mirror of
https://github.com/element-hq/synapse.git
synced 2024-11-24 10:35:46 +03:00
94 lines
3.6 KiB
Markdown
94 lines
3.6 KiB
Markdown
# OpenTracing
|
|
|
|
## Background
|
|
|
|
OpenTracing is a semi-standard being adopted by a number of distributed
|
|
tracing platforms. It is a common api for facilitating vendor-agnostic
|
|
tracing instrumentation. That is, we can use the OpenTracing api and
|
|
select one of a number of tracer implementations to do the heavy lifting
|
|
in the background. Our current selected implementation is Jaeger.
|
|
|
|
OpenTracing 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
|
|
OpenTracing 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 OpenTracing concepts see
|
|
<https://opentracing.io/docs/overview/what-is-tracing/>.
|
|
|
|
For more information about Jaeger's implementation see
|
|
<https://www.jaegertracing.io/docs/>
|
|
|
|
## Setting up OpenTracing
|
|
|
|
To receive OpenTracing spans, start up a Jaeger server. This can be done
|
|
using docker like so:
|
|
|
|
```sh
|
|
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 OpenTracing in Synapse
|
|
|
|
OpenTracing is not enabled by default. It must be enabled in the
|
|
homeserver config by adding the `opentracing` option to your config file. You can find
|
|
documentation about how to do this in the [config manual under the header 'Opentracing'](usage/configuration/config_documentation.md#opentracing).
|
|
See below for an example Opentracing configuration:
|
|
|
|
```yaml
|
|
opentracing:
|
|
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 to OpenTracing's logs.
|
|
|
|
## Configuring Jaeger
|
|
|
|
Sampling strategies can be set as in this document:
|
|
<https://www.jaegertracing.io/docs/latest/sampling/>.
|