Hi the Node js SDK

Question

Hi the Node js SDK <https docs growthbook io lib js nodejs|docs> mention to gt Create a separate GrowthBook instance for every incoming request This is easiest if you use a middleware May I know why a separate instance needs to be instantiated for each request Wouldn t this create many instances proportional to the traffic the server is handling Is it possible to have a cached GrowthBook instance across the server and we just add it to the `req` property and override the attributes if required like `req growthbook setAttributes ` for incoming requests

happy-autumn-40938 · Answer

Your intuition is correct in wanting a cached version of features experiments across all requests and in fact that s exactly how our SDK works behind the scenes Our JS and React SDKs create a shared cached singleton across all SDK instances and each new SDK instance is just a thin short lived wrapper around that singleton This allows each SDK instance to be user specific based on attributes while still tapping into the central feature repository

limited-ram-77544 · Answer

Ah I see Got it thanks for explaining < happy autumn 40938>

limited-ram-77544 · Answer

< happy autumn 40938> I have a follow up question for backend usage The recommended approach creates and introduces a `growthbook` property within the Express `req` object at a middleware level Let s say you have a downstream service e g serviceC depending on a feature flag E g middleware gt controller gt serviceA gt serviceB gt serviceC Does this mean that I need the pass the `growthbook` object from req from the controller level all the way down to serviceC This seems similar to prop drilling Is there a way serviceC can just obtain the singleton from the SDK I considered just initialising a new SDK instance in serviceC and reading the value of the flag This would work for normal usage but not for percentage based rollouts which would depend on the attributes set by the middleware layer e g an authorization middleware that sets user specific attributes like email and role Is there any recommended approach here

happy-autumn-40938 · Answer

The issue here is that your singleton does not have anything user specific in it So if you re simply using feature flags as global on off switches for everybody then instantiating a new SDK instance within serviceC and reading from the singleton works fine But ultimately you ll probably want user based targeting especially for experimentation There s no good way to avoid prop drilling in this case although you can smooth it over a bit by putting either the evaluated feature results or even just the user s SDK instance into a shared `ctx` object and passing that between services

limited-ram-77544 · Answer

I guess I will inject the user s SDK instance into a `ctx` as you suggested Thanks for the input < happy autumn 40938>