The Q Capture and Q Apply programs

It is the Q Capture and Q Apply programs, which form the heart of Q replication as it is these two programs, which read transactions from the source system and apply them to the target table.

The Q replication constituent components section from Chapter 1, gave a brief introduction to the Q Capture and Q Apply programs. What we will do in this section is examine at a deeper level how these programs work and communicate with each other.

Q Capture internals

Let’s review what Q Capture does. Essentially, Q Capture reads transactions for tables that it is interested in from the DB2 log by its transaction thread calling the DB2 log interface API db2ReadLog. It builds complete transactions in memory until it detects a commit or rollback statement in the log. If it detects a rollback statement, then the transaction is flushed from memory. If it detects a commit statement, then Q Capture places the transaction in compressed XML format onto a WebSphere MQ queue called...