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93 lines
3.4 KiB
Markdown
93 lines
3.4 KiB
Markdown
# OpenTracing
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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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OpenTracing 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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roughly represents a single piece of work that was done and the time at
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which it occurred. A span can have child spans, meaning that the work of
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the child had to be completed for the parent span to complete, or it can
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have follow-on spans which represent work that is undertaken as a result
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of the parent but is not depended on by the parent to in order to
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finish.
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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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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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It is up to the remote server to decide what it does with the spans it
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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 more information about Jaeger's implementation see
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<https://www.jaegertracing.io/docs/>
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## Setting up OpenTracing
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To receive OpenTracing 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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docker run -d --name jaeger
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-p 6831:6831/udp \
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-p 6832:6832/udp \
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-p 5778:5778 \
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-p 16686:16686 \
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-p 14268:14268 \
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jaegertracing/all-in-one:1.13
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```
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Latest documentation is probably at
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<https://www.jaegertracing.io/docs/1.13/getting-started/>
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## Enable OpenTracing 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 uncommenting the config options under `opentracing`
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as shown in the [sample config](./sample_config.yaml). For example:
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```yaml
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opentracing:
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tracer_enabled: true
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homeserver_whitelist:
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- "mytrustedhomeserver.org"
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- "*.myotherhomeservers.com"
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```
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## Homeserver whitelisting
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The homeserver whitelist is configured using regular expressions. A list
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of regular expressions can be given and their union will be compared
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when propagating any spans contexts to another homeserver.
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Though it's mostly safe to send and receive span contexts to and from
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untrusted users since span contexts are usually opaque ids it can lead
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to two problems, namely:
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- If the span context is marked as sampled by the sending homeserver
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the receiver will sample it. Therefore two homeservers with wildly
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different sampling policies could incur higher sampling counts than
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intended.
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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/1.13/sampling/>
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