# Cadence

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[Documentation](https://docs.rs/cadence/)

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, histograms, 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,no_run
// 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. The examples start
simple and work up to how you should be using Cadence in a production
application.

### 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};

// 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);
```

### Buffered UDP Sink

While sending a metric over UDP is very fast, the overhead of frequent
network calls can start to add up. This is especially true if you are
writing a high performance application that emits a lot of metrics.

To make sure that metrics aren't interfering with the performance of
your application, you may want to use a `MetricSink` implementation that
buffers multiple metrics before sending them in a single network
operation. For this, there's `BufferedUdpMetricSink`. An example of
using this sink is given below.

``` rust,no_run
use std::net::UdpSocket;
use cadence::prelude::*;
use cadence::{StatsdClient, BufferedUdpMetricSink, DEFAULT_PORT};

let socket = UdpSocket::bind("0.0.0.0:0").unwrap();
socket.set_nonblocking(true).unwrap();

let host = ("metrics.example.com", DEFAULT_PORT);
let sink = BufferedUdpMetricSink::from(host, socket).unwrap();
let client = StatsdClient::from_sink("my.prefix", sink);

client.count("my.counter.thing", 29);
client.time("my.service.call", 214);
client.incr("some.event");
```

As you can see, using this buffered UDP sink is no more complicated
than using the regular, non-buffered, UDP sink.

The only downside to this sink is that metrics aren't written to the
Statsd server until the buffer is full. If you have a busy application
that is constantly emitting metrics, this shouldn't be a problem.
However, if your application only occasionally emits metrics, this sink
might result in the metrics being delayed for a little while until the
buffer fills.

### Queuing Asynchronous Metric Sink

To make sure emitting metrics doesn't interfere with the performance
of your application (even though emitting metrics is generally quite
fast), it's probably a good idea to make sure metrics are emitted in
in a different thread than your application thread.

To allow you do this, there is `QueuingMetricSink`. This sink allows
you to wrap any other metric sink and send metrics to it via a queue,
as it emits metrics in another thread, asynchronously from the flow of
your application.

The requirements for the wrapped metric sink are that it is thread
safe, meaning that it implements the `Send` and `Sync` traits. If
you're using the `QueuingMetricSink` with another sink from Cadence,
you don't need to worry: they are all thread safe.

An example of using the `QueuingMetricSink` to wrap a buffered UDP
metric sink is given below. This is the preferred way to use Cadence
in production.

``` rust,no_run
use std::net::UdpSocket;
use cadence::prelude::*;
use cadence::{StatsdClient, QueuingMetricSink, BufferedUdpMetricSink,
              DEFAULT_PORT};

let socket = UdpSocket::bind("0.0.0.0:0").unwrap();
socket.set_nonblocking(true).unwrap();

let host = ("metrics.example.com", DEFAULT_PORT);
let udp_sink = BufferedUdpMetricSink::from(host, socket).unwrap();
let queuing_sink = QueuingMetricSink::from(udp_sink);
let client = StatsdClient::from_sink("my.prefix", queuing_sink);

client.count("my.counter.thing", 29);
client.time("my.service.call", 214);
client.incr("some.event");
```

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

Each of the methods that the Cadence `StatsdClient` struct uses to send
metrics are implemented as a trait. There is also a trait that combines
all of these other traits. If we want, we can just use one of the trait
types 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 or want to abstract away all the
implementation details of the client being used behind a trait and
pointer.

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 metric client to keep track of the number of times the
// 'getUserById' method gets called.
pub struct MyUserDao {
    metrics: Box<MetricClient>
}

impl MyUserDao {
    // Create a new instance that will use the StatsdClient
    pub fn new<T: MetricClient + 'static>(metrics: T) -> MyUserDao {
        MyUserDao { metrics: Box::new(metrics) }
    }

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

// Create a new Statsd client that writes to "metrics.example.com"
let host = ("metrics.example.com", DEFAULT_PORT);
let metrics = 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(metrics);

// 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 `AsyncMetricSink` wrapping an instance
of the `BufferedMetricSink`.

However, maybe you want to do something not covered by an existing sink.
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)
    }
}

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");
```

### Custom UDP Socket

Most users of the Cadence `StatsdClient` will be using it to send metrics
over a UDP socket. If you need to customize the socket, for example you
want to use the socket in blocking mode but set a write timeout, you can
do that as demonstrated below.

``` rust,no_run
use std::net::UdpSocket;
use std::time::Duration;
use cadence::prelude::*;
use cadence::{StatsdClient, UdpMetricSink, DEFAULT_PORT};

let socket = UdpSocket::bind("0.0.0.0:0").unwrap();
socket.set_write_timeout(Some(Duration::from_millis(1))).unwrap();

let host = ("metrics.example.com", DEFAULT_PORT);
let sink = UdpMetricSink::from(host, socket).unwrap();
let client = StatsdClient::from_sink("my.prefix", sink);

client.count("my.counter.thing", 29);
client.time("my.service.call", 214);
client.incr("some.event");
```

## Documentation

The documentation is available at https://docs.rs/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
```

## License

Licensed under either of
* Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
* MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.

### Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you shall be dual licensed as above, without any
additional terms or conditions.
