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You need to make sure all of your DB changes are backwards compatible. For example, adding new tables, adding columns (with defaults), and adding indexes can all be done without breaking existing code. The code does need to do the proper thing to make this work, such as INSERTs with the column list.

We normally disallow non-backwards compatible changes, such as renaming columns. We only drop tables after renaming and waiting a while (so we can quickly rename back).

When you have a lot of database servers this is pretty important since trying to keep them all exactly in-sync with the same schema at all times in the process is pretty much impossible. While doing the change, you are always going to end up with some finishing earlier than others.

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