On 04/25/2016 08:30 PM, Amit Saha wrote:
Hi all, In my scenario, DB migrations (via alembic) will be run as part of the app deployment and multiple app deployments will happen near simultaneously (multiple EC2 instances talking to the same MySQL DB). Let's, for simplicity's sake assume two instances: Instance #1: Alembic sees alembic_version table and finds that we do need the migrations to happen, and starts the migration. Instance #2: Alembic sees alembic_version table and finds that we do need the migrations to happen, and starts the migration. This doesn't obviously happen since MySQL will lock the table during the migration.
this scenario is entirely unsupported. MySQL does not support transactional DDL and migration #1 will be hitting some tables and migration #2 another. The version table is not updated until after a particular migration takes place so concurrent processes will both be doing the same migration at the same time.
Now, let's say instance #1 finishes the migration first and updates the alembic_version table. Then, the migration triggered by instance #2 starts to run, and errors out because the schema changes are already in place. Is this what will happen or does alembic get a read/write lock on alembic_version table for the entire migration?
there are no "read/write locks" on the alembic_version table. What you're doing will not work at all. Only one process should be running migrations against a single target database at a time.
(And hence instance #2
never will need to run the migration since when it reads alembic_version, the DB already has the migrated schema). Thanks for your insights. Best Wishes, Amit. -- You received this message because you are subscribed to the Google Groups "sqlalchemy-alembic" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy-alembic+unsubscr...@googlegroups.com <mailto:sqlalchemy-alembic+unsubscr...@googlegroups.com>. For more options, visit https://groups.google.com/d/optout.
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