Request pipeline in Apollo iOS

In Apollo iOS, most ApolloClient instances use the RequestChainNetworkTransport to execute GraphQL queries and mutations on a remote server. Appropriately, this network transport uses a structure called a request chain to process each operation in individual steps.

Request chains

A request chain defines a sequence of interceptors that handle the lifecycle of a particular GraphQL operation's execution. One interceptor might add custom HTTP headers to a request, while the next might be responsible for actually sending the request to a GraphQL server over HTTP. A third interceptor might then write the operation's result to the Apollo iOS cache.

When an operation is executed, an object called an InterceptorProvider generates a RequestChain for the operation. Then, kickoff is called on the request chain, which runs the first interceptor in the chain:

An interceptor can perform arbitrary, asynchronous logic on any thread. When an interceptor finishes running, it calls proceedAsync on its RequestChain, which advances to the next interceptor.

By default when the last interceptor in the chain finishes, if a parsed operation result is available, that result is returned to the operation's original caller. Otherwise, error-handling logic is called.

Each request has its own short-lived RequestChain. This means that the sequence of interceptors can differ for each operation.

Interceptor providers

To generate a request chain for each GraphQL operation, Apollo iOS passes operations to an object called an interceptor provider. This object conforms to the InterceptorProvider protocol.

Default provider

DefaultInterceptorProvider is a default implementation of an interceptor provider. It works with the Apollo iOS parsing and caching system and tries to replicate the experience of using the old HTTPNetworkTransport as closely as possible. It takes a URLSessionClient and an ApolloStore to pass into the interceptors it creates.

DefaultInterceptorProvider is recommended for most applications. If necessary, you can create a custom interceptor provider.

Default interceptors

The DefaultInterceptorProvider creates a request chain with the following interceptors for every operation, as shown in the source:

Custom interceptor providers

If your use case requires it, you can create a custom struct or class that conforms to the InterceptorProvider protocol.

If you define a custom InterceptorProvider, it should almost always create a RequestChain that uses a similar structure to the default, but that includes additions or modifications as needed for particular operations.

When creating request chains in your custom interceptor provider, note the following:

• Interceptors are designed to be short-lived. Your interceptor provider should provide a completely new set of interceptors for each request to avoid having multiple calls use the same interceptor instance simultaneously.
• Holding references to individual interceptors (outside of test verification) is generally not recommended. Instead, you can create an interceptor that holds onto a longer-lived object, and the provider can pass this object into each new set of interceptors. This way, each interceptor is disposable, but you don't have to recreate the underlying object that does heavier work.

If you do create your own InterceptorProvider, you can use any of the built-in interceptors that are included in Apollo iOS:

Built-in interceptors

Apollo iOS provides a collection of built-in interceptors you can create in a custom interceptor provider. You can also create a custom interceptor by defining a class that conforms to the ApolloInterceptor protocol.

additionalErrorInterceptor

An InterceptorProvider can optionally provide an additionalErrorInterceptor that's called before an error is returned to the caller. This is mostly useful for logging and tracing errors. This interceptor must conform to the ApolloErrorInterceptor protocol.

The additionalErrorInterceptor is not part of the request chain. Instead, any other interceptor can invoke this interceptor by calling chain.handleErrorAsync.

Note that for expected errors with a clear resolution (such as renewing an expired authentication token), you should define an interceptor within your request chain that can resolve the issue and retry the operation.

Interceptor flow

Most interceptors execute their logic and then call chain.proceedAsync to proceed to the next interceptor in the request chain. However, interceptors can call other methods to override this default flow.

Retrying an operation

Any interceptor can call chain.retry to immediately restart the current request chain from the beginning. This can be helpful if the interceptor needed to refresh an access token or modify other configuration for the operation to succeed.

Returning a value

An interceptor can directly return a value to the operation's original caller, instead of waiting for the request chain to complete. To do so, the interceptor can call chain.returnValueAsync.

This does not prevent the rest of the request chain from executing. An interceptor can still call chain.proceedAsync as usual after calling chain.returnValueAsync.

You can even call chain.returnValueAsync multiple times within a request chain! This is helpful when initially returning a locally cached value before returning a value returned by the GraphQL server.

Returning an error

If an interceptor encounters an error, it can return the details of that error by calling chain.handleErrorAsync.

This does not prevent the rest of the request chain from executing. An interceptor can still call chain.proceedAsync as usual after calling chain.handleErrorAsync. However, if the encountered error will cause the operation to fail, you can skip calling chain.proceedAsync to end the request chain.

Examples

The following example snippets demonstrate how to use an advanced request pipeline with custom interceptors. This code assumes you have the following hypothetical classes in your own code (these classes are not part of Apollo iOS):

• UserManager: Checks whether the active user is logged in, performs associated checks on errors and responses to see if they need to renew their token, and performs that renewal when necessary.
• Logger: Handles printing logs based on their level. Supports .debug, .error, and .always log levels.

Example interceptors

UserManagementInterceptor

This example interceptor checks whether the active user is logged in. If so, it asynchronously renews that user's access token if it's expired. Finally, it adds the access token to an Authorization header before proceeding to the next interceptor in the request chain.

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import Apollo
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class UserManagementInterceptor: ApolloInterceptor {
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  enum UserError: Error {
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    case noUserLoggedIn
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  }
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  /// Helper function to add the token then move on to the next step
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  private func addTokenAndProceed<Operation: GraphQLOperation>(
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    _ token: Token,
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    to request: HTTPRequest<Operation>,
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    chain: RequestChain,
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    response: HTTPResponse<Operation>?,
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    completion: @escaping (Result<GraphQLResult<Operation.Data>, Error>) -> Void) {
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    request.addHeader(name: "Authorization", value: "Bearer \(token.value)")
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    chain.proceedAsync(request: request,
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                        response: response,
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                        completion: completion)
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  }
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  func interceptAsync<Operation: GraphQLOperation>(
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    chain: RequestChain,
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    request: HTTPRequest<Operation>,
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    response: HTTPResponse<Operation>?,
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    completion: @escaping (Result<GraphQLResult<Operation.Data>, Error>) -> Void) {
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    guard let token = UserManager.shared.token else {
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      // In this instance, no user is logged in, so we want to call
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      // the error handler, then return to prevent further work
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      chain.handleErrorAsync(UserError.noUserLoggedIn,
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                              request: request,
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                              response: response,
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                              completion: completion)
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      return
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    }
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    // If we've gotten here, there is a token!
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    if token.isExpired {
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      // Call an async method to renew the token
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      UserManager.shared.renewToken { [weak self] tokenRenewResult in
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        guard let self = self else {
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            return
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        }
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        switch tokenRenewResult {
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        case .failure(let error):
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          // Pass the token renewal error up the chain, and do
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          // not proceed further. Note that you could also wrap this in a
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          // `UserError` if you want.
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          chain.handleErrorAsync(error,
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                                  request: request,
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                                  response: response,
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                                  completion: completion)
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        case .success(let token):
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          // Renewing worked! Add the token and move on
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          self.addTokenAndProceed(token,
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                                  to: request,
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                                  chain: chain,
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                                  response: response,
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                                  completion: completion)
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        }
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      }
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    } else {
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      // We don't need to wait for renewal, add token and move on
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      self.addTokenAndProceed(token,
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                              to: request,
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                              chain: chain,
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                              response: response,
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                              completion: completion)
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    }
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  }
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}

RequestLoggingInterceptor

This example interceptor logs the outgoing request using the hypothetical Logger class, then proceeds to the next interceptor in the request chain:

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import Apollo
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class RequestLoggingInterceptor: ApolloInterceptor {
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  func interceptAsync<Operation: GraphQLOperation>(
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    chain: RequestChain,
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    request: HTTPRequest<Operation>,
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    response: HTTPResponse<Operation>?,
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    completion: @escaping (Result<GraphQLResult<Operation.Data>, Error>) -> Void) {
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    Logger.log(.debug, "Outgoing request: \(request)")
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    chain.proceedAsync(request: request,
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                        response: response,
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                        completion: completion)
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  }
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}

‌ResponseLoggingInterceptor

This example interceptor uses the hypothetical Logger class to log the request's response if it exists, then proceeds to the next interceptor in the request chain.

This is an example of an interceptor that can both proceed and throw an error. We don't necessarily want to stop processing if this interceptor was added in wrong place, but we do want to know about that error.

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import Apollo
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class ResponseLoggingInterceptor: ApolloInterceptor {
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  enum ResponseLoggingError: Error {
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    case notYetReceived
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  }
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  func interceptAsync<Operation: GraphQLOperation>(
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    chain: RequestChain,
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    request: HTTPRequest<Operation>,
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    response: HTTPResponse<Operation>?,
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    completion: @escaping (Result<GraphQLResult<Operation.Data>, Error>) -> Void) {
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    defer {
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      // Even if we can't log, we still want to keep going.
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      chain.proceedAsync(request: request,
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                          response: response,
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                          completion: completion)
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    }
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    guard let receivedResponse = response else {
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      chain.handleErrorAsync(ResponseLoggingError.notYetReceived,
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                              request: request,
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                              response: response,
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                              completion: completion)
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      return
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    }
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    Logger.log(.debug, "HTTP Response: \(receivedResponse.httpResponse)")
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    if let stringData = String(bytes: receivedResponse.rawData, encoding: .utf8) {
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      Logger.log(.debug, "Data: \(stringData)")
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    } else {
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      Logger.log(.error, "Could not convert data to string!")
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    }
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  }
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}

Example interceptor provider

This InterceptorProvider creates request chains using all of the default interceptors in their usual order, with all of the example interceptors defined above added at the appropriate points in the request pipeline:

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import Foundation
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import Apollo
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struct NetworkInterceptorProvider: InterceptorProvider {
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  // These properties will remain the same throughout the life of the `InterceptorProvider`, even though they
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  // will be handed to different interceptors.
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  private let store: ApolloStore
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  private let client: URLSessionClient
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  init(store: ApolloStore,
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        client: URLSessionClient) {
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    self.store = store
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    self.client = client
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  }
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  func interceptors<Operation: GraphQLOperation>(for operation: Operation) -> [ApolloInterceptor] {
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    return [
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      MaxRetryInterceptor(),
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      CacheReadInterceptor(store: self.store),
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      UserManagementInterceptor(),
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      RequestLoggingInterceptor(),
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      NetworkFetchInterceptor(client: self.client),
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      ResponseLoggingInterceptor(),
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      ResponseCodeInterceptor(),
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      JSONResponseParsingInterceptor(cacheKeyForObject: self.store.cacheKeyForObject),
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      AutomaticPersistedQueryInterceptor(),
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      CacheWriteInterceptor(store: self.store)
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    ]
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  }
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}

Example Network singleton

As when initializing a basic client, it's recommended to create a Network singleton to use a single ApolloClient instance across your app.

Here's what that singleton might look like for an advanced client:

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import Foundation
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import Apollo
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class Network {
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  static let shared = Network()
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  private(set) lazy var apollo: ApolloClient = {
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    // The cache is necessary to set up the store, which we're going to hand to the provider
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    let cache = InMemoryNormalizedCache()
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    let store = ApolloStore(cache: cache)
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    let client = URLSessionClient()
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    let provider = NetworkInterceptorProvider(store: store, client: client)
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    let url = URL(string: "https://apollo-fullstack-tutorial.herokuapp.com/graphql")!
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    let requestChainTransport = RequestChainNetworkTransport(interceptorProvider: provider,
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                                                              endpointURL: url)
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    // Remember to give the store you already created to the client so it
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    // doesn't create one on its own
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    return ApolloClient(networkTransport: requestChainTransport,
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                        store: store)
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  }()
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}

An example of setting up a client that can handle WebSocket and subscriptions is included in the subscriptions documentation.

RequestChainNetworkTransport API reference

The initializer for RequestChainNetworkTransport accepts the following properties, which provide you with fine-grained control of your HTTP requests and responses:

The URLSessionClient class

Because URLSession only supports use in the background using the delegate-based API, Apollo iOS provides a URLSessionClient class that helps configure that.

By default, instances of URLSessionClient use URLSessionConfiguration.default to set up their URL session, and instances of DefaultInterceptorProvider use the default initializer for URLSessionClient.

The URLSessionClient class and most of its methods are open, so you can subclass it if you need to override any of the delegate methods for the URLSession delegates we're using, or if you need to handle additional delegate scenarios.