//! `tracing-actix-web` provides [`TracingLogger`], a middleware to collect telemetry data from applications
//! built on top of the [`actix-web`] framework.
//!
//! # How to install
//!
//! Add `tracing-actix-web` to your dependencies:
//!
//! ```toml
//! [dependencies]
//! # ...
//! tracing-actix-web = "0.4.0-beta.13"
//! tracing = "0.1"
//! actix-web = "4.0.0-beta.9"
//! ```
//!
//! `tracing-actix-web` exposes three feature flags:
//!
//! - `opentelemetry_0_13`: attach [OpenTelemetry](https://github.com/open-telemetry/opentelemetry-rust)'s context to the root span using `opentelemetry` 0.13;
//! - `opentelemetry_0_14`: same as above but using `opentelemetry` 0.14;
//! - `opentelemetry_0_15`: same as above but using `opentelemetry` 0.15;
//! - `opentelemetry_0_16`: same as above but using `opentelemetry` 0.16;
//! - `emit_event_on_error`: emit a [`tracing`] event when request processing fails with an error (enabled by default).
//!
//! `tracing-actix-web` will release `0.4.0`, going out of beta, as soon as `actix-web` releases a stable `4.0.0`.
//!
//! # Getting started
//!
//! ```rust,compile_fail
//! use actix_web::{App, web, HttpServer};
//! use tracing_actix_web::TracingLogger;
//!
//! let server = HttpServer::new(|| {
//!     App::new()
//!         // Mount `TracingLogger` as a middleware
//!         .wrap(TracingLogger::default())
//!         .service( /*  */ )
//! });
//! ```
//!
//! Check out [the examples on GitHub](https://github.com/LukeMathWalker/tracing-actix-web/tree/main/examples) to get a taste of how [`TracingLogger`] can be used to observe and monitor your
//! application.  
//!
//! # `tracing`: who art thou?
//!
//! [`TracingLogger`] is built on top of [`tracing`], a modern instrumentation framework with
//! [a vibrant ecosystem](https://github.com/tokio-rs/tracing#related-crates).
//!
//! `tracing-actix-web`'s documentation provides a crash course in how to use [`tracing`] to instrument an `actix-web` application.  
//! If you want to learn more check out ["Are we observable yet?"](https://www.lpalmieri.com/posts/2020-09-27-zero-to-production-4-are-we-observable-yet/) -
//! it provides an in-depth introduction to the crate and the problems it solves within the bigger picture of [observability](https://docs.honeycomb.io/learning-about-observability/).
//!
//! # The root span
//!
//! [`tracing::Span`] is the key abstraction in [`tracing`]: it represents a unit of work in your system.  
//! A [`tracing::Span`] has a beginning and an end. It can include one or more **child spans** to represent sub-unit
//! of works within a larger task.
//!
//! When your application receives a request, [`TracingLogger`] creates a new span - we call it the **[root span]**.  
//! All the spans created _while_ processing the request will be children of the root span.
//!
//! [`tracing`] empowers us to attach structured properties to a span as a collection of key-value pairs.  
//! Those properties can then be queried in a variety of tools (e.g. ElasticSearch, Honeycomb, DataDog) to
//! understand what is happening in your system.  
//!
//! # Customisation via [`RootSpanBuilder`]
//!
//! Troubleshooting becomes much easier when the root span has a _rich context_ - e.g. you can understand most of what
//! happened when processing the request just by looking at the properties attached to the corresponding root span.  
//!
//! You might have heard of this technique as the [canonical log line pattern](https://stripe.com/blog/canonical-log-lines),
//! popularised by Stripe. It is more recently discussed in terms of [high-cardinality events](https://www.honeycomb.io/blog/observability-a-manifesto/)
//! by Honeycomb and other vendors in the observability space.
//!
//! [`TracingLogger`] gives you a chance to use the very same pattern: you can customise the properties attached
//! to the root span in order to capture the context relevant to your specific domain.
//!
//! [`TracingLogger::default`] is equivalent to:
//!
//! ```rust
//! use tracing_actix_web::{TracingLogger, DefaultRootSpanBuilder};
//!
//! // Two ways to initialise TracingLogger with the default root span builder
//! let default = TracingLogger::default();
//! let another_way = TracingLogger::<DefaultRootSpanBuilder>::new();
//! ```
//!
//! We are delegating the construction of the root span to [`DefaultRootSpanBuilder`].  
//! [`DefaultRootSpanBuilder`] captures, out of the box, several dimensions that are usually relevant when looking at an HTTP
//! API: method, version, route, etc. - check out its documentation for an extensive list.
//!
//! You can customise the root span by providing your own implementation of the [`RootSpanBuilder`] trait.  
//! Let's imagine, for example, that our system cares about a client identifier embedded inside an authorization header.
//! We could add a `client_id` property to the root span using a custom builder, `DomainRootSpanBuilder`:
//!
//! ```rust
//! use actix_web::dev::{ServiceResponse, ServiceRequest};
//! use actix_web::Error;
//! use tracing_actix_web::{TracingLogger, DefaultRootSpanBuilder, RootSpanBuilder};
//! use tracing::Span;
//!
//! pub struct DomainRootSpanBuilder;
//!
//! impl RootSpanBuilder for DomainRootSpanBuilder {
//!     fn on_request_start(request: &ServiceRequest) -> Span {
//!         let client_id: &str = todo!("Somehow extract it from the authorization header");
//!         tracing::info_span!("Request", client_id)
//!     }
//!
//!     fn on_request_end<B>(_span: Span, _outcome: &Result<ServiceResponse<B>, Error>) {}
//! }
//!
//! let custom_middleware = TracingLogger::<DomainRootSpanBuilder>::new();
//! ```
//!
//! There is an issue, though: `client_id` is the _only_ property we are capturing.  
//! With `DomainRootSpanBuilder`, as it is, we do not get any of that useful HTTP-related information provided by
//! [`DefaultRootSpanBuilder`].  
//!
//! We can do better!
//!
//! ```rust
//! use actix_web::dev::{ServiceResponse, ServiceRequest};
//! use actix_web::Error;
//! use tracing_actix_web::{TracingLogger, DefaultRootSpanBuilder, RootSpanBuilder};
//! use tracing::Span;
//!
//! pub struct DomainRootSpanBuilder;
//!
//! impl RootSpanBuilder for DomainRootSpanBuilder {
//!     fn on_request_start(request: &ServiceRequest) -> Span {
//!         let client_id: &str = todo!("Somehow extract it from the authorization header");
//!         tracing_actix_web::root_span!(request, client_id)
//!     }
//!
//!     fn on_request_end<B>(span: Span, outcome: &Result<ServiceResponse<B>, Error>) {
//!         DefaultRootSpanBuilder::on_request_end(span, outcome);
//!     }
//! }
//!
//! let custom_middleware = TracingLogger::<DomainRootSpanBuilder>::new();
//! ```
//!
//! [`root_span!`] is a macro provided by `tracing-actix-web`: it creates a new span by combining all the HTTP properties tracked
//! by [`DefaultRootSpanBuilder`] with the custom ones you specify when calling it (e.g. `client_id` in our example).  
//!
//! We need to use a macro because `tracing` requires all the properties attached to a span to be declared upfront, when the span is created.  
//! You cannot add new ones afterwards. This makes it extremely fast, but it pushes us to reach for macros when we need some level of
//! composition.
//!
//! # The [`RootSpan`] extractor
//!
//! It often happens that not all information about a task is known upfront, encoded in the incoming request.  
//! You can use the [`RootSpan`] extractor to grab the root span in your handlers and attach more information
//! to your root span as it becomes available:
//!
//! ```rust
//! use actix_web::dev::{ServiceResponse, ServiceRequest};
//! use actix_web::{Error, HttpResponse};
//! use tracing_actix_web::{RootSpan, DefaultRootSpanBuilder, RootSpanBuilder};
//! use tracing::Span;
//! use actix_web::get;
//! use tracing_actix_web::RequestId;
//! use uuid::Uuid;
//!
//! #[get("/")]
//! async fn handler(root_span: RootSpan) -> HttpResponse {
//!     let application_id: &str = todo!("Some domain logic");
//!     // Record the property value against the root span
//!     root_span.record("application_id", &application_id);
//!
//!     // [...]
//!     # todo!()
//! }
//!
//! pub struct DomainRootSpanBuilder;
//!
//! impl RootSpanBuilder for DomainRootSpanBuilder {
//!     fn on_request_start(request: &ServiceRequest) -> Span {
//!         let client_id: &str = todo!("Somehow extract it from the authorization header");
//!         // All fields you want to capture must be declared upfront.
//!         // If you don't know the value (yet), use tracing's `Empty`
//!         tracing_actix_web::root_span!(
//!             request,
//!             client_id, application_id = tracing::field::Empty
//!         )
//!     }
//!
//!     fn on_request_end<B>(span: Span, response: &Result<ServiceResponse<B>, Error>) {
//!         DefaultRootSpanBuilder::on_request_end(span, response);
//!     }
//! }
//! ```
//!
//! # The [`RequestId`] extractor
//!
//! `tracing-actix-web` generates a unique identifier for each incoming request, the **request id**.  
//!
//! You can extract the request id using the [`RequestId`] extractor:
//!
//! ```rust
//! use actix_web::get;
//! use tracing_actix_web::RequestId;
//! use uuid::Uuid;
//!
//! #[get("/")]
//! async fn index(request_id: RequestId) -> String {
//!   format!("{}", request_id)
//! }
//! ```
//!
//! # OpenTelemetry integration
//!
//! `tracing-actix-web` follows [OpenTelemetry's semantic convention](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/overview.md#spancontext)
//! for field names.  
//! Furthermore, if you have not disabled the `opentelemetry_0_13` or `opentelemetry_0_14` or `opentelemetry_0_15` feature flags, `tracing-actix-web` automatically
//! performs trace propagation according to the OpenTelemetry standard.
//! It tries to extract the OpenTelemetry context out of the headers of incoming requests and, when it finds one, it sets
//! it as the remote context for the current root span.
//!
//! If you add [`tracing-opentelemetry::OpenTelemetryLayer`](https://docs.rs/tracing-opentelemetry/0.12.0/tracing_opentelemetry/struct.OpenTelemetryLayer.html)
//! in your `tracing::Subscriber` you will be able to export the root span (and all its children) as OpenTelemetry spans.
//!
//! Check out the [relevant example in the GitHub repository](https://github.com/LukeMathWalker/tracing-actix-web/tree/main/examples/opentelemetry) for reference.
//!
//! You can find an alternative integration of `actix-web` with OpenTelemetry in [`actix-web-opentelemetry`](https://github.com/OutThereLabs/actix-web-opentelemetry).
//! Parts of this project were heavily inspired by their implementation. They provide support for metrics
//! and instrumentation for the `awc` HTTP client, both out of scope for `tracing-actix-web`.
//!
//! [root span]: crate::RootSpan
//! [`actix-web`]: https://docs.rs/actix-web/4.0.0-beta.6/actix_web/index.html
mod middleware;
mod request_id;
mod root_span;
mod root_span_builder;

pub use middleware::TracingLogger;
pub use request_id::RequestId;
pub use root_span::RootSpan;
pub use root_span_builder::{DefaultRootSpanBuilder, RootSpanBuilder};

#[doc(hidden)]
pub mod root_span_macro;

#[cfg(any(
    feature = "opentelemetry_0_13",
    feature = "opentelemetry_0_14",
    feature = "opentelemetry_0_15",
    feature = "opentelemetry_0_16"
))]
mod otel;

#[cfg(all(feature = "opentelemetry_0_13", feature = "opentelemetry_0_14"))]
compile_error!("feature \"opentelemetry_0_13\" and feature \"opentelemetry_0_14\" cannot be enabled at the same time");

#[cfg(all(feature = "opentelemetry_0_13", feature = "opentelemetry_0_15"))]
compile_error!("feature \"opentelemetry_0_13\" and feature \"opentelemetry_0_15\" cannot be enabled at the same time");

#[cfg(all(feature = "opentelemetry_0_13", feature = "opentelemetry_0_16"))]
compile_error!("feature \"opentelemetry_0_13\" and feature \"opentelemetry_0_16\" cannot be enabled at the same time");

#[cfg(all(feature = "opentelemetry_0_14", feature = "opentelemetry_0_15"))]
compile_error!("feature \"opentelemetry_0_14\" and feature \"opentelemetry_0_15\" cannot be enabled at the same time");

#[cfg(all(feature = "opentelemetry_0_14", feature = "opentelemetry_0_16"))]
compile_error!("feature \"opentelemetry_0_14\" and feature \"opentelemetry_0_16\" cannot be enabled at the same time");

#[cfg(all(feature = "opentelemetry_0_15", feature = "opentelemetry_0_16"))]
compile_error!("feature \"opentelemetry_0_15\" and feature \"opentelemetry_0_16\" cannot be enabled at the same time");
