streamingpollinglaunchdarkly

When to use polling and streaming in launch darkly

I have started using launch darkly(LD) recently. And I was exploring how LD updates its feature flags.

As mentioned Here, there are two ways.

  1. Streaming
  2. Polling

I was just thinking which implementation will be better in what cases. After a little research about streaming vs polling, It was found Streaming has the following advantages over polling.

I am pretty sure all of the above advantages comes at a cost. So,

  1. Are there any downsides of using streaming over polling?
  2. In what scenarios polling should be preferred? or the other way around?
  3. On what factors should I decide whether to stream or poll?
Solution

  • Streaming

    Streaming requires your application to be always alive. This might not be the case in a serverless environment. Furthermore, a streaming solution usually relies on a connection that is always open in the background. This might be costly, so feature flag providers tend to limit the number of concurrent connections you can keep open to their infrastructure. This might be not a problem if you use feature flags only in a few application instances. But you will easily reach the limit if you want to stream feature flag updates to mobile apps or a ton of microservices.

    Polling

    Polling sounds less fancy, but it's a reliable & robust old-school pattern that will work in almost all environments.

    Webhooks

    There is a third option too: webhooks. The basic idea is that you create an HTTP endpoint on your end and he feature flag service will call that endpoint whenever a feature flag value update happens. This way you get a "notification" about feature flag value changes. For example ConfigCat supports this model. ConfigCat can notify your infrastructure by calling your webhooks and (optionally) pushing new values to your end. Webhooks have the advantage over streaming that they are cheap to maintain, so feature flag service providers don't limit them as much (for example ConfigCat can give you unlimited webhooks).

    How to decide

    How I would use the above 3 option really depends on your use-case. A general rule of thumb is: use polling by default and add quasi real-time notifications (by streaming or by webhooks) to the components where it's critical to know about feature flag value updates.