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0 pointsrattray4y ago0 comments

You can't _only_ do that, but it seems like a reasonable place to start for REST API's. I don't think there's anything to change in your product, just that the docs should recommend ways to reroute as early as possible in a request lifespan (eg, at the routing layer or before).

I'd worry about a POST route that does some expensive/slow reads/computations in the first half of the request, and then only writes at the end – lots of lost time! Would have been much better to say, hey, for this route (or for any POST route in _my_ application), please bounce to the region with the primary db instance.

Actually, while that could be done at the application level with reasonable latency, it'd probably still be better to allow the user to write some rules at the proxy layer for the "first guess".

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tptacek4y ago· 1 in thread

I get what you're trying to say, but what I'm saying is that the approach you're proposing simply doesn't work. Sure, it'll route POSTs and PUTs to the write master, that part will work fine. But GETs will randomly bomb when they throw an UPDATE at their read replica. We can't predict which of those GETs will break on behalf of our users, and the one thing we're trying to avoid asking our users to do is to do complicated surgery on their applications to make them run well on us.

Like, I think it would make sense to have the feature that routes by HTTP verb! But it would be dangerous to promote that as a write-steering feature.

rattrayOP4y ago

No, I'm saying you do both - proactively steer routes that are obviously going to write to the primary, and use the clever retry technique for everything else.

The retry technique is something you must do as a fallback, but it causes additional latency and should be avoided when you know ahead of time that the request involves a write.

Again, the application engineer should be the one responsible for writing these rules, whether in their application's routing/middleware layer (easier) or in flys proxy layer with a rules engine (faster).

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