Instrumenting Ruby code in AWS Lambda

Lambda integrates with AWS X-Ray to enable you to trace, debug, and optimize Lambda applications. You can use X-Ray to trace a request as it traverses resources in your application, from the frontend API to storage and database on the backend. By simply adding the X-Ray SDK library to your build configuration, you can record errors and latency for any call that your function makes to an AWS service.

After you've configured active tracing, you can observe specific requests through your application. The X-Ray service graph shows information about your application and all its components. The following example shows an application with two functions. The primary function processes events and sometimes returns errors. The second function at the top processes errors that appear in the first's log group and uses the AWS SDK to call X-Ray, Amazon Simple Storage Service (Amazon S3), and Amazon CloudWatch Logs.

To toggle active tracing on your Lambda function with the console, follow these steps:

Your function needs permission to upload trace data to X-Ray. When you activate 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 doesn't trace all requests to your application. X-Ray applies a sampling algorithm to ensure that tracing is efficient, while still providing a representative sample of all requests. The sampling rate is 1 request per second and 5 percent of additional requests. You can't configure the X-Ray sampling rate for your functions.

In X-Ray, a trace records information about a request that is processed by one or more services. Lambda records 2 segments per trace, which creates two nodes on the service graph. The following image highlights these two nodes:

The first node on the left represents the Lambda service, which receives the invocation request. The second node represents your specific Lambda function. The following example shows a trace with these two segments. Both are named my-function, but one has an origin of AWS::Lambda and the other has an origin of AWS::Lambda::Function. If the AWS::Lambda segment shows an error, the Lambda service had an issue. If the AWS::Lambda::Function segment shows an error, your function had an issue.

This example expands the AWS::Lambda::Function segment to show its three subsegments.

The old-style function segment contains the following subsegments:

The new-style function segment doesn't contain an Invocation subsegment. Instead, customer subsegments are attached directly to the function segment. For more information about the structure of the old- and new-style function segments, see Understanding X-Ray traces.

You can instrument your handler code to record metadata and trace downstream calls. To record detail about calls that your handler makes to other resources and services, use the X-Ray SDK for Ruby. To get the SDK, add the aws-xray-sdk package to your application's dependencies.

# Gemfile
source 'https://rubygems.org'

gem 'aws-xray-sdk', '0.11.4'
gem 'aws-sdk-lambda', '1.39.0'
gem 'test-unit', '3.3.5'

To instrument AWS SDK clients, require the aws-xray-sdk/lambda module after creating a client in initialization code.

# lambda_function.rb
require 'logger'
require 'json'
require 'aws-sdk-lambda'
$client = Aws::Lambda::Client.new()
$client.get_account_settings()

require 'aws-xray-sdk/lambda'

def lambda_handler(event:, context:)
  logger = Logger.new($stdout)
  ...

In X-Ray, a trace records information about a request that is processed by one or more services. Lambda records 2 segments per trace, which creates two nodes on the service graph. The following image highlights these two nodes:

The first node on the left represents the Lambda service, which receives the invocation request. The second node represents your specific Lambda function. The following example shows a trace with these two segments. Both are named my-function, but one has an origin of AWS::Lambda and the other has an origin of AWS::Lambda::Function. If the AWS::Lambda segment shows an error, the Lambda service had an issue. If the AWS::Lambda::Function segment shows an error, your function had an issue.

This example expands the AWS::Lambda::Function segment to show its three subsegments.

The old-style function segment contains the following subsegments:

The new-style function segment doesn't contain an Invocation subsegment. Instead, customer subsegments are attached directly to the function segment. For more information about the structure of the old- and new-style function segments, see Understanding X-Ray traces.

You can also instrument HTTP clients, record SQL queries, and create custom subsegments with annotations and metadata. For more information, see The X-Ray SDK for Ruby in the AWS 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 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 activate tracing on an AWS::Lambda::Function resource in an AWS CloudFormation template, use the TracingConfig property.

Resources:
  function:
    Type: AWS::Lambda::Function
    Properties:
      TracingConfig:
        Mode: Active
      ...

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

Resources:
  function:
    Type: AWS::Serverless::Function
    Properties:
      Tracing: Active
      ...

Storing runtime dependencies in a layer

If you use the X-Ray SDK to instrument AWS SDK clients your function code, your deployment package can become quite large. To avoid uploading runtime dependencies every time you update your function code, package the X-Ray SDK in a Lambda layer.

The following example shows an AWS::Serverless::LayerVersion resource that stores X-Ray SDK for Ruby.

Resources:
  function:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: function/.
      Tracing: Active
      Layers:
        - !Ref libs
      ...
  libs:
    Type: AWS::Serverless::LayerVersion
    Properties:
      LayerName: blank-ruby-lib
      Description: Dependencies for the blank-ruby sample app.
      ContentUri: lib/.
      CompatibleRuntimes:
        - ruby2.5

With this configuration, you update the library layer only if you change your runtime dependencies. Since the function deployment package contains only your code, this can help reduce upload times.

Creating a layer for dependencies requires build changes to generate the layer archive prior to deployment. For a working example, see the blank-ruby sample application.