Using the @defer directive in Apollo Kotlin

Apollo Kotlin provides experimental support of the @defer directive, which enables your queries to receive data for specific fields asynchronously. This is helpful whenever some fields in a query take much longer to resolve than the others.

For example, let's say we're building a social media application that can quickly fetch a user's basic profile information, but retrieving that user's friends takes longer. If we include all of those fields in a single query, we want to be able to display the profile information as soon as it's available, instead of waiting for the friend fields to resolve.

To achieve this, we can apply the @defer directive to an inline fragment that contains all slow-resolving fields related to friend data:

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query PersonQuery($personId: ID!) {
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  person(id: $personId) {
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    # Basic fields (fast)
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    id
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    firstName
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    lastName
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    # Friend fields (slower)
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    ... on User @defer {
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      friends {
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        id
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      }
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    }
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  }
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}

In the generated code for this query, the onUser field for the fragment will be nullable. That is because when the initial payload is received from the server, the fields of the fragment are not yet present. A Person will be emitted with only the basic fields filled in.

When the fields of the fragment are available, a new Person will be emitted, this time with the onUser field present and filled with the fields of the fragment.

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apolloClient.query(PersonQuery(personId)).toFlow().collect {
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  println("Received: $it")
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  if (it.dataOrThrow().person.onUser == null) {
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    // Initial payload: basic info only
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    // ...
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  } else {
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    // Subsequent payload: with friends
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    // ...
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  }
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}

Will print something like this:

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Received: Person(id=1, firstName=John, lastName=Doe, onUser=null))
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Received: Person(id=1, firstName=John, lastName=Doe, onUser=OnUser(friends=[Friend(id=2), Friend(id=3)]))

Limitations/known issues

• @defer cannot be used with responseBased codegen.
• Some servers might send an empty payload to signal the end of the stream. In such a case you will receive an extra terminal emission. You can filter it out by using distinctUntilChangedBy():

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apolloClient.query(MyQuery()).toFlow()
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    .distinctUntilChangedBy { it.data }  // filter out duplicates
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    .collect { /* ... */ }