# URQL GraphQL Client Documentation
# Source: https://raw.githubusercontent.com/urql-graphql/urql/main/docs/basics/document-caching.md
# Path: docs/basics/document-caching.md

---
title: Document Caching
order: 4
---

# Document Caching

By default, `urql` uses a concept called _Document Caching_. It will avoid sending the same requests
to a GraphQL API repeatedly by caching the result of each query.

This works like the cache in a browser. `urql` creates a key for each request that is sent based on
a query and its variables.

The default _document caching_ logic is implemented in the default `cacheExchange`. We'll learn more
about ["Exchanges" on the "Architecture" page.](../architecture.md)

## Operation Keys

![Keys for GraphQL Requests](../assets/urql-operation-keys.png)

Once a result comes in it's cached indefinitely by its key. This means that each unique request
can have exactly one cached result.

However, we also need to invalidate the cached results so that requests are sent again and updated,
when we know that some results are out-of-date. With document caching we assume that a result may
be invalidated by a mutation that executes on data that has been queried previously.

In GraphQL the client can request additional type information by adding the `__typename` field to a
query's _selection set_. This field returns the name of the type for an object in the results, and
we use it to detect commonalities and data dependencies between queries and mutations.

![Document Caching](../assets/urql-document-caching.png)

In short, when we send a mutation that contains types that another query's results contains as well,
that query's result is removed from the cache.

This is an aggressive form of cache invalidation. However, it works well for content-driven sites,
while it doesn't deal with normalized data or IDs.

## Request Policies

The _request policy_ that is defined will alter what the default document cache does. By default, the
cache will prefer cached results and will otherwise send a request, which is called `cache-first`.
In total there are four different policies that we can use:

- `cache-first` (the default) prefers cached results and falls back to sending an API request when
  no prior result is cached.
- `cache-and-network` returns cached results but also always sends an API request, which is perfect
  for displaying data quickly while keeping it up-to-date.
- `network-only` will always send an API request and will ignore cached results.
- `cache-only` will always return cached results or `null`.

The `cache-and-network` policy is particularly useful, since it allows us to display data instantly
if it has been cached, but also refreshes data in our cache in the background. This means though
that `fetching` will be `false` for cached results although an API request may still be ongoing in
the background.

For this reason there's another field on results, `result.stale`, which indicates that the cached
result is either outdated or that another request is being sent in the background.

[Read more about which request policies are available in the API
docs.](../api/core.md#requestpolicy-type)

## Document Cache Gotchas

This cache has a small trade-off! If we request a list of data, and the API returns an empty list,
then the cache won't be able to see the `__typename` of said list and invalidate it.

There are two ways to fix this issue, supplying `additionalTypenames` to the context of your query or [switch to "Normalized Caching"
instead](../graphcache/normalized-caching.md).

### Adding typenames

This will elaborate about the first fix for empty lists, the `additionalTypenames`.

Example where this would occur:

```js
const query = `query { todos { id name } }`;
const result = { todos: [] };
```

At this point we don't know what types are possible for this query, so a best practice when using
the default cache is to add `additionalTypenames` for this query.

```js
// Keep the reference stable.
const context = useMemo(() => ({ additionalTypenames: ['Todo'] }), []);
const [result] = useQuery({ query, context });
```

Now the cache will know when to invalidate this query even when the list is empty.

We may also use this feature for mutations, since occasionally mutations must invalidate data that
isn't directly connected to a mutation by a `__typename`.

```js
const [result, execute] = useMutation(`mutation($name: String!) { createUser(name: $name) }`);

const onClick = () => {
  execute({ name: 'newName' }, { additionalTypenames: ['Wallet'] });
};
```

Now our `mutation` knows that when it completes it has an additional type to invalidate.