The third part module, which clutters up any serches you make for the built-in one.
Each transaction isolation level specifies what can or cannot happen when two queries are being run in parallel, i.e.: the memory semantics of the system.
Remember that queries can affects thousands of rows, and database systems like PostgreSQL can run multiple such queries at the same time.
Good summary on the PostgreSQL page: www.postgresql.org/docs/14/transaction-iso.html
Implementation specifics:
@cirosantilli/_file/nodejs/sequelize/raw/nodejs/sequelize/raw/parallel_create_delete_empty_tag.js by 
Ciro Santilli 35 Updated 2025-04-05 +Created 1970-01-01
Ciro Santilli 35 Updated 2025-04-05 +Created 1970-01-01In 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:which would result in the new post incorrectly not having the
- thread 2: delete old post
- thread 2: find all tags with 0 posts. Finds
tag0from 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'sINSERT OR IGNORE INTOor PostgreSQL'sINSERT ... ON CONFLICT DO NOTHING - thread 1: assign
tag0to 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
tag0is empty, and deletes it, which then cascades into the join table
tag0.Failure case 2:which leads to a foreign key failure, because the tag does not exist anymore when the assignment happens.
- 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
tag0is empty, and deletes it - thread 1: assign
tag0to the new post
Failure case 3:which leads to a foreign key failure, because the tag does not exist anymore when the assignment happens.
- 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
tag0to the new post
Sample executions:All executions use 2 threads.
node --unhandled-rejections=strict ./parallel_create_delete_empty_tag.js p 9 1000 'READ COMMITTED': PostgreSQL, 9 tags, DELETE/CREATE thetag0test tag 1000 times, useREAD COMMITTEDExecution often fails, although not always. The failure is always:because the:error: insert or update on table "PostTag" violates foreign key constraint "PostTag_tagId_fkey"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.INSERT INTO "PostTag"TODO: we've never managed to observe the failure case in whichtag0is 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 aSELECT ... FOR UPDATEbefore trying toINSERT.This is likely correct and the fastest correct method according to our quick benchmarking, about 20% faster thanREPEATABLE READ.We are just now 100% sure it is corret becase we can't find out if theSELECTin theDELETEsubquery could first select some rows, which are then locked by the tag creator, and only then locked byDELETEafter selection. Or does it re-evaludate theSELECTeven though it is in a subquery?node --unhandled-rejections=strict ./parallel_create_delete_empty_tag.js p 9 1000 'REPEATABLE READ': repeatable readWe'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.
- the
node --unhandled-rejections=strict ./parallel_create_delete_empty_tag.js p 9 1000 'SERIALIZABLE': serializablenode --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 thanREAD COMMITTED
Some theoretical notes:
- Failure case 3 is averted by a
READ COMMITTEDtransaction, 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.
sqlite3 ':memory:' 'WITH t (i) AS (VALUES (-1), (-1), (-2)) SELECT *, row_number() over () FROM t'-1|1
-1|2
-2|3With a possible output:
partition by:sqlite3 ':memory:' 'WITH t (i) AS (VALUES (-1), (-1), (-2)) SELECT *, row_number() over ( partition by i ) FROM t'-2|1
-1|1
-1|2
The most powerful GUI file manager ever?? Infinite configurability??
Ciro Santilli wasted some time on it before he gave up on file managers altogether and started using only the CLI with a few aliases.
Law enforcement in the United States by Ciro Santilli 35 Updated 2025-04-05 +Created 1970-01-01
Ciro Santilli 35 Updated 2025-04-05 +Created 1970-01-01Very hot stuff! It's like ISA-portable assembly, but with types! In particular it also it deals with calling conventions for us (since it is ISA-portable). TODO: isn't that exactly what C does? :-) LLVM IR vs C
Documentation: llvm.org/docs/LangRef.html
Pinned article: ourbigbook/introduction-to-the-ourbigbook-project
Welcome to the OurBigBook Project! Our goal is to create the perfect publishing platform for STEM subjects, and get university-level students to write the best free STEM tutorials ever.
Everyone is welcome to create an account and play with the site: ourbigbook.com/go/register. We belive that students themselves can write amazing tutorials, but teachers are welcome too. You can write about anything you want, it doesn't have to be STEM or even educational. Silly test content is very welcome and you won't be penalized in any way. Just keep it legal!
We have two killer features:
- topics: topics group articles by different users with the same title, e.g. here is the topic for the "Fundamental Theorem of Calculus" ourbigbook.com/go/topic/fundamental-theorem-of-calculusArticles of different users are sorted by upvote within each article page. This feature is a bit like:
- a Wikipedia where each user can have their own version of each article
- a Q&A website like Stack Overflow, where multiple people can give their views on a given topic, and the best ones are sorted by upvote. Except you don't need to wait for someone to ask first, and any topic goes, no matter how narrow or broad
This feature makes it possible for readers to find better explanations of any topic created by other writers. And it allows writers to create an explanation in a place that readers might actually find it.
Figure 1. Screenshot of the "Derivative" topic page. View it live at: ourbigbook.com/go/topic/derivative - local editing: you can store all your personal knowledge base content locally in a plaintext markup format that can be edited locally and published either:This way you can be sure that even if OurBigBook.com were to go down one day (which we have no plans to do as it is quite cheap to host!), your content will still be perfectly readable as a static site.
- to OurBigBook.com to get awesome multi-user features like topics and likes
- as HTML files to a static website, which you can host yourself for free on many external providers like GitHub Pages, and remain in full control
Figure 2. You can publish local OurBigBook lightweight markup files to either OurBigBook.com or as a static website.
Figure 3. Visual Studio Code extension installation.
Figure 4. Visual Studio Code extension tree navigation.
Figure 5. . You can also edit articles on the Web editor without installing anything locally. Video 3. Edit locally and publish demo. Source. This shows editing OurBigBook Markup and publishing it using the Visual Studio Code extension.Video 4. OurBigBook Visual Studio Code extension editing and navigation demo. Source. - Internal cross file references done right:
- Infinitely deep tables of contents:
Figure 6. Dynamic article tree with infinitely deep table of contents.Live URL: ourbigbook.com/cirosantilli/chordateDescendant pages can also show up as toplevel e.g.: ourbigbook.com/cirosantilli/chordate-subclade
All our software is open source and hosted at: github.com/ourbigbook/ourbigbook
Further documentation can be found at: docs.ourbigbook.com
Feel free to reach our to us for any help or suggestions: docs.ourbigbook.com/#contact