Use @connections commands in your backend service

Your backend service can use the following WebSocket connection HTTP requests to send a callback message to a connected client, get connection information, or disconnect the client.

In the following command, you need to replace {api-id} with the actual API ID, which is displayed in the API Gateway console or returned by the Amazon CLI create-api command. You must establish the connection before using this command.

To send a callback message to the client, use:

POST https://{api-id}.execute-api.us-east-1.amazonaws.com/{stage}/@connections/{connection_id}

You can test this request by using Postman or by calling awscurl as in the following example:

awscurl --service execute-api -X POST -d "hello world" https://{prefix}.execute-api.us-east-1.amazonaws.com/{stage}/@connections/{connection_id}

You need to URL-encode the command as in the following example:

awscurl --service execute-api -X POST -d "hello world" https://aabbccddee.execute-api.us-east-1.amazonaws.com/prod/%40connections/R0oXAdfD0kwCH6w%3D

To get the latest connection status of the client, use:

GET https://{api-id}.execute-api.us-east-1.amazonaws.com/{stage}/@connections/{connection_id}

To disconnect the client, use:

DELETE https://{api-id}.execute-api.us-east-1.amazonaws.com/{stage}/@connections/{connection_id}

You can dynamically build a callback URL by using the $context variables in your integration. For example, if you use Lambda proxy integration with a Node.js Lambda function, you can build the URL and send a message to a connected client as follows:

import {
  ApiGatewayManagementApiClient,
  PostToConnectionCommand,
} from "@aws-sdk/client-apigatewaymanagementapi";

export const handler = async (event) => {
  const domain = event.requestContext.domainName;
  const stage = event.requestContext.stage;
  const connectionId = event.requestContext.connectionId;
  const callbackUrl = `https://${domain}/${stage}`;
  const client = new ApiGatewayManagementApiClient({ endpoint: callbackUrl });

  const requestParams = {
    ConnectionId: connectionId,
    Data: "Hello!",
  };

  const command = new PostToConnectionCommand(requestParams);

  try {
    await client.send(command);
  } catch (error) {
    console.log(error);
  }

  return {
    statusCode: 200,
  };
};

When sending a callback message, your Lambda function must have permission to call the API Gateway Management API. You might receive an error that contains GoneException if you post a message before the connection is established, or after the client has disconnected.