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Understand Urql's Exchanges and Request Policies

Ian Jones
I build software on the web.
・2 min read

Watch "Understand Urql's Document Cache Exchange and Request Policies" (community resource) on egghead.

When you are setting up an urql client, the client comes with default exchanges that your operations go through.

These exchanges are [dedupExchange, cacheExchange, fetchExchange]. The order of the array matters.

const client = new Client({
  url: 'http://graphql.org/swapi-graphql/',
  exchanges: [dedupExchange, cacheExchange, fetchExchange],
})

In this case, the dedupExchange is the first exchange that graphql operation will pass through. This exchange has one job: to remove accidental duplicate requests. If you didn't manage your inputs correctly a user can cause many requests at once.

The next exchange the operation goes through is the cacheExchange. This exchange caches any successful requests. This is a Document Cache[01]. This sort of cache takes the query you've written and the variables and hashes them together. Whenever urql finds an operation hash coming through that matches a hash in the cacheExchange, it will return that value based on the requestPolicy.

The requestPolicy for the cacheExchange can be 1 of 4 things:

  • cache-first
  • cache-only
  • network-only
  • cache-and-network

cache-first is the default policy. It tells urql that if an operation passed to the cacheExchange has been stored already, the data found should be returned and the operation should not be passed to the fetchExchange. If the operation hash hasn't been stored in the cache, then its passed to the fetchExchange

cache-only operates like it sounds: it will only look in the cache for data matching the operation hash and will not pass the operation to the `fetchExchange.

network-only will always skip the cache and pass the operation to the fetchExchange.

cache-and-network will return any value in the cache and pass the operation to the fetchExchange even if there was a successful cache hit. The fetchExchange will then update the cache when the request comes back.

To change the default requestPolicy you can pass one of these values through the Client configuration object.

js
const client = new Client({
url: 'http://graphql.org/swapi-graphql/',
exchanges: [dedupExchange, cacheExchange, fetchExchange],
requestPolicy: 'cache-and-network',
})

The last default exchange that your graphql operation passes through is the fetchExchange. This exchanges uses fetch to make an http request to the url defined on the client.

Other exchanges to look at per the documentation.

  • retryExchange: Allows operations to be retried
  • devtoolsExchange: Provides the ability to use the urql-devtools
  • multipartFetchExchange: Provides multipart file upload capability
  • suspenseExchange (experimental): Allows the use of React Suspense on the client-side with urql's built-in suspense mode
  • graph cache cacheExchange

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