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Possible Duplicate:
Django cache.set() causing duplicate key error

I ran into this problem using django core's database cache:

ERROR:  duplicate key value violates unique constraint "cache_pkey"
STATEMENT:  INSERT INTO "cache" (cache_key, value, expires) VALUES (E':1:cms-menu_nodes_en-us_1', E'gAJdcQEoY21lbnVzLmJhc2UKTmF2aW
LOG:  server process (PID 8453) was terminated by signal 9: Killed
LOG:  terminating any other active server processes
LOG:  all server processes terminated; reinitializing
FATAL:  could not create shared memory segment: Cannot allocate memory
DETAIL:  Failed system call was shmget(key=5432001, size=29278208, 03600).

I looked in the table and sure enough, there is an entry for the key ':1:cms-menu_nodes_en-us_1'. I found a similar issue here, but was unable to exactly understand what the issue is.

Anyone have any ideas or suggestions? Sounds like a bug in django core, since if a key exist, it should update the record.

edit: I should have clarified that the DB was PostgreSQL 8.4.7. Thanks lazerscience.

edit @ Jack M: I haven't been able to replicate this error, but believe the code is in django.core.cache.backends.db.DatabaseCache in a method called set() that calls _base_set().

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  • With what database backend did you try it? Try it with others as well... Commented Jul 12, 2011 at 16:39
  • What is the code which is adding this entry to the cache? Commented Jul 12, 2011 at 20:12

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Sounds like a bug in django core, since if a key exist, it should update the record.

Indeed, but I'd suggest that said bug is related to a concurrency issue, in which case it could be fixed at the app level. As in two neighboring calls to the same asset/page/whatever run an exist() statement, find no row, and proceed to insert as a result -- without issuing a lock of any kind, and without wrapping the thing in a transaction to discard the offending call and (since it's just a cache) continuing.

It also begs one question: are you sure that you should be caching in your database in the first place? The database typically is a bottleneck in a web application (especially when using an ORM), and the whole point of caching is to avoid that bottleneck. Shouldn't you be using memcache instead?

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My issue is that I'm memory constrained, so adding memcache is not an option. The webpage makes an in-ordinate number of sql calls so instead of optimizing in code, a single database call significantly sped things up. That's assuming the cache works....

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