Tracing

Background

OpenTelemetry is a semi-standard being adopted by a number of distributed tracing platforms. It is a common API for facilitating vendor-agnostic tracing instrumentation.

Tracing is a tool which gives an insight into the causal relationship of work done in and between servers. The servers each track events and report them to a centralised server - in Synapse's case: Jaeger. The basic unit used to represent events is the span. The span roughly represents a single piece of work that was done and the time at which it occurred. A span can have child spans, meaning that the work of the child had to be completed for the parent span to complete, or it can have follow-on spans which represent work that is undertaken as a result of the parent but is not depended on by the parent to in order to finish.

Since this is undertaken in a distributed environment a request to another server, such as an RPC or a simple GET, can be considered a span (a unit or work) for the local server. This causal link is what tracing aims to capture and visualise. In order to do this metadata about the local server's span, i.e the 'span context', needs to be included with the request to the remote.

It is up to the remote server to decide what it does with the spans it creates. This is called the sampling policy and it can be configured through Jaeger's settings.

For OpenTelemetry concepts, see https://opentelemetry.io/docs/concepts/.

For more information about the Python implementation of OpenTelemetry we're using, see https://opentelemetry.io/docs/instrumentation/python/

For more information about Jaeger, see https://www.jaegertracing.io/docs/

Setting up tracing

To receive tracing spans, start up a Jaeger server. This can be done using docker like so:

docker run -d --name jaeger \
  -p 6831:6831/udp \
  -p 6832:6832/udp \
  -p 5778:5778 \
  -p 16686:16686 \
  -p 14268:14268 \
  jaegertracing/all-in-one:1

Latest documentation is probably at https://www.jaegertracing.io/docs/latest/getting-started.

Enable tracing in Synapse

Tracing is not enabled by default. It must be enabled in the homeserver config by adding the tracing option to your config file. You can find documentation about how to do this in the config manual under the header 'Tracing'. See below for an example tracing configuration:

tracing:
  enabled: true
  homeserver_whitelist:
    - "mytrustedhomeserver.org"
    - "*.myotherhomeservers.com"

Homeserver whitelisting

The homeserver whitelist is configured using regular expressions. A list of regular expressions can be given and their union will be compared when propagating any spans contexts to another homeserver.

Though it's mostly safe to send and receive span contexts to and from untrusted users since span contexts are usually opaque ids it can lead to two problems, namely:

• If the span context is marked as sampled by the sending homeserver the receiver will sample it. Therefore two homeservers with wildly different sampling policies could incur higher sampling counts than intended.
• Sending servers can attach arbitrary data to spans, known as 'baggage'. For safety this has been disabled in Synapse but that doesn't prevent another server sending you baggage which will be logged in the trace.