Instrumenting Go using AWS X-Ray

Observability of production workloads can often be a challenge — particularly so when you run a microservice based architecture. When each service performs one or more operations (such as database queries, file operations, publishing messages, etc) and one user request could invoke any amount of services, how do you debug issues and identify potential bottlenecks?

That’s where distributed tracing comes in.

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From A Cloud Guru’s “I’m here to tell you the truth, the good, the bad and the ugly of AWS X-Ray and Lambda”.

Distributed tracing is a concept in which requests are tracked as they traverse services for the purpose of debugging and root cause analysis. Typically, a unique request identifier is assigned to the incoming request at the point of ingress, which is passed from service to service. Each service records segments and subsegments (often otherwise called “spans”) of data about the request and the behaviour of the service handling the request. The end-to-end tracking of a request is known as a “trace”.

Usually request segment data contains information such as timestamps, user or authentication information, response codes, metrics such as DNS lookup time or TLS negotiation time, etc. It’s also possible for applications to record custom additional segments and subsegments of information by “instrumenting” your code.

Distributed tracing can be used to help answer questions such as:

  • Which services are not performing well enough? What’s the average response time for a request from service A to service B?
  • Am I making unnecessary API calls or database queries?
  • Do I have bottlenecks? If so, where are they?
  • Can I optimise requests and/or operations using concurrency? Where are the blocking processes?
  • Why is a particular request unsuccessful? Which component isn’t working?


AWS X-Ray is Amazon’s managed distributed tracing tool and is available in most regions. X-Ray is designed to help developers debug production distributed applications and identify errors. X-Ray currently works with EC2, ECS, Lambda and Elastic Beanstalk.

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An example trace from the AWS X-Ray documentation

The above screenshot, from Amazon’s documentation, shows an example X-Ray trace, in which a game state is saved using DynamoDB. It shows the parent request segment (the dark blue segment at the top) and each instrumented operation subsegment (the following segments) below. Although the example above shows a request to a single service X-Ray is capable of tracing a single request throughout many services.

AWS X-Ray is made up of two components — the daemon and the API.

The X-Ray daemon is an application responsible for listening for raw segment data emitted by applications and relaying it to the X-Ray API. Within AWS the X-Ray daemon is available to Lambda functions and (when enabled) Elastic Beanstalk environments. Outside of AWS, and in AWS environments which don’t provide the X-Ray daemon, like EC2, it can be installed and run locally.

The X-Ray API provides access to all X-Ray functionality via AWS SDKs, CLI tools and over HTTPS.

X-Ray Daemon

There are two options for installing the X-Ray daemon. Either download and install the X-Ray daemon or pull the official Docker image.

Personally, I prefer to use the daemon Docker image as it’s portable and allows for an easier deployment. This is what we’ll use for the purposes of this article.

If you prefer, you can install the X-Ray daemon on your host machine and skip this section.

Here’s an example Makefile which runs the X-Ray daemon Docker container:

Let’s break this down, line by line:

docker run --rm

Line 4: This line tells Docker that we want to run the following container. By default, a container’s file system persists even after the container exists — using the --rm flag tells Docker to remove it.

--env AWS_ACCESS_KEY_ID=$$(aws configure get aws_access_key_id) \  
--env AWS_SECRET_ACCESS_KEY=$$(aws configure get aws_secret_access_key) \
--env AWS_REGION=eu-west-2 \

Lines 5–7: These lines tell Docker to set these environment variables inside the running container. These environment variables are used by the X-Ray daemon to authenticate with the AWS X-Ray API. We can use aws configure shell commands to retrieve our AWS credentials at runtime.

--name xray-daemon \

Line 8: This line tells Docker to name the running container “xray-daemon”, instead of a random name.

--publish 2000:2000/udp \

Line 9: This line tells Docker to bind the host port 2000/udp to port 2000/udp inside the container. This allows us to access the daemon at

amazon/aws-xray-daemon -o

Line 10: This line tells Docker to run the “amazon/aws-xray-daemon” container. If Docker doesn’t already have this image cached it will download the latest version from the Docker Hub. The -o flag is passed to the container entrypoint and tells the X-Ray daemon to run in local mode (which doesn’t check for EC2 instance metadata).

The terminal session you use must have access to your AWS credentials. You can test this by running aws configure get aws_access_key_id. You should see your access key ID.

To start the X-Ray daemon Docker container, simply run make xray. You should see output similar to the following:

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The X-Ray daemon is now listening for raw segment data on

Instrumenting Go

Now the X-Ray daemon is listening we’re ready to send it raw segment data. We can use the AWS provided X-Ray SDK to instrument our code, which sends segment data to by default.

Out-of-the-box the SDK provides:

The inbound HTTP wrapper records HTTP method, client address, response code, timing, user agent and content length for each sampled request.

Sampling controls the rate at which requests are traced by the X-Ray SDK. By default the first request in every second is sampled, then 5% of all requests thereafter. You can configure the daemon to suit your needs.

The outbound HTTP wrapper records similar data to the inbound HTTP traffic wrapper. Requests made using a wrapped http.Client must use the http.Request WithContext() method.

Similarly to the outbound HTTP wrapper, all calls made using a wrapped client.Client must use the WithContext() version of the method invoked. X-Ray records downstream calls in trace subsegments and displays the services used in the Service Map.

The xray.Open function replaces sql.Open. Similarly to outbound HTTP and AWS SDK clients, context must also be passed to instrumented client methods. xray.DB method signatures are the same as sql.DB, except they expect a context.Context as the first parameter.

The X-Ray SDK provides an xray.Capture function which is used for recording custom segments of data. The xray.Capture function is actually used internally by the SDK for recording most segments of data.

It’s also possible to record additional fields of data in segments and subsegments by using annotations and metadata. Annotations are indexed and searchable in the X-Ray console, metadata is not. The X-Ray SDK provides methods for recording key-value pairs of data as both annotations and metadata.

In a previous article, I created a small Go app which submitted photos taken with your webcam to AWS’ Rekognition API and displayed the results. For the purposes of this article I’ve made a slight change; instead of taking photos with your webcam, two random images are retrieved from

We’ll use the AWS X-Ray SDK to instrument the web server, outbound calls to and the Rekognition API client.

If you want to follow along with this article you can grab the accompanying code from GitHub.

Inbound HTTP

A typical HTTP handler might look like the following:

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
// [...]

Only a slight change is necessary to instrument this handler — and we can use the xray.Handler() function from the X-Ray SDK for this. The xray.Handler() function accepts two parameters; an xray.SegmentNamer and a http.Handler.

The first parameter sets the service name. The service name is used to identify the particular service amongst others within the X-Ray console — it shows in both X-Ray traces and the Service Map. The X-Ray documentation contains a more in-depth section about segment naming strategies.

The second parameter is the http.Handler we want to instrument. We can simply take the second parameter (the func) from the earlier code and move it here, wrapped in http.HandlerFunc.

Now we’ve satisfied the xray.Handler function, we can use it in the http.Handle function to serve instrumented requests.

http.Handle("/", xray.Handler(xray.NewFixedSegmentNamer("..."), http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
// [...]

Outbound HTTP

Usually outbound HTTP requests look similar to the following:

client := &http.Client{}
req, _ := http.NewRequest(http.MethodGet, "...", nil)
res, _ := client.Do(req)

Instrumenting the http.Client type is as simple as wrapping it with the xray.Client() function. The X-Ray SDK creates a shallow copy of the http.Client but wraps Transport with xray.RoundTripper().

Requests made using an instrumented client must use context. To retrieve context from the initial request we can use r.Context().

client := xray.Client(&http.Client{})
req, _ := http.NewRequest(http.MethodGet, "...", nil)
res, _ := client.Do(req.WithContext(r.Context()))

AWS Services

Most AWS SDK clients are created in the same way — (service-name).New(&aws.Config{}). Here’s a typical example of how you could use the Rekognition SDK client:

svc := rekognition.New(session.New(&aws.Config{}))labels, err := svc.DetectLabels(&rekognition.DetectLabelsInput{Image: &rekognition.Image{Bytes: v}})

Similarly to outbound HTTP traffic, AWS SDK clients are instrumented by wrapping the client.Client in xray.AWS() and passing context. Most client methods have a WithContext() version.

svc := rekognition.New(session.New(&aws.Config{}))
labels, err := svc.DetectLabelsWithContext(r.Context(), &rekognition.DetectLabelsInput{Image: &rekognition.Image{Bytes: v}})

Let’s try it out!

We’re ready to try it out. Firstly, make sure the X-Ray Docker daemon is running (make xray) and run our Go program with go run main.go.

Visiting the page in the browser should show something like the following:

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Nature %99.7065200805664
Outdoors %99.4658432006836
Ocean %99.4658432006836
Sea %99.4658432006836
Water %99.4658432006836
Bird %94.42656707763672
Animal %94.42656707763672
Sky %69.50778198242188
Sunrise %66.34564971923828
Weather %59.46468734741211
Sand %56.408626556396484
Soil %55.84718322753906

It still works — great!

You should see output from the X-Ray Docker daemon, similar to the following:

[Info] Successfully sent batch of 21 segments (0.148 seconds)

This means that the daemon was able to successfully send collected traces to the X-Ray API.

X-Ray Console

We should now be able to see traces in the X-Ray console. X-Ray appears under “Developer Tools” in the AWS Console.

Clicking on “Traces”, on the left hand side, shows us a table of reported traces and some useful information such as average response time, HTTP response code and URL.

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By clicking into a particular trace we can drill down into the instrumented operations.

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The first segment, named “VisionService.Analyse”, represents the lifespan of the request made to the app. Below that are subsegments which represent instrumented operations which occurred during the request.

Each new subsegment is a child of the previous subsegment, and occurred during the parent operation. For example “dns” occurred during “”, which occurred during the main segment. The graph can be read from the top left to bottom right.

Since X-Ray is aware of outbound calls within our app, it is able to identify service boundaries, and can therefore generate a Service Map showing the flow of traffic within our application. We’re able to easily see how one client request actually invokes three services.

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We’ve seen how quick and easy it is to instrument applications using the AWS X-Ray SDK, and how distributed tracing could be used to identify bottlenecks and debug errors. We’ve also seen how by instrumenting our code we’re able to easily trace requests throughout a distributed system using a Service Map.

Go software engineer interested in all things devops. I write about Go, containers and tooling.

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