Propagating PHP Trace Context

The Datadog APM Tracer supports B3 and W3C Trace Context headers extraction and injection for distributed tracing.

You can configure injection and extraction styles for distributed headers.

The PHP Tracer supports the following styles:

  • Datadog: Datadog
  • W3C Trace Context: tracecontext
  • B3 Multi Header: b3multi (B3 alias is deprecated)
  • B3 Single Header: B3 single header

You can use the following environment variables to configure the PHP tracing library injection and extraction styles. For instance:

  • DD_TRACE_PROPAGATION_STYLE_INJECT=Datadog,tracecontext,B3 single header
  • DD_TRACE_PROPAGATION_STYLE_EXTRACT=Datadog,tracecontext,B3 single header

The environment variable values are comma-separated lists of header styles enabled for injection or extraction. By default, only the tracecontext and Datadog injection styles are enabled.

If multiple extraction styles are enabled, the extraction attempt is completed with the following priorities: tracecontext has priority, then Datadog, then B3.

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);
    }
);

Alternatively, starting with version 0.87.0, if the raw headers are available, a function DDTrace\consume_distributed_tracing_headers(array|callable $headersOrCallback) is provided. Note that the header names must be in lowercase.

$headers = [
	"x-datadog-trace-id" => "1234567890",
	"x-datadog-parent-id" => "987654321",
];

\DDTrace\consume_distributed_tracing_headers($headers);

To extract the trace context directly as headers, a function DDTrace\generate_distributed_tracing_headers(?array $inject = null): array is provided. Its sole optional argument accepts an array of injection style names. It defaults to the configured injection style.

$headers = DDTrace\generate_distributed_tracing_headers();
// Store headers somewhere, inject them in an outbound request, ...
// These $headers can also be read back by \DDTrace\consume_distributed_tracing_headers from another process.

RabbitMQ

Although the PHP tracer supports automatic tracing of the php-amqplib/php-amqplib library starting with version 0.87.0, there are some known cases where your distributed trace can be disconnected. Most notably, when reading messages from a distributed queue using the basic_get method while not already in a trace, you would need to add a custom trace surrounding a basic_get call and the corresponding message processing.

Here is an example:

// Create a surrounding trace
$newTrace = \DDTrace\start_trace_span();
$newTrace->name = 'basic_get.process';
$newTrace->service = 'amqp';


// basic_get call(s) + message(s) processing
$msg = $channel->basic_get($queue);
if ($msg) {
   $messageProcessing($msg);
}


// Once done, close the span
\DDTrace\close_span();

Creating this surrounding trace to your consuming-processing logic ensures observability of your distributed queue.

Further Reading

Additional helpful documentation, links, and articles:

Monitor OpenTelemetry-instrumented apps with support for W3C Trace ContextBLOG more more