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Update docs/tracing.md
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# OpenTracing
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# Tracing
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## Background
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OpenTracing is a semi-standard being adopted by a number of distributed
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tracing platforms. It is a common api for facilitating vendor-agnostic
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tracing instrumentation. That is, we can use the OpenTracing api and
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select one of a number of tracer implementations to do the heavy lifting
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in the background. Our current selected implementation is Jaeger.
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OpenTelemetry is a semi-standard being adopted by a number of distributed
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tracing platforms. It is a common API for facilitating vendor-agnostic
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tracing instrumentation.
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OpenTracing is a tool which gives an insight into the causal
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Tracing is a tool which gives an insight into the causal
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relationship of work done in and between servers. The servers each track
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events and report them to a centralised server - in Synapse's case:
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Jaeger. The basic unit used to represent events is the span. The span
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Since this is undertaken in a distributed environment a request to
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another server, such as an RPC or a simple GET, can be considered a span
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(a unit or work) for the local server. This causal link is what
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OpenTracing aims to capture and visualise. In order to do this metadata
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tracing aims to capture and visualise. In order to do this metadata
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about the local server's span, i.e the 'span context', needs to be
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included with the request to the remote.
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creates. This is called the sampling policy and it can be configured
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through Jaeger's settings.
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For OpenTracing concepts see
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<https://opentracing.io/docs/overview/what-is-tracing/>.
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For OpenTelemetry concepts, see
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<https://opentelemetry.io/docs/concepts/>.
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For more information about Jaeger's implementation see
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For more information about the Python implementation of OpenTelemetry we're using, see
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<https://opentelemetry.io/docs/instrumentation/python/>
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For more information about Jaeger, see
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<https://www.jaegertracing.io/docs/>
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## Setting up OpenTracing
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## Setting up tracing
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To receive OpenTracing spans, start up a Jaeger server. This can be done
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To receive tracing spans, start up a Jaeger server. This can be done
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using docker like so:
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```sh
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Latest documentation is probably at
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https://www.jaegertracing.io/docs/latest/getting-started.
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## Enable OpenTracing in Synapse
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## Enable tracing in Synapse
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OpenTracing is not enabled by default. It must be enabled in the
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homeserver config by adding the `opentracing` option to your config file. You can find
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documentation about how to do this in the [config manual under the header 'Opentracing'](usage/configuration/config_documentation.md#opentracing).
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See below for an example Opentracing configuration:
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Tracing is not enabled by default. It must be enabled in the
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homeserver config by adding the `tracing` option to your config file. You can find
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documentation about how to do this in the [config manual under the header 'Tracing'](usage/configuration/config_documentation.md#tracing).
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See below for an example tracing configuration:
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```yaml
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opentracing:
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tracing:
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enabled: true
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homeserver_whitelist:
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- "mytrustedhomeserver.org"
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- Sending servers can attach arbitrary data to spans, known as
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'baggage'. For safety this has been disabled in Synapse but that
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doesn't prevent another server sending you baggage which will be
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logged to OpenTracing's logs.
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## Configuring Jaeger
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Sampling strategies can be set as in this document:
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<https://www.jaegertracing.io/docs/latest/sampling/>.
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logged in the trace.
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Sub-options include:
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* `enabled`: whether tracing is enabled. Set to true to enable. Disabled by default.
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* `homeserver_whitelist`: The list of homeservers we wish to send and receive span contexts and span baggage.
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See [here](../../tracing.md) for more.
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See [here](../../tracing.md#homeserver-whitelisting) for more.
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This is a list of regexes which are matched against the `server_name` of the homeserver.
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By default, it is empty, so no servers are matched.
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* `sample_rate`: The probability that a given span and subsequent child spans in the trace will be
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