Give me /events, not webhooks

🔗 Give me /events, not webhooks
blog.sequinstream.com

Webhooks are a supplement to many APIs. With a webhook system in place, System B can register to receive notifications about certain changes to System A. When a change occurs, System A pushes the change to System B, usually in the form of making an HTTP POST request.

Webhooks are intended to eliminate or reduce the need to constantly poll for data. But in my experience, webhooks come with a few challenges.

In general, you can't rely on webhooks alone to keep two systems consistent. Every integration I've ever worked on has realized this fact by eventually augmenting webhooks with polling. This is due to a few problem areas.

First, there are risks when you go down. Yes, senders typically retry undelivered webhooks with some exponential back-off. But the guarantees are often loose or unclear. And the last thing your system probably needs after recovering from a disaster is a deluge of backed-up webhooks to handle.

Second, webhooks are ephemeral. They are too easy to mishandle or lose. If you realize after deploying a code change that you fat-fingered a JSON field and are inserting nulls into your database, there is no way to play the webhooks back. Or, you might handle part of the webhook processing pipeline out-of-band with the webhook request - like the database insert. But then you risk that failing and you losing the webhook.

To mitigate both of these issues, many developers end up buffering webhooks onto a message bus system like Kafka, which feels like a cumbersome compromise.

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