mirror of
https://github.com/matrix-org/dendrite.git
synced 2024-11-26 16:21:55 -06:00
113 lines
4.8 KiB
Markdown
113 lines
4.8 KiB
Markdown
Opentracing
|
|
===========
|
|
|
|
Dendrite extensively uses the [opentracing.io](http://opentracing.io) framework
|
|
to trace work across the different logical components.
|
|
|
|
At its most basic opentracing tracks "spans" of work; recording start and end
|
|
times as well as any parent span that caused the piece of work.
|
|
|
|
A typical example would be a new span being created on an incoming request that
|
|
finishes when the response is sent. When the code needs to hit out to a
|
|
different component a new span is created with the initial span as its parent.
|
|
This would end up looking roughly like:
|
|
|
|
```
|
|
Received request Sent response
|
|
|<───────────────────────────────────────>|
|
|
|<────────────────────>|
|
|
RPC call RPC call returns
|
|
```
|
|
|
|
This is useful to see where the time is being spent processing a request on a
|
|
component. However, opentracing allows tracking of spans across components. This
|
|
makes it possible to see exactly what work goes into processing a request:
|
|
|
|
|
|
```
|
|
Component 1 |<─────────────────── HTTP ────────────────────>|
|
|
|<──────────────── RPC ─────────────────>|
|
|
Component 2 |<─ SQL ─>| |<── RPC ───>|
|
|
Component 3 |<─ SQL ─>|
|
|
```
|
|
|
|
This is achieved by serializing span information during all communication
|
|
between components. For HTTP requests, this is achieved by the sender
|
|
serializing the span into a HTTP header, and the receiver deserializing the span
|
|
on receipt. (Generally a new span is then immediately created with the
|
|
deserialized span as the parent).
|
|
|
|
A collection of spans that are related is called a trace.
|
|
|
|
|
|
Spans are passed through the code via contexts, rather than manually. It is
|
|
therefore important that all spans that are created are immediately added to the
|
|
current context. Thankfully the opentracing library gives helper functions for
|
|
doing this:
|
|
|
|
```golang
|
|
span, ctx := opentracing.StartSpanFromContext(ctx, spanName)
|
|
defer span.Finish()
|
|
```
|
|
|
|
This will create a new span, adding any span already in `ctx` as a parent to the
|
|
new span.
|
|
|
|
|
|
Adding Information
|
|
------------------
|
|
|
|
Opentracing allows adding information to a trace via three mechanisms:
|
|
- "tags" ─ A span can be tagged with a key/value pair. This is typically
|
|
information that relates to the span, e.g. for spans created for incoming HTTP
|
|
requests could include the request path and response codes as tags, spans for
|
|
SQL could include the query being executed.
|
|
- "logs" ─ Key/value pairs can be looged at a particular instance in a trace.
|
|
This can be useful to log e.g. any errors that happen.
|
|
- "baggage" ─ Arbitrary key/value pairs can be added to a span to which all
|
|
child spans have access. Baggage isn't saved and so isn't available when
|
|
inspecting the traces, but can be used to add context to logs or tags in child
|
|
spans.
|
|
|
|
|
|
See
|
|
[specification.md](https://github.com/opentracing/specification/blob/master/specification.md)
|
|
for some of the common tags and log fields used.
|
|
|
|
|
|
Span Relationships
|
|
------------------
|
|
|
|
Spans can be related to each other. The most common relation is `childOf`, which
|
|
indicates the child span somehow depends on the parent span ─ typically the
|
|
parent span cannot complete until all child spans are completed.
|
|
|
|
A second relation type is `followsFrom`, where the parent has no dependence on
|
|
the child span. This usually indicates some sort of fire and forget behaviour,
|
|
e.g. adding a message to a pipeline or inserting into a kafka topic.
|
|
|
|
|
|
Jaeger
|
|
------
|
|
|
|
Opentracing is just a framework. We use
|
|
[jaeger](https://github.com/jaegertracing/jaeger) as the actual implementation.
|
|
|
|
Jaeger is responsible for recording, sending and saving traces, as well as
|
|
giving a UI for viewing and interacting with traces.
|
|
|
|
To enable jaeger a `Tracer` object must be instansiated from the config (as well
|
|
as having a jaeger server running somewhere, usually locally). A `Tracer` does
|
|
several things:
|
|
- Decides which traces to save and send to the server. There are multiple
|
|
schemes for doing this, with a simple example being to save a certain fraction
|
|
of traces.
|
|
- Communicating with the jaeger backend. If not explicitly specified uses the
|
|
default port on localhost.
|
|
- Associates a service name to all spans created by the tracer. This service
|
|
name equates to a logical component, e.g. spans created by clientapi will have
|
|
a different service name than ones created by the syncapi. Database access
|
|
will also typically use a different service name.
|
|
|
|
This means that there is a tracer per service name/component.
|