---
title: Overview
description: Overview of distributed tracing in Istio.
weight: 1
keywords: [telemetry,tracing]
aliases:
 - /zh/docs/tasks/telemetry/distributed-tracing/overview/
---

Distributed tracing enables users to track a request through mesh that is distributed across multiple services.
This allows a deeper understanding about request latency, serialization and parallelism via visualization.

Istio leverages [Envoy's distributed tracing](https://www.envoyproxy.io/docs/envoy/v1.10.0/intro/arch_overview/tracing) feature
to provide tracing integration out of the box. Specifically, Istio provides options to install various tracing backend
and configure proxies to send trace spans to them automatically.
See [Zipkin](../zipkin/), [Jaeger](../jaeger/) and [LightStep](/docs/tasks/observability/distributed-tracing/lightstep/) task docs about how Istio works with those tracing systems.

## Trace context propagation

Although Istio proxies are able to automatically send spans, they need some hints to tie together the entire trace.
Applications need to propagate the appropriate HTTP headers so that when the proxies send span information,
the spans can be correlated correctly into a single trace.

To do this, an application needs to collect and propagate the following headers from the incoming request to any outgoing requests:

* `x-request-id`
* `x-b3-traceid`
* `x-b3-spanid`
* `x-b3-parentspanid`
* `x-b3-sampled`
* `x-b3-flags`
* `x-ot-span-context`

If you look at the sample Python `productpage` service, for example,
you see that the application extracts the required headers from an HTTP request
using [OpenTracing](https://opentracing.io/) libraries:

{{< text python >}}
def getForwardHeaders(request):
    headers = {}

    # x-b3-*** headers can be populated using the opentracing span
    span = get_current_span()
    carrier = {}
    tracer.inject(
        span_context=span.context,
        format=Format.HTTP_HEADERS,
        carrier=carrier)

    headers.update(carrier)

    # ...

    incoming_headers = ['x-request-id']

    # ...

    for ihdr in incoming_headers:
        val = request.headers.get(ihdr)
        if val is not None:
            headers[ihdr] = val

    return headers
{{< /text >}}

The reviews application (Java) does something similar:

{{< text java >}}
@GET
@Path("/reviews/{productId}")
public Response bookReviewsById(@PathParam("productId") int productId,
                            @HeaderParam("end-user") String user,
                            @HeaderParam("x-request-id") String xreq,
                            @HeaderParam("x-b3-traceid") String xtraceid,
                            @HeaderParam("x-b3-spanid") String xspanid,
                            @HeaderParam("x-b3-parentspanid") String xparentspanid,
                            @HeaderParam("x-b3-sampled") String xsampled,
                            @HeaderParam("x-b3-flags") String xflags,
                            @HeaderParam("x-ot-span-context") String xotspan) {

  if (ratings_enabled) {
    JsonObject ratingsResponse = getRatings(Integer.toString(productId), user, xreq, xtraceid, xspanid, xparentspanid, xsampled, xflags, xotspan);
{{< /text >}}

When you make downstream calls in your applications, make sure to include these headers.

## Trace sampling

Istio captures a trace for all requests by default when installing with the demo profile.
For example, when using the Bookinfo sample application above, every time you access
`/productpage` you see a corresponding trace in the
dashboard. This sampling rate is suitable for a test or low traffic
mesh. For a high traffic mesh you can lower the trace sampling
percentage in one of two ways:

* During the mesh setup, use the option `values.pilot.traceSampling` to
  set the percentage of trace sampling. See the
  [Installing with {{< istioctl >}}](/docs/setup/install/istioctl/) documentation for
  details on setting options.
* In a running mesh, edit the `istio-pilot` deployment and
  change the environment variable with the following steps:

    1. To open your text editor with the deployment configuration file
       loaded, run the following command:

        {{< text bash >}}
        $ kubectl -n istio-system edit deploy istio-pilot
        {{< /text >}}

    1. Find the `PILOT_TRACE_SAMPLING` environment variable, and change
       the `value:` to your desired percentage.

In both cases, valid values are from 0.0 to 100.0 with a precision of 0.01.