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