Data consistency and concurrency

Data consistency involves readers which want to access data currently modified by writers and concurrency data access involves several writers accessing the same data for modification. Locking granularity defines the amount of data concerned when a lock is set (row, page, table, ...).

Informix^®

Informix uses a locking mechanism to handle data consistency and concurrency. When a process changes database information with UPDATE, INSERT or DELETE, an exclusive lock is set on the touched rows. The lock remains active until the end of the transaction. Statements performed outside a transaction are treated as a transaction containing a single operation and therefore release the locks immediately after execution. SELECT statements can set shared locks according to the isolation level. In case of locking conflicts (for example, when two processes want to acquire an exclusive lock on the same row for modification or when a writer is trying to modify data protected by a shared lock), the behavior of a process can be changed by setting the lock wait mode.

Control:

Lock wait mode: SET LOCK MODE TO ...
Isolation level: SET ISOLATION TO ...
Locking granularity: CREATE TABLE ... LOCK MODE {PAGE|ROW}
Explicit exclusive lock: SELECT ... FOR UPDATE

Defaults:

The default isolation level is read committed.
The default lock wait mode is "not wait".
The default locking granularity is per page.

MySQL

When data is modified, exclusive locks are set and held until the end of the transaction. For data consistency, MySQL uses a locking mechanism. Readers must wait for writers as in Informix.

Control:

No lock wait mode control is provided.
Isolation level: SET TRANSACTION ISOLATION LEVEL ...
Explicit exclusive lock: SELECT ... FOR UPDATE

Defaults:

The default isolation level is Read Committed.
The default locking granularity is per table (pre page when using BDB tables).

Solution

The SET ISOLATION TO ... Informix syntax is replaced by SET SESSION TRANSACTION ISOLATION LEVEL ... in MySQL. The next table shows the isolation level mappings done by the MySQL database driver:

Table 1. Isolation level mappings done by the MySQL database driver
SET ISOLATION instruction in program	Native SQL command
SET ISOLATION TO DIRTY READ	SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
SET ISOLATION TO COMMITTED READ [READ COMMITTED] [RETAIN UPDATE LOCKS]	SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED
SET ISOLATION TO CURSOR STABILITY	SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED
SET ISOLATION TO REPEATABLE READ	SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ

For portability, it is recommended that you work with Informix in the read committed isolation level, make processes wait for each other (lock mode wait), and create tables with the "lock mode row" option.

See Informix and MySQL documentation for more details about data consistency, concurrency and locking mechanisms.

Data consistency and concurrency

Informix®

MySQL

Solution

Informix^®