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AWS Lambda is usually used for short-lived processes like the request/response lifecycle. The default Lambda timeout of 3 seconds reflects this common use-case.
\n\nHowever the maximum execution duration of AWS Lambda functions was increased from 1 minute to 5 minutes a few years ago.
\nNot only is it possible to connect to AWS Lambda via WebSockets using MQTT or AWS IoTData but this strategy works surprisingly well.
\nMQTT is both a general publish-subscribe messaging protocol and a popular npm package. MQTT (the npm package) has a clean interface that enables initiating a WebSocket connection, subscribing to topics, listening for messages, and publishing messages.
\nOn the client side, we fetch a WebSocket url that formally looks something like this:
\nwss://XXXXX.iot.us-east-1.amazonaws.com/mqtt?X-Amz-Date=20180812T133259Z&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=ZZZ%2F20180812%2Fus-east-1%2Fiotdevicegateway%2Faws4_request&X-Amz-SignedHeaders=host&X-Amz-Signature=PPP&X-Amz-Security-Token=QQQ
\nThis can be generated using aws-sign-mqtt:
\n// set the AWS_IOT_HOST environmental variable:\n// export AWS_IOT_HOST=$(aws --region us-east-1 iot describe-endpoint --output text)\nconst url = require('aws-sign-mqtt')()
\nThis is just a WebSocket URL signed using AWS signature v4.
\nThis WebSocket URL should obviously be generated server-side (lest you expose your AWS credentials).
\nTo demonstrate how AWS Lambda can be used for long-lived WebSocket connections, we'll create two simple Lambda functions. The first is a simple HTTP endpoint that returns a signed WebSocket url. The second lambda function is triggered when the client subscribes to an AWS IoT topic.
\nconst AWS = require('aws-sdk')\nconst createPresignedURL = require('aws-sign-mqtt')\nconst { randomBytes } = require('crypto')\n// Lambda function #1 - session\n// Simple HTTP endpoint that returns the WebSocket URL and channel ID\nmodule.exports.session = (event, context, callback) => {\n const endpointUrl = createPresignedURL()\n // create an arbitrary channel ID to identify the current session\n const channelId = randomBytes(5).toString('hex') \n callback(null, {\n statusCode: 200,\n body: JSON.stringify({ endpointUrl, channelId }),\n headers: {\n 'Access-Control-Allow-Origin': '*'\n }\n })\n}\n// Lambda function #2 - mqtt\n// Sends messages to the client via AWS IotData.\n// You must manually set the AWS_IOT_HOST environmental variable which is account-specific.\n// From the command line run: aws iot describe-endpoint --output text\nmodule.exports.mqtt = async (payload, context) => {\n // The payload is whatever we send from the browser\n const { channelId } = payload\n const iot = new AWS.IotData({\n endpoint: process.env.AWS_IOT_HOST,\n region: process.env.AWS_REGION\n })\n // we respond with a different topic that uses the channelID\n // to avoid broadcasting the same message to all users\n await iot.publish({\n topic: `foobar/${channelId}/response`,\n payload: JSON.stringify({ message: 'hello' }),\n qos: 1\n }).promise()\n // at this point the Lambda function ends because the Node event loop is closed\n}\n
\nHere's what our serverless.yml might look like:
\nservice: foobar\nprovider:\n name: aws\n runtime: nodejs8.10\n region: us-east-1\n environment: \n AWS_IOT_HOST: ''\n iamRoleStatements:\n - Effect: \"Allow\"\n Action: \n - iot:*\n Resource: \"*\"\nfunctions:\n session:\n handler: handler.session\n events:\n - http:\n method: GET\n path: /\n cors: true # change this in production to \"credentails: true\" to secure endpoint\n mqtt:\n handler: handler.mqtt\n events: \n - iot\n sql: 'SELECT * FROM foobar/session'\n
\nAfter deploying our function (sls deploy), serverless will print the session endpoint that we need to generate the WebSocket URL:
\nService Information\nservice: foobar\nregion: us-east-1\nstack: foobar-dev\napi keys:\n None\nendpoints:\n GET - https://xxxxx.execute-api.us-east-1.amazonaws.com/dev\n GET - https://xxxxx.execute-api.us-east-1.amazonaws.com/dev\nfunctions:\n mqtt: foobar-dev-mqtt\n session: foobar-dev-session
\nIf you have the AWS command line interface installed you can try running something like the following from terminal to test your functions:
\naws --region us-east-1 iot-data publish --topic 'foobar/session' --payload '{ \"channelId\": \"1\" }'
\nThe above command should trigger foobar-dev-mqtt.
\nCopy the HTTP endpoint (https://xxxxx.execute-api.us-east-1.amazonaws.com/dev) to use in the client-side code.
\nIn the browser, we fetch the WebSocket URL and initiate a connection using mqtt. Here's an example:
\n// client.js\nconst mqtt = require('mqtt')\nconst ENDPOINT_URL = 'https://xxxxx.execute-api.us-east-1.amazonaws.com/dev'\nasync function connect () {\n // fetch URL and arbitrary channelId from Lambda function #1 (session)\n let { channelId, endpointUrl } = await fetch(ENDPOINT_URL)\n .then(resp => resp.json())\n let channel = mqtt.connect(endpointUrl)\n channel.on('connect', () => {\n channel.subscribe(`foobar/${channelId}/response`, () => {\n // publish a message to the 'foobar/session' topic \n // which triggers Lambda function #2 (mqtt)\n channel.publish('foobar/session', {\n payload: JSON.stringify({ channelId }),\n qos: 1,\n })\n // Listen for messages\n channel.on('message', (topic, buffer) => {\n console.log({ topic, message: buffer.toString() })\n })\n })\n })\n}\nconnect()
\nSo far we have deployed two lambda functions: the first generates a signed WebSocket URL and the second responds by publishing messages using AWS.IotData.
\nHowever, responding in AWS Lambda using IotData is limited because we can only send one message from the client (the first message that initiates the session). What if we want to respond to a stream of messages? In that case, we'll use the MQTT package on the server side as well.
\nOur new version of Lambda function #2 (mqtt) might look like this:
\n// handler.js\nconst mqtt = require('mqtt')\nconst createPresignedURL = require('aws-sign-mqtt')\nconst connect = ({ channel }) =>\n new Promise((resolve, reject) => channel.on('connect', resolve))\nconst subscribe = ({ channel, topic }) =>\n new Promise((resolve, reject) => channel.subscribe(topic, resolve))\n \nmodule.exports.mqtt = async (payload, context, callback) => {\n const { channelId } = payload\n const TOPIC_REQUEST = `foobar/${channelId}/request`\n const TOPIC_RESPONSE = `foobar/${channelId}/response`\n const channel = mqtt.connect(createPresignedURL())\n await connect({ channel })\n await subscribe({ channel, topic: TOPIC_REQUEST })\n let resolve\n let promise = new Promise(res => {\n resolve = res\n })\n const sessionTimeout = () => setTimeout(() => {\n channel.end(resolve)\n }, 30000)\n let timeout = sessionTimeout()\n channel.on('message', (topic, buffer) => {\n console.log({ topic, message: buffer.toString() })\n clearTimeout(timeout)\n timeout = sessionTimeout()\n channel.publish(TOPIC_RESPONSE, {\n qos: 1,\n payload: JSON.stringify({ message: 'PONG' })\n })\n })\n // this promise is resolved after 30 seconds of inactivity\n // ending function execution. If we don't promisify these event\n // handlers, then the Lambda function will end prematurely.\n await promise.then(() => callback(null))\n}
\nThe primary difference between this version and what we had before is that now we're using MQTT to listen for messages and respond rather than AWS.IotData. This is necessary for two-way communication between client and server.
\nWe can then revise our client-side code as follows:
\n// client.js\nconst mqtt = require('mqtt')\nconst ENDPOINT_URL = 'https://xxxxx.execute-api.us-east-1.amazonaws.com/dev'\nasync function connect () {\n // fetch URL and arbitrary channelId from Lambda function #1 (session)\n let { endpointUrl, channelId } = await fetch(ENDPOINT_URL)\n .then(resp => resp.json())\n let channel = mqtt.connect(endpointUrl)\n channel.on('connect', () => {\n channel.subscribe(`foobar/${channelId}/response`, async () => {\n // publish a message to the 'foobar/session' topic \n // which triggers Lambda function #2 (mqtt)\n await new Promise((resolve, reject) => \n channel.publish(\n 'foobar/session', {\n payload: JSON.stringify({ channelId }),\n qos: 1,\n },\n resolve\n )\n )\n \n // Listen for messages\n channel.on('message', (topic, buffer) => {\n console.log({ topic, message: buffer.toString() })\n })\n \n channel.publish(\n `foobar/${channelId}/request`,\n JSON.stringify({ message: 'hello' }),\n { qos: 1 }\n )\n })\n })\n}\nconnect()
\nWhat's different? This time we issue a request using the topic foobar/{channelId}/request. As before, we listen for messages using the topic foobar/{channelId}/response.
\nWhen we run the above client-side code, here's what happens:
\nSometimes it's best to look at the source code of real projects rather than muddle through half-baked tutorials. To that end, here are some examples:
\n