Synchronous calls to another serverless function


I love serverless, but I’m not sure how to architect the following:
Occasionally I have a function in one service that needs to call a function in another service.
Normally if there is no time sensitivity, and I don’t need a response from that other function - I just use events / AWS SNS to subscribe the other function to an event that I trigger.

But what options do I have for synchronous calls to another function, where I need a response in < 500ms?
I don’t want to have to use the REST API that serverless will provide… as that feels inefficient.

Any ideas? Something like RPC?


The simplest solution is to invoke the Lambda directly.