Ciro Santilli @cirosantilli 37

In this example, posts have tags. When a post is deleted, we check to see if there are now any empty tags, and now we want to delete any empty tags that the post deletion may have created.

If we are creating and deleting posts concurrently, a naive implementation might wrongly delete the tags of a newly created post.

This could be due to a concurrency issue of the following types.

Failure case 1:

thread 2: delete old post
thread 2: find all tags with 0 posts. Finds tag0 from the deleted old post which is now empty.
thread 1: create new post, which we want to have tag tag0
thread 1: try to create a new tag tag0, but don't because it already exists, this is done using SQLite's INSERT OR IGNORE INTO or PostgreSQL's INSERT ... ON CONFLICT DO NOTHING
thread 1: assign tag0 to the new post by adding an entry to the join table
thread 2: delete all tags with 0 posts. It still sees from its previous search that tag0 is empty, and deletes it, which then cascades into the join table

which would result in the new post incorrectly not having the tag0.

Failure case 2:

thread 2: delete old post
thread 2: find all tags with 0 posts
thread 1: create new post
thread 1: try to create a new tag tag0, but don't because it already exists
thread 2: delete all tags with 0 posts. It still sees from its previous search that tag0 is empty, and deletes it
thread 1: assign tag0 to the new post

which leads to a foreign key failure, because the tag does not exist anymore when the assignment happens.

Failure case 3:

thread 2: delete old post
thread 1: create new post, which we want to have tag tag0
thread 1: try to create a new tag tag0, and succeed because it wasn't present
thread 2: find all tags with 0 posts, finds the tag that was just created
thread 2: delete all tags with 0 posts, deleting the new tag
thread 1: assign tag0 to the new post

which leads to a foreign key failure, because the tag does not exist anymore when the assignment happens.

Sample executions:

node --unhandled-rejections=strict ./parallel_create_delete_empty_tag.js p 9 1000 'READ COMMITTED': PostgreSQL, 9 tags, DELETE/CREATE the tag0 test tag 1000 times, use READ COMMITTED
Execution often fails, although not always. The failure is always:
```
error: insert or update on table "PostTag" violates foreign key constraint "PostTag_tagId_fkey"
```
because the:
```
INSERT INTO "PostTag"
```
tries to insert a tag that was deleted in the other thread, as it didn't have any corresponding posts, so this is the foreign key failure.
TODO: we've never managed to observe the failure case in which tag0 is deleted. Is it truly possible? And if not, by which guarantee?
node --unhandled-rejections=strict ./parallel_create_delete_empty_tag.js p 9 1000 'READ COMMITTED' 'FOR UPDATE': do a SELECT ... FOR UPDATE before trying to INSERT.
This is likely correct and the fastest correct method according to our quick benchmarking, about 20% faster than REPEATABLE READ.
We are just now 100% sure it is corret becase we can't find out if the SELECT in the DELETE subquery could first select some rows, which are then locked by the tag creator, and only then locked by DELETE after selection. Or does it re-evaludate the SELECT even though it is in a subquery?
node --unhandled-rejections=strict ./parallel_create_delete_empty_tag.js p 9 1000 'REPEATABLE READ': repeatable read
We've never observed any failures with this level. This should likely fix the foreign key issue according to the PostgreSQL docs, since:
- the DELETE "Post" commit cannot start to be seen only in the middle of the thread 1 transaction
- and then if DELETE happened, the thread 1 transaction will detect it, ROLLBACK, and re-run. TODO how does it detect the need rollback? Is it because of the foreign key? It is very hard to be sure about this kind of thing, just can't find the information. Related: postgreSQL serialization failure.
node --unhandled-rejections=strict ./parallel_create_delete_empty_tag.js p 9 1000 'SERIALIZABLE': serializable
node --unhandled-rejections=strict ./parallel_create_delete_empty_tag.js p 9 1000 'NONE': magic value, don't use any transaction. Can blow up of course, since even less restrictions than READ COMMITTED

All executions use 2 threads.

Some theoretical notes:

Failure case 3 is averted by a READ COMMITTED transaction, because thread 2 won't see the uncommitted tag that thread 1 created, and therefore won't be able to delete it

stackoverflow.com/questions/10935850/when-to-use-select-for-update from SELECT FOR UPDATE also talks about a similar example, and has relevant answers.

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