTransactionOptions().setSetSnapshot(true);
final Transaction txn =
txnDb.beginTransaction(writeOptions, txnOptions)) {
// Do some reads and writes to key "x"
Snapshot snapshot = txnDb.getSnapshot();
readOptions.setSnapshot(snapshot);
byte[] value = txn.get(readOptions, keyX);
txn.put(valueX, valueX);
// Do a write outside of the transaction to key "y"
txnDb.put(writeOptions, keyY, valueY);
// Set a new snapshot in the transaction
txn.setSnapshot();
snapshot = txnDb.getSnapshot();
readOptions.setSnapshot(snapshot);
// Do some reads and writes to key "y"
// Since the snapshot was advanced, the write done outside of the
// transaction does not conflict.
value = txn.getForUpdate(readOptions, keyY, true);
txn.put(keyY, valueY);
// Commit. Since the snapshot was advanced, the write done outside of the
// transaction does not prevent this transaction from Committing.
txn.commit();
} finally {
// Clear snapshot from read options since it is no longer valid
readOptions.setSnapshot(null);
}
}
}