#35702: Clarify mysqlclient does not provide connection pooling
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Reporter: Hisham Mahmood | Owner: (none)
Type: Cleanup/optimization | Status: new
Component: Documentation | Version: dev
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by Simon Charette):
* stage: Unreviewed => Accepted
Comment:
The note about `mysqlclient` being thread safe and implementing connection
pooling was inherited in #23446
(7f089ac2e3e6a7f4b2b41085a37d35e074fad805). The thread safety and
connection pooling claims were added for `MySQLDB`
[https://github.com/django/django/pull/1906/files originally] but they are
certainly not true for either `pymysql` or `mysqlclient`.
As pointed out in [https://mysqlclient.readthedocs.io/user_guide.html
their docs]
> The MySQL protocol can not handle multiple threads using the same
connection at once. Some earlier versions of MySQLdb utilized locking to
achieve a threadsafety of 2. While this is not terribly hard to accomplish
using the standard Cursor class (which uses mysql_store_result()), it is
complicated by SSCursor (which uses mysql_use_result(); with the latter
you must ensure all the rows have been read before another query can be
executed. It is further complicated by the addition of transactions, since
transactions start when a cursor executes a query, but end when COMMIT or
ROLLBACK is executed by the Connection object. Two threads simply cannot
share a connection while a transaction is in progress, in addition to not
being able to share it during query execution. This excessively
complicated the code to the point where it just isn’t worth it.
>
> The general upshot of this is: Don’t share connections between threads.
It’s really not worth your effort or mine, and in the end, will probably
hurt performance, since the MySQL server runs a separate thread for each
connection. You can certainly do things like cache connections in a pool,
and give those connections to one thread at a time. **If you let two
threads use a connection simultaneously, the MySQL client library will
probably upchuck and die. You have been warned**.
I would remove the thread safety claims as well.
--
Ticket URL: <https://code.djangoproject.com/ticket/35702#comment:1>
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