Executing a transaction means evaluating the body of the transaction to produce the intended reads and writes. There is still notable variety across systems as to how the body of a transaction is executed. Writes might be applied to storage during this phase, or they might be buffered locally and submitted as a batch at the end. A transaction might be executed more than once for different purposes.
Ordering a transaction means assigning the transaction some notion of a time at which it occurred. This could be a version, a timestamp, a log sequence number, or a more complex description of transaction IDs it happened before or after. MVCC databases may assign two versions: an initial read version, and a final commit version. In this case, we’re mainly focused on the specific point at which the commit version is chosen — the time at which the database claims all reads and writes occurred atomically.
Validating a transaction means enforcing concurrency control, or more rarely, domain-specific semantics. If a transaction is going to be rejected for a system defined reason, such as having serializability conflicts with existing transactions, it will happen here. When validation happens after ordering, it checks to see if the assigned order is valid. When validation happens before ordering, it provides a range of acceptable commit versions, and the ordering step chooses one of them.
Persisting a transaction makes making it durable, generally to disk. Sometimes writes are incrementally made durable during transaction execution, but the key point in persistence is when all writes and the commit record marking the transaction as committed are durable. Often this is noting how the system performs replication and persists the outcome of its atomic commitment protocol. (And sometimes the lines between those two aren’t very clear.)
All four of these things must be done before the system may acknowledge a transaction’s result to a client. However, these steps can be done in any order. They can be done concurrently. Different systems achieve different tradeoffs by reordering these steps. ... A classic optimistic concurrency control database will execute a transaction, and record the read and write sets. Once execution finishes, a commit version is allocated, and concurrent transactions are checked for conflicts. If no conflicts were found, the transaction is made durable, and acknowledged as committed. ... A classic pessimistic concurrency control database executes a transaction and acquires locks as it runs to exclude conflicting transactions. When the transaction finishes, it acquires a commit version, and then is persisted to disk. Then it releases the locks.
Last modified 31 October 2025