All these pieces of functionality follow the same pattern: We want an abstraction that is composable. But we don’t want to write TracingRetryableClient, TracingRetryableCachingHttpClient, RetryableTracingCachingHttpClient (order matters!) and all the other possible combinations. There is other functionality we want our HTTP clients to handle: propagation of distributed tracing headers, caching, logging. We could write a RetryHttpClient that wraps a standard client to augment it with retry functionality - but retries are not the full story. At the same time, we’d prefer to keep our domain code free of this network-level concern - it’d be ideal to have retries transparently implemented in the HTTP client itself. We have tens of clients across our codebases: we don’t want to re-implement retries in an ad-hoc fashion for each of them. The mechanics are fairly simple: wrap each request in a loop and retry until you get a successful response or you run out of attempts.

Retries are a common strategy to improve reliability. The problem Running applications at scale requires built-in resilience when communicating over the network with internal and external services, because services fail.

This post is about reqwest-middleware, a crate built on top of the reqwest HTTP client to provide middleware functionality. The first one was gRPC load balancing in Rust, in case you missed it. This is the second post in our open source series, where we talk about engineering challenges at TrueLayer and open source our solutions to them.

How to get rust for free 2017