ghost_adapter VS PaperTrail

At least for Rails there are several gems available (ie https://github.com/WeTransfer/ghost_adapter or https://github.com/departurerb/departure) that seamlessly hook into the existing migration system and will run all eligible migrations through gh-ost or pt-osc as needed. You're right that it's not free but it isn't all that far off either.

That said, online schema migrations are a specialized tool designed for very big tables that take hours to run an ALTER TABLE on. If all your tables are small enough that alterations take less than a second or so, don't bother and just block the table for a bit. It's fine.

https://github.com/WeTransfer/ghost_adapter integrates gh-ost with activerecord and the departure gem does the same but for pt-osc.

if the source (web) application makes their own audit tables. ex: our ruby on rails application uses the paper-trail gem

Start with https://github.com/paper-trail-gem/paper_trail

We use paper_trail for this

I think you could use that in conjunction with the paper_trail gem, as /u/GreenCalligrapher571 mentioned, which is also a good suggestion. As an additional note, when changing records in production while using the paper_trail gem, I suggest wrapping your database-mutating statements executed in the rails console within a whodunnit block, so PaperTrail.request(whodunnit: 'Dorian Marié') { widget.update name: 'Wibble' } or something rather than just widget.update name: 'Wibble'. Or, if you have some sort of issue-tracking / ticketing system, you could set the whodunnit value to the ticket number or whatever, and then anyone who wants to know why the records are in the state they're in can consult that ticket, which hopefully has additional relevant context.

For a while we did this using the paper-trail gem. This was a very simple way to add a few lines of code to keep track of all of the changes made to an ActiveRecord model. But it came with one drawback. Every change to the data had to be done through ActiveRecord. There are often times when this makes an app vulnerable to a race condition. I’ll use a contrived example so as not to share any real code from our client’s app.

In most designs, this would probably not be possible unless a table tracking extension is being used in an ORM. Even with change tracking enabled through extensions like paper trail or Django simple history, it can be tricky to restore deleted entities. Object tracking would need to have been enabled before it is needed to ensure the data is still around to be restored.

Maybe you are looking for the papertrail gem? https://github.com/paper-trail-gem/paper_trail