# 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/1A63zSWX0x2CyCK_LoNhmQC4rqhLpYXJzXbEPDUQ2n6w/edit#heading=h.9tngw7jdwd6b)
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.

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`. You
can alternatively 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 would put the project in the `opentelemetry-go` directory in
current working 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`.

### 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 to be **ready to merge** when:

* It has received two approvals from Collaborators/Maintainers (at
  different companies). This is not enforced through technical means
  and a PR may be **ready to merge** with a single approval if the change
  and its approach have been discussed and consensus reached.
* Feedback has been addressed.
* Any substantive changes to your PR will require that you clear any prior
  Approval reviews, this includes changes resulting from other feedback. Unless
  the approver explicitly stated that their approval will persist across
  changes it should be assumed that the PR needs their review again. Other
  project members (e.g. approvers, maintainers) can help with this if there are
  any questions or if you forget to clear reviews.
* It has been open for review for at least one working day. This gives
  people reasonable time to review.
* Trivial changes (typo, cosmetic, doc, etc.) do not have to wait for
  one day and may be merged with a single Maintainer's approval.
* `CHANGELOG.md` has been updated to reflect what has been
  added, changed, removed, or fixed.
* `README.md` has been updated if necessary.
* Urgent fix can take exception as long as it has been actively
  communicated.

Any Maintainer can merge the PR once it is **ready to merge**.

## Design Choices

As with other OpenTelemetry clients, opentelemetry-go follows the
[opentelemetry-specification](https://github.com/open-telemetry/opentelemetry-specification).

It's especially valuable to read through the [library
guidelines](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/library-guidelines.md).

### Focus on Capabilities, Not Structure Compliance

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

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

## Documentation

Each non-example Go Module should have its own `README.md` containing:

- A pkg.go.dev badge which can be generated [here](https://pkg.go.dev/badge/).
- Brief description.
- Installation instructions (and requirements if applicable).
- Hyperlink to an example. Depending on the component the example can be:
  - An `example_test.go` like [here](exporters/stdout/stdouttrace/example_test.go).
  - A sample Go application with its own `README.md`, like [here](example/zipkin).
- Additional documentation sections such us:
  - Configuration,
  - Contributing,
  - References.

[Here](exporters/jaeger/README.md) is an example of a concise `README.md`.

Moreover, it should be possible to navigate to any `README.md` from the
root `README.md`.

## 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.

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 therefor 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)
	}
	// Preform any validation here.
	return config
}
```

If validation of the `config` options is also preformed 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
doumentation are allowed to be extended with additional methods.

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

Otherwise, stable interfaces MUST NOT be modified.

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.

## Approvers and Maintainers

Approvers:

- [Evan Torrie](https://github.com/evantorrie), Verizon Media
- [Josh MacDonald](https://github.com/jmacd), LightStep
- [Sam Xie](https://github.com/XSAM), Cisco/AppDynamics
- [David Ashpole](https://github.com/dashpole), Google
- [Gustavo Silva Paiva](https://github.com/paivagustavo), LightStep
- [Robert Pająk](https://github.com/pellared), Splunk

Maintainers:

- [Aaron Clawson](https://github.com/MadVikingGod), LightStep
- [Anthony Mirabella](https://github.com/Aneurysm9), AWS
- [Tyler Yahn](https://github.com/MrAlias), Splunk

### Become an Approver or a Maintainer

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