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Complexity

Warning This chapter applies only to the code first approach.

Query complexity allows you to define how complex certain fields are, and to restrict queries with a maximum complexity. The idea is to define how complex each field is by using a simple number. A common default is to give each field a complexity of 1. In addition, the complexity calculation of a GraphQL query can be customized with so-called complexity estimators. A complexity estimator is a simple function that calculates the complexity for a field. You can add any number of complexity estimators to the rule, which are then executed one after another. The first estimator that returns a numeric complexity value determines the complexity for that field.

The @nestjs/graphql package integrates very well with tools like graphql-query-complexity that provides a cost analysis-based solution. With this library, you can reject queries to your GraphQL server that are deemed too costly to execute.

Installation#

To begin using it, we first install the required dependency.

$ npm install --save graphql-query-complexity

Getting started#

Once the installation process is complete, we can define the ComplexityPlugin class:

import { GraphQLSchemaHost, Plugin } from '@nestjs/graphql';
import {
  ApolloServerPlugin,
  GraphQLRequestListener,
} from 'apollo-server-plugin-base';
import { GraphQLError } from 'graphql';
import {
  fieldExtensionsEstimator,
  getComplexity,
  simpleEstimator,
} from 'graphql-query-complexity';

@Plugin()
export class ComplexityPlugin implements ApolloServerPlugin {
  constructor(private gqlSchemaHost: GraphQLSchemaHost) {}

  requestDidStart(): GraphQLRequestListener {
    const { schema } = this.gqlSchemaHost;

    return {
      didResolveOperation({ request, document }) {
        const complexity = getComplexity({
          schema,
          operationName: request.operationName,
          query: document,
          variables: request.variables,
          estimators: [
            fieldExtensionsEstimator(),
            simpleEstimator({ defaultComplexity: 1 }),
          ],
        });
        if (complexity >= 20) {
          throw new GraphQLError(
            `Query is too complex: ${complexity}. Maximum allowed complexity: 20`,
          );
        }
        console.log('Query Complexity:', complexity);
      },
    };
  }
}

For demonstration purposes, we specified the maximum allowed complexity as 20. In the example above, we used 2 estimators, the simpleEstimator and the fieldExtensionsEstimator.

• simpleEstimator: the simple estimator returns a fixed complexity for each field
• fieldExtensionsEstimator: the field extensions estimator extracts the complexity value for each field of your schema

Hint Remember to add this class to the providers array in any module.

Field-level complexity#

With this plugin in place, we can now define the complexity for any field by specifying the complexity property in the options object passed into the @Field() decorator, as follows:

@Field({ complexity: 3 })
title: string;

Alternatively, you can define the estimator function:

@Field({ complexity: (options: ComplexityEstimatorArgs) => ... })
title: string;

Query/Mutation-level complexity#

In addition, @Query() and @Mutation() decorators may have a complexity property specified like so:

@Query({ complexity: (options: ComplexityEstimatorArgs) => options.args.count * options.childComplexity })
items(@Args('count') count: number) {
  return this.itemsService.getItems({ count });
}