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.
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.
Just pull the library in using pip install django-dbconn-retry. Then add
django_dbconn_retry to INSTALLED_APPS in your settings.py.
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.
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.
Set to 0 to disable retries entirely. |
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. |
Many thanks to people who have contributed to this library:
- durant (@zhangzhw8)
- Shawn Zivontsis (@shawnz)
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:
- Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
- 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.
- 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.