Share a cache between two instances

Share a cache between two instances#

In this tutorial, we will run Redis and two Plone instances on your computer. One instance will compute a cached value, and the other will read it from Redis instead of computing it again.

You need git, make, uv, Docker, and curl. It takes about fifteen minutes, most of it spent installing Plone.

Get the code#

Clone the repository and install it:

git clone https://github.com/collective/collective.redismemoize.git
cd collective.redismemoize
make install

make install creates a virtual environment with Plone and the package, and the configuration of a first instance in instance/.

Create the first site#

Create a Plone site in the first instance:

make create-site

The command ends without output when the site exists.

Create the second instance#

Create the configuration of a second instance, listening on port 8081, in instance-two/:

uvx cookiecutter -f --no-input -c 3.1.0 --config-file instance.yaml gh:plone/cookiecutter-zope-instance target=instance-two wsgi_listen=localhost:8081

Then create a site in it:

.venv/bin/zconsole run instance-two/etc/zope.conf scripts/create_site.py

The two instances have separate databases. The only thing they share will be Redis.

Start Redis#

Start Redis in a container, listening on port 6379:

docker run --rm --detach --name redismemoize-tutorial --publish 6379:6379 redis:7-alpine

Start both instances#

In one terminal, start the first instance with the Redis URL in its environment:

REDIS_MEMOIZE_URL=redis://localhost:6379/0 make start

In a second terminal, in the same directory, start the second instance:

REDIS_MEMOIZE_URL=redis://localhost:6379/0 .venv/bin/runwsgi instance-two/etc/zope.ini

Wait until both terminals show Serving on.

Compute a value on the first instance#

Plone caches each translation catalog it sends to the browser. Ask the first instance for the English catalog:

curl "http://localhost:8080/Plone/plonejsi18n?domain=plone&language=en"

You see a long JSON object of translations.

Now look at the first instance's status:

curl -u admin:admin -H "Accept: application/json" http://localhost:8080/Plone/@redismemoize

Find plone.app.content.browser.i18n._gettext_catalog under functions. Its route is redis, and its counters show one redis_misses: the instance did not find the catalog in Redis, computed it, and stored it there.

Read it on the second instance#

Ask the second instance for the same catalog:

curl "http://localhost:8081/Plone/plonejsi18n?domain=plone&language=en"

Look at the second instance's status:

curl -u admin:admin -H "Accept: application/json" http://localhost:8081/Plone/@redismemoize

This time, the counters of plone.app.content.browser.i18n._gettext_catalog show one redis_hits and no redis_misses. The second instance never computed the catalog: it read the one the first instance stored.

Notice also server, with Redis's version, and breaker, with open set to false.

Clean up#

Stop both instances with Control-C, then stop Redis:

docker stop redismemoize-tutorial

You have now shared a cached value between two Plone instances that share nothing else.

Next steps#

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