DB2 Thread

A DB2 thread is the DB2-side agent that executes a CICS task's SQL. Threads are created, reused and terminated as transactions come and go.

What a DB2 thread is

• When a CICS task issues its first SQL statement, CICS gets a DB2 thread for it through the DFHD2EX1 exit.
• The thread holds the plan allocation, locks and cursors for that task's unit of work.
• Creating a thread costs CPU, so CICS reuses threads instead of creating a new one for every transaction.
• Too few threads means transactions wait. Too many wastes storage. Sizing is the main tuning job.

Thread types

• Pool threads - shared threads any transaction can use when its own entry threads are exhausted.
• Protected threads - threads kept alive after a transaction ends, ready for immediate reuse. Controlled by PROTECTNUM.
• Command threads - threads used for DB2 commands issued from CICS, not for application SQL.
• Protected threads avoid the create and terminate cost on every transaction, which matters at high volume.

Thread reuse and limits

• After a transaction commits, its thread can return to the pool or stay protected for the next transaction.
• THREADLIMIT - on a DB2ENTRY, the maximum threads transactions using that entry may hold at once.
• THREADWAIT - what a transaction does when the limit is reached: wait, abend, or overflow to pool threads.
• PROTECTNUM - how many threads stay protected (kept for reuse) for this entry.

Example

• A DB2ENTRY definition giving one transaction group its own plan, thread limit and protected threads.
• CREATE DB2ENTRY(CUSTENT) PLAN(CUSTPLAN) THREADLIMIT(25) THREADWAIT(POOL) PROTECTNUM(10)
• Up to 25 threads may be active for this entry, overflow goes to pool threads, and 10 threads are kept protected for fast reuse.