Read committed is a consistency model which strengthens read uncommitted by preventing dirty reads: transactions are not allowed to observe writes from transactions which do not commit.
Read committed is a transactional model: operations (usually termed “transactions”) can involve several primitive sub-operations performed in order. It is also a multi-object property: operations can act on multiple objects in the system.
Read committed can be totally available: in the presence of network partitions, every node can make progress. Without sacrificing availability, you can also ensure that transactions effects are observed together by choosing the stronger monotonic atomic view. For performance, you may wish to allow dirty reads, and relax to read uncommitted.
Note that read committed does not impose any real-time constraints. If process A completes write w, then process B begins a read r, r is not necessarily guaranteed to observe w. For a transactional model that provides real-time constraints, consider strict serializability.
Moreover, read committed does not require a per-process order between transactions. A process can observe a write, then fail to observe that same write in a subsequent transaction. In fact, a process can fail to observe its own prior writes, if those writes occurred in different transactions.
Like serializability, read committed allows pathological orderings. For instance, a read commmitted database can always return the empty state for any reads, by appearing to execute those reads at time 0. It can also discard write-only transactions by reordering them to execute at the very end of the history, after any reads. Operations like increments can also be discarded, assuming the result of the increment is never observed. Luckily, most implementations don’t seem to take advantage of these optimization opportunities.
Formally
The ANSI SQL 1999 spec specifies repeatable read by disallowing phenomenon P1:
P1 (“Dirty read”): SQL-transaction T1 modifies a row. SQL-transaction T2 then reads that row before T1 performs a COMMIT. If T1 then performs a ROLLBACK, T2 will have read a row that was never committed and that may thus be considered to have never existed.
However, as Berenson, Bernstein, et al observed, the ANSI specification allows multiple intepretations, and one of those interpretations (the "anomaly interpretation) still admits nonserializable histories for “serializable” systems. Instead, we prefer Adya’s formalization of transactional isolation levels, which provides a concise definition of the preventative interpretation. In this model, read committed prohibits:
- P0 (Dirty Write): w1(x) … w2(x)
- P1 (Dirty Read): w1(x) … r2(x)
but allows:
- P2 (Fuzzy Read): r1(x) … w2(x)
- P3 (Phantom): r1(P) … w2(y in P)
Here w denotes a write, r denotes a read, and subscripts indicate the transaction which executed that operation. The notation “…” indicates a series of micro-operations except for a commit or abort. P indicates a predicate.
For a state-based formalization, see Crooks, Pu, Alvisi, & Clement: Seeing is Believing: A Client-Centric Specification of Database Isolation.