status-go/protocol/migrations/sqlite/README.md

### How to write migrations?

We only write `up` migrations, `down` migrations are not always possible in sqlite or 
too complex/too expensive. For example to remove a column you would have to duplicate
the table, copy over the data, delete and recreated.
This can be very expensive for some tables (user_messages for example), so should not be attempted.


### Notes

One issue we faced multiple times is that updates to `user_messages` can be very 
expensive, leading to slow upgrade times and interrupted migrations. So avoid
writes if not necessary.
Fix communities migration In some instances the communities migration would be skipped but not marked as `dirty`. This commit addresses the issue by: - Making sure that if dirty is set the migration is not skipped but replayed - If the version is on the communities migration and dirty is false, we check for the presence of the communities table. If not present we replay the communities migration. - Make community_id field in user_messages nullable It also removes all the `down` migration, as we can't use them effectively, as explained in the README.md added. 2021-01-07 12:16:19 +01:00			`### How to write migrations?`

			We only write `up` migrations, `down` migrations are not always possible in sqlite or
			`too complex/too expensive. For example to remove a column you would have to duplicate`
			`the table, copy over the data, delete and recreated.`
			`This can be very expensive for some tables (user_messages for example), so should not be attempted.`


			`### Notes`

			One issue we faced multiple times is that updates to `user_messages` can be very
			`expensive, leading to slow upgrade times and interrupted migrations. So avoid`
			`writes if not necessary.`