Tracing

Prerequisites

1. Ensure Jaeger is running.

  • Single Docker container: Jaeger will be integrated into the Sourcegraph single Docker container starting in 3.16.
  • Docker Compose: Jaeger is deployed if you use the provided docker-compose.yaml. Access it at port 16686 on the Sourcegraph node. One way to do this is to add an Ingress rule exposing port 16686 to public Internet traffic from your IP, then navigate to http://${NODE_IP}:16686 in your browser.
  • Kubernetes: Jaeger is already deployed, unless you explicitly removed it from the Sourcegraph manifest. Access it by running kubectl port-forward svc/jaeger-query 16686 and going to http://localhost:16686 in your browser.

The Jaeger UI should look something like this:

Jaeger UI

2. Turn on sending traces to Jaeger from Sourcegraph:

  1. Go to site configuration, add the following, and save:

    "observability.tracing": {
  "sampling": "selective"
}

  2. Go to Sourcegraph in your browser and do a search.

  3. Open Chrome dev tools.

  4. Append &trace=1 to the end of the URL and hit Enter.

  5. In the Chrome dev tools Network tab, find the graphql?Search request. Click it and click on the Headers tab. The value of the x-trace Response Header should be a trace ID, e.g., 7edb43f744c42fbf.

Using Jaeger

In site configuration, you can configure the Jaeger client to use different sampling modes. There are currently two modes:

  • "selective" (recommend) will cause a trace to be recorded only when trace=1 is present as a URL parameter.
  • "all" will cause a trace to be recorded on every request.

"selective" is the recommended default, because collecting traces on all requests can be quite memory- and network-intensive. If you have a large Sourcegraph instance (e.g,. more than 10k repositories), turn this on with caution. You may need to increase the memory/CPU quota for the Jaeger instance or set a downsampling rate in Jaeger itself, and even then, the volume of network traffic caused by Jaeger spans being sent to the collector may disrupt the performance of the overall Sourcegraph instance.

Jaeger debugging algorithm

Jaeger is a powerful debugging tool that can break down where time is spent over the lifecycle of a request and help pinpoint the source of high latency or errors. We generally follow the following algorithm to root-cause issues with Jaeger:

  1. Reproduce a slower user request (e.g., a search query that takes too long or times out).
  2. Add trace=1 to the slow URL and reload the page, so that traces will be collected.
  3. Open Chrome developer tools to the Network tab and find the corresponding GraphQL request that takes a long time. If there are multiple requests that take a long time, investigate them one by one.
  4. In the Response Headers for the slow GraphQL request, find the x-trace header. It should contain a trace ID like 7edb43f744c42fbf.
  5. Go to the Jaeger UI and paste in the trace ID to the "Lookup by Trace ID" input in the top menu bar.
  6. Explore the breakdown of the request tree in the Jaeger UI. Look for items near the leaves that take up a significant portion of the overall request time.
  7. Report this information to Sourcegraph by screenshotting the relevant trace or by downloading the trace JSON.

net/trace

Sourcegraph uses the net/trace package in its backend services. This provides simple tracing information within a single process. It can be used as an alternative when Jaeger is not available or as a supplement to Jaeger.

Site admins can access net/trace information at https://sourcegraph.example.com/-/debug/. From there, click Requests to view the traces for that service.