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Error handling


ApolloResponse

Use ApolloResponse.data to check if the server returned data:

if (response.data != null) {
// Handle (potentially partial) data
} else {
// Something wrong happened
if (response.exception != null) {
// Handle non-GraphQL errors
} else {
// Handle GraphQL errors in response.errors
}
}

This is also true when calling toFlow() (e.g. with subscriptions) and watch() (with the normalized cache).

apolloClient.subscription(subscription).toFlow().collect { response ->
if (response.data != null) {
// Handle (potentially partial) data
} else {
if (response.exception != null) {
// Handle non-GraphQL errors
} else {
// Handle GraphQL errors in response.errors
}
}
}

If you prefer throwing, you can use dataOrThrow() to get a non-null data:

val data = apolloClient.query(ExampleQuery()).execute().dataOrThrow()
// data is non-null

Different types of errors

Whenever you execute a with (or any other ), two types of errors can occur:

Fetch errors

Fetch errors happen when it's impossible to fetch a GraphQL response. They can have a wide variety of causes (non-exhaustive list):

  • The app is offline or doesn't have access to the network.
  • A DNS error occurred, making it impossible to look up the host.
  • An SSL error occurred (e.g., the server certificate isn't trusted).
  • The connection was closed.
  • The server responded with a non-successful HTTP code.
  • The server didn't respond with valid JSON.
  • The response JSON doesn't satisfy the schema and cannot be parsed.
  • A request was specified as CacheOnly but the data wasn't cached.
  • And more...

You can display an error based on the exception in response.exception

GraphQL errors

GraphQL errors happen when a GraphQL response is successfully fetched but contains GraphQL errors. In that case, the response may contain partial data.

For example, the following query uses an invalid id to look up a Person:

query FilmAndPersonQuery {
film(id: "ZmlsbXM6MQ==") {
title
}
person(id: "badId") {
name
}
}

The server will return the following response:

{
"data": {
"film": {
"title": "A New Hope"
},
"person": null
},
"errors": [
{
"message": "Cannot find person with id 'badId'",
"path": ["person"]
}
]
}

person is null:

// there was an error fetching data
println(response.data?.person) // null
// read the error from response.errors
println(response.errors?.first()?.message) // "Cannot find person with id 'badId'"
// partial data is also returned
println(response.data?.film?.title) // "A New Hope"
// exception is null
println(response.exception) // null

This allows to display as much data as possible without having to do a new network round trip.

Because GraphQL models both errors and semantic nulls as nullable , you must check errors to determine whether the field is an error or a true null.

For an example, it is possible for a person to not have a starship:

{
"data": {
"person": {
"starship": null
}
}
}

In that case, starship is a true null and not an error.

Handling GraphQL errors at parsing time

Because making the difference between true nulls and error nulls is cumbersome, Apollo Kotlin offers a way to handle errors automatically at parsing time. that are nullable only for error purposes can also be generated as non-null in Kotlin. Read "handling nullability" for more details.

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