Serverless Trace Propagation

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Serverless Distributed Non-HTTP Trace

Required setup

Additional instrumentation is sometimes required to see a single, connected trace in Node and Python serverless applications asynchronously triggering Lambda functions. If you are just getting started with monitoring serverless applications in Datadog, follow our main installation steps and read this page on choosing your tracing library. Once you are sending traces from your Lambda functions to Datadog using the Datadog Lambda Library, you may want to follow these steps to connect traces between two Lambda functions in cases such as:

  • Triggering Lambda functions via Step Functions
  • Invoking Lambda functions via non-HTTP protocols such as MQTT

Tracing many AWS Managed services (listed here) is supported out-of-the-box and does not require following the steps outlined on this page.

To successfully connect trace context between resources sending traces, you need to:

  • Include Datadog trace context in outgoing events. The outgoing event can originate from a host or Lambda function with dd-trace installed.
  • Extract the trace context in the consumer Lambda function.

Passing trace context

The following code samples outline how to pass trace context in outgoing payloads to services which do not support HTTP headers, or managed services not supported natively by Datadog in Node and Python:

In Python, you can use the get_dd_trace_context helper function to pass tracing context to outgoing events in a Lambda functions:

import json
import boto3
import os

from datadog_lambda.tracing import get_dd_trace_context  # Datadog tracing helper function

def handler(event, context):
    my_custom_client.sendRequest(
        {
          'myCustom': 'data',
          '_datadog': {
              'DataType': 'String',
              'StringValue': json.dumps(get_dd_trace_context()) # Includes trace context in outgoing payload.
          },
        },
    )

In Node, you can use the getTraceHeaders helper function to pass tracing context to outgoing events in a Lambda function:

const { getTraceHeaders } = require("datadog-lambda-js"); // Datadog tracing helper function

module.exports.handler = async event => {
  const _datadog = getTraceHeaders(); // Captures current Datadog trace context.

  var payload = JSON.stringify({ data: 'sns', _datadog });
  await myCustomClient.sendRequest(payload)

From hosts

If you aren’t passing trace context from your Lambda functions, you can use the following code template in place of the getTraceHeaders and get_dd_trace_context helper functions to get the current span context. Instructions on how to do this in every runtime are outlined here.

const tracer = require("dd-trace");

exports.handler = async event => {
  const span = tracer.scope().active();
  const _datadog = {}
  tracer.inject(span, 'text_map', _datadog)

  // ...

Extracting trace context

To extract the above trace context from the consumer Lambda function, you need to define an extractor function that captures trace context before the execution of your Lambda function handler. To do this, configure the DD_TRACE_EXTRACTOR environment variable to point to the location of your extractor function. The format for this is <FILE NAME>.<FUNCTION NAME>. For example, extractors.json if the json extractor is in the extractors.js file. Datadog recommends you place your extractor methods all in one file, as extractors can be re-used across multiple Lambda functions. These extractors are completely customizable to fit any use case.

Notes:

  • If you are using TypeScript or a bundler like webpack, you must import or require your Node.js module where the extractors are defined. This ensures the module gets compiled and bundled into your Lambda deployment package.
  • If your Node.js Lambda function runs on arm64, you must define the extractor in your function code instead of using the DD_TRACE_EXTRACTOR environment variable.

Sample extractors

The following code samples outline sample extractors you might use for propagating trace context across a third party system, or an API which does not support standard HTTP headers.

def extractor(payload):
    trace_headers = json.loads(payload["_datadog"]);
    trace_id = trace_headers["x-datadog-trace-id"];
    parent_id = trace_headers["x-datadog-parent-id"];
    sampling_priority = trace_headers["x-datadog-sampling-priority"];
    return trace_id, parent_id, sampling_priority
exports.json = (payload) => {
    const traceData = payload._datadog
    const traceID = traceData["x-datadog-trace-id"];
    const parentID = traceData["x-datadog-parent-id"];
    const sampledHeader = traceData["x-datadog-sampling-priority"];
    const sampleMode = parseInt(sampledHeader, 10);

    return {
      parentID,
      sampleMode,
      source: 'event',
      traceID,
    };
};
var exampleSQSExtractor = func(ctx context.Context, ev json.RawMessage) map[string]string {
	eh := events.SQSEvent{}

	headers := map[string]string{}

	if err := json.Unmarshal(ev, &eh); err != nil {
		return headers
	}

	// Using SQS as a trigger with a batchSize=1 so it's important we check
  // for this as a single SQS message will drive the execution of the handler.
	if len(eh.Records) != 1 {
		return headers
	}

	record := eh.Records[0]

	lowercaseHeaders := map[string]string{}
	for k, v := range record.MessageAttributes {
		if v.StringValue != nil {
			lowercaseHeaders[strings.ToLower(k)] = *v.StringValue
		}
	}

	return lowercaseHeaders
}

cfg := &ddlambda.Config{
    TraceContextExtractor: exampleSQSExtractor,
}
ddlambda.WrapFunction(handler, cfg)

Further Reading