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:

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:

Another alternative is not making the session global and using a context manager, as described at the bottom of this section: 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 running 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:

Tuesday, June 21, 2016

Javascript's Round vs PHP's Round

Today I learned that by default PHP rounds differently than javascript.

PHP (using the default PHP_ROUND_HALF_UP)

php > echo round(-1.5);
"Round val up to precision decimal places away from zero, when it is half way there. Making 1.5 into 2 and -1.5 into -2."


"For negative numbers, if the decimal portion is exactly -0.5, the return value is the smallest integer that is greater than the number."

Saturday, May 21, 2016


It would be nice if there were a sqlalchemy/flask equivalent to this:

The equivalent might use sqlalchemy_utils for its generic relationships:

Thursday, May 19, 2016

Modified Preorder Tree Traversal

This was a great explanation of Modified Preorder Tree Traversal (or MPTT):

A common use-case for MPTT is categorization data, like with the way django-oscar uses django-mptt:

Sunday, May 8, 2016

Datatables - Uncaught TypeError: Cannot read property 'style' of undefined

If you see this error in datatables: "Uncaught TypeError: Cannot read property 'style' of undefined"

It's likely that you're missing a header column in your table. Make sure you have the same number of <th> elements as items in the "columns" section of your datatables initialization.