Get Query String & Path parameters in Lambda with API Gateway

In this article, we're going to discuss how to get query string and path parameters from API Gateway in AWS Lambda.

What is the problem that we're trying to solve?

When you're building Serverless API using API Gateway and Lambda, there may be scenarios where you want to access query string parameters or path parameters.

For example, you may want to fetch the user information based on the user id parameter passed to the API. As this is a GET request, there are primarily 2 ways to pass this information via URL - path parameter or query string parameter

In the below URL,  user id information is passed as a path parameter 1

https://your-api-gateway-url/users/1

If you want to use a query string, you would pass the parameter after ? as shown below

https://your-api-gateway-url/users?userId=1

Note: If you want to pass multiple values, it is better to use query string .

https://your-api-gateway-url/users?param1=value1&param2=value2

If you want to expose additional operations for that parameter( user id in this case), it is better to use the path parameter as shown below.

You can use the first URL  when want to get the user groups for the user and you can use the second URL when you want to get the metadata about the user.

https://your-api-gateway-url/users/1/groups
https://your-api-gateway-url/users/1/metadata

Infrastructure Creation

We'll be using AWS CDK for creating all necessary AWS resources in this article. It's an open-source software development framework that lets you define cloud infrastructure. AWS CDK supports many languages including TypeScript, Python, C#, Java, and others. You can learn more about AWS CDK from a beginner's guide here. We're going to use TypeScript in this article.

Creating REST API

We can use the below code to create REST API

   const api = new RestApi(this, "queryStringPathParamApi", {
      restApiName: "query-string-path-param-api",
    });

Lambda function properties

We're going to use node 18 runtime for the lambda - as shown below

 const nodeJsFunctionProps: NodejsFunctionProps = {
      bundling: {
        externalModules: [
          "aws-sdk", // Use the 'aws-sdk' available in the Lambda runtime
        ],
      },
      runtime: Runtime.NODEJS_18_X,
      timeout: Duration.seconds(30), // Api Gateway timeout
    };
    const qsPathLambdaFn = new NodejsFunction(this, "qsPathLambdaFn", {
      entry: path.join(__dirname, "../lambdas", "users.ts"),
      ...nodeJsFunctionProps,
      functionName: "qsPathLambdaFn",
    });

Lambda function

In this lambda function, we're going to access the query string and path parameters from the URL. We're returning JSON with query parameters and path parameters populated from the incoming APIGateway event itself.

import { APIGatewayEvent } from "aws-lambda";
export const handler = async (event: APIGatewayEvent): Promise<any> => {
  return {
    statusCode: 200,
    body: JSON.stringify({
      queryParameters: event.queryStringParameters || {},
      pathParameters: event.pathParameters || {},
    }),
  };
};

Integrate with API Gateway

We can integrate the lambda with API Gateway, as below.

const qsPathLambdaIntegration = new LambdaIntegration(qsPathLambdaFn);

const usersResource = api.root.addResource("users");
usersResource.addMethod("GET", qsPathLambdaIntegration);

const userResource = usersResource.addResource("{id}");
userResource.addMethod("GET", qsPathLambdaIntegration);

Please note that we've used the same lambda function for the root API resource and its child resource {id}

Deployment & Testing

You can deploy the stack using the below command

cdk deploy

Getting the query string value

If you're using query string, you can get the value of query string parameter values from event.queryParameters?.userId

Get the path parameter values

You can access the path parameter from event.pathParameters?.id if you've created a child resource with id

You can use these values to query the database and return the appropriate results to the user.

Please let me know your thoughts in the comments.