3b6dc049f7
Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/9629 Pessimistic transactions use pessimistic concurrency control, i.e. locking. Keys are locked upon first operation that writes the key or has the intention of writing. For example, `PessimisticTransaction::Put()`, `PessimisticTransaction::Delete()`, `PessimisticTransaction::SingleDelete()` will write to or delete a key, while `PessimisticTransaction::GetForUpdate()` is used by application to indicate to RocksDB that the transaction has the intention of performing write operation later in the same transaction. Pessimistic transactions support two-phase commit (2PC). A transaction can be `Prepared()`'ed and then `Commit()`. The prepare phase is similar to a promise: once `Prepare()` succeeds, the transaction has acquired the necessary resources to commit. The resources include locks, persistence of WAL, etc. Write-committed transaction is the default pessimistic transaction implementation. In RocksDB write-committed transaction, `Prepare()` will write data to the WAL as a prepare section. `Commit()` will write a commit marker to the WAL and then write data to the memtables. While writing to the memtables, different keys in the transaction's write batch will be assigned different sequence numbers in ascending order. Until commit/rollback, the transaction holds locks on the keys so that no other transaction can write to the same keys. Furthermore, the keys' sequence numbers represent the order in which they are committed and should be made visible. This is convenient for us to implement support for user-defined timestamps. Since column families with and without timestamps can co-exist in the same database, a transaction may or may not involve timestamps. Based on this observation, we add two optional members to each `PessimisticTransaction`, `read_timestamp_` and `commit_timestamp_`. If no key in the transaction's write batch has timestamp, then setting these two variables do not have any effect. For the rest of this commit, we discuss only the cases when these two variables are meaningful. read_timestamp_ is used mainly for validation, and should be set before first call to `GetForUpdate()`. Otherwise, the latter will return non-ok status. `GetForUpdate()` calls `TryLock()` that can verify if another transaction has written the same key since `read_timestamp_` till this call to `GetForUpdate()`. If another transaction has indeed written the same key, then validation fails, and RocksDB allows this transaction to refine `read_timestamp_` by increasing it. Note that a transaction can still use `Get()` with a different timestamp to read, but the result of the read should not be used to determine data that will be written later. commit_timestamp_ must be set after finishing writing and before transaction commit. This applies to both 2PC and non-2PC cases. In the case of 2PC, it's usually set after prepare phase succeeds. We currently require that the commit timestamp be chosen after all keys are locked. This means we disallow the `TransactionDB`-level APIs if user-defined timestamp is used by the transaction. Specifically, calling `PessimisticTransactionDB::Put()`, `PessimisticTransactionDB::Delete()`, `PessimisticTransactionDB::SingleDelete()`, etc. will return non-ok status because they specify timestamps before locking the keys. Users are also prompted to use the `Transaction` APIs when they receive the non-ok status. Reviewed By: ltamasi Differential Revision: D31822445 fbshipit-source-id: b82abf8e230216dc89cc519564a588224a88fd43
RocksDB: A Persistent Key-Value Store for Flash and RAM Storage
RocksDB is developed and maintained by Facebook Database Engineering Team. It is built on earlier work on LevelDB by Sanjay Ghemawat (sanjay@google.com) and Jeff Dean (jeff@google.com)
This code is a library that forms the core building block for a fast key-value server, especially suited for storing data on flash drives. It has a Log-Structured-Merge-Database (LSM) design with flexible tradeoffs between Write-Amplification-Factor (WAF), Read-Amplification-Factor (RAF) and Space-Amplification-Factor (SAF). It has multi-threaded compactions, making it especially suitable for storing multiple terabytes of data in a single database.
Start with example usage here: https://github.com/facebook/rocksdb/tree/main/examples
See the github wiki for more explanation.
The public interface is in include/. Callers should not include or
rely on the details of any other header files in this package. Those
internal APIs may be changed without warning.
Questions and discussions are welcome on the RocksDB Developers Public Facebook group and email list on Google Groups.
License
RocksDB is dual-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). You may select, at your option, one of the above-listed licenses.