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Online schema changes

Online schema changes

Mar 25, 2009

  • One of the problems with databases is schema management. Changes to a database schema are highly disruptive:

    • Bring the system down.

    • Make a collection of changes to the schema.

    • Bring the system back up again.

    • Discover you made a mistake, at which point your system is unusable until you can fix it.

  • For RAMCloud the system must be able to evolve with no downtime.

  • A fixed scheme that probably doesn't make sense.

  • One alternative: make the individual data items self-identifying enough that the schema can evolve gracefully:

    • Perhaps there is no schema: individual records are variable-length with variable columns and completely self-identifying.

    • Store a version number in each record?

  • What is the state-of-the-art for schema changes in relational databases today? Can they be done without bringing the system down at all?