django-dbconn-retry 0.3.1

Patch Django to retry a database connection first before failing.

Django Database Connection Autoreconnect

This library monkeypatches django.db.backends.base.BaseDatabaseWrapper so that when a database operation fails because the underlying TCP connection was already closed, it first tries to reconnect, instead of immediately raising an OperationException.

Why is this useful?

I use HAProxy as a load-balancer in front of my PostgreSQL databases all the time, sometimes in addition to pgbouncer. Even though you can mostly prevent surprises by enabling TCP keep-alive packets through tcpka, clitcpka and srvtcpka, I still encounter situations where the underlying TCP connection has been closed through the load-balancer. Most often this results in

django.db.utils.OperationalError: server closed the connection unexpectedly
This probably means the server terminated abnormally before or while
processing the request.

This library patches Django such that it will try to reconnect once before failing.

Another application of this is when using Hashicorp Vault, where credentials for a database connection can expire at any time and then need to be refreshed from Vault.

How to install?

Just pull the library in using pip install django-dbconn-retry. Then add django_dbconn_retry to INSTALLED_APPS in your settings.py.

Signals

The library provides an interface for other code to plug into the process to, for example, allow 12factor-vault to refresh the database credentials before the code tries to reestablish the database connection. These are implemented using Django Signals.

Signal	Description
pre_reconnect	Installs a hook of the type Callable[[type, BaseDatabaseWrapper], None] that will be called before the library tries to reestablish a connection. 12factor-vault uses this to refresh the database credentials from Vault.
post_reconnect	Installs a hook of the type Callable[[type, BaseDatabaseWrapper], None] that will be called after the library tried to reestablish the connection. Success or failure has not been tested at this point. So the connection may be in any state.

Both signals send a parameter dbwrapper which points to the current instance of django.db.backends.base.BaseDatabaseWrapper which allows the signal receiver to act on the database connection.

Settings

Here’s a list of settings available in django-dbconn-retry and their default values. You can change the value in your settings.py.

Setting	Description
MAX_DBCONN_RETRY_TIMES	Default: 1 The max times which django-dbconn-retry will try.
DBCONN_RETRY_DELAY	Default: 0 The base delay in seconds before each retry attempt. Used as the initial delay and, together with DBCONN_RETRY_BACKOFF, controls the delay for subsequent retries. Set to 0 to retry immediately.
DBCONN_RETRY_BACKOFF	Default: 1 The multiplier applied to the delay after each retry. For example, with DBCONN_RETRY_DELAY=1 and DBCONN_RETRY_BACKOFF=2, the delays will be 1s, 2s, 4s, etc. Note that delays grow exponentially and can become very large for higher values. For example, with DBCONN_RETRY_DELAY=10, DBCONN_RETRY_BACKOFF=3 and MAX_DBCONN_RETRY_TIMES=10, the final retry would wait on the order of tens of hours. Choose these settings carefully if very long waits are not acceptable for your deployment.

Contributors

Many thanks to people who have contributed to this library:

• durant (@zhangzhw8)

• Shawn Zivontsis (@shawnz)

License

Copyright (c) 2018, Jonas Maurus All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

3. Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.