Preventing Cache Stampedes Using python-redis-lock

This is a companion repo for the blog post, "Preventing Cache Stampedes Using python-redis-lock" on the EFL Technology Blog.

It contains three example scripts that show how to use locks to prevent "cache stampedes" when cached values become stale:

• cache_stampede/example_no_protection.py shows what happens during a cache stampede.
• cache_stampede/example_threading.py proves the concept by using threading.Lock to control access to the cache among multiple threads in a single process.
• cache_stampede/example_redis.py takes it a step further by moving the lock into Redis itself, allowing access to be controlled across multiple processes, even multiple servers.

Running the Examples

This project is compatible with Python 3.6 and 2.7.

Set up the Django project (only has to be done once):

1. Create a virtualenv, if desired.
2. pip install -r requirements.txt
3. Ensure Redis is running, and configure cache_stampede/settings.py:CACHES if necessary (by default it is configured to connect to localhost:6379).
4. cd into the cache_stampede directory of the project.

To run the example scripts, execute the following commands (tested in bash and zsh):

Cache Stampede Example:

This is the "control group", showing what happens during a cache stampede.

redis-cli -n 1 FLUSHDB
DJANGO_SETTINGS_MODULE="settings" python example_no_protection.py

threading.Lock Example:

This example uses threading.Lock to prevent a cache stampede.

redis-cli -n 1 FLUSHDB
DJANGO_SETTINGS_MODULE="settings" python example_threading.py

python-redis-lock Example:

This example uses python-redis-lock to prevent a cache stampede.

redis-cli -n 1 FLUSHDB
for _ in {1..5}; do DJANGO_SETTINGS_MODULE="settings" python example_redis.py & done

You may see a single UNLOCK_SCRIPT not cached. warning in the output.

Releasing a lock requires several commands that must run atomically (this requires a LUA script). For better performance, python-redis-lock uses the EVALSHA command so that the LUA script gets cached after the first time it is eval'd.

UNLOCK_SCRIPT not cached. simply means that the LUA script hasn't been cached by Redis yet. It is safe to ignore this warning, as long as it only appears once.