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Apollo Server plugin event reference

New in Apollo Server 3: All plugin lifecycle methods are async, except for willResolveField.

This reference describes the lifecycle events that your custom Apollo Server plugin can respond to.

Apollo Server fires two types of events that plugins can hook into: server lifecycle events and request lifecycle events.

  • Server lifecycle events are high-level events related to the lifecycle of Apollo Server itself (e.g., serverWillStart).

  • Request lifecycle events are associated with the lifecycle of a specific request.

With one exception, all plugin methods in Apollo Server 3 are async. The exception is willResolveField, which is called much more frequently than other plugin methods.

Server lifecycle events

serverWillStart

The serverWillStart event fires when Apollo Server is preparing to start serving GraphQL requests. The server doesn't start until this asynchronous method completes. If it throws (i.e., if the Promise it returns is rejected), startup fails and your server does not serve GraphQL operations. This helps you make sure all of your server's dependencies are available before attempting to begin serving requests.

This event is fired at different times depending on which Apollo Server middleware you're using:

  • In apollo-server, it's fired from the listen() method.
  • In non-serverless middleware libraries like apollo-server-express, it's fired from the start() method.
  • In serverless middleware libraries like apollo-server-lambda, it's fired in response to the first incoming request.

Example

const server = new ApolloServer({
  /* ... other necessary configuration ... */

  plugins: [
    {
      async serverWillStart() {
        console.log('Server starting!');
      }
    }
  ]
})

serverWillStop

The serverWillStop event fires when Apollo Server is starting to shut down because ApolloServer.stop() has been invoked (either explicitly by your code, or by one of the termination signal handlers). If your plugin is running any background tasks, this is a good place to shut them down.

You define your serverWillStop handler in the object returned by your serverWillStart handler, because the two handlers usually interact with the same data. Currently, serverWillStop handlers do not take arguments (this might change in the future).

Example

const server = new ApolloServer({
  /* ... other necessary configuration ... */

  plugins: [
    {
      async serverWillStart() {
        const interval = setInterval(doSomethingPeriodically, 1000);
        return {
          async serverWillStop() {
            clearInterval(interval);
          }
        }
      }
    }
  ]
})

renderLandingPage

This event enables you to serve a custom landing page from Apollo Server's base URL. The event is fired once by Apollo Server after all serverWillStart events run. At most one installed plugin can define a renderLandingPage handler. Otherwise, Apollo Server throws an error on startup.

You define your plugin's renderLandingPage handler in the object returned by your serverWillStart handler, which enables it to read values passed to serverWillStart:

index.js
const server = new ApolloServer({
  // ... other configuration ...

  plugins: [
    {      async serverWillStart() {        return {          async renderLandingPage() {            const html = `<!DOCTYPE html><html>  <head>  </head>  <body>    <h1>Hello world!</h1>  </body></html>`;            return { html };          }        }      }    }  ]
});

The handler should return an object with a string html field. The value of that field is served as HTML for any requests with accept: text/html headers.

For more landing page options, see Changing the landing page.

Example

const server = new ApolloServer({
  /* ... other necessary configuration ... */

  plugins: [
    {
      async serverWillStart() {
        return {
          async renderLandingPage() {
            return { html: `<html><body>Welcome to your server!</body></html>` };
          }
        }
      }
    }
  ]
})

requestDidStart

The requestDidStart event fires whenever Apollo Server begins fulfilling a GraphQL request.

requestDidStart?(
  requestContext: WithRequired<
    GraphQLRequestContext<TContext>,
    'request' | 'context' | 'logger'
  >
): Promise<GraphQLRequestListener<TContext> | void>;

This function can optionally return an object that includes functions for responding to request lifecycle events that might follow requestDidStart.

const server = new ApolloServer({
  /* ... other necessary configuration ... */

  plugins: [
    {
      async requestDidStart(requestContext) {
        // Within this returned object, define functions that respond
        // to request-specific lifecycle events.
        return {
          // The `parsingDidStart` request lifecycle event fires
          // when parsing begins. The event is scoped within an
          // associated `requestDidStart` server lifecycle event.
          async parsingDidStart(requestContext) {
            console.log('Parsing started!')
          },
        }
      }
    }
  ],
})

If your plugin doesn't need to respond to any request lifecycle events, requestDidStart should not return a value.

Request lifecycle events

If you're using TypeScript to create your plugin, implement the GraphQLRequestListener interface from the apollo-server-plugin-base module to define functions for request lifecycle events.

When Apollo Server processes a request, these events fire in the order listed (with the exception of didEncounterErrors, which might fire in one of a few places depending on when errors occur). See the flow diagram

Note that not every event fires for every request (for example, parsingDidStart doesn't fire for an operation that Apollo Server has cached and doesn't need to parse again).

didResolveSource

The didResolveSource event is invoked after Apollo Server has determined the String-representation of the incoming operation that it will act upon. In the event that this String was not directly passed in from the client, this may be retrieved from a cache store (e.g., Automated Persisted Queries).

At this stage, there is not a guarantee that the operation is not malformed.

didResolveSource?(
  requestContext: WithRequired<
    GraphQLRequestContext<TContext>, 'source' | 'logger'>,
  >,
): Promise<void>;

parsingDidStart

The parsingDidStart event fires whenever Apollo Server will parse a GraphQL request to create its associated document AST.

If Apollo Server receives a request with a query string that matches a previous request, the associated document might already be available in Apollo Server's cache. In this case, parsingDidStart is not called for the request, because parsing does not occur.

parsingDidStart?(
  requestContext: WithRequired<
    GraphQLRequestContext<TContext>,
    'metrics' | 'source' | 'logger'
  >,
): Promise<void | (err?: Error) => Promise<void>>;

validationDidStart

The validationDidStart event fires whenever Apollo Server will validate a request's document AST against your GraphQL schema.

Like parsingDidStart, this event does not fire if a request's document is already available in Apollo Server's cache (only successfully validated documents are cached by Apollo Server).

The document AST is guaranteed to be available at this stage, because parsing must succeed for validation to occur.

validationDidStart?(
  requestContext: WithRequired<
    GraphQLRequestContext<TContext>,
    'metrics' | 'source' | 'document' | 'logger'
  >,
): Promise<void | (err?: ReadonlyArray<Error>) => Promise<void>>;

didResolveOperation

The didResolveOperation event fires after the graphql library successfully determines the operation to execute from a request's document AST. At this stage, both the operationName string and operation AST are available.

This event is not associated with your GraphQL server's resolvers. When this event fires, your resolvers have not yet executed (they execute after executionDidStart).

If the operation is anonymous (i.e., the operation is query { ... } instead of query NamedQuery { ... }), then operationName is null.

didResolveOperation?(
  requestContext: WithRequired<
    GraphQLRequestContext<TContext>,
    'metrics' | 'source' | 'document' | 'operationName' | 'operation' | 'logger'
  >,
): Promise<void>;

responseForOperation

The responseForOperation event is fired immediately before GraphQL execution would take place. If its return value resolves to a non-null GraphQLResponse, that result is used instead of executing the query. Hooks from different plugins are invoked in series, and the first non-null response is used.

responseForOperation?(
  requestContext: WithRequired<
    GraphQLRequestContext<TContext>,
    'metrics' | 'source' | 'document' | 'operationName' | 'operation' | 'logger'
  >,
): Promise<GraphQLResponse | null>;

executionDidStart

The executionDidStart event fires whenever Apollo Server begins executing the GraphQL operation specified by a request's document AST.

executionDidStart?(
  requestContext: WithRequired<
    GraphQLRequestContext<TContext>,
    'metrics' | 'source' | 'document' | 'operationName' | 'operation' | 'logger'
  >,
): Promise<GraphQLRequestExecutionListener | void>;

executionDidStart may return an object with one or both of the methods executionDidEnd and willResolveField. executionDidEnd is treated like an end hook: it is called after execution with any errors that occurred. willResolveField is documented in the next section. (In Apollo Server 2, executionDidStart could return also return an end hook directly.)

willResolveField

The willResolveField event fires whenever Apollo Server is about to resolve a single field during the execution of an operation. The handler is passed an object with four fields (source, args, context, and info) that correspond to the four positional arguments passed to resolvers. (Note that source corresponds to the argument often called parent in these docs.)

You provide your willResolveField handler in the object returned by your executionDidStart handler.

Your willResolveField handler can optionally return an "end hook" function that's invoked with the resolver's result (or the error that it throws). The end hook is called when your resolver has fully resolved (e.g., if the resolver returns a Promise, the hook is called with the Promise's eventual resolved result).

willResolveField and its end hook are the only synchronous plugin APIs (ie, they do not return Promises).

Example

const server = new ApolloServer({
  /* ... other necessary configuration ... */

  plugins: [
    {
      async requestDidStart(initialRequestContext) {
        return {
          async executionDidStart(executionRequestContext) {
            return {
              willResolveField({source, args, context, info}) {
                const start = process.hrtime.bigint();
                return (error, result) => {
                  const end = process.hrtime.bigint();
                  console.log(`Field ${info.parentType.name}.${info.fieldName} took ${end - start}ns`);
                  if (error) {
                    console.log(`It failed with ${error}`);
                  } else {
                    console.log(`It returned ${result}`);
                  }
                };
              }
            }
          }
        }
      }
    }
  ]
})

didEncounterErrors

The didEncounterErrors event fires when Apollo Server encounters errors while parsing, validating, or executing a GraphQL operation.

didEncounterErrors?(
  requestContext: WithRequired<
    GraphQLRequestContext<TContext>,
    'metrics' | 'source' | 'errors' | 'logger'
  >,
): Promise<void>;

willSendResponse

The willSendResponse event fires whenever Apollo Server is about to send a response for a GraphQL operation. This event fires (and Apollo Server sends a response) even if the GraphQL operation encounters one or more errors.

willSendResponse?(
  requestContext: WithRequired<
    GraphQLRequestContext<TContext>,
    'metrics' | 'response' | 'logger'
  >,
): Promise<void>;
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