# Cadence

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An extensible Statsd client for Rust!

[Statsd](https://github.com/etsy/statsd) is a network server that listens for
metrics (things like counters and timers) sent over UDP and sends aggregates of
these metrics to a backend service of some kind (often
[Graphite](http://graphite.readthedocs.org/)).

Cadence is a client written in Rust for interacting with a Statsd server. You
might want to emit metrics (using Cadence, sending them to a Statsd server) in
your Rust server application.

For example, if you are running a Rust web service you might want to record:

* Number of succesful requests
* Number of error requests
* Time taken for each request

Cadence is a flexible and easy way to do this!

## Features

* Support for emitting counters, timers, gauges, and meters to Statsd over UDP.
* Support for alternate backends via the `MetricSink` trait.
* A simple yet flexible API for sending metrics.

## Install

To make use of Cadence in your project, add it as a dependency in your `Cargo.toml`
file.

``` toml
[dependencies]
cadence = "x.y.z"
```

Then, link to it in your library or application.

``` rust
// bin.rs or lib.rs
extern crate cadence;

// rest of your library or application
```

## Usage

Some examples of how to use Cadence are shown below.

### Simple Use

Simple usage of Cadence is shown below. In this example, we just import the client,
create an instance that will write to some imaginary metrics server, and send a few
metrics.

``` rust,no_run
// Import the client.
use cadence::prelude::*;
use cadence::{StatsdClient, UdpMetricSink, DEFAULT_PORT};

fn main() {
    // Create client that will write to the given host over UDP.
    //
    // Note that you'll probably want to actually handle any errors creating the client
    // when you use it for real in your application. We're just using .unwrap() here
    // since this is an example!
    let host = ("metrics.example.com", DEFAULT_PORT);
    let client = StatsdClient::<UdpMetricSink>::from_udp_host(
        "my.metrics", host).unwrap();

    // Emit metrics!
    client.incr("some.counter");
    client.time("some.methodCall", 42);
    client.gauge("some.thing", 7);
    client.meter("some.value", 5);
}
```

### Counted, Timed, Gauged, and Metered Traits

Each of the methods that the Cadence `StatsdClient` struct uses to send metrics are
implemented as a trait. If we want, we can just use the trait type to refer to the
client instance. This might be useful to you if you'd like to swap out the actual
Cadence client with a dummy version when you are unit testing your code.

Each of these traits are exported in the prelude module. They are also available
in the main module but aren't typically used like that.

``` rust,no_run
use cadence::prelude::*;
use cadence::{StatsdClient, UdpMetricSink, DEFAULT_PORT};


pub struct User {
    id: u64,
    username: String,
    email: String
}


// Here's a simple DAO (Data Access Object) that doesn't do anything but
// uses a counter to keep track of the number of times the 'getUserById'
// method gets called.
pub struct MyUserDao<T: Counted> {
    counter: T
}


impl<T: Counted> MyUserDao<T> {
    // Create a new instance that will use the counter / client
    pub fn new(counter: T) -> MyUserDao<T> {
        MyUserDao{counter: counter}
    }

    /// Get a new user by their ID
    pub fn get_user_by_id(&self, id: u64) -> Option<User> {
        self.counter.incr("getUserById");
        None
    }
}


fn main() {
    // Create a new Statsd client that writes to "metrics.example.com"
    let host = ("metrics.example.com", DEFAULT_PORT);
    let counter = StatsdClient::<UdpMetricSink>::from_udp_host(
        "counter.example", host).unwrap();

    // Create a new instance of the DAO that will use the client
    let dao = MyUserDao::new(counter);

    // Try to lookup a user by ID!
    match dao.get_user_by_id(123) {
        Some(u) => println!("Found a user!"),
        None => println!("No user!")
    };
}
```

### Custom Metric Sinks

The Cadence `StatsdClient` uses implementations of the `MetricSink` trait to
send metrics to a metric server. Most users of the Candence library probably
want to use the `UdpMetricSink` implementation. This is the way people typically
interact with a Statsd server, sending packets over UDP.

However, maybe you'd like to do something custom: use a thread pool, send multiple
metrics at the same time, or something else. An example of creating a custom sink
is below.

``` rust,no_run
use std::io;
use cadence::prelude::*;
use cadence::{StatsdClient, MetricSink, DEFAULT_PORT};

pub struct MyMetricSink;


impl MetricSink for MyMetricSink {
    fn emit(&self, metric: &str) -> io::Result<usize> {
        // Your custom metric sink implementation goes here!
        Ok(0)
    }
}


fn main() {
    let sink = MyMetricSink;
    let client = StatsdClient::from_sink("my.prefix", sink);

    client.count("my.counter.thing", 42);
    client.time("my.method.time", 25);
    client.incr("some.other.counter");
}
```


## Documentation

The documentation is available at https://tshlabs.github.io/cadence/

## Source

The source code is available on GitHub at https://github.com/tshlabs/cadence

## Changes

Release notes for Cadence can be found in the [CHANGES.md](CHANGES.md) file.

## Development

Cadence uses Cargo for performing various development tasks.

To build Cadence:

```
$ cargo build
```

To run tests:

```
$ cargo test
```

or:

```
$ cargo test -- --ignored
```

To run benchmarks:

```
$ cargo bench
```

To build documentation:

```
$ cargo doc
```
