Data consistency and concurrency

Informix^®

Informix uses a locking mechanism to manage data consistency and concurrency. When a process modifies data with UPDATE, INSERT or DELETE, an exclusive lock is set on the affected rows. The lock is held 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 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:

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

Defaults:

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

IBM^® DB2^®

As in Informix, IBM DB2 uses locks to manage data consistency and concurrency. The database manager sets exclusive locks on the modified rows and shared locks when data is read, according to the isolation level. The locks are held until the end of the transaction. When multiple processes want to access the same data, the latest processes must wait until the first finishes its transaction. The lock granularity is at the row or table level. For more details, see DB2's Administration Guide, "Application Consideration".

Control:

• Lock wait mode: Always WAIT. Only the Lock Timeout can be changed, but this is a global database parameter.
• Isolation level: Can be set through an API function call or with a database client configuration parameter.
• Locking granularity: Row level or Table level.
• Explicit locking: SELECT ... FOR UPDATE

Defaults:

• The default isolation level is Cursor Stability (readers cannot see uncommitted data, no shared lock is set when reading data).

Solution

The SET ISOLATION TO ... Informix syntax is replaced by an ODBC API call setting the SQL_ATTR_TXN_ISOLATION connection attribute. The next table shows the isolation level mappings done by the database driver:

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

See Informix and IBM DB2 documentation for more details about data consistency, concurrency and locking mechanisms.