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Using AWS Lambda with AWS X-Ray

You can use AWS X-Ray to visualize the components of your application, identify performance bottlenecks, and troubleshoot requests that resulted in an error. Your Lambda functions send trace data to X-Ray, and X-Ray processes the data to generate a service map and searchable trace summaries.

If you've enabled X-Ray tracing in a service that invokes your function, Lambda sends traces to X-Ray automatically. The upstream service, such as Amazon API Gateway, or an application hosted on Amazon EC2 that is instrumented with the X-Ray SDK, samples incoming requests and adds a tracing header that tells Lambda to send traces or not.

To trace requests that don't have a tracing header, enable active tracing in your function's configuration.

To enable active tracing

  1. Open the Functions page of the Lambda console.

  2. Choose a function.

  3. Choose Configuration and then choose Monitoring tools.

  4. Choose Edit.

  5. Under X-Ray, enable Active tracing.

  6. Choose Save.

Pricing
X-Ray has a perpetual free tier. Beyond the free tier threshold, X-Ray charges for trace storage and retrieval. For details, see AWS X-Ray pricing.

Your function needs permission to upload trace data to X-Ray. When you enable active tracing in the Lambda console, Lambda adds the required permissions to your function's execution role. Otherwise, add the AWSXRayDaemonWriteAccess policy to the execution role.

X-Ray applies a sampling algorithm to ensure that tracing is efficient, while still providing a representative sample of the requests that your application serves. The default sampling rule is 1 request per second and 5 percent of additional requests. This sampling rate cannot be configured for Lambda functions.

In X-Ray, a trace records information about a request that is processed by one or more services. Services record segments that contain layers of subsegments. Lambda records a segment for the Lambda service that handles the invocation request, and one for the work done by the function. The function segment comes with subsegments for Initialization, Invocation and Overhead. For more information see Lambda execution environment lifecycle.

The Initialization subsegment represents the init phase of the Lambda execution environment lifecycle. During this phase, Lambda creates or unfreezes an execution environment with the resources you have configured, downloads the function code and all layers, initializes extensions, initializes the runtime, and runs the function's initialization code.

The Invocation subsegment represents the invoke phase where Lambda invokes the function handler. This begins with runtime and extension registration and it ends when the runtime is ready to send the response.

The Overhead subsegment represents the phase that occurs between the time when the runtime sends the response and the signal for the next invoke. During this time, the runtime finishes all tasks related to an invoke and prepares to freeze the sandbox.

Note
If your Lambda function uses provisioned concurrency, your X-Ray trace might display a function initialization with a very long duration.
Provisioned concurrency initializes function instances in advance, to reduce lag at the time of invocation. Over time, provisioned concurrency refreshes these instances by creating new instances to replace the old ones. For workloads with steady traffic, the new instances are initialized well in advance of their first invocation. The time gap gets recorded in the X-Ray trace as the initialization duration.

Important
In Lambda, you can use the X-Ray SDK to extend the Invocation subsegment with additional subsegments for downstream calls, annotations, and metadata. You can't access the function segment directly or record work done outside of the handler invocation scope.

See the following topics for a language-specific introduction to tracing in Lambda:

For a full list of services that support active instrumentation, see Supported AWS services in the AWS X-Ray Developer Guide.

Topics

Execution role permissions

Lambda needs the following permissions to send trace data to X-Ray. Add them to your function's execution role.

These permissions are included in the AWSXRayDaemonWriteAccess managed policy.

The AWS X-Ray daemon

Instead of sending trace data directly to the X-Ray API, the X-Ray SDK uses a daemon process. The AWS X-Ray daemon is an application that runs in the Lambda environment and listens for UDP traffic that contains segments and subsegments. It buffers incoming data and writes it to X-Ray in batches, reducing the processing and memory overhead required to trace invocations.

The Lambda runtime allows the daemon to up to 3 percent of your function's configured memory or 16 MB, whichever is greater. If your function runs out of memory during invocation, the runtime terminates the daemon process first to free up memory.

The daemon process is fully managed by Lambda and cannot be configured by the user. All segments generated by function invocations are recorded in the same account as the Lambda function. The daemon cannot be configured to redirect them to any other account.

For more information, see The X-Ray daemon in the X-Ray Developer Guide.

Enabling active tracing with the Lambda API

To manage tracing configuration with the AWS CLI or AWS SDK, use the following API operations:

The following example AWS CLI command enables active tracing on a function named my-function.

aws lambda update-function-configuration --function-name my-function \
--tracing-config Mode=Active

Tracing mode is part of the version-specific configuration that is locked when you publish a version of your function. You can't change the tracing mode on a published version.

Enabling active tracing with AWS CloudFormation

To enable active tracing on an AWS::Lambda::Function resource in an AWS CloudFormation template, use the TracingConfig property.

Example function-inline.yml – Tracing configuration

Resources:
  function:
    Type: [AWS::Lambda::Function](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-lambda-function.html)
    Properties:
      TracingConfig:
        Mode: Active
      ...

For an AWS Serverless Application Model (AWS SAM) AWS::Serverless::Function resource, use the Tracing property.

Example template.yml – Tracing configuration

Resources:
  function:
    Type: [AWS::Serverless::Function](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-resource-function.html)
    Properties:
      Tracing: Active
      ...