The different types of Q Replication

There are four basic types of Q replication:

• Unidirectional
• Bidirectional
• Peer-to-peer
• Event Publishing

Replicating to a stored procedure or a Consistent Change Data (CCD) table are a subset of unidirectional replication. Let's look at each of these in more detail.

Unidirectional replication

In unidirectional replication, we can replicate all of the rows and columns of a source table or we can just replicate a subset of the rows and columns. We cannot really perform any transformation on this data. If we want to perform some sort of transformation, then we would need to replicate to a stored procedure.

Replicating to a stored procedure

Stored procedure replication is a subset of unidirectional replication in which the target is not a table as such, but a stored procedure, as shown in the following diagram:

A stored procedure can transform the data and output the results to a target table. This target table is not known to Q Apply. These stored procedures can be written in SQL, C, or Java.

Prior to DB2 9.7 the source table and the stored procedure must have the same name, and the target table name can be any name we like.

Bidirectional replication

In bidirectional replication, we replicate copies of tables between two servers, each of which has a copy of the table. Note that we can only set up bidirectional replication between two servers. Unlike unidirectional replication, where we can replicate a subset of rows and columns, this is not possible in bidirectional replication. The tables on both servers can have different names, but must have the same number of rows and columns. The columns must have identical column names of compatible data types. It is not possible to do any data transformation using this type of replication.

Because we are updating records on both servers, it is possible that the same record will be updated at the same time on both servers.

Although Q replication provides a conflict detection mechanism, we strongly advise that the driving application should be written or modified in such a way that such conflicts be avoided. The conflict detection provided by Q replication should be treated as a safety net and not the primary conflict resolution mechanism.

This mechanism allows us to choose which data values are used to detect conflicts (key column values only, changed column values, or all column values) and which server should win if such a conflict is detected. The row in the losing system is rolled back and the record is written to the IBMQSNAP_EXCEPTIONS table for review.

One of the related subjects to conflict detection is the concept of which server takes precedence in a conflict, or to put it more bluntly, which server is the master and which is the slave! If there is a conflict, then whichever server takes precedence will not apply changes from the other server. This ensures that the servers remain in sync. There is a more egalitarian option, which is that no server takes precedence. In this situation, rows are applied irrespective of whether or not there is a conflict, which ultimately leads to a divergence of the contents of the databases, which is not good!

There are two types of bidirectional replication—the first type is where we have an active/passive setup and the second type is where we have an active/active setup. The type of replication you choose will have implications on which server is defined as the master and which as the slave and what to do if a Q subscription is inadvertently inactivated.

In an active/passive setup, the passive server should be made the master. In an active/active setup, the choice of which system is the master is a decision you have to make.

Peer-to-peer replication

Peer-to-peer replication allows us to replicate data between two or more servers. This is different from bidirectional replication, which is only between two servers. Each server has a copy of the table (which can have a different schema and name), but must have the same number of rows and columns and these columns must have identical column names and compatible data types. It is not possible to do any data transformation using this type of replication.

In peer-to-peer replication, there is no such thing as a master or slave server—each server will have the most recent copy of the table—eventually! What this means is that there will be a slight delay between the first server having a copy of the table and the last server having that copy. This is an asynchronous process, so at any one time the tables might be different, but once applications stop updating them, then the tables will converge to the most recently updated value. This type of processing means that there isn't any "manual" conflict detection as such (it is handled automatically by Q Apply), because the latest update will always win.

If two applications update the same record at exactly the same time, then Q replication uses the server number allocated when the peer-to-peer environment was set up to determine the winner. This type of processing means that two columns are added to each of the tables in the Q replication environment, where the first column is a timestamp of when the row was last updated (GMT) and the second column is the machine number. These updates are performed through triggers on the tables.