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AWS X-Ray
Developer Guide

Tracing Incoming Requests with the X-Ray SDK for Node.js

You can use the X-Ray SDK for Node.js to trace incoming HTTP requests that your Express application serves on an EC2 instance in Amazon EC2, AWS Elastic Beanstalk, or Amazon ECS.

The X-Ray SDK for Node.js provides middleware for applications that use the Express framework. When you add the X-Ray middleware to your application, the X-Ray SDK for Node.js creates a segment for each sampled request. Any segments created by additional instrumentation become subsegments of the request-level segment. The request-level segment provides information about the HTTP request and response including timing, method, and disposition of the request.

Each segment has a name that identifies your application in the service map. The segment can be named statically, or you can configure the SDK to name it dynamically based on the host header in the incoming request. Dynamic naming lets you group traces based on the domain name in the request, and apply a default name if the name doesn't match an expected pattern (for example, if the host header is forged).

Forwarded Requests

If a request is forwarded to your application by a load balancer or other intermediary, The client IP in the segment is taken from the X-Forwarded-For header in the request instead of the source IP in the IP packet. The client IP recorded for a forwarded request can be forged so should not be trusted.

When a request is forwarded, the SDK sets an additional field in the segment to indicate this. If the segment contains the field x_forwarded_for set to true, the client IP was taken from the X-Forwarded-For header in the HTTP request.

The message handler creates a segment for each incoming request with an http block that contains the following information:

  • HTTP method – GET, POST, PUT, DELETE, etc.

  • Client address – The IP address of the client that sent the request.

  • Response code – The HTTP response code for the completed request.

  • Timing – The start time (when the request was received) and end time (when the response was sent).

Adding a Tracing Filter to your Application

To use the middleware, initialize the SDK client and use the middleware returned by the express.openSegment function before you define your routes.

Example app.js

var app = express();

var AWSXRay = require('aws-xray-sdk');
app.use(AWSXRay.express.openSegment('MyApp'));

app.get('/', function (req, res) {
  res.render('index');
});

app.use(AWSXRay.express.closeSegment());

After you define your routes, use the output of express.closeSegment as shown to handle any errors returned by the X-Ray SDK for Node.js.

Configuring a Segment Naming Strategy

The X-Ray SDK can name segments after the host name in the HTTP request header, but this header can be forged, which could result in unexpected nodes in your service map. To prevent the SDK from naming segments incorrectly due to requests with forged host headers, you must specify a name to use for all segments, or configure a dynamic naming strategy. A dynamic naming strategy allows the SDK to use the host name for names that match an expected pattern, and apply a default name to names that don't.

To use the same name for all request segments, specify the name of your application when you initialize the Express middleware, as shown in the previous section.

Note

You can override the service name that you define in code with the AWS_XRAY_TRACING_NAME environment variable.

A dynamic naming strategy defines a pattern that host names should match, and a default name to use if the host name in the HTTP request does not match the pattern. To name segments dynamically, use AWSXRay.middleware.enableDynamicNaming.

Example app.js - Dynamic Segment Names

If the hostname in the request matches the pattern *.example.com, use the hostname. Otherwise, use MyApp.

var app = express();

var AWSXRay = require('aws-xray-sdk');
app.use(AWSXRay.express.openSegment('MyApp'));
AWSXRay.middleware.enableDynamicNaming('*.example.com');
        
app.get('/', function (req, res) {
  res.render('index');
});

app.use(AWSXRay.express.closeSegment());