gh-ost VS wal2json

I’ve been through multiple incidents where everything worked fine in the testing environment but ended up locking the production database for minutes when deployed. A category of open-source tools called OSC (Online Schema Change) exists to mitigate such pain, like gh-ost used by GitHub and OSC used by Meta. They work by creating a set of "ghost tables" to apply the migrations, copy over old data from the original tables, and catch up with new writes simultaneously. When all old data is migrated, you can trigger a cutover to make the "ghost tables" production. Check the post below for a great introduction and comparison:

Sounds like it's basically explained in the gh-ost readme https://github.com/github/gh-ost#how

I think it amounts to "use views to decouple access to the table with a fixed interface" and "use triggers for migrating data between tables"

Gh-ost is the new hotness. Simple to use and lots of great features: https://github.com/github/gh-ost

If the performance of the db is a concern during migrations (locking, high cpu consumption for large writes) there are tools that can help and do similiar to what your describing but with the benefit that they are battle tested tools. This one spring to mind https://github.com/github/gh-ost there are other options as well and its worth reading the trade off docs

Not sure which version of MySQL you're using, but one approach would be to use a tool like pt-online-schema-change (from Percona) or g-host -- which will create a duplicate table and then swap it in place of the original table. It's a safer approach when operating in production environments. Here's a good comparison of the tools many people use https://planetscale.com/docs/learn/online-schema-change-tools-comparison

No, especially on large tables with billions of records. They make online schema changes impossible. More details: https://github.com/github/gh-ost/issues/331#issuecomment-266...

Tip #4: Consider slow-running migrations. Some tables can be so large that the traditional migration way is simply not a viable option for them. In such cases, you can consider embedding the data migration code right into your application, or use a special utility like GitHub's online schema migration for MySQL. A slow-running migration can work in production for days or even weeks. It gradually converts the data by small chunks, so you can carefully balance the load on the database while making sure that it doesn't cause slowness or downtime.

GitHub gh-ost

Actually I never tried but I was scared by the small print of GH not using RDS themselves [1] and Ghost relying on lower-level features that might be not easily available in RDS. Also I had the impression you have to setup a normal non-RDS replica attached to your RDS master?

[1] https://github.com/github/gh-ost/blob/master/doc/rds.md

To illustrate the above, let’s look at an example, using wal2json for convenience. An update event looks something like this:

You could also use tools in conjunction with logical replication to stream changes, like debezium or wal2json.

I've done CDC before for mysql and postgres, in both cases, rolled my own 200~ loc python scripts. No experience with DMS. Debezium, as I understand, needs a kafka topic to stream to, so wasn't workable for us. Rolling our own proved to be fairly trivial. For postgres, there's wal2json https://github.com/eulerto/wal2json

I was researching for the whole day and found out https://github.com/eulerto/wal2json and postgres bult-in `pgoutput` plugin help me to catch the changes.

I don't think this is something for "rails" to do per se, and more the underlying DB. replication tools to capture a "change log". I have use postgres Write Ahead Logs (WAL) shipped to s3 in json format https://github.com/eulerto/wal2json

[0] https://supabase.com [1] https://github.com/supabase/realtime [2] https://github.com/eulerto/wal2json [3] https://github.com/supabase/walrus [4] https://gsd.di.uminho.pt/members/cbm/ps/delta-crdt-draft16ma... [5] https://supabase.com/blog/2022/04/01/supabase-realtime-with-... [6] https://fly.io [7] https://github.com/supabase/realtime/tree/multiplayer [8] https://github.com/supabase/supabase/tree/master/docker

wal2json Output Plugin that converts WAL output to JSON objects [Open Source] pg_recvlogical Postgres app that can consume update stream [Out-of-the-box with Postgres] decoderbufs Output Plugin that delivers data as protobuf [Open Source, Used in Debezium]