Austria Updated 2025-07-16
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nodejs/sequelize/raw/parallel_create_delete_empty_tag.js Updated 2025-07-16
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: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"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.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:
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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- knowyourmeme.com/memes/high-expectations-asian-father
- www.reddit.com/r/AsianParentStories/ 100k+ subs Reddit sub about it
Program in C++. No. Program in A++
. 
Jáchymov Updated 2025-07-16
"Joachimsthal" is the German for it. Note how it is just near the modern frontier between Germany and the Czech Republic.
en.wikipedia.org/w/index.php?title=Uranium&oldid=1243907294#Pre-discovery_use:
In the early 19th century, the world's only known sources of uranium ore were these mines.
Apparently the region was a silver mining center:
Starting in the late Middle Ages, pitchblende was extracted from the Habsburg silver mines in Joachimsthal, Bohemia (now Jáchymov in the Czech Republic), and was used as a coloring agent in the local glassmaking industry
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Leading Chinese lyrics website: mojim.com/twznew.htm
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Literally: "capital of the East". Note that the Chinese term 东京 (dong1 jing1) is highly ambiguous in historical contexts, as many places throughout history have been known as "the capital of the East".Of of those other places is the Kaifeng, which is the Capital in Water Margin and in real life during the Song dynasty. The novel is also likely highly popular in Japan apparently as well BTW, e.g. it merited a well funded adaptation: The Water Margin.
São Paulo City Updated 2025-07-16
Although Ciro Santilli lived in São Paulo City nominally during his studies, it would be more precise to say that he lived in University of São Paulo-land, because Ciro was cheap, didn't have a car, and did nothing but study, stay at home, go back to Santos to see his parents and the beach.
But the little he saw of the city made a deep impression on him.
The unreasonable size.
The unbearable contrasts.
Caetano's Sampa is the ultimate description of the city!
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