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Websockets example (Chat):

Getting started:

Install:

npm i

Deploy:

npx cdk deploy

How to test it?

Prerequisite

  • Two terminals opened
  • Websocket-CLI installed (e.g. Websocat)

Connect to Websocket:

websocat <StackOutput.WebSocketUrl>

Log connection ID:

in one of the terminals enter:

{"action":"whoAmI"}

this should return something like: bUPgldy-FiACE_Q=

Send a message:

in the other terminal enter:

{"action":"sendMessage","connectionId":"<ID-FROM-ABOVE>","message":"Hello!"}

If everything worked successfully you should now see your message in the first terminal.

How does this solution work?

  • When you establish a websocket connection, the lambda connectHandler will be executed.
    • This is saving the connectionId into a dynamodb table, which could be used for different purposes like broadcasting.
  • When you close the websocket connection, the lambda disconnectHandler will be executed.
    • This is removes the connectionId from a dynamodb table.
  • When you have a connection open and post a message with an action property, it will route the message to the configured lamdba.
    • See Diagram

diagram.png

How do Websockets with Lambda work?

First of all, Lambdas are not constantly running!
When you send a message into the opened connection API-Gateway tries to route the message to the corresponding lambda, based on the action argument. When someone tries to send a message to a open connection, you need the use the PostToConnection-Command.