graphql-engine/server/lib/incremental

incremental

A library for caching intermediate results in Arrow computations. Used by graphql-engine to optimise schema cache updates by avoiding redundant recomputation.

Schema cache?

The SchemaCache is is a very complex object that, among other things, keeps track of the GraphQL schema and its parser for each user role in a graphql-engine instance. Its computation is based on both user-supplied metadata and the schema of each data source configured to work with Hasura. When one of these dependencies is updated (perhaps the user makes a metadata change), we'd like to recompile the schema cache as quickly (and efficiently with regards to memory) as possible.

For our specific uses of incremental, see the implementation of buildSchemaCacheRule and its Inc.cache calls, as well as the note titled, Avoiding GraphQL schema rebuilds when changing irrelevant Metadata.

Arrow 101 and ArrowCache

The Arrow abstraction describes a computation from some input to some output. A very simple example of an Arrow is regular Haskell functions a -> b: a computation that transforms some a into some b. Another good example is functions of the shape a -> m b for some Monad m: a computation that transforms some a into some b with potential side-effects. The Arrow abstraction forms the basis of a whole hierarchy of classes, such as ArrowChoice and ArrowLoop, each of which build on this basic idea. We can think of these as analogous to Monad being the basis of classes like MonadPlus and MonadIO: each class describes further specific capabilities of the generic Monad interface.

This library introduces the ArrowCache class. Sometimes, an Arrow represents a computation that is computationally expensive, but is also consistent in its output for any given input. For example, a compiler takes a collection of source files and produces a binary. This computation can be very expensive if we have a lot of source files and/or a complicated compiler, but the result should be deterministic. In this case, ArrowCache allows us to decorate an Arrow with a caching mechanism, avoiding recompilation of unchanged files.

When we cache an arrow, we keep track of the last input/output pair. If the next input matches the last input, we can skip the computation and return the last output, avoiding the need for the expensive computation. If the next input doesn't match the last input, we replace the stored pair with the new input/output pair. We can cache any arrow or composition of arrows in a pipeline, allowing us to express different granularities of caching¹.

If thinking of programs as a composition of Arrows is unfamiliar, we can instead think of our program as a directed, acyclic graph of intermediate results. For example, our compiler might first compile the individual modules, and then bundle them together. Any one of these intermediate results can be cached to avoid expensive recomputation when its dependencies remain unchanged.

How does it work?

Rule and cache

Rule is the interesting implementation of ArrowCache. Its instance in Hasura.Incremental.Internal.Cache explains the system above, specifically in the cached definition of its where clause: if the input has not changed, then we return the last result. Otherwise, we recompute a result for the new input. Note that this is not memoisation: we don't store an input/output map. We only ever store the last input/output pair.

Keyed Dependencies

The ArrowCache also features two other functions: newDependency and dependOn. Consider an arrow that takes Metadata as input, but only ever accesses a small set of specific fields within it. In this case, we'd ideally only like to consider those particular fields when we determine whether we should recompute!

The Dependency type gives us a way to keep track of the parts of a structure on which we'd like to depend, and Rule then uses this information to cache specifically based on our Accesses, rather than just naïvely checking for equality between the last and next input. The functions for working with Dependency live in Hasura.Incremental.Internal.Dependency.

Invalidation keys

Sometimes, we want to cache arrows that are not deterministic. In these cases, we might want to re-compute some value despite the inputs being the same. For example, data source introspection is an inherently non-deterministic action: what if the database schema changes? In these cases, the provided InvalidationKey helper type can be used. When we invalidate an InvalidationKey, the resulting value does not equal the previous value, which should force a recomputation of the arrow when it is given as part of the input.

Gotchas

Monadic parameter-passing in Rule

The reason for using the Arrow abstraction is to talk about the computational relationship between the input and the output. In our specific case, what we want to talk about is whether or not we've seen this pairing before. However, this relies on the Arrow input being the only input to the computation.

When we build or rebuild a Rule m a b computation, we do so inside some Applicative m. If this m happens to be, say, (->) r, then we have access to an r parameter that isn't explicitly given as the a parameter, and it will not be considered part of the input that determines whether or not we need to recompute. This can lead to problems: if the result of the computation meaningfully changes depending on this value, then repeated calls may yield incorrect results. In general, we should be careful to make sure that our chosen m does not affect our decision to cache.

On the other hand, this is an occasionally helpful loophole. For example, in Hasura.RQL.Types.SchemaCache.Build, we use a MonadReader BuildReason m constraint to determine whether we need to update, say, the event trigger catalogue. If the input (i.e. the schema) hasn't changed, then the BuildReason doesn't matter: the fact that it is accessed via the m means that caching works the way we'd like it to work.

Another example is when m happens to be IO: in these cases, we have no guarantees about values being introduced during the computation. As mentioned in the InvalidationKey section, non-determinism may be desired, and in these cases the onus is on the user to invalidate the cache when recomputations are required.

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1. It might be tempting to cache every single arrow. However, this quite quickly leads to a large amount of memory being consumed, and often for no good reason. Sometimes, a result is so simple to recompute that we're better off recomputing rather than allocating more memory. ↩︎