PHP Custom Instrumentation

Writing custom instrumentation

If you do need to write your own custom instrumentation, consider the following sample application and walk through the coding examples.

A sample application to be instrumented

Assume the following directory structure:

.
|-- composer.json
|-- docker-compose.yml
|-- index.php
`-- src
    |-- Exceptions
    |   `-- NotFound.php
    |-- Services
    |   `-- SampleRegistry.php
    `-- utils
        `-- functions.php

Within this, two files contain the functions and methods that are interesting to instrument. The most relevant files are src/utils/functions.php:

namespace App;

function some_utility_function($someArg)
{
    return 'result';
}

And src/Services/SampleRegistry.php:

namespace App\Services;

use App\Exceptions\NotFound;
use Exception;

class SampleRegistry
{
    public function put($key, $value)
    {
        \App\some_utility_function('some argument');
        // Return the id of the item inserted
        return 456;
    }

    public function faultyMethod()
    {
        throw new Exception('Generated at runtime');
    }

    public function get($key)
    {
        // The service uses an exception to report a key not found.
        throw new NotFound('The key was not found');
    }

    public function compact()
    {
        // This function executes some operations on the registry and
        // returns nothing. In the middle of the function, we have an
        // interesting value that is not returned but can be related
        // to the slowness of the function

        $numberOfItemsProcessed = 123;

        // ...
    }
}

Writing the custom instrumentation

To avoid mixing application or service business logic with instrumentation code, write the required code in a separate file.

1. Create a file datadog/instrumentation.php and add it to the composer autoloader.

   composer.json

   Copy

   {
       ...
       "autoload": {
           ...
           "files": [
               ...
               "datadog/instrumentation.php"
           ]
       },
       ...
   }
      

2. Dump the autoloader, for example by running composer update.

   Note: The file that contains the custom instrumentation code and the actual classes that are instrumented are not required to be in the same code base and package. By separating them, you can publish an open source composer package, for example to GitHub, containing only your instrumentation code, which others might find useful. Registering the instrumentation entry point in the composer.json's autoload.files array ensures that the file is always executed when the composer autoloader is required.

3. In the datadog/instrumentation.php file, check if the extension is loaded. If the extension is not loaded then all the functions used in this file do not exist.

   datadog/instrumentation.php

   Copy

   if (!extension_loaded('ddtrace')) {
       return;
   }
      
   

4. Instrument a function, \App\some_utility_function. If you are not interested in any specific aspect of the function other than the execution time, then this is all that is required:

   datadog/instrumentation.php

   Copy

   \DDTrace\trace_function('App\some_utility_function', function (\DDTrace\SpanData $span, $args, $ret, $exception) {});
      
   

5. Suppose for the SampleRegistry::put method, you not only want to generate a span, you also want to add a tag with the value of the returned item identifier, and a tag for the key. Because put is a method, use \DDTrace\trace_method instead of \DDTrace\trace_function:

   datadog/instrumentation.php

   Copy

   ...
   \DDTrace\trace_method(
       'App\Services\SampleRegistry',
       'put',
       function (\DDTrace\SpanData $span, $args, $ret, $exception) {
           $span->meta['app.cache.key'] = $args[0]; // The first argument is the 'key'
           $span->meta['app.cache.item_id'] = $ret; // The returned value
       }
   );
      
   

   When you set tags, to avoid overwriting existing tags automatically added by the Datadog core instrumentation, do write $span->meta['mytag'] = 'value'. Do not write $span->meta = ['mytag' => 'value'].

6. In the sample code, SampleRegistry::faultyMethod generates an exception. There is nothing you have to do with regards to custom instrumentation. If the method is instrumented, the default exception reporting mechanism takes care of attaching the exception message and the stack trace.

   datadog/instrumentation.php

   Copy

   ...
   \DDTrace\trace_method(
       'App\Services\SampleRegistry',
       'faultyMethod',
       function (\DDTrace\SpanData $span, $args, $ret, $exception) {
       }
   );
      
   

7. The SampleRegistry::get method uses a NotFound exception to notify that an item was not found. This exception is an expected part of the business logic and you do not want to mark the span as an error. You just want to change the resource name to add it to a pool of not_found operations. To achieve that, you unset the exception for the span:

   datadog/instrumentation.php

   Copy

   ...
   \DDTrace\trace_method(
       'App\Services\SampleRegistry',
       'get',
       function (\DDTrace\SpanData $span, $args, $ret, $exception) {
           if ($exception instanceof \App\Exceptions\NotFound) {
               unset($span->exception);
               $span->resource = 'cache.get.not_found';
           }
       }
   );
      
   

8. The SampleRegistry::compact method demonstrates an interesting use case. You are interested in adding a tag with a value that is neither an argument nor the value returned by the function. To do this, edit both datadog/instrumentation.php and the class file src/Services/SampleRegistry.php:

   datadog/instrumentation.php

   Copy

   ...
   \DDTrace\trace_method(
       'App\Services\SampleRegistry',
       'compact',
       function (\DDTrace\SpanData $span, $args, $ret, $exception) {
       }
   );
      
   

   In src/Services/SampleRegistry.php edit the body of the method:

   src/Services/SampleRegistry.php

   Copy

   ...
       public function compact()
       {
           // This function execute some operations on the registry and
           // returns nothing. In the middle of the function, we have an
           // interesting value that is not returned but can be related
           // to the slowness of the function
   
           $numberOfItemsProcessed = 123;
   
           // Add instrumenting code within your business logic.
           if (\function_exists('\DDTrace\active_span') && $span = \DDTrace\active_span()) {
               $span->meta['registry.compact.items_processed'] = $numberOfItemsProcessed;
           }
   
           // ...
       }
      
   

Details about trace_function and trace_method

The DDTrace\trace_function and DDTrace\trace_method functions instrument (trace) specific function and method calls. These functions automatically handle the following tasks:

• Open a span before the code executes.
• Set any errors from the instrumented call on the span.
• Close the span when the instrumented call is done.

Additional tags are set on the span from the closure (called a tracing closure).

For example, the following snippet traces the CustomDriver::doWork method and adds custom tags. Exceptions are automatically tracked on the span.

<?php
\DDTrace\trace_method(
    'CustomDriver',
    'doWork',
    function (\DDTrace\SpanData $span, array $args, $retval, $exception) {
        // This closure runs after the instrumented call
        // Span was automatically created before the instrumented call

        // SpanData::$name defaults to 'ClassName.methodName' if not set
        $span->name = 'CustomDriver.doWork';
        // SpanData::$resource defaults to SpanData::$name if not set
        $span->resource = 'CustomDriver.doWork';
        $span->service = 'php';

        // If an exception was thrown from the instrumented call, return value is null
        $span->meta['doWork.size'] = $exception ? 0 : count($retval),
        // Access object members via $this
        $span->meta['doWork.thing'] = $this->workToDo;

        // The span will automatically close
    }
);

// For functions
\DDTrace\trace_function(
    'doCustomDriverWork',
    function (\DDTrace\SpanData $span, array $args, $retval, $exception) {
        // Same as DDTrace\trace_method tracing closure
    }
);
?>

Accessing active spans

The built-in instrumentation and your own custom instrumentation will create spans around meaningful operations. You can access the active span in order to include meaningful data.

<?php
$span = \DDTrace\active_span();
if ($span) {
    $span->meta['customer.id'] = get_customer_id();
}
?>

<?php
$span = \DDTrace\root_span();
if ($span) {
    $span->meta['customer.id'] = get_customer_id();
}
?>

Adding tags

<?php

\DDTrace\trace_function(
    'myRandFunc',
    function(\DDTrace\SpanData $span, array $args, $retval) {
        // ...
        $span->meta['rand.range'] = $args[0] . ' - ' . $args[1];
        $span->meta['rand.value'] = $retval;
    }
);

DD_TAGS=key1:value1,<TAG_KEY>:<TAG_VALUE>

<?php

function doRiskyThing() {
    throw new Exception('Oops!');
}

\DDTrace\trace_function(
    'doRiskyThing',
    function() {
        // Span will be flagged as erroneous and have
        // the stack trace and exception message attached as tags
    }
);

<?php

function doRiskyThing() {
    return SOME_ERROR_CODE;
}

\DDTrace\trace_function(
    'doRiskyThing',
    function(\DDTrace\SpanData $span, $args, $retval) {
        if ($retval === SOME_ERROR_CODE) {
            $span->meta['error.msg'] = 'Foo error';
            // Optional:
            $span->meta['error.type'] = 'CustomError';
            $span->meta['error.stack'] = (new \Exception)->getTraceAsString();
        }
    }
);

Distributed tracing

When a new PHP script is launched, the tracer automatically checks for the presence of datadog headers for distributed tracing:

• x-datadog-trace-id (environment variable: HTTP_X_DATADOG_TRACE_ID)
• x-datadog-parent-id (environment variable: HTTP_X_DATADOG_PARENT_ID)
• x-datadog-origin (environment variable: HTTP_X_DATADOG_ORIGIN)
• x-datadog-tags (environment variable: HTTP_X_DATADOG_TAGS)

To manually set this information in a CLI script on new traces or an existing trace a function DDTrace\set_distributed_tracing_context(string $trace_id, string $parent_id, ?string $origin = null, ?array $tags = null) is provided.

<?php

function processIncomingQueueMessage($message) {
}

\DDTrace\trace_function(
    'processIncomingQueueMessage',
    function(\DDTrace\SpanData $span, $args) {
        $message = $args[0];
        \DDTrace\set_distributed_tracing_context($message->trace_id, $message->parent_id);
    }
);

Resource filtering

Traces can be excluded based on their resource name, to remove synthetic traffic such as health checks from reporting traces to Datadog. This and other security and fine-tuning configurations can be found on the Security page.

API reference

Parameters of the tracing closure

The tracing closure provided to DDTrace\trace_method() and DDTrace\trace_function() has four parameters:

function(
    DDTrace\SpanData $span,
    array $args,
    mixed $retval,
    Exception|null $exception
);

1. $span: An instance of DDTrace\SpanData to write to the span properties
2. $args: An array of arguments from the instrumented call
3. $retval: The return value of the instrumented call
4. $exception: An instance of the exception that was thrown in the instrumented call or null if no exception was thrown

Parameter 1: DDTrace\SpanData $span

The DDTrace\SpanData instance contains the same span information that the Agent expects. A few exceptions are trace_id, span_id, parent_id, start, and duration which are set at the C level and not exposed to userland via DDTrace\SpanData. Exceptions from the instrumented call are automatically attached to the span.

<?php

use DDTrace\SpanData;

function myRandFunc($min, $max) {
    return mt_rand($min, $max);
}

\DDTrace\trace_function(
    'myRandFunc',
    function(SpanData $span, $args, $retval) {
        // SpanData::$name defaults to 'functionName' if not set (>= v0.47.0)
        $span->name = 'myRandFunc';
        // SpanData::$resource defaults to SpanData::$name if not set (>= v0.47.0)
        $span->resource = 'myRandFunc';
        $span->service = 'php';
        // The following are optional
        $span->type = 'web';
        $span->meta['rand.range'] = $args[0] . ' - ' . $args[1];
        $span->meta['rand.value'] = $retval;
        $span->metrics['some_metric'] = 0.9;
    }
);

Parameter 2: array $args

The second parameter to the tracing closure is an array of arguments from the instrumented call. It functions similarly to func_get_args().

By default the tracing closure is executed after the instrumented call which means any arguments passed by reference could be a different value when they reach the tracing closure.

<?php

use DDTrace\SpanData;

function argsByRef(&$a) {
    return ++$a;
}

\DDTrace\trace_function(
    'argsByRef',
    function(SpanData $span, $args) {
        var_dump($args);
    }
);

$foo = 10;
var_dump(argsByRef($foo));
// array(1) {
//   [0]=>
//   int(11)
// }
// int(11)

On PHP 7, the tracing closure has access to the same arguments passed to the instrumented call. If the instrumented call mutates an argument, including arguments passed by value, the posthook tracing closure will receive the mutated argument.

This is the expected behavior of arguments in PHP 7 as illustrated in the following example:

<?php

function foo($a) {
    var_dump(func_get_args());
    $a = 'Dogs';
    var_dump(func_get_args());
}

foo('Cats');

/*
array(1) {
  [0]=>
  string(4) "Cats"
}
array(1) {
  [0]=>
  string(4) "Dogs"
}
*/

The following example demonstrates this effect on posthook tracing closures.

<?php

function foo($a) {
    $a = 'Dogs';
}

\DDTrace\trace_function('foo', function ($span, array $args) {
    var_dump($args[0]);
});

foo('Cats');

// string(4) "Dogs"

If an argument needs to be accessed before mutation, the tracing closure can be marked as prehook to access the arguments before the instrumented call.

Parameter 3: mixed $retval

The third parameter of the tracing closure is the return value of the instrumented call. Functions or methods that declare a void return type or ones that do not return a value will have a value of null.

<?php

use DDTrace\SpanData;

function message(): void {
    echo "Hello!\n";
}

\DDTrace\trace_function(
    'message',
    function(SpanData $span, $args, $retval) {
        echo "Traced\n";
        var_dump($retval);
    }
);

var_dump(message());
// Hello!
// Traced
// NULL
// NULL

Parameter 4: Exception|null $exception

The final parameter of the tracing closure is an instance of the exception that was thrown in the instrumented call or null if no exception was thrown.

<?php

use DDTrace\SpanData;

function mightThrowException() {
  throw new Exception('Oops!');
  return 'Hello';
}

\DDTrace\trace_function(
  'mightThrowException',
  function(SpanData $span, $args, $retval, $ex) {
    if ($ex) {
      echo 'Exception from instrumented call: ';
      echo $ex->getMessage() . PHP_EOL;
    }
  }
);

mightThrowException();

/*
Exception from instrumented call: Oops!
NULL
PHP Fatal error:  Uncaught Exception: Oops! ...
*/

As exceptions are attached to spans automatically, there is no need to manually set SpanData::$meta['error.*'] metadata. But having access to the exception instance enables you to check for a thrown exception before accessing the return value.

<?php

use DDTrace\SpanData;

\DDTrace\trace_function(
    'mightThrowException',
    function(SpanData $span, $args, $retval, $ex) {
        if (null === $ex) {
            // Do something with $retval
        }
    }
);

To manually remove an exception from a span, use unset, for example: unset($span->exception).

Advanced configurations

Tracing internal functions and methods

An optimization was added starting in 0.46.0 to ignore all internal functions and methods for instrumentation. Internal functions and methods can still be instrumented by setting the DD_TRACE_TRACED_INTERNAL_FUNCTIONS environment variable. This takes a CSV of functions or methods that is to be instrumented. For example, DD_TRACE_TRACED_INTERNAL_FUNCTIONS=array_sum,mt_rand,DateTime::add. Once a function or method has been added to the list, it can be instrumented using DDTrace\trace_function() and DDTrace\trace_method() respectively.

Running the tracing closure before the instrumented call

By default, tracing closures are treated as posthook closures meaning they will be executed after the instrumented call. Some cases require running the tracing closure before the instrumented call. In that case, tracing closures are marked as prehook using an associative configuration array.

\DDTrace\trace_function('foo', [
    'prehook' => function (\DDTrace\SpanData $span, array $args) {
        // This tracing closure will run before the instrumented call
    }
]);

Debugging sandboxed errors

Tracing closures are “sandboxed” in that exceptions thrown and errors raised inside of them do no impact the instrumented call.

<?php

function my_func() {
  echo 'Hello!' . PHP_EOL;
}

\DDTrace\trace_function(
  'my_func',
  function() {
    throw new \Exception('Oops!');
  }
);

my_func();
echo 'Done.' . PHP_EOL;

/*
Hello!
Done.
*/

To debug, set the environment variable DD_TRACE_DEBUG=1 to expose any exceptions or errors that may have occurred in a tracing closure.

/*
Hello!
Exception thrown in tracing closure for my_func: Oops!
Done.
*/

Zend framework 1 manual instrumentation

Zend framework 1 is automatically instrumented by default, so you are not required to modify your ZF1 project. However, if automatic instrumentation is disabled, enable the tracer manually.

First, download the latest source code from the releases page. Extract the zip file and copy the src/DDTrace folder to your application’s /library folder. Then add the following to your application/configs/application.ini file:

autoloaderNamespaces[] = "DDTrace_"
pluginPaths.DDTrace = APPLICATION_PATH "/../library/DDTrace/Integrations/ZendFramework/V1"
resources.ddtrace = true

PHP code optimization

Prior to PHP 7, some frameworks provided ways to compile PHP classes (for example, through the Laravel’s php artisan optimize command).

While this has been deprecated if you are using PHP 7.x, you still may use this caching mechanism in your app prior to version 7.x. In this case, Datadog suggests you use the OpenTracing API instead of adding datadog/dd-trace to your Composer file.

Further Reading

Additional helpful documentation, links, and articles:

Instrument a custom method to get deep visibility into your business logicGUIDE Connect your Logs and Traces togetherDOCUMENTATION Explore your services, resources, and tracesDOCUMENTATION