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Zipkin instrumentation for PHP

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# Zipkin PHP

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Zipkin PHP is the official PHP Tracer implementation for Zipkin, supported by the OpenZipkin community.

## Installation

```bash
composer require openzipkin/zipkin
```

## Setup

```php
use Zipkin\Annotation;
use Zipkin\Endpoint;
use Zipkin\Samplers\BinarySampler;
use Zipkin\TracingBuilder;
use Zipkin\Reporters\Http;

// First we create the endpoint that describes our service
$endpoint = Endpoint::create('my_service');

$reporter = new Http(['endpoint_url' => 'http://myzipkin:9411/api/v2/spans']);
$sampler = BinarySampler::createAsAlwaysSample();
$tracing = TracingBuilder::create()
    ->havingLocalEndpoint($endpoint)
    ->havingSampler($sampler)
    ->havingReporter($reporter)
    ->build();

$tracer = $tracing->getTracer();

...

$tracer->flush();
```

**Obs.** for a more complete frontend/backend example, check [this repository](https://github.com/openzipkin/zipkin-php-example).

## Tracing

The tracer creates and joins spans that model the latency of potentially
distributed work. It can employ sampling to reduce overhead in process
or to reduce the amount of data sent to Zipkin.

Spans returned by a tracer report data to Zipkin when finished, or do
nothing if unsampled. After starting a span, you can annotate events of
interest or add tags containing details or lookup keys.

Spans have a context which includes trace identifiers that place it at
the correct spot in the tree representing the distributed operation.

### Local Tracing

When tracing local code, just run it inside a span

```php
$span = $tracer->newTrace();
$span->setName('encode');
$span->start();

try {
  doSomethingExpensive();
} finally {
  $span->finish();
}
```

In the above example, the span is the root of the trace. In many cases,
you will be a part of an existing trace. When this is the case, call
`newChild` instead of `newTrace`

```php
$span = $tracer->newChild($root->getContext());
$span->setName('encode');
$span->start();
try {
  doSomethingExpensive();
} finally {
  $span->finish();
}
```

### Customizing spans

Once you have a span, you can add tags to it, which can be used as lookup
keys or details. For example, you might add a tag with your runtime
version.

```php
$span->tag('http.status_code', '200');
```

### RPC tracing

RPC tracing is often done automatically by interceptors. Under the scenes,
they add tags and events that relate to their role in an RPC operation.

Here's an example of a client span:

```php
// before you send a request, add metadata that describes the operation
$span = $tracer->newTrace();
$span->setName('get');
$span->setKind(Kind\CLIENT);
$span->tag('http.status_code', '200');
$span->tag(Tags\HTTP_PATH, '/api');
$span->setRemoteEndpoint(Endpoint::create('backend', 127 << 24 | 1, null, 8080));

// when the request is scheduled, start the span
$span->start();

// if you have callbacks for when data is on the wire, note those events
$span->annotate(Annotation\WIRE_SEND);
$span->annotate(Annotation\WIRE_RECV);

// when the response is complete, finish the span
$span->finish();
```

## Sampling

Sampling may be employed to reduce the data collected and reported out
of process. When a span isn't sampled, it adds no overhead (noop).

Sampling is an up-front decision, meaning that the decision to report
data is made at the first operation in a trace, and that decision is
propagated downstream.

By default, there's a global sampler that applies a single rate to all
traced operations. `Sampler` is how you indicate this,
and it defaults to trace every request.

### Custom sampling

You may want to apply different policies depending on what the operation
is. For example, you might not want to trace requests to static resources
such as images, or you might want to trace all requests to a new api.

Most users will use a framework interceptor which automates this sort of
policy. Here's how they might work internally.

```php
private function newTrace(Request $request) {
  $flags = SamplingFlags::createAsEmpty();
  if (strpos($request->getUri(), '/experimental') === 0) {
    $flags = DefaultSamplingFlags::createAsSampled();
  } else if (strpos($request->getUri(), '/static') === 0) {
    $flags = DefaultSamplingFlags::createAsSampled();
  }
  return $tracer->newTrace($flags);
}
```

## Propagation

Propagation is needed to ensure activity originating from the same root
are collected together in the same trace. The most common propagation
approach is to copy a trace context from a client sending an RPC request
to a server receiving it.

For example, when an downstream Http call is made, its trace context is
sent along with it, encoded as request headers:

```
   Client Span                                                Server Span
┌──────────────────┐                                       ┌──────────────────┐
│                  │                                       │                  │
│   TraceContext   │           Http Request Headers        │   TraceContext   │
│ ┌──────────────┐ │          ┌───────────────────┐        │ ┌──────────────┐ │
│ │ TraceId      │ │          │ X-B3-TraceId      │        │ │ TraceId      │ │
│ │              │ │          │                   │        │ │              │ │
│ │ ParentSpanId │ │ Extract  │ X-B3-ParentSpanId │ Inject │ │ ParentSpanId │ │
│ │              ├─┼─────────>│                   ├────────┼>│              │ │
│ │ SpanId       │ │          │ X-B3-SpanId       │        │ │ SpanId       │ │
│ │              │ │          │                   │        │ │              │ │
│ │ Sampled      │ │          │ X-B3-Sampled      │        │ │ Sampled      │ │
│ └──────────────┘ │          └───────────────────┘        │ └──────────────┘ │
│                  │                                       │                  │
└──────────────────┘                                       └──────────────────┘
```

The names above are from [B3 Propagation](https://github.com/openzipkin/b3-propagation),
which is built-in to Brave and has implementations in many languages and
frameworks.

Most users will use a framework interceptor which automates propagation.
Here's how they might work internally.

Here's what client-side propagation might look like

```php
// configure a function that injects a trace context into a request
$injector = $tracing->getPropagation()->getInjector(new RequestHeaders);

// before a request is sent, add the current span's context to it
$injector($span->getContext(), $request);
```

Here's what server-side propagation might look like

```php
// configure a function that extracts the trace context from a request
$extractor = $tracing->getPropagation()->getExtractor(new RequestHeaders);
$extracted = $extractor($request);

$span = $tracer->newChild($extracted);
$span->setKind(Kind\SERVER);
```

If you aren't using a framework or don't have access to the Request object, you
can extract the context from the $_SERVER variable

```php
$extractor = $tracing->getPropagation()->getExtractor(new ServerHeaders);
$extracted = $extractor($_SERVER);
```

### Extracting a propagated context

The `Extractor` reads trace identifiers and sampling status
from an incoming request or message. The carrier is usually a request object
or headers.

`SamplingFlags|TraceContext` is usually only used with `$tracer->newChild(extracted)`, unless you are
sharing span IDs between a client and a server.

### Implementing Propagation

`Extractor` will output a `SamplingFlags|TraceContext` with one of the following:

* `TraceContext` if trace and span IDs were present.
* `SamplingFlags` if no identifiers were present

## Current Span

Zipkin supports a "current span" concept which represents the in-flight
operation. `Tracer::currentSpan()` can be used to add custom tags to a
span and `Tracer::nextSpan()` can be used to create a child of whatever
is in-flight.

A common use case for the current span is to instrument RPC clients. For example:

```php
/**
  * This http clients composes an http client using PSR7
  */
class TraceClient implements ClientInterface
{
    public function request($method, $uri = '', array $options = [])
    {
        /* Gets the child Span of the current one */
        $span = $this->tracer->nextSpan();
        $span->setKind(Zipkin\Kind\CLIENT);
        $span->tag(Tags\HTTP_PATH, $uri);

try {
            $response = $this->client->request($method, $uri, $options);
            $span->tag(Tags\HTTP_STATUS_CODE, (string) $response->getStatusCode());

return $response;
        catch (Throwable $e) {
            $span->setError($e);
            throw $e;
        } finally {
            $span->finish();
        }
    }
}
```

### Setting a span in scope manually

When writing new instrumentation, it is important to place a span you
created in scope as the current span.

In edge cases, you may need to clear the current span temporarily. For
example, launching a task that should not be associated with the current
request. To do this, simply pass null to `openScope`.

## Instrumentation

- [PSR18 HTTP Client](src/Zipkin/Instrumentation/Http/Client/Psr18)
- [PSR15 HTTP Server](src/Zipkin/Instrumentation/Http/Server/Psr15)

## Tests

Tests can be run by

```bash
composer test
```

Whereas static checks can be run by:

```bash
composer static-check
```

## Reference

* [Instrumenting a library](http://zipkin.io/pages/instrumenting.html)
* [openzipkin/zipkin-api](https://github.com/openzipkin/zipkin-api)

zipkin-php

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