Thursday, January 19, 2017

Python - David Beazley's "Understanding The Python GIL"

This talk explains a lot about Python's multithreading: http://www.dabeaz.com/python/UnderstandingGIL.pdf

In Python 2.7, it knows to switch to different thread whenever it starts waiting on IO. And, it will check if it needs to switch threads every 100 bytecode instructions.

Monday, January 16, 2017

SQLAlchemy - Is is necessary to commit after session.execute?

The docs for session.execute say:
Execute a SQL expression construct or string statement within the current transaction.
So, the answer is yes, you need to issue a commit after running session.execute().

Here's a code example along with the output from "echo=True" showing it begins the session but doesn't end it unless you run session.commit():

Thursday, November 17, 2016

SQLAlchemy 1.1.4's New "server_side_cursors" Option

Before SQLAlchemy 1.1.4, if you wanted to stream your MySQL query using server side cursors, you would need to run your query using "execution_options(stream_results=True)" and pass SSCursor into create_engine's connect_args. Now, all you need to do is pass "server_side_cursors=True" into create_engine and it will automatically stream the results for your select queries.

http://docs.sqlalchemy.org/en/latest/dialects/mysql.html#server-side-cursors

Sunday, October 23, 2016

Is it necessary to run SQLAlchemy's session.remove()?

If you're using flask-sqlalchemy, then the answer is no - it's already doing it for you when the request finishes: https://github.com/mitsuhiko/flask-sqlalchemy/blob/2.1/flask_sqlalchemy/__init__.py#L823

If you're using scoped_session and not explicitly running session.remove(), then your connections will only be returned to the pool after your thread finishes and garbage collection occurs. It's not a good idea to leave this to garbage collection, because you can't guarantee your connections will be returned to the pool when your application is busy. This will often lead to errors like this: "TimeoutError: QueuePool limit of size 5 overflow 10 reached, connection timed out, timeout 30"

The solution is to make sure session.remove() runs when the work finishes (like flask-sqlalchemy does), as described here: http://docs.sqlalchemy.org/en/latest/orm/contextual.html#using-thread-local-scope-with-web-applications

Another alternative is not making the session global and using a context manager, as described at the bottom of this section: http://docs.sqlalchemy.org/en/latest/orm/session_basics.html#when-do-i-construct-a-session-when-do-i-commit-it-and-when-do-i-close-it However, this can make things difficult when you need to access the query results outside of the context manager. If you access the query results outside of the context manager without running session.expunge_all(), you'll see all kinds of errors like this: "DetachedInstanceError: Instance <ReportingJob at 0xa41cd8c> is not bound to a Session; attribute refresh operation cannot proceed"

Another option is setting autocommit to True, but that has quite a few gotchas. Turning on autocommit will acquire connections from the engine on an as-needed basis and return them immediately after their use. You will have to explicitly start transactions with session.begin() if you enable it. It seems like this setting should be called "autotransaction" instead. It's also not very efficient, because it needs to get and return a connection each time you run a query.

Friday, September 2, 2016

PHP - max_execution_time doesn't work for socket operations

In new relic transaction logs, I was seeing some PHP requests last as long as an hour. It turns out that time spent waiting on sockets doesn't apply toward max_execution_time, and loops that involved waiting on sockets could end up taking a really long time without timing out.

PHP docs have this to say about max_execution_time:
"The set_time_limit() function and the configuration directive max_execution_time only affect the execution time of the script itself. Any time spent on activity that happens outside the execution of the script such as system calls using system(), stream operations, database queries, etc. is not included when determining the maximum time that the script has been running. This is not true on Windows where the measured time is real."

The solution ended up being setting request_terminate_timeout in php-fpm.

Saturday, August 27, 2016

Python - Cachetools LRUCache KeyError

If your application is threaded and you're getting a "KeyError" while using the non-decorator version of cachetool's LRUCache, then you need to put whatever is manipulating the cache object inside of a lock. Also, since LRUCache is modified when values are gotten from it, you will also need to make sure you're locking when you get values from cache too. If you can use the decorator version of LRUCache, that's preferred since it has built-in locking.

Here's an example of the error:

And an example of the fix: https://bitbucket.org/zzzeek/dogpile.cache/pull-requests/32/add-a-cachetools-lru-lfu-in-memory-backend/diff#comment-22242704