# Contributing to opentelemetry-go

The Go special interest group (SIG) meets regularly. See the
OpenTelemetry
[community](https://github.com/open-telemetry/community#golang-sdk)
repo for information on this and other language SIGs.

See the [public meeting
notes](https://docs.google.com/document/d/1E5e7Ld0NuU1iVvf-42tOBpu2VBBLYnh73GJuITGJTTU/edit)
for a summary description of past meetings. To request edit access,
join the meeting or get in touch on
[Slack](https://cloud-native.slack.com/archives/C01NPAXACKT).

## Development

You can view and edit the source code by cloning this repository:

```sh
git clone https://github.com/open-telemetry/opentelemetry-go.git
```

Run `make test` to run the tests instead of `go test`.

There are some generated files checked into the repo. To make sure
that the generated files are up-to-date, run `make` (or `make
precommit` - the `precommit` target is the default).

The `precommit` target also fixes the formatting of the code and
checks the status of the go module files.

Additionally, there is a `codespell` target that checks for common
typos in the code. It is not run by default, but you can run it
manually with `make codespell`. It will set up a virtual environment
in `venv` and install `codespell` there.

If after running `make precommit` the output of `git status` contains
`nothing to commit, working tree clean` then it means that everything
is up-to-date and properly formatted.

## Pull Requests

### How to Send Pull Requests

Everyone is welcome to contribute code to `opentelemetry-go` via
GitHub pull requests (PRs).

To create a new PR, fork the project in GitHub and clone the upstream
repo:

```sh
go get -d go.opentelemetry.io/otel
```

(This may print some warning about "build constraints exclude all Go
files", just ignore it.)

This will put the project in `${GOPATH}/src/go.opentelemetry.io/otel`.
Alternatively, you can use `git` directly with:

```sh
git clone https://github.com/open-telemetry/opentelemetry-go
```

(Note that `git clone` is *not* using the `go.opentelemetry.io/otel` name -
that name is a kind of a redirector to GitHub that `go get` can
understand, but `git` does not.)

This will add the project as `opentelemetry-go` within the current directory.

Enter the newly created directory and add your fork as a new remote:

```sh
git remote add <YOUR_FORK> git@github.com:<YOUR_GITHUB_USERNAME>/opentelemetry-go
```

Check out a new branch, make modifications, run linters and tests, update
`CHANGELOG.md`, and push the branch to your fork:

```sh
git checkout -b <YOUR_BRANCH_NAME>
# edit files
# update changelog
make precommit
git add -p
git commit
git push <YOUR_FORK> <YOUR_BRANCH_NAME>
```

Open a pull request against the main `opentelemetry-go` repo. Be sure to add the pull
request ID to the entry you added to `CHANGELOG.md`.

Avoid rebasing and force-pushing to your branch to facilitate reviewing the pull request.
Rewriting Git history makes it difficult to keep track of iterations during code review.
All pull requests are squashed to a single commit upon merge to `main`.

### How to Receive Comments

* If the PR is not ready for review, please put `[WIP]` in the title,
  tag it as `work-in-progress`, or mark it as
  [`draft`](https://github.blog/2019-02-14-introducing-draft-pull-requests/).
* Make sure CLA is signed and CI is clear.

### How to Get PRs Merged

A PR is considered **ready to merge** when:

* It has received two qualified approvals[^1].

  This is not enforced through automation, but needs to be validated by the
  maintainer merging.
  * At least one of the qualified approvals needs to be from an
    [Approver]/[Maintainer] affiliated with a different company than the author
    of the PR.
  * PRs introducing changes that have already been discussed and consensus
    reached only need one qualified approval. The discussion and resolution
    needs to be linked to the PR.
  * Trivial changes[^2] only need one qualified approval.

* All feedback has been addressed.
  * All PR comments and suggestions are resolved.
  * All GitHub Pull Request reviews with a status of "Request changes" have
    been addressed. Another review by the objecting reviewer with a different
    status can be submitted to clear the original review, or the review can be
    dismissed by a [Maintainer] when the issues from the original review have
    been addressed.
  * Any comments or reviews that cannot be resolved between the PR author and
    reviewers can be submitted to the community [Approver]s and [Maintainer]s
    during the weekly SIG meeting. If consensus is reached among the
    [Approver]s and [Maintainer]s during the SIG meeting the objections to the
    PR may be dismissed or resolved or the PR closed by a [Maintainer].
  * Any substantive changes to the PR require existing Approval reviews be
    cleared unless the approver explicitly states that their approval persists
    across changes. This includes changes resulting from other feedback.
    [Approver]s and [Maintainer]s can help in clearing reviews and they should
    be consulted if there are any questions.

* The PR branch is up to date with the base branch it is merging into.
  * To ensure this does not block the PR, it should be configured to allow
    maintainers to update it.

* It has been open for review for at least one working day. This gives people
  reasonable time to review.
  * Trivial changes[^2] do not have to wait for one day and may be merged with
    a single [Maintainer]'s approval.

* All required GitHub workflows have succeeded.
* Urgent fix can take exception as long as it has been actively communicated
  among [Maintainer]s.

Any [Maintainer] can merge the PR once the above criteria have been met.

[^1]: A qualified approval is a GitHub Pull Request review with "Approve"
  status from an OpenTelemetry Go [Approver] or [Maintainer].
[^2]: Trivial changes include: typo corrections, cosmetic non-substantive
  changes, documentation corrections or updates, dependency updates, etc.

## Design Choices

As with other OpenTelemetry clients, opentelemetry-go follows the
[OpenTelemetry Specification](https://opentelemetry.io/docs/specs/otel).

It's especially valuable to read through the [library
guidelines](https://opentelemetry.io/docs/specs/otel/library-guidelines).

### Focus on Capabilities, Not Structure Compliance

OpenTelemetry is an evolving specification, one where the desires and
use cases are clear, but the methods to satisfy those use cases are
not.

As such, Contributions should provide functionality and behavior that
conforms to the specification, but the interface and structure are
flexible.

It is preferable to have contributions follow the idioms of the
language rather than conform to specific API names or argument
patterns in the spec.

For a deeper discussion, see
[this](https://github.com/open-telemetry/opentelemetry-specification/issues/165).

## Tests

Each functionality should be covered by tests.

Performance-critical functionality should also be covered by benchmarks.

- Pull requests adding a performance-critical functionality
should have `go test -bench` output in their description.
- Pull requests changing a performance-critical functionality
should have [`benchstat`](https://pkg.go.dev/golang.org/x/perf/cmd/benchstat)
output in their description.

## Dependencies

This project uses [Go Modules] for dependency management. All modules will use
`go.mod` to explicitly list all direct and indirect dependencies, ensuring a
clear dependency graph. The `go.sum` file for each module will be committed to
the repository and used to verify the integrity of downloaded modules,
preventing malicious tampering.

This project uses automated dependency update tools (i.e. dependabot,
renovatebot) to manage updates to dependencies. This ensures that dependencies
are kept up-to-date with the latest security patches and features and are
reviewed before being merged. If you would like to propose a change to a
dependency it should be done through a pull request that updates the `go.mod`
file and includes a description of the change.

See the [versioning and compatibility](./VERSIONING.md) policy for more details
about dependency compatibility.

[Go Modules]: https://pkg.go.dev/cmd/go#hdr-Modules__module_versions__and_more

### Environment Dependencies

This project does not partition dependencies based on the environment (i.e.
`development`, `staging`, `production`).

Only the dependencies explicitly included in the released modules have been
tested and verified to work with the released code. No other guarantee is made
about the compatibility of other dependencies.

## Documentation

Each (non-internal, non-test) package must be documented using
[Go Doc Comments](https://go.dev/doc/comment),
preferably in a `doc.go` file.

Prefer using [Examples](https://pkg.go.dev/testing#hdr-Examples)
instead of putting code snippets in Go doc comments.
In some cases, you can even create [Testable Examples](https://go.dev/blog/examples).

You can install and run a "local Go Doc site" in the following way:

  ```sh
  go install golang.org/x/pkgsite/cmd/pkgsite@latest
  pkgsite
  ```

[`go.opentelemetry.io/otel/metric`](https://pkg.go.dev/go.opentelemetry.io/otel/metric)
is an example of a very well-documented package.

### README files

Each (non-internal, non-test, non-documentation) package must contain a
`README.md` file containing at least a title, and a `pkg.go.dev` badge.

The README should not be a repetition of Go doc comments.

You can verify the presence of all README files with the `make verify-readmes`
command.

## Style Guide

One of the primary goals of this project is that it is actually used by
developers. With this goal in mind the project strives to build
user-friendly and idiomatic Go code adhering to the Go community's best
practices.

For a non-comprehensive but foundational overview of these best practices
the [Effective Go](https://golang.org/doc/effective_go.html) documentation
is an excellent starting place.

We also recommend following the
[Go Code Review Comments](https://go.dev/wiki/CodeReviewComments)
that collects common comments made during reviews of Go code.

As a convenience for developers building this project the `make precommit`
will format, lint, validate, and in some cases fix the changes you plan to
submit. This check will need to pass for your changes to be able to be
merged.

In addition to idiomatic Go, the project has adopted certain standards for
implementations of common patterns. These standards should be followed as a
default, and if they are not followed documentation needs to be included as
to the reasons why.

### Configuration

When creating an instantiation function for a complex `type T struct`, it is
useful to allow variable number of options to be applied. However, the strong
type system of Go restricts the function design options. There are a few ways
to solve this problem, but we have landed on the following design.

#### `config`

Configuration should be held in a `struct` named `config`, or prefixed with
specific type name this Configuration applies to if there are multiple
`config` in the package. This type must contain configuration options.

```go
// config contains configuration options for a thing.
type config struct {
	// options ...
}
```

In general the `config` type will not need to be used externally to the
package and should be unexported. If, however, it is expected that the user
will likely want to build custom options for the configuration, the `config`
should be exported. Please, include in the documentation for the `config`
how the user can extend the configuration.

It is important that internal `config` are not shared across package boundaries.
Meaning a `config` from one package should not be directly used by another. The
one exception is the API packages.  The configs from the base API, eg.
`go.opentelemetry.io/otel/trace.TracerConfig` and
`go.opentelemetry.io/otel/metric.InstrumentConfig`, are intended to be consumed
by the SDK therefore it is expected that these are exported.

When a config is exported we want to maintain forward and backward
compatibility, to achieve this no fields should be exported but should
instead be accessed by methods.

Optionally, it is common to include a `newConfig` function (with the same
naming scheme). This function wraps any defaults setting and looping over
all options to create a configured `config`.

```go
// newConfig returns an appropriately configured config.
func newConfig(options ...Option) config {
	// Set default values for config.
	config := config{/* […] */}
	for _, option := range options {
		config = option.apply(config)
	}
	// Perform any validation here.
	return config
}
```

If validation of the `config` options is also performed this can return an
error as well that is expected to be handled by the instantiation function
or propagated to the user.

Given the design goal of not having the user need to work with the `config`,
the `newConfig` function should also be unexported.

#### `Option`

To set the value of the options a `config` contains, a corresponding
`Option` interface type should be used.

```go
type Option interface {
	apply(config) config
}
```

Having `apply` unexported makes sure that it will not be used externally.
Moreover, the interface becomes sealed so the user cannot easily implement
the interface on its own.

The `apply` method should return a modified version of the passed config.
This approach, instead of passing a pointer, is used to prevent the config from being allocated to the heap.

The name of the interface should be prefixed in the same way the
corresponding `config` is (if at all).

#### Options

All user configurable options for a `config` must have a related unexported
implementation of the `Option` interface and an exported configuration
function that wraps this implementation.

The wrapping function name should be prefixed with `With*` (or in the
special case of a boolean options `Without*`) and should have the following
function signature.

```go
func With*(…) Option { … }
```

##### `bool` Options

```go
type defaultFalseOption bool

func (o defaultFalseOption) apply(c config) config {
	c.Bool = bool(o)
    return c
}

// WithOption sets a T to have an option included.
func WithOption() Option {
	return defaultFalseOption(true)
}
```

```go
type defaultTrueOption bool

func (o defaultTrueOption) apply(c config) config {
	c.Bool = bool(o)
    return c
}

// WithoutOption sets a T to have Bool option excluded.
func WithoutOption() Option {
	return defaultTrueOption(false)
}
```

##### Declared Type Options

```go
type myTypeOption struct {
	MyType MyType
}

func (o myTypeOption) apply(c config) config {
	c.MyType = o.MyType
    return c
}

// WithMyType sets T to have include MyType.
func WithMyType(t MyType) Option {
	return myTypeOption{t}
}
```

##### Functional Options

```go
type optionFunc func(config) config

func (fn optionFunc) apply(c config) config {
	return fn(c)
}

// WithMyType sets t as MyType.
func WithMyType(t MyType) Option {
	return optionFunc(func(c config) config {
		c.MyType = t
        return c
	})
}
```

#### Instantiation

Using this configuration pattern to configure instantiation with a `NewT`
function.

```go
func NewT(options ...Option) T {…}
```

Any required parameters can be declared before the variadic `options`.

#### Dealing with Overlap

Sometimes there are multiple complex `struct` that share common
configuration and also have distinct configuration. To avoid repeated
portions of `config`s, a common `config` can be used with the union of
options being handled with the `Option` interface.

For example.

```go
// config holds options for all animals.
type config struct {
	Weight      float64
	Color       string
	MaxAltitude float64
}

// DogOption apply Dog specific options.
type DogOption interface {
	applyDog(config) config
}

// BirdOption apply Bird specific options.
type BirdOption interface {
	applyBird(config) config
}

// Option apply options for all animals.
type Option interface {
	BirdOption
	DogOption
}

type weightOption float64

func (o weightOption) applyDog(c config) config {
	c.Weight = float64(o)
	return c
}

func (o weightOption) applyBird(c config) config {
	c.Weight = float64(o)
	return c
}

func WithWeight(w float64) Option { return weightOption(w) }

type furColorOption string

func (o furColorOption) applyDog(c config) config {
	c.Color = string(o)
	return c
}

func WithFurColor(c string) DogOption { return furColorOption(c) }

type maxAltitudeOption float64

func (o maxAltitudeOption) applyBird(c config) config {
	c.MaxAltitude = float64(o)
	return c
}

func WithMaxAltitude(a float64) BirdOption { return maxAltitudeOption(a) }

func NewDog(name string, o ...DogOption) Dog    {…}
func NewBird(name string, o ...BirdOption) Bird {…}
```

### Interfaces

To allow other developers to better comprehend the code, it is important
to ensure it is sufficiently documented. One simple measure that contributes
to this aim is self-documenting by naming method parameters. Therefore,
where appropriate, methods of every exported interface type should have
their parameters appropriately named.

#### Interface Stability

All exported stable interfaces that include the following warning in their
documentation are allowed to be extended with additional methods.

> Warning: methods may be added to this interface in minor releases.

These interfaces are defined by the OpenTelemetry specification and will be
updated as the specification evolves.

Otherwise, stable interfaces MUST NOT be modified.

#### How to Change Specification Interfaces

When an API change must be made, we will update the SDK with the new method one
release before the API change. This will allow the SDK one version before the
API change to work seamlessly with the new API.

If an incompatible version of the SDK is used with the new API the application
will fail to compile.

#### How Not to Change Specification Interfaces

We have explored using a v2 of the API to change interfaces and found that there
was no way to introduce a v2 and have it work seamlessly with the v1 of the API.
Problems happened with libraries that upgraded to v2 when an application did not,
and would not produce any telemetry.

More detail of the approaches considered and their limitations can be found in
the [Use a V2 API to evolve interfaces](https://github.com/open-telemetry/opentelemetry-go/issues/3920)
issue.

#### How to Change Other Interfaces

If new functionality is needed for an interface that cannot be changed it MUST
be added by including an additional interface. That added interface can be a
simple interface for the specific functionality that you want to add or it can
be a super-set of the original interface. For example, if you wanted to a
`Close` method to the `Exporter` interface:

```go
type Exporter interface {
	Export()
}
```

A new interface, `Closer`, can be added:

```go
type Closer interface {
	Close()
}
```

Code that is passed the `Exporter` interface can now check to see if the passed
value also satisfies the new interface. E.g.

```go
func caller(e Exporter) {
	/* ... */
	if c, ok := e.(Closer); ok {
		c.Close()
	}
	/* ... */
}
```

Alternatively, a new type that is the super-set of an `Exporter` can be created.

```go
type ClosingExporter struct {
	Exporter
	Close()
}
```

This new type can be used similar to the simple interface above in that a
passed `Exporter` type can be asserted to satisfy the `ClosingExporter` type
and the `Close` method called.

This super-set approach can be useful if there is explicit behavior that needs
to be coupled with the original type and passed as a unified type to a new
function, but, because of this coupling, it also limits the applicability of
the added functionality. If there exist other interfaces where this
functionality should be added, each one will need their own super-set
interfaces and will duplicate the pattern. For this reason, the simple targeted
interface that defines the specific functionality should be preferred.

See also:
[Keeping Your Modules Compatible: Working with interfaces](https://go.dev/blog/module-compatibility#working-with-interfaces).

### Testing

We allow using [`testify`](https://github.com/stretchr/testify) even though
it is seen as non-idiomatic according to
the [Go Test Comments](https://go.dev/wiki/TestComments#assert-libraries) page.

The tests should never leak goroutines.

Use the term `ConcurrentSafe` in the test name when it aims to verify the
absence of race conditions. The top-level tests with this term will be run
many times in the `test-concurrent-safe` CI job to increase the chance of
catching concurrency issues. This does not apply to subtests when this term
is not in their root name.

### Internal packages

The use of internal packages should be scoped to a single module. A sub-module
should never import from a parent internal package. This creates a coupling
between the two modules where a user can upgrade the parent without the child,
and if the internal package API has changed, it will fail to upgrade[^3].

There are two known exceptions to this rule:

- `go.opentelemetry.io/otel/internal/global`
  - This package manages global state for all of opentelemetry-go. It needs to
  be a single package in order to ensure the uniqueness of the global state.
- `go.opentelemetry.io/otel/internal/baggage`
  - This package provides values in a `context.Context` that need to be
  recognized by `go.opentelemetry.io/otel/baggage` and
  `go.opentelemetry.io/otel/bridge/opentracing` but remain private.

If you have duplicate code in multiple modules, make that code into a Go
template stored in `go.opentelemetry.io/otel/internal/shared` and use [gotmpl]
to render the templates in the desired locations. See [#4404] for an example of
this.

[^3]: https://github.com/open-telemetry/opentelemetry-go/issues/3548

### Ignoring context cancellation

OpenTelemetry API implementations need to ignore the cancellation of the context that is
passed when recording a value (e.g. starting a span, recording a measurement, emitting a log).
Recording methods should not return an error describing the cancellation state of the context
when they complete, nor should they abort any work.

This rule may not apply if the OpenTelemetry specification defines a timeout mechanism for
the method. In that case the context cancellation can be used for the timeout with the
restriction that this behavior is documented for the method. Otherwise, timeouts
are expected to be handled by the user calling the API, not the implementation.

Stoppage of the telemetry pipeline is handled by calling the appropriate `Shutdown` method
of a provider. It is assumed the context passed from a user is not used for this purpose.

Outside of the direct recording of telemetry from the API (e.g. exporting telemetry,
force flushing telemetry, shutting down a signal provider) the context cancellation
should be honored. This means all work done on behalf of the user provided context
should be canceled.

### Observability

OpenTelemetry Go SDK components should be instrumented to enable users observability for the health and performance of the telemetry pipeline itself.
This allows operators to understand how well their observability infrastructure is functioning and to identify potential issues before they impact their applications.

This section outlines the best practices for building instrumentation in OpenTelemetry Go SDK components.

#### Environment Variable Activation

Observability features are currently experimental.
They should be disabled by default and activated through the `OTEL_GO_X_OBSERVABILITY` environment variable.
This follows the established experimental feature pattern used throughout the SDK.

Components should check for this environment variable using a consistent pattern:

```go
import "go.opentelemetry.io/otel/*/internal/x"

if x.Observability.Enabled() {
    // Initialize observability metrics
}
```

**References**:

- [stdouttrace exporter](./exporters/stdout/stdouttrace/internal/x/x.go)
- [sdk](./sdk/internal/x/x.go)

#### Encapsulation

Instrumentation should be encapsulated within a dedicated `struct` (e.g. `instrumentation`).
It should not be mixed into the instrumented component.

Prefer this:

```go
type SDKComponent struct {
    inst *instrumentation
}

type instrumentation struct {
	inflight otelconv.SDKComponentInflight
	exported otelconv.SDKComponentExported
}
```

To this:

```go
// ❌ Avoid this pattern.
type SDKComponent struct {
	/* other SDKComponent fields... */

	inflight otelconv.SDKComponentInflight
	exported otelconv.SDKComponentExported
}
```

The instrumentation code should not bloat the code being instrumented.
Likely, this means its own file, or its own package if it is complex or reused.

#### Initialization

Instrumentation setup should be explicit, side-effect free, and local to the relevant component.
Avoid relying on global or implicit [side effects][side-effect] for initialization.

Encapsulate setup in constructor functions, ensuring clear ownership and scope:

```go
import (
	"errors"

	semconv "go.opentelemetry.io/otel/semconv/v1.40.0"
	"go.opentelemetry.io/otel/semconv/v1.40.0/otelconv"
)

type SDKComponent struct {
    inst *instrumentation
}

func NewSDKComponent(config Config) (*SDKComponent, error) {
    inst, err := newInstrumentation()
    if err != nil {
        return nil, err
    }
    return &SDKComponent{inst: inst}, nil
}

type instrumentation struct {
    inflight otelconv.SDKComponentInflight
    exported otelconv.SDKComponentExported
}

func newInstrumentation() (*instrumentation, error) {
    if !x.Observability.Enabled() {
        return nil, nil
    }
 
    meter := otel.GetMeterProvider().Meter(
        "<component-package-name>",
        metric.WithInstrumentationVersion(sdk.Version()),
        metric.WithSchemaURL(semconv.SchemaURL),
    )

	inst := &instrumentation{}

	var err, e error
    inst.inflight, e = otelconv.NewSDKComponentInflight(meter)
	err = errors.Join(err, e)

    inst.exported, e = otelconv.NewSDKComponentExported(meter)
	err = errors.Join(err, e)

    return inst, err
}
```

```go
// ❌ Avoid this pattern.
func (c *Component) initObservability() {
	// Initialize observability metrics
	if !x.Observability.Enabled() {
		return
	}

	// Initialize observability metrics
	c.inst = &instrumentation{/* ... */}
}
```

[side-effect]: https://en.wikipedia.org/wiki/Side_effect_(computer_science)

#### Performance

When observability is disabled or the instrument is not `Enabled`, there should be little to no overhead.

```go
func (e *Exporter) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan) error {
	if e.inst != nil && e.inst.Enabled(ctx) {
		attrs := expensiveOperation()
		e.inst.recordSpanInflight(ctx, int64(len(spans)), attrs...)
	}
    // Export spans...
}
```

```go
// ❌ Avoid this pattern.
func (e *Exporter) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan) error {
	attrs := expensiveOperation()
	e.inst.recordSpanInflight(ctx, int64(len(spans)), attrs...)
    // Export spans...
}

func (i *instrumentation) recordSpanInflight(ctx context.Context, count int64, attrs ...attribute.KeyValue) {
	if i == nil || i.inflight == nil || !i.inflight.Enabled(ctx) {
		return
	}
	i.inflight.Add(ctx, count, metric.WithAttributes(attrs...))
}
```

When observability is enabled, the instrumentation code paths should be optimized to reduce allocation and computation overhead.

##### Attribute and Option Allocation Management

Pool attribute slices and options with [`sync.Pool`] to minimize allocations in measurement calls with dynamic attributes.

```go
var (
	attrPool = sync.Pool{
        New: func() any {
		    // Pre-allocate common capacity
    		knownCap := 8 // Adjust based on expected usage
            s := make([]attribute.KeyValue, 0, knownCap)
    		// Return a pointer to avoid extra allocation on Put().
            return &s
        },
    }

	addOptPool = &sync.Pool{
		New: func() any {
			const n = 1 // WithAttributeSet
			o := make([]metric.AddOption, 0, n)
    		// Return a pointer to avoid extra allocation on Put().
			return &o
		},
	}
)

func (i *instrumentation) record(ctx context.Context, value int64, baseAttrs ...attribute.KeyValue) {
    if !i.counter.Enabled(ctx) {
        return
    }
    attrs := attrPool.Get().(*[]attribute.KeyValue)
    defer func() {
        *attrs = (*attrs)[:0] // Reset.
        attrPool.Put(attrs)
    }()

    *attrs = append(*attrs, baseAttrs...)
    // Add any dynamic attributes.
    *attrs = append(*attrs, semconv.OTelComponentName("exporter-1"))

	addOpt := addOptPool.Get().(*[]metric.AddOption)
	defer func() {
		*addOpt = (*addOpt)[:0]
		addOptPool.Put(addOpt)
	}()

	set := attribute.NewSet(*attrs...)
	*addOpt = append(*addOpt, metric.WithAttributeSet(set))

    i.counter.Add(ctx, value, *addOpt...)
}
```

Pools are most effective when there are many pooled objects of the same sufficiently large size, and the objects are repeatedly used.
This amortizes the cost of allocation and synchronization.
Ideally, the pools should be scoped to be used as widely as possible within the component to maximize this efficiency while still ensuring correctness.

[`sync.Pool`]: https://pkg.go.dev/sync#Pool

##### Cache common attribute sets for repeated measurements

If a static set of attributes are used for measurements and they are known at compile time, pre-compute and cache these attributes.

```go
type spanLiveSetKey struct {
	sampled bool
}

var spanLiveSetCache = map[spanLiveSetKey]attribute.Set{
	{true}: attribute.NewSet(
		otelconv.SDKSpanLive{}.AttrSpanSamplingResult(
			otelconv.SpanSamplingResultRecordAndSample,
		),
	),
	{false}: attribute.NewSet(
		otelconv.SDKSpanLive{}.AttrSpanSamplingResult(
			otelconv.SpanSamplingResultRecordOnly,
		),
	),
}

func spanLiveSet(sampled bool) attribute.Set {
	key := spanLiveSetKey{sampled: sampled}
	return spanLiveSetCache[key]
}
```

##### Benchmarking

Always provide benchmarks when introducing or refactoring instrumentation.
Demonstrate the impact (allocs/op, B/op, ns/op) in enabled/disabled scenarios:

```go
func BenchmarkExportSpans(b *testing.B) {
    scenarios := []struct {
        name           string
        obsEnabled bool
    }{
        {"ObsDisabled", false},
        {"ObsEnabled", true},
    }
 
    for _, scenario := range scenarios {
        b.Run(scenario.name, func(b *testing.B) {
            b.Setenv(
				"OTEL_GO_X_OBSERVABILITY",
				strconv.FormatBool(scenario.obsEnabled),
			)

            exporter := NewExporter()
            spans := generateTestSpans(100)

            b.ResetTimer()
            b.ReportAllocs()

            for i := 0; i < b.N; i++ {
                _ = exporter.ExportSpans(context.Background(), spans)
            }
        })
    }
}
```

#### Error Handling and Robustness

Errors should be reported back to the caller if possible, and partial failures should be handled as gracefully as possible.

```go
func newInstrumentation() (*instrumentation, error) {
    if !x.Observability.Enabled() {
        return nil, nil
    }
 
    m := otel.GetMeterProvider().Meter(/* initialize meter */)
    counter, err := otelconv.NewSDKComponentCounter(m)
	// Use the partially initialized counter if available.
	i := &instrumentation{counter: counter}
	// Return any error to the caller.
    return i, err
}
```

```go
// ❌ Avoid this pattern.
func newInstrumentation() *instrumentation {
    if !x.Observability.Enabled() {
        return nil, nil
    }
 
    m := otel.GetMeterProvider().Meter(/* initialize meter */)
    counter, err := otelconv.NewSDKComponentCounter(m)
	if err != nil {
		// ❌ Do not dump the error to the OTel Handler. Return it to the
		// caller.
		otel.Handle(err)
		// ❌ Do not return nil if we can still use the partially initialized
		// counter.
		return nil
	}
    return &instrumentation{counter: counter}
}
```

If the instrumented component cannot report the error to the user, let it report the error to `otel.Handle`.

#### Context Propagation

Ensure observability measurements receive the correct context, especially for trace exemplars and distributed context:

```go
func (e *Exporter) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan) error {
    // Use the provided context for observability measurements
    if e.inst.Enabled(ctx) {
        e.inst.recordSpanExportStarted(ctx, len(spans))
    }
 
    err := e.doExport(ctx, spans)

    if e.inst.Enabled(ctx) {
        if err != nil {
            e.inst.recordSpanExportFailed(ctx, len(spans), err)
        } else {
            e.inst.recordSpanExportSucceeded(ctx, len(spans))
        }
    }

    return err
}
```

```go
// ❌ Avoid this pattern.
func (e *Exporter) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan) error {
    // ❌ Do not break the context propagation.
    e.inst.recordSpanExportStarted(context.Background(), len(spans))
 
    err := e.doExport(ctx, spans)

	/* ... */
 
    return err
}
```

#### Semantic Conventions Compliance

All observability metrics should follow the [OpenTelemetry Semantic Conventions for SDK metrics](https://github.com/open-telemetry/semantic-conventions/blob/1cf2476ae5e518225a766990a28a6d5602bd5a30/docs/otel/sdk-metrics.md).

Use the metric semantic conventions convenience package [otelconv](./semconv/v1.40.0/otelconv/metric.go).

##### Component Identification

Component names and types should follow [semantic convention](https://github.com/open-telemetry/semantic-conventions/blob/1cf2476ae5e518225a766990a28a6d5602bd5a30/docs/registry/attributes/otel.md#otel-component-attributes).

If a component is not a well-known type specified in the semantic conventions, use the package path scope type as a stable identifier.

```go
componentType := "go.opentelemetry.io/otel/sdk/trace.Span"
```

```go
// ❌ Do not do this.
componentType := "trace-span"
```

The component name should be a stable unique identifier for the specific instance of the component.

Use a global counter to ensure uniqueness if necessary.

```go
// Unique 0-based ID counter for component instances.
var componentIDCounter atomic.Int64

// nextID returns the next unique ID for a component.
func nextID() int64 {
	return componentIDCounter.Add(1) - 1
}

// componentName returns a unique name for the component instance.
func componentName() attribute.KeyValue {
	id := nextID()
	name := fmt.Sprintf("%s/%d", componentType, id)
	return semconv.OTelComponentName(name)
}
```

The component ID will need to be resettable for deterministic testing.
If tests are in a different package than the component being tested (i.e. a `<component package>_test` package name), use a generated `counter` internal package to manage the counter.
See [stdouttrace exporter example](./exporters/stdout/stdouttrace/internal/gen.go) for reference.

#### Testing

Use deterministic testing with isolated state:

```go
func TestObservability(t *testing.T) {
	// Restore state after test to ensure this does not affect other tests.
    prev := otel.GetMeterProvider()
    t.Cleanup(func() { otel.SetMeterProvider(prev) })

    // Isolate the meter provider for deterministic testing
    reader := metric.NewManualReader()
    meterProvider := metric.NewMeterProvider(metric.WithReader(reader))
    otel.SetMeterProvider(meterProvider)

	// Use t.Setenv to ensure environment variable is restored after test.
    t.Setenv("OTEL_GO_X_OBSERVABILITY", "true")

	// Reset component ID counter to ensure deterministic component names.
	componentIDCounter.Store(0)
 
	/* ... test code ... */
}
```

Test order should not affect results.
Ensure that any global state (e.g. component ID counters) is reset between tests.

### Experimental Features

To support the development of new features in the specification, we use the following patterns to implement in-development features without adding new public artifacts in stable modules.

#### Experimental behavior with no API artifacts

Features that change behavior without changing the API (e.g., exemplar collection, auto-generation of identifiers) are implemented behind a feature gate.
The implementation resides in an `/internal/x` package and is activated through environment variables with the `OTEL_GO_X_` prefix (e.g., `OTEL_GO_X_OBSERVABILITY`).
The feature must be documented in a `README.md` file in the `/internal/x` package.

#### Experimental methods on SDK-only interfaces

Features that require new methods on SDK interfaces are defined as a new interface in an experimental module (e.g., `go.opentelemetry.io/otel/sdk/x`).
The SDK uses type assertions (without importing the unstable package) to check if passing types implement these experimental interfaces.
The SDK must not depend on the experimental module.

#### Experimental structs, functions, or interfaces

Features that don't need any changes to the existing stable package are implemented in an experimental module (e.g., `go.opentelemetry.io/otel/sdk/x`).

#### Experimental signals and components

New telemetry signals (e.g., Logs before stabilization) and components (e.g. bridges) are hosted in new, unstable modules (e.g., `go.opentelemetry.io/otel/log` before 1.0.0).
The package should have the final name it will use once stabilized (i.e. not `/x`), and is released at a v0.x.y version to indicate it is not stable.
Most new components are hosted in [opentelemetry-go-contrib](https://github.com/open-telemetry/opentelemetry-go-contrib).

#### Experimental options for API or SDK functions

Experimental Options functions are implemented in an experimental module (e.g., `go.opentelemetry.io/otel/sdk/x`).
The return type of the Option function must embed the option's type (e.g. `metric.InstrumentOption`), and have an `Experimental()` method to prevent the API from panicking when the option is used.
The SDK uses type assertions (without importing the unstable package) to check if passing types implement these experimental interfaces.
The SDK must not depend on the experimental module.

For example:

```go
type myOption struct {
    // Embed the stable option type.
    metric.InstrumentOption
    value string
}

// Experimental prevents the API from panicking when the option is used.
func (o myOption) Experimental() {}

// The SDK can use type assertions to use this function.
func (o myOption) Value() string { return o.value }

func WithMyOption(value string) metric.InstrumentOption {
    return myOption{value: value}
}
```

#### Not Supported

The following kinds of experimental features are **not currently supported** on stable interfaces:

- Experimental methods on API interfaces
- Experimental fields for API or SDK exported structs

In some cases forks or long-lived branches may be used for prototyping these features.

## Approvers and Maintainers

### Maintainers

- [Damien Mathieu](https://github.com/dmathieu), Elastic ([GPG](https://keys.openpgp.org/search?q=5A126B972A81A6CE443E5E1B408B8E44F0873832))
- [David Ashpole](https://github.com/dashpole), Google ([GPG](https://keys.openpgp.org/search?q=C0D1BDDCAAEAE573673085F176327DA4D864DC70))
- [Robert Pająk](https://github.com/pellared), Splunk ([GPG](https://keys.openpgp.org/search?q=CDAD3A60476A3DE599AA5092E5F7C35A4DBE90C2))
- [Sam Xie](https://github.com/XSAM), Splunk ([GPG](https://keys.openpgp.org/search?q=AEA033782371ABB18EE39188B8044925D6FEEBEA))
- [Tyler Yahn](https://github.com/MrAlias), Splunk ([GPG](https://keys.openpgp.org/search?q=0x46B0F3E1A8B1BA5A))

For more information about the maintainer role, see the [community repository](https://github.com/open-telemetry/community/blob/main/guides/contributor/membership.md#maintainer).

### Approvers

- [Flc](https://github.com/flc1125), Independent

For more information about the approver role, see the [community repository](https://github.com/open-telemetry/community/blob/main/guides/contributor/membership.md#approver).

### Triagers

- [Alex Kats](https://github.com/akats7), Capital One

For more information about the triager role, see the [community repository](https://github.com/open-telemetry/community/blob/main/guides/contributor/membership.md#triager).

### Emeritus

- [Aaron Clawson](https://github.com/MadVikingGod)
- [Anthony Mirabella](https://github.com/Aneurysm9)
- [Cheng-Zhen Yang](https://github.com/scorpionknifes)
- [Chester Cheung](https://github.com/hanyuancheung)
- [Evan Torrie](https://github.com/evantorrie)
- [Gustavo Silva Paiva](https://github.com/paivagustavo)
- [Josh MacDonald](https://github.com/jmacd)
- [Liz Fong-Jones](https://github.com/lizthegrey)

For more information about the emeritus role, see the [community repository](https://github.com/open-telemetry/community/blob/main/guides/contributor/membership.md#emeritus-maintainerapprovertriager).

### Become an Approver or a Maintainer

See the [community membership document in OpenTelemetry community
repo](https://github.com/open-telemetry/community/blob/main/guides/contributor/membership.md).

[Approver]: #approvers
[Maintainer]: #maintainers
[gotmpl]: https://pkg.go.dev/go.opentelemetry.io/build-tools/gotmpl
[#4404]: https://github.com/open-telemetry/opentelemetry-go/pull/4404
