The Strangler Pattern for Database Migration (With XA Transactions)

How XA can help you migrate databases…

The Problem

So you are using a DBMS from one vendor and want to migrate to a DBMS from another vendor. You need some time to migrate all DBMS access code to the new database platform - so how do you do this without a big-bang release after X months (almost a guarantee for failure)?

The Solution

If both DBMS support XA then the solution is relatively easy:

• Setup the initial instance of the target DBMS
• Change all DBMS updates to go to the two DBMS with XA transactions (so updates are guaranteed to happen on both instances during the migration)
• Gradually migrate your applications to use only the target DBMS
• When no accesses remain to the original DBMS, take out the dual writes with XA transactions

Limitations

This works only if both DBMS support XA. Why complicate your life by selecting a storage technology without XA support? You will effectively lock yourself in because you know you won't be able to migrate away easily.