overview.md

Overview

Distributed Tracing

A distributed trace is a set of events, triggered as a result of a single logical operation, consolidated across various components of an application. A distributed trace contains events that cross process, network and security boundaries. A distributed trace may be initiated when someone presses a button to start an action on a website - in this example, the trace will represent calls made between the downstream services that handled the chain of requests initiated by this button being pressed.

Trace

Traces in OpenTelemetry are defined implicitly by their Spans. In particular, a Trace can be thought of as a directed acyclic graph (DAG) of Spans, where the edges between Spans are defined as parent/child relationship.

For example, the following is an example Trace made up of 6 Spans:

Causal relationships between Spans in a single Trace

        [Span A]  ←←←(the root span)
            |
     +------+------+
     |             |
 [Span B]      [Span C] ←←←(Span C is a `child` of Span A)
     |             |
 [Span D]      +---+-------+
               |           |
           [Span E]    [Span F]

Sometimes it's easier to visualize Traces with a time axis as in the diagram below:

Temporal relationships between Spans in a single Trace

––|–––––––|–––––––|–––––––|–––––––|–––––––|–––––––|–––––––|–> time

 [Span A···················································]
   [Span B··············································]
      [Span D··········································]
    [Span C········································]
         [Span E·······]        [Span F··]

Span

Each Span encapsulates the following state:

• An operation name
• A start and finish timestamp
• A set of zero or more key:value Attributes. The keys must be strings. The values may be strings, bools, or numeric types.
• A set of zero or more Events, each of which is itself a key:value map paired with a timestamp. The keys must be strings, though the values may be of the same types as Span Attributes.
• Parent's Span identifier.
• Links to zero or more causally-related Spans (via the SpanContext of those related Spans).
• SpanContext identification of a Span. See below.

SpanContext

Represents all the information that identifies Span in the Trace and MUST be propagated to child Spans and across process boundaries. A SpanContext contains the tracing identifiers and the options that are propagated from parent to child Spans.

• TraceId is the identifier for a trace. It is worldwide unique with practically sufficient probability by being made as 16 randomly generated bytes. TraceId is used to group all spans for a specific trace together across all processes.
• SpanId is the identifier for a span. It is globally unique with practically sufficient probability by being made as 8 randomly generated bytes. When passed to a child Span this identifier becomes the parent span id for the child Span.
• TraceFlags represents the options for a trace. It is represented as 1 byte (bitmap).
  • Sampling bit - Bit to represent whether trace is sampled or not (mask 0x1).
• Tracestate carries tracing-system specific context in a list of key value pairs. Tracestate allows different vendors propagate additional information and inter-operate with their legacy Id formats. For more details see this.

Links between spans

A Span may be linked to zero or more other Spans (defined by SpanContext) that are causally related. Links can point to SpanContexts inside a single Trace or across different Traces. Links can be used to represent batched operations where a Span was initiated by multiple initiating Spans, each representing a single incoming item being processed in the batch.

Another example of using a Link is to declare the relationship between the originating and following trace. This can be used when a Trace enters trusted boundaries of a service and service policy requires the generation of a new Trace rather than trusting the incoming Trace context. The new linked Trace may also represent a long running asynchronous data processing operation that was initiated by one of many fast incoming requests.

When using the scatter/gather (also called fork/join) pattern, the root operation starts multiple downstream processing operations and all of them are aggregated back in a single Span. This last Span is linked to many operations it aggregates. All of them are the Spans from the same Trace. And similar to the Parent field of a Span. It is recommended, however, to not set parent of the Span in this scenario as semantically the parent field represents a single parent scenario, in many cases the parent Span fully encloses the child Span. This is not the case in scatter/gather and batch scenarios.

Metrics

OpenTelemetry allows to record raw measurements or metrics with predefined aggregation and set of labels.

Recording raw measurements using OpenTelemetry API allows to defer to end-user the decision on what aggregation algorithm should be applied for this metric as well as defining labels (dimensions). It will be used in client libraries like gRPC to record raw measurements "server_latency" or "received_bytes". So end user will decide what type of aggregated values should be collected out of these raw measurements. It may be simple average or elaborate histogram calculation.

Recording of metrics with the pre-defined aggregation using OpenTelemetry API is not less important. It allows to collect values like cpu and memory usage, or simple metrics like "queue length".

Recording raw measurements

The main classes used to record raw measurements are Measure and Measurement. List of Measurements alongside the additional context can be recorded using OpenTelemetry API. So user may define to aggregate those Measurements and use the context passed alongside to define additional dimensions of the resulting metric.

Measure

Measure describes the type of the individual values recorded by a library. It defines a contract between the library exposing the measurements and an application that will aggregate those individual measurements into a Metric. Measure is identified by name, description and a unit of values.

Measurement

Measurement describes a single value to be collected for a Measure. Measurement is an empty interface in API surface. This interface is defined in SDK.

Recording metrics with predefined aggregation

The base class for all types of pre-aggregated metrics is called Metric. It defines basic metric properties like a name and labels. Classes inheriting from the Metric define their aggregation type as well as a structure of individual measurements or Points. API defines the following types of pre-aggregated metrics:

• Counter metric to report instantaneous measurement. Counter values can go up or stay the same, but can never go down. Counter values cannot be negative. There are two types of counter metric values - double and long.
• Gauge metric to report instantaneous measurement of a numeric value. Gauges can go both up and down. The gauges values can be negative. There are two types of gauge metric values - double and long.

API allows to construct the Metric of a chosen type. SDK defines the way to query the current value of a Metric to be exported.

Every type of a Metric has it's API to record values to be aggregated. API supports both - push and pull model of setting the Metric value.

Metrics data model and SDK

Metrics data model is defined in SDK and is based on metrics.proto. This data model is used by all the OpenTelemetry exporters as an input. Different exporters have different capabilities (e.g. which data types are supported) and different constraints (e.g. which characters are allowed in label keys). Metrics is intended to be a superset of what's possible, not a lowest common denominator that's supported everywhere. All exporters consume data from Metrics Data Model via a Metric Producer interface defined in OpenTelemetry SDK.

Because of this, Metrics puts minimal constraints on the data (e.g. which characters are allowed in keys), and code dealing with Metrics should avoid validation and sanitization of the Metrics data. Instead, pass the data to the backend, rely on the backend to perform validation, and pass back any errors from the backend.

OpenTelemetry defines the naming convention for metric names as well as a well-known metric names in Semantic Conventions document.

DistributedContext

The DistributedContext exists to store labels that describe the context of an operation an application performs. It is intended to enable context that are custom to the application or integrations in contrast to other contexts, such as SpanContext. Only one DistributedContext should be associated with any particular operation.

For example, a web service can benefit from including context around what service has sent the request. Or a SaaS provider can include context about the API user or token that is responsible for that request. These values can be consumed from DistributedContext and used as an additional dimension for a metric, or additional context for logs and traces.

DistributedContext is a collection of key-value Entry pairs, with each key of associated with exactly one value. DistributedContext is serializable, to facilitate propagating it not only inside the process but also across process boundaries.

DistributedContext is a recommended name but languages can have more language-specific names like dctx.

Entry

An Entry is used to represent the labels that are contained inside the DistributedContext, representing values such as the service that originated the request, or vendor-specific data. It consists of an EntryKey, an EntryValue and an EntryMetadata.

• EntryKey is the name of the Entry. EntryKey along with EntryValue can be used to aggregate and group stats, annotate traces and logs, etc. EntryKey is a string that contains only printable ASCII (codes between 32 and 126 inclusive) and with a length greater than zero and less than 256.
• EntryValue is a string that contains only printable ASCII (codes between 32 and 126).
• EntryMetadata contains properties associated with an Entry. For now only the property EntryTTL is defined.
• EntryTTL is an integer that represents number of hops an entry can propagate. Anytime a sender serializes an entry, sends it over the wire and a receiver deserializes the entry then the entry is considered to have travelled one hop.

Resources

Resource captures information about the entity for which telemetry is recorded. For example, metrics exposed by a Kubernetes container can be linked to a resource that specifies the cluster, namespace, pod, and container name.

Resource may capture an entire hierarchy of entity identification. It may describe the host in the cloud and specific container or an application running in the process.

Note, that some of the process identification information can be associated with telemetry automatically by OpenTelemetry SDK or specific exporter. See OpenTelemetry proto for an example.

TODO: Better describe the difference between the resource and a Node open-telemetry/opentelemetry-proto#17

Propagators

OpenTelemetry uses Propagators to serialize and deserialize SpanContext and DistributedContext into a binary or text format. Currently there are two types of propagators:

• BinaryFormat which is used to serialize and deserialize a value into a binary representation.
• HTTPTextFormat which is used to inject and extract a value as text into carriers that travel in-band across process boundaries.

Collector

The OpenTelemetry collector is a set of components that can collect traces, metrics and eventually other telemetry data (e.g. logs) from processes instrumented by OpenTelementry or other monitoring/tracing libraries (Jaeger, Prometheus, etc.), do aggregation and smart sampling, and export traces and metrics to one or more monitoring/tracing backends. The collector will allow to enrich and transform collected telemetry (e.g. add additional attributes or scrub personal information).

The OpenTelemetry collector has two primary modes of operation: Agent (a daemon running locally with the application) and Collector (a standalone running service).

Read more at OpenTelemetry Service Long-term Vision.

Instrumentation adapters

The inspiration of the project is to make every library and application manageable out of the box by instrumenting it with OpenTelemetry. However on the way to this goal there will be a need to enable instrumentation by plugging instrumentation adapters into the library of choice. These adapters can be wrapping library APIs, subscribing to the library-specific callbacks or translating telemetry exposed in other formats into OpenTelemetry model.

Instrumentation adapters may be called different names. It is often referred as plugin, collector or auto-collector, telemetry module, bridge, etc. It is always recommended to follow the library and language standards. For instance, if instrumentation adapter is implemented as "log appender" - it will probably be called an appender, not an instrumentation adapter. However if there is no established name - the recommendation is to call packages "Instrumentation Adapter" or simply "Adapter".

Code injecting adapters

TODO: fill out as a result of SIG discussion.