Flask-SQLAlchemy sets up a connection pool with 5 connections and will
create up to 10 additional connections if all the pool ones are in use.
If all connections in the pool and all overflow connections are in
use, SQLAlchemy will block new DB sessions until a connection becomes
available. If a session can't acquire a connections for a specified
time (we set it to 30s) then a TimeoutError is raised.
By default db.session is deleted with the related context object
(so when the request is finished or app context is discarded).
This effectively limits the number of concurrent requests/tasks with
multithreaded gunicorn/celery workers to the maximum DB connection pool
size. Most of the time these limits are fine since the API requests are
relatively quick and are mainly interacting with the database anyway.
Service callbacks however have to make an HTTP request to a third party.
If these requests start taking a long time and the number of threads is
larger than the number of DB connections then remaining threads will
start blocking and potentially failing if it takes more than 30s to
acquire a connection. For example if a 100 threads start running tasks
that take 20s each with a max DB connection pool size of 10 then first 10
threads will acquire a connection right away, next 10 tasks will block for
20 seconds before the initial connections are released and all other tasks
will raise a TimeoutError after 30 seconds.
To avoid this, we perform all database operations at the beginning of
the task and then explicitly close the DB session before sending the
HTTP request to the service callback URL. Closing the session ends
the transaction and frees up the connection, making it available for
other tasks. Making calls to the DB after calling `close` will acquire
a new connection. This means that tasks are still limited to running
at most 15 queries at the same time, but can have a lot more concurrent
HTTP requests in progress.
- check if service_callback_api exist before putting tasks on queue
- create_service_callback_api in tests before asserting if send_delivery_status_to_service has been called.
