[btilly]

Set up a Redis pub-sub: https://redis.io/docs/manual/pubsub/

The code for the websocket subscribes to Redis. The webhook will publish to Redis.

The sequence of actions is now that you send the client to the PSP, which will hit the webhook, that publishes to Redis, that the server side of the websocket is receiving, that realizes it should send the message to the client, that receives the message from the websocket, that then tells the client to redirect.

[Mavvie]

I was about to talk about issues with scaling Redis pubsub but apparently they just recently published a newer version (7) with proper sharding support in cluster mode!

[btilly]

You do have to be aware of Redis' limits. But for most sites, "people paying at the moment" is well within them. And Redis is the easiest solution to implement from scratch.

That said, if you're deployed in a cloud, look up cloud specific messaging services. Specifically AWS SNS, Google PubSub and Azure Web PubSub. But the pattern remains the same. The webhook publishes to the PubSub which finds the other end of the websocket which sends information to the client that does something with it.