Having a large number of post-deploy migrations running out-of-numerical-sequence with pre-deploy migrations can be problematic. For example, if we have the sequence
- db/migrate/2017... - add column
- db/post_migrate/2018... - drop the column
- db/migrate/2021... - add the same column again
It will work fine in numerical order. But if you run the pre-deploy migrations **followed by** the post-deploy migrations, you will not get the same result.
Our post-deploy system is designed to allow for seamless upgrades of Discourse. However, it is reasonable for us to only support this totally seamless experience for a limited period of time. This commit moves all post_deploy migrations which are more than 1 year old (i.e. more than 2 major Discourse versions ago) into the regular pre-deploy migrations directory. This limits the impact of any edge cases caused by out-of-numerical-sequence migrations.
This reverts commit 8b46f14744.
It corrects the reason for the revert:
We rely on SafeMigrate existing cause we call it from migrations,
Zeitwerk will autoload it.
Instead of previous pattern we explicitly bypass all the hacks in
production mode.
We need to disable SafeMigrate cause it is not thread safe.
A thread safe implementation is possible but not worth the effort,
we catch the issues in dev and test.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
This moves us away from the delayed drops pattern which
was problematic on two counts. First, it uses a hardcoded "delay for"
duration which may be too short for certain deployment strategies.
Second, delayed drop doesn't ensure that it only runs after
the latest application code has been deployed. If the migration runs
and the application code fails to deploy, running the migration after
"delay for" has been met will cause the application to blow up.
The new strategy allows post deployment migrations to be skipped if the
env `SKIP_POST_DEPLOYMENT_MIGRATIONS` is provided.
```
SKIP_POST_DEPLOYMENT_MIGRATIONS=1 rake db:migrate
-> deploy app servers
SKIP_POST_DEPLOYMENT_MIGRATIONS=0 rake db:migrate
```
To aid with the generation of a post deployment migration, a generator
has been added. Simply run `rails generate post_migration`.
Often we need to amend our schema, it is tempting to use
drop_table, rename_column and drop_column to amned schema
trouble though is that existing code that is running in production
can depend on the existance of previous schema leading to application
breaking until new code base is deployed.
The commit enforces new rules to ensure we can never drop tables or
columns in migrations and instead use Migration::ColumnDropper and
Migration::TableDropper to defer drop the db objects
