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Caching in Zulip

Like any product with good performance characteristics, Zulip makes extensive use of caching. This article talks about our caching strategy, focusing on how we use memcached (since it's the thing people generally think about when they ask about how a server does caching).

Backend caching with memcached

On the backend, Zulip uses memcached, a popular key-value store, for caching. Our memcached caching helps let us optimize Zulip's performance and scalability, since most requests don't need to talk to the database (which, even for a trivial query with everything on the same machine, usually takes 3-10x as long as a memcached fetch).

We use Django's built-in caching integration to manage talking to memcached, and then a small application-layer library (zerver/lib/cache.py).

It's common for projects using a caching system like memcached to either have the codebase littered with explicit requests to interact with the cache (or flush data from a cache), or (worse) be littered with weird bugs that disappear after you flush memcached.

Caching bugs are a pain to track down, because they generally require an extra and difficult-to-guess step to reproduce (namely, putting the wrong data into the cache).

So we've designed our backend to ensure that if we write a small amount of Zulip's core caching code correctly, then the code most developers naturally write will both benefit from caching and not create any cache consistency problems.

The overall result of this design is that in the vast majority of Zulip's Django codebase, all one needs to do is call the standard accessor functions for data (like get_user or get_stream to fetch user and stream objects, or for view code, functions like access_stream_by_id, which checks permissions), and everything will work great. The data fetches automatically benefit from memcached caching, since those accessor methods have already been written to transparently use Zulip's memcached caching system, and the developer doesn't need to worry about whether the data returned is up-to-date: it is. In the following sections, we'll talk about how we make this work.

As a side note, the policy of using these accessor functions wherever possible is a good idea, regardless of caching, because the functions also generally take care of details you might not think about (e.g. case-insensitive matching of stream names or email addresses). It's amazing how slightly tricky logic that's duplicated in several places invariably ends up buggy in some of those places, and in aggregate we call these accessor functions hundreds of times in Zulip. But the caching is certainly a nice bonus.

The core implementation

The get_user function is a pretty typical piece of code using this framework; as you can see, it's very little code on top of our cache_with_key decorator:

def user_profile_cache_key_id(email: str, realm_id: int) -> str:
    return u"user_profile:%s:%s" % (make_safe_digest(email.strip()), realm_id,)

def user_profile_cache_key(email: str, realm: 'Realm') -> str:
    return user_profile_cache_key_id(email, realm.id)

@cache_with_key(user_profile_cache_key, timeout=3600*24*7)
def get_user(email: str, realm: Realm) -> UserProfile:
    return UserProfile.objects.select_related().get(
        email__iexact=email.strip(), realm=realm)

This decorator implements a pretty classic caching paradigm:

The user_profile_cache_key function defines a unique map from a canonical form of its arguments to a string. These strings are namespaced (the user_profile: part) so that they won't overlap with other caches, and encode the arguments so that two uses of this cache won't overlap. In this case, a hash of the email address and realm ID are those canonicalized arguments. (The make_safe_digest is important to ensure we don't send special characters to memcached). And we have two versions, depending whether the caller has access to a Realm or just a realm_id.
When get_user is called, cache_with_key will compute the key, and do a Django cache_get query for the key (which goes to memcached). If the key is in the cache, it just returns the value. Otherwise, it fetches the value from the database (using the actual code in the body of get_user), and then stores the value back to that memcached key before returning the result to the caller.
Cache entries expire after the timeout; in this case, a week. Though in frequently deployed environments like chat.zulip.org, often cache entries will stop being used long before that, because KEY_PREFIX is rotated every time we deploy to production; see below for details.

We use this decorator in more than 30 places in Zulip, and it saves a huge amount of otherwise very self-similar caching code.

Cautions

The one thing to be really careful with in using cache_with_key is that if an item is in the cache, the body of get_user (above) is never called. This means some things that might seem like clever code reuse are actually a really bad idea. For example:

Don't add a get_active_user function that uses the same cache key function as get_user (but with a different query that filters our deactivated users). If one called get_active_user to access a deactivated user, the right thing would happen, but if you called get_user to access that user first, then the get_active_user function would happily return the user from the cache, without ever doing your more restrictive query.

So remember: Use separate cache key functions for different data sets, even if they feature the same objects.

Cache invalidation after writes

The caching strategy described above works pretty well for anything where the state it's storing is immutable (i.e. never changes). With mutable state, one needs to do something to ensure that the Python processes don't end up fetching stale data from the cache after a write to the database.

We handle this using Django's longstanding post_save signals feature. Django signals let you configure some code to run every time Django does something (for post_save, right after any write to the database using Django's .save()).

There's a handful of lines in zerver/models.py like these that configure this:

post_save.connect(flush_realm, sender=Realm)
post_save.connect(flush_user_profile, sender=UserProfile)

Once this post_save hook is registered, whenever one calls user_profile.save(...) with a UserProfile object in our Django project, Django will call the flush_user_profile function. Zulip is systematic about using the standard Django .save() function for modifying user_profile objects (and passing the update_fields argument to .save() consistently, which encodes which fields on an object changed). This means that all we have to do is write those cache-flushing functions correctly, and people writing Zulip code won't need to think about (or even know about!) the caching.

Each of those flush functions basically just computes the list of cache keys that might contain data that was modified by the .save(...) call (based on the object changed and the update_fields data), and then sends a bulk delete request to memcached to remove those keys from the cache (if present).

Maintaining these flush functions requires some care (every time we add a new cache, we need to look through them), but overall it's a pretty simple algorithm: If the changed data appears in any form in a given cache key, that cache key needs to be cleared. E.g. the active_user_ids_cache_key cache for a realm needs to be flushed whenever a new user is created in that realm, or user is deactivated/reactivated, even though it's just a list of IDs and thus doesn't explicitly contain the is_active flag.

Once you understand how that works, it's pretty easy to reason about when a particular flush function should clear a particular cache; so the main thing that requires care is making sure we remember to reason about that when changing cache semantics.

But the overall benefit of this cache system is that almost all the code in Zulip just needs to modify Django model objects and call .save(), and the caching system will do the right thing.

Production deployments and database migrations

When upgrading a Zulip server, it's important to avoid having one version of the code interact with cached objects from another version that has a different data layout. In Zulip, we avoid this through some clever caching strategies. Each "deployment directory" for Zulip in production has inside it a var/remote_cache_prefix file, containing a cache prefix (KEY_PREFIX in the code) that is automatically appended to the start of any cache keys accessed by that deployment directory (this is all handled internally by zerver/lib/cache.py).

This completely solves the problem of potentially having contamination from inconsistent versions of the source code / data formats in the cache.

Automated testing and memcached

For Zulip's test-backend unit tests, we use the same strategy. In particular, we just edit KEY_PREFIX before each unit test; this means each of the thousands of test cases in Zulip has its own independent memcached key namespace on each run of the unit tests. As a result, we never have to worry about memcached caching causing problems across multiple tests.

This is a really important detail. It makes it possible for us to do assertions in our tests on the number of database queries or memcached queries that are done as part of a particular function/route, and have those checks consistently get the same result (those tests are great for catching bugs where we accidentally do database queries in a loop). And it means one can debug failures in the test suite without having to consider the possibility that memcached is somehow confusing the situation.

Further, this KEY_PREFIX model means that running the backend tests won't potentially conflict with whatever you're doing in a Zulip development environment on the same machine, which also saves a ton of time when debugging, since developers don't need to think about things like whether some test changed Hamlet's email address and that's why login is broken.

More full-stack test suites like test-js-with-puppeteer or test-api use a similar strategy (set a random KEY_PREFIX at the start of the test run).

Manual testing and memcached

Zulip's development environment will automatically flush (delete all keys in) memcached when provisioning and when starting run-dev. You can run the server with that behavior disabled using tools/run-dev --no-clear-memcached.

Performance

One thing be careful about with memcached queries is to avoid doing them in loops (the same applies for database queries!). Instead, one should use a bulk query. We have a fancy function, generate_bulk_cached_fetch, which is super magical and handles this for us, with support for a bunch of fancy features like marshalling data before/after going into the cache (e.g. to compress message objects to minimize data transfer between Django and memcached).

In-process caching in Django

We generally try to avoid in-process backend caching in Zulip's Django codebase, because every Zulip production installation involves multiple servers. We do have a few, however:

per_request_display_recipient_cache: A cache flushed at the start of every request; this simplifies correctly implementing our goal of not repeatedly fetching the "display recipient" (e.g. stream name) for each message in the GET /messages codebase.
Caches of various data, like the SourceMap object, that are expensive to construct, not needed for most requests, and don't change once a Zulip server has been deployed in production.

Browser caching of state

Zulip makes extensive use of caching of data in the browser and mobile apps; details like which users exist, with metadata like names and avatars, similar details for streams, recent message history, etc.

This data is fetched in the /register endpoint (or page_params for the web app), and kept correct over time. The key to keeping these state up to date is Zulip's real-time events system, which allows the server to notify clients whenever state that might be cached by clients is changed. Clients are responsible for handling the events, updating their state, and rerendering any UI components that might display the modified state.

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