wal2json VS gh-ost

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 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]

The format of these change events will be determined based on the Logical Decoding Output Plugin. For example, the wal2json output plugin allows you to output the changes in JSON, which are easier to parse than the test_decoding plugin output.

We do this by installing this plugin called wal2json, which is done by following it's README's instructions, that describes how to enable logical replication.

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

As they have said it depends on the size of the table on disk and the number of rows, but an alter in production is not difficult to last from seconds to days. I don't know if you are doing the alter as is but try to check https://docs.percona.com/percona-toolkit/pt-online-schema-change.html or https://github.com/github/gh-ost , they usually simplify a lot the alters.

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

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

MySQL has some robust tooling in this space. Some of the tools use triggers to copy to a new table. GitHub's gh-ost[1] is probably the state of the art, and uses the binary log stream to replicate the data.

[1] https://github.com/github/gh-ost

https://github.com/github/gh-ost/issues/331#issuecomment-266...) it does become a little bit of a "you do not have google problems" type discussion.

(Perhaps you do have such problems, I don't know where you work! But 99%+ of companies don't have such problems and never will.)

GitHub doesn't use foreign keys[1], and there's likely many tables related to all the users, notifications, permissions, etc... that would need to be cleaned up. Without foreign keys they likely have some system process that does this instead of a simple `DELETE FROM` which cascades.

1. https://github.com/github/gh-ost/issues/331#issuecomment-266...