Plugins

Plugins enable you to extend Apollo Server's core functionality by performing custom operations in response to certain events. Currently, these events correspond to individual phases of the GraphQL request lifecycle, and to the startup of Apollo Server itself.

For example, a basic logging plugin might log the GraphQL query string associated with each request that's sent to Apollo Server.

Creating a plugin

Plugins are JavaScript objects that implement one or more functions that respond to events. Here's a basic plugin that responds to the serverWillStart event:

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const myPlugin = {
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  serverWillStart() {
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    console.log('Server starting up!');
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  },
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};

You can define a plugin in the same file where you initialize Apollo Server, or you can export it as a separate module:

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module.exports = {
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  serverWillStart() {
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    console.log('Server starting up!');
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  },
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};

To create a plugin that accepts options, create a function that accepts an options object and returns a properly structured plugin object, like so:

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module.exports = (options) => {
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  return {
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    serverWillStart() {
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      console.log(options.logMessage);
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    },
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  };
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};

Responding to events

A plugin specifies exactly which events it responds to by implementing functions that correspond to those events. The plugin in the examples above responds to the serverWillStart event, which fires when Apollo Server is preparing to start up.

A plugin can respond to any combination of supported events.

Responding to request lifecycle events

Plugins can respond to the following events associated with the GraphQL request lifecycle:

• parsingDidStart
• validationDidStart
• didResolveOperation
• responseForOperation
• executionDidStart
• didEncounterErrors
• willSendResponse

However, the way you define these functions is slightly different from the serverWillStart example above. First, your plugin must define the requestDidStart function:

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const myPlugin = {
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  requestDidStart() {
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    console.log('Request started!');
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  },
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};

The requestDidStart event fires whenever Apollo Server receives a GraphQL request, before any of the lifecycle events listed above. You can respond to this event just like you respond to serverWillStart, but you also use this function to define responses for a request's lifecycle events, like so:

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const myPlugin = {
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  requestDidStart(requestContext) {
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    console.log('Request started!');
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    return {
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      parsingDidStart(requestContext) {
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        console.log('Parsing started!');
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      },
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      validationDidStart(requestContext) {
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        console.log('Validation started!');
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      }
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    }
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  },
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};

As shown, the requestDidStart function can optionally return an object that defines functions that respond to request lifecycle events. This structure organizes and encapsulates all of your plugin's request lifecycle logic, making it easier to reason about.

Request lifecycle event flow

The following diagram illustrates the sequence of events that fire for each request. Each of these events is documented below.

^{*The indicated events also support end hooks that are called when their associated step completes.}

End hooks

Event handlers for the following events can optionally return a function that is invoked after the corresponding lifecycle phase ends:

• parsingDidStart
• validationDidStart
• executionDidStart (this handler can alternatively return an object containing an executionDidEnd function)
• willResolveField

These end hooks are passed any errors that occurred during the execution of that lifecycle phase. For example, the following plugin logs any errors that occur during any of the above lifecycle events:

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const myPlugin = {
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  requestDidStart() {
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    return {
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      parsingDidStart() {
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        return (err) => {
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          if (err) {
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            console.error(err);
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          }
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        }
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      },
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      validationDidStart() {
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        // This end hook is unique in that it can receive an array of errors,
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        // which will contain every validation error that occurred.
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        return (errs) => {
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          if (errs) {
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            errs.forEach(err => console.error(err));
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          }
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        }
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      },
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      executionDidStart() {
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        return (err) => {
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          if (err) {
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            console.error(err);
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          }
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        }
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      }
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    }
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  }
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}

Note that the validationDidStart end hook receives an array of errors that contains every validation error that occurred (if any). The willResolveField end hook receives the error thrown by the resolver as the first argument and the result of the resolver as the second argument. The arguments to each end hook are documented in the type definitions in Request lifecycle events.

Inspecting request and response details

As the example above shows, requestDidStart and request lifecycle functions accept a requestContext parameter. This parameter is of type GraphQLRequestContext, which includes a request (of type GraphQLRequest), along with a response field (of type GraphQLResponse) if it's available.

These types and their related subtypes are all defined in apollo-server-types/src/index.ts.

Installing a plugin

Add your plugin to Apollo Server by providing a plugins configuration option to the ApolloServer constructor, like so:

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const { ApolloServer } = require('apollo-server');
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const ApolloServerOperationRegistry =
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  require('apollo-server-plugin-operation-registry');
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/* This example doesn't provide `typeDefs` or `resolvers`,
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   both of which are required to start the server. */
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const { typeDefs, resolvers } = require('./separatelyDefined');
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const server = new ApolloServer({
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  typeDefs,
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  resolvers,
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  // You can import plugins or define them in-line, as shown:
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  plugins: [
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    /* This plugin is from a package that's imported above. */
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    ApolloServerOperationRegistry({ /* options */ }),
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    /* This plugin is imported in-place. */
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    require('./localPluginModule'),
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    /* This plugin is defined in-line. */
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    {
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      serverWillStart() {
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        console.log('Server starting up!');
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      },
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    }
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  ],
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})

Apollo Server event reference

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. Currently, two server lifecycle events are supported: serverWillStart and requestDidStart.

Request lifecycle events are associated with a specific request. You define responses to these events within the response to a requestDidStart event, as described in Responding to request lifecycle events.

Server lifecycle events

serverWillStart

The serverWillStart event fires when Apollo Server is preparing to start serving GraphQL requests. If you respond to this event with an async function (or if the function returns a Promise), the server doesn't start until the asynchronous operation completes. If the Promise is rejected, startup fails (unless you're using Express middleware). This helps you make sure all of your server's dependencies are available before attempting to begin serving requests.

Example

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const server = new ApolloServer({
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  /* ... other necessary configuration ... */
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  plugins: [
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    {
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      serverWillStart() {
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        console.log('Server starting!');
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      }
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    }
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  ]
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})

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

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const server = new ApolloServer({
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  /* ... other necessary configuration ... */
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  plugins: [
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    {
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      serverWillStart() {
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        const interval = setInterval(doSomethingPeriodically, 1000);
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        return {
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          serverWillStop() {
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            clearInterval(interval);
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          }
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        }
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      }
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    }
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  ]
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})

requestDidStart

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

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

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

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

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

Request lifecycle events

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.

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didResolveSource?(
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  requestContext: WithRequired<
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    GraphQLRequestContext<TContext>, 'source' | 'logger'>,
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  >,
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): ValueOrPromise<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.

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

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

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

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

executionDidStart

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

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

executionDidStart may return an "end hook" function. Alternatively, it may return an object with one or both of the methods executionDidEnd and willResolveField. executionDidEnd is treated identically to an end hook: it is called after execution with any errors that occurred. willResolveField is documented in the next section.

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).

Example

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

didEncounterErrors

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

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didEncounterErrors?(
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  requestContext: WithRequired<
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    GraphQLRequestContext<TContext>,
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    'metrics' | 'source' | 'errors' | 'logger'
5
  >,
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): ValueOrPromise<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.

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