Documentation

Overview

Package metric implements the OpenTelemetry metric API.

This package is currently in a pre-GA phase. Backwards incompatible changes may be introduced in subsequent minor version releases as we work to track the evolving OpenTelemetry specification and user feedback.

The Accumulator type supports configurable metrics export behavior through a collection of export interfaces that support various export strategies, described below.

The OpenTelemetry metric API consists of methods for constructing synchronous and asynchronous instruments. There are two constructors per instrument for the two kinds of number (int64, float64).

Synchronous instruments are managed by a sync.Map containing a *record with the current state for each synchronous instrument. A bound instrument encapsulates a direct pointer to the record, allowing bound metric events to bypass a sync.Map lookup. A lock-free algorithm is used to protect against races when adding and removing items from the sync.Map.

Asynchronous instruments are managed by an internal AsyncInstrumentState, which coordinates calling batch and single instrument callbacks.

Internal Structure

Each observer also has its own kind of record stored in the SDK. This record contains a set of recorders for every specific label set used in the callback.

A sync.Map maintains the mapping of current instruments and label sets to internal records. To create a new bound instrument, the SDK consults the Map to locate an existing record, otherwise it constructs a new record. The SDK maintains a count of the number of references to each record, ensuring that records are not reclaimed from the Map while they are still active from the user's perspective.

Metric collection is performed via a single-threaded call to Collect that sweeps through all records in the SDK, checkpointing their state. When a record is discovered that has no references and has not been updated since the prior collection pass, it is removed from the Map.

Both synchronous and asynchronous instruments have an associated aggregator, which maintains the current state resulting from all metric events since its last checkpoint. Aggregators may be lock-free or they may use locking, but they should expect to be called concurrently. Aggregators must be capable of merging with another aggregator of the same type.

Export Pipeline

While the SDK serves to maintain a current set of records and coordinate collection, the behavior of a metrics export pipeline is configured through the export types in go.opentelemetry.io/otel/sdk/export/metric. It is important to keep in mind the context these interfaces are called from. There are two contexts, instrumentation context, where a user-level goroutine that enters the SDK resulting in a new record, and collection context, where a system-level thread performs a collection pass through the SDK.

Descriptor is a struct that describes the metric instrument to the export pipeline, containing the name, units, description, metric kind, number kind (int64 or float64). A Descriptor accompanies metric data as it passes through the export pipeline.

The AggregatorSelector interface supports choosing the method of aggregation to apply to a particular instrument, by delegating the construction of an Aggregator to this interface. Given the Descriptor, the AggregatorFor method returns an implementation of Aggregator. If this interface returns nil, the metric will be disabled. The aggregator should be matched to the capabilities of the exporter. Selecting the aggregator for Adding instruments is relatively straightforward, but many options are available for aggregating distributions from Grouping instruments.

Aggregator is an interface which implements a concrete strategy for aggregating metric updates. Several Aggregator implementations are provided by the SDK. Aggregators may be lock-free or use locking, depending on their structure and semantics. Aggregators implement an Update method, called in instrumentation context, to receive a single metric event. Aggregators implement a Checkpoint method, called in collection context, to save a checkpoint of the current state. Aggregators implement a Merge method, also called in collection context, that combines state from two aggregators into one. Each SDK record has an associated aggregator.

Processor is an interface which sits between the SDK and an exporter. The Processor embeds an AggregatorSelector, used by the SDK to assign new Aggregators. The Processor supports a Process() API for submitting checkpointed aggregators to the processor, and a CheckpointSet() API for producing a complete checkpoint for the exporter. Two default Processor implementations are provided, the "defaultkeys" Processor groups aggregate metrics by their recommended Descriptor.Keys(), the "simple" Processor aggregates metrics at full dimensionality.

LabelEncoder is an optional optimization that allows an exporter to provide the serialization logic for labels. This allows avoiding duplicate serialization of labels, once as a unique key in the SDK (or Processor) and once in the exporter.

CheckpointSet is an interface between the Processor and the Exporter. After completing a collection pass, the Processor.CheckpointSet() method returns a CheckpointSet, which the Exporter uses to iterate over all the updated metrics.

Record is a struct containing the state of an individual exported metric. This is the result of one collection interface for one instrument and one label set.

Labels is a struct containing an ordered set of labels, the corresponding unique encoding, and the encoder that produced it.

Exporter is the final stage of an export pipeline. It is called with a CheckpointSet capable of enumerating all the updated metrics.

Controller is not an export interface per se, but it orchestrates the export pipeline. For example, a "push" controller will establish a periodic timer to regularly collect and export metrics. A "pull" controller will await a pull request before initiating metric collection. Either way, the job of the controller is to call the SDK Collect() method, then read the checkpoint, then invoke the exporter. Controllers are expected to implement the public metric.MeterProvider API, meaning they can be installed as the global Meter provider.

Index

Constants

This section is empty.

Variables

var (
	ErrUninitializedInstrument = fmt.Errorf("use of an uninitialized instrument")
)

Functions

func AtomicFieldOffsets

func AtomicFieldOffsets() map[string]uintptr

Types

type Accumulator

type Accumulator struct {
	// contains filtered or unexported fields
}

Accumulator implements the OpenTelemetry Meter API. The Accumulator is bound to a single export.Processor in `NewAccumulator()`.

The Accumulator supports a Collect() API to gather and export current data. Collect() should be arranged according to the processor model. Push-based processors will setup a timer to call Collect() periodically. Pull-based processors will call Collect() when a pull request arrives.

func NewAccumulator

func NewAccumulator(processor export.Processor, resource *resource.Resource) *Accumulator

NewAccumulator constructs a new Accumulator for the given processor. This Accumulator supports only a single processor.

The Accumulator does not start any background process to collect itself periodically, this responsbility lies with the processor, typically, depending on the type of export. For example, a pull-based processor will call Collect() when it receives a request to scrape current metric values. A push-based processor should configure its own periodic collection.

func (*Accumulator) Collect

func (m *Accumulator) Collect(ctx context.Context) int

Collect traverses the list of active records and observers and exports data for each active instrument. Collect() may not be called concurrently.

During the collection pass, the export.Processor will receive one Export() call per current aggregation.

Returns the number of records that were checkpointed.

func (*Accumulator) CollectAsync

func (m *Accumulator) CollectAsync(kv []label.KeyValue, obs ...metric.Observation)

CollectAsync implements internal.AsyncCollector.

func (*Accumulator) NewAsyncInstrument

func (m *Accumulator) NewAsyncInstrument(descriptor metric.Descriptor, runner metric.AsyncRunner) (metric.AsyncImpl, error)

NewAsyncInstrument implements metric.MetricImpl.

func (*Accumulator) NewSyncInstrument

func (m *Accumulator) NewSyncInstrument(descriptor metric.Descriptor) (metric.SyncImpl, error)

NewSyncInstrument implements metric.MetricImpl.

func (*Accumulator) RecordBatch

func (m *Accumulator) RecordBatch(ctx context.Context, kvs []label.KeyValue, measurements ...metric.Measurement)

RecordBatch enters a batch of metric events.