Use w/ 3rd party logging crate

[kpeters-cbsi]

I'm working on a Rust project using Fastly's Compute@Edge platform. C@E allows you to choose a variety of logging backends; I'm using New Relic, which takes JSON input. The nice thing is that, if you use Fastly's log_fastly crate Fastly will handle your New Relic authentication for you, so all you do is log::info!(...) and you're good. The not-so-nice thing is that while the crate will automagically format a simple string as JSON for you (in the form {"message":"whatever you put in your simple string"}) it doesn't support adding fields to every message, so you have to do something like log::info!(r#"{"message":"something", "source":"myapp"}"#); every time. This is a pain in the ass. Is there a way to get tracing to call Fastly's log!() macro?

[hawkw]

log_fastly appears to re-export the log crate's macros. If you want to use tracing to export to the log crate, you can use the "log" feature flag on the tracing crate to send tracing events to the log crate. However, this won't actually get you structured fields reported as JSON; they will just be formatted as key=value in a string.

If you want to actually use tracing to emit structured JSON logs, you would need to actually use a tracing::Subscriber implementation that consumes tracing's structured fields. Unfortunately, I don't think that will work nicely out of the box with log_fastly. If you want to continue using Fastly's logging infrastructure, rather than setting up your own tracing subscriber, the best solution is probably to ask the Fastly team if they'd consider adding tracing support to log_fastly. I know some of the Compute@Edge team (hi @acfoltzer, @pchickey, and @cratelyn!) are tracing enthusiasts, so it's possible they could add a tracing::Subscriber implementation in log_fastly!

[acfoltzer]

Hi @hawkw! Yes indeed, a tracing subscriber backed by the lower-level fastly::log::Endpoint type is what's needed here. It should be possible in the mean time to implement without any internal changes to the existing fastly crate, but I've made sure to pass this along as a potential TODO to provide a first-party solution.

[hawkw]

I think all that's necessary is to add a MakeWriter implementation for fastly::log::Endpoint (or a wrapper type, in downstream code), and then it could be passed to tracing_subscriber::fmt's SubscriberBuilder::with_writer method. It would then be necessary to configure the subscriber to format events as JSON.

Eventually, fastly could probably provide a wrapper that configures tracing to do that, so that it just works out of the box, similarly to the log crate.

[kpeters-cbsi]

So... how would I go about doing that? I'm still pretty new to Rust.

[kpeters-cbsi]

Well, I couldn't figure out the MakeWriter business, but I did find a lovely blog post at https://burgers.io/custom-logging-in-rust-using-tracing that helped a great deal. I took the CustomLayer created there and in place of println!("{}", serde_json::to_string_pretty(&output).unwrap());, I did writeln!(endpoint, "{}", serde_json::to_string(&output).unwrap()).unwrap(); (endpoint being an instance of Fastly::log::Endpoint), and that looks to have solved my problem.