4 ms·

For that exact reason, we have a rule that every DDL migration must be a separate migration in the framework. So if migration #2 out of 3 fails, only 1 migratio

by kgeist 2y ago

For that exact reason, we have a rule that every DDL migration must be a separate migration in the framework. So if migration #2 out of 3 fails, only 1 migration will be marked as completed. If we see migration failed in CI/CD logs, it's immediately clear what exactly failed, and we can quickly revert what has managed to run (by applying the corresponding down migration), to return the DB to the previous valid state (if we decide to abort the release and fix the migrations first). Or we can rerun individual DDL migrations again, if it's a timeout issue. We design migrations in such a way so that new schemas/data never conflict with already running (old) code. And new code is activated only after all migrations completed successfully. So in practice half completed migrations is rarely an issue for us. But yeah, it requires more boilerplate. However, there's fewer surprises, compared to IF NOT EXISTS.

arp242 2y ago
There's lots of cases where that doesn't really work, and can't logically work. An obvious example is: drop index old; create index new [..]; There's tons of examples like this where things can kind-of work with enough effort, but also not really – if the query takes 5 minutes without that index it's just going to timeout, so not that different from having broken code, and not much you can do about that too. You really want those two combined to be atomic. And in cases of "create table new; drop table old" using the old table is often 1) a major hassle, and 2) often doesn't actually work all that well as it's not a tested path. Either way, it seems MariaDB does support transactions for all of this if you disable "autocommit mode", judging from their docs? I haven't used MySQL/MariaDB in quite a few years, but I've never seen migrations for them done well without tons of hassle and it's a major reason I've typically recommended PostgreSQL. It's just eliminates an entire class of common headaches and errors.
- kgeist 2y ago
  For your index example, why not just do: create index new [..] drop index old; At every point, there's always an index to use. We usually have to split releases into several "subreleases": Release #1 (automatic migration step) - alter the table - add a new index which uses new columns (etc.) (code release step) - new code goes live and starts using the new index Release #2 (we call it "post release") - remove the old unused index Automation, CI/CD help here. Yeah, it's somewhat error prone, but you usually get used to it. Not saying it's perfect, but manageable. In any case, you can never make both SQL migration and code release atomic (if you use rolling updates with zero downtime) so you already have to split into such subreleases anyway.
  - arp242 2y ago
    > Yeah, it's somewhat error prone, but you usually get used to it. Not saying it's perfect, but manageable. Of course; I'm just saying this is why people want idempotency and use "if not exists", especially in MariaDB/MySQL land.
- evanelias 2y ago
  > it seems MariaDB does support transactions for all of this No, DDL cannot be used in a transaction in any version of MySQL or MariaDB. > I've typically recommended PostgreSQL. It's just eliminates an entire class of common headaches and errors That's fair, but it does create new headaches, for example the notion of an invalid index (as described in the post) simply does not exist in MySQL or MariaDB. Index creation does not block concurrent writes, and won't fail mid-stream due to a lock conflict with old row purging (equivalent to PG's vacuum). And if a DDL statement somehow does fail, e.g. due to an ill-timed host crash, it is fully cleaned up -- DDL is atomic, just not transactional. One way to avoid the "half a migration" problem you mentioned up-thread is to use declarative schema management, which doesn't require tracking completion state of "migrations" at all: https://www.skeema.io/blog/2019/01/18/declarative/ https://www.skeema.io/blog/2019/01/18/declarative/

by kgeist 2y ago

arp242 2y ago

There's lots of cases where that doesn't really work, and can't logically work. An obvious example is: drop index old; create index new [..]; There's tons of examples like this where things can kind-of work with enough effort, but also not really – if the query takes 5 minutes without that index it's just going to timeout, so not that different from having broken code, and not much you can do about that too. You really want those two combined to be atomic. And in cases of "create table new; drop table old" using the old table is often 1) a major hassle, and 2) often doesn't actually work all that well as it's not a tested path. Either way, it seems MariaDB does support transactions for all of this if you disable "autocommit mode", judging from their docs? I haven't used MySQL/MariaDB in quite a few years, but I've never seen migrations for them done well without tons of hassle and it's a major reason I've typically recommended PostgreSQL. It's just eliminates an entire class of common headaches and errors.

kgeist 2y ago
For your index example, why not just do: create index new [..] drop index old; At every point, there's always an index to use. We usually have to split releases into several "subreleases": Release #1 (automatic migration step) - alter the table - add a new index which uses new columns (etc.) (code release step) - new code goes live and starts using the new index Release #2 (we call it "post release") - remove the old unused index Automation, CI/CD help here. Yeah, it's somewhat error prone, but you usually get used to it. Not saying it's perfect, but manageable. In any case, you can never make both SQL migration and code release atomic (if you use rolling updates with zero downtime) so you already have to split into such subreleases anyway.
- arp242 2y ago
  > Yeah, it's somewhat error prone, but you usually get used to it. Not saying it's perfect, but manageable. Of course; I'm just saying this is why people want idempotency and use "if not exists", especially in MariaDB/MySQL land.
evanelias 2y ago
> it seems MariaDB does support transactions for all of this No, DDL cannot be used in a transaction in any version of MySQL or MariaDB. > I've typically recommended PostgreSQL. It's just eliminates an entire class of common headaches and errors That's fair, but it does create new headaches, for example the notion of an invalid index (as described in the post) simply does not exist in MySQL or MariaDB. Index creation does not block concurrent writes, and won't fail mid-stream due to a lock conflict with old row purging (equivalent to PG's vacuum). And if a DDL statement somehow does fail, e.g. due to an ill-timed host crash, it is fully cleaned up -- DDL is atomic, just not transactional. One way to avoid the "half a migration" problem you mentioned up-thread is to use declarative schema management, which doesn't require tracking completion state of "migrations" at all: https://www.skeema.io/blog/2019/01/18/declarative/ https://www.skeema.io/blog/2019/01/18/declarative/