Thanks! I think you're right. To go beyond my original question: In terms of how I have thought about them before, webhooks are similar, but not quite the sam

Thanks! I think you're right. To go beyond my original question: In terms of how I have thought about them before, webhooks are similar, but not quite the same thing. I've really only dealt with webhooks when the response was delayed because of a need for human interaction, for example: sending a document for e-signature and then later receiving notifications when the document is viewed and then signed or rejected. I haven't experienced much need to make REST APIs async, only in cases where the processing was generally always very slow for reasons that couldn't be optimized away. I haven't seen much advocacy for it either. However, if we think about lambda-based clients, then it makes a lot more sense to provide async apis. Why have both sides paying for the duration, even if the duration is relatively short? update: Whether or not it's cheaper for the client depends on the granularity of the pricing of the client's provider. For AWS, it seems to be per second of duration, so async would be more expensive for APIs that execute quickly.