# ``CoreMetrics``

A Metrics API package for Swift.

## Overview

Almost all production server software needs to emit metrics information for observability. Because it's unlikely that all parties can agree on one specific metrics backend implementation, this API is designed to establish a standard that can be implemented by various metrics libraries which then post the metrics data to backends like [Prometheus](https://prometheus.io/), [Graphite](https://graphiteapp.org), publish over [statsd](https://github.com/statsd/statsd), write to disk, etc.

This is the beginning of a community-driven open-source project actively seeking contributions, be it code, documentation, or ideas. Apart from contributing to SwiftMetrics itself, we need metrics compatible libraries which send the metrics over to backend such as the ones mentioned above. What SwiftMetrics provides today is covered in the [API docs](https://apple.github.io/swift-metrics/), but it will continue to evolve with community input.

## Getting started

If you have a server-side Swift application, or maybe a cross-platform (e.g. Linux, macOS) application or library, and you would like to emit metrics, targeting this metrics API package is a great idea. Below you'll find all you need to know to get started.

### Adding the dependency

To add a dependency on the metrics API package, you need to declare it in your `Package.swift`:

```swift
// swift-metrics 1.x and 2.x are almost API compatible, so most clients should use
.package(url: "https://github.com/apple/swift-metrics.git", "1.0.0" ..< "3.0.0"),
```

and to your application/library target, add "Metrics" to your dependencies:

```swift
.target(
    name: "BestExampleApp",
    dependencies: [
        // ...
        .product(name: "Metrics", package: "swift-metrics"),
    ]
),
```

###  Emitting metrics information

```swift
// 1) let's import the metrics API package
import Metrics

// 2) we need to create a concrete metric object, the label works similarly to a `DispatchQueue` label
let counter = Counter(label: "com.example.BestExampleApp.numberOfRequests")

// 3) we're now ready to use it
counter.increment()
```

### Selecting a metrics backend implementation (applications only)

Note: If you are building a library, you don't need to concern yourself with this section. It is the end users of your library (the applications) who will decide which metrics backend to use. Libraries should never change the metrics implementation as that is something owned by the application.

SwiftMetrics only provides the metrics system API. As an application owner, you need to select a metrics backend (such as the ones mentioned above) to make the metrics information useful.

Selecting a backend is done by adding a dependency on the desired backend client implementation and invoking the `MetricsSystem.bootstrap` function at the beginning of the program:

```swift
MetricsSystem.bootstrap(SelectedMetricsImplementation())
```

This instructs the `MetricsSystem` to install `SelectedMetricsImplementation` (actual name will differ) as the metrics backend to use.

> Tip: Refer to the project's [README](https://github.com/apple/swift-metrics) for an up-to-date list of backend implementations.

### Swift Metrics Extras

You may also be interested in some "extra" modules which are collected in the [Swift Metrics Extras](https://github.com/apple/swift-metrics-extras) repository.

## Detailed design

### Architecture

We believe that for the Swift on Server ecosystem, it's crucial to have a metrics API that can be adopted by anybody so a multitude of libraries from different parties can all provide metrics information. More concretely this means that we believe all the metrics events from all libraries should end up in the same place, be one of the backends mentioned above or wherever else the application owner may choose.

In the real world, there are so many opinions over how exactly a metrics system should behave, how metrics should be aggregated and calculated, and where/how to persist them. We think it's not feasible to wait for one metrics package to support everything that a specific deployment needs while still being simple enough to use and remain performant. That's why we decided to split the problem into two:

1. a metrics API
2. a metrics backend implementation

This package only provides the metrics API itself, and therefore, SwiftMetrics is a "metrics API package." SwiftMetrics can be configured (using `MetricsSystem.bootstrap`) to choose any compatible metrics backend implementation. This way, packages can adopt the API, and the application can choose any compatible metrics backend implementation without requiring any changes from any of the libraries.

This API was designed with the contributors to the Swift on Server community and approved by the SSWG (Swift Server Work Group) to the "sandbox level" of the SSWG's incubation process.

[pitch](https://forums.swift.org/t/metrics/19353) |
[discussion](https://forums.swift.org/t/discussion-server-metrics-api/) |
[feedback](https://forums.swift.org/t/feedback-server-metrics-api/)


## Topics

### Metric types

- ``Counter``
- ``FloatingPointCounter``
- ``Meter``
- ``Recorder``
- ``Gauge``
- ``Timer``