# 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
[Gitter](https://gitter.im/open-telemetry/opentelemetry-go).

## Development

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

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

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).
* Major feedbacks are resolved.
* It has been open for review for at least one working day. This gives
  people reasonable time to review.
* Trivial change (typo, cosmetic, doc, etc.) doesn't have to wait for
  one day.
* `CHANGELOG.md` has been updated to reflect what has been
  added, changed, removed, or fixed.
* Urgent fix can take exception as long as it has been actively
  communicated.

Any Collaborator/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/master/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:
https://github.com/open-telemetry/opentelemetry-specification/issues/165

## 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 `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 `struct` must contain configuration options.

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

In general the `config` `struct` 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 `config` are not shared across package boundaries.
Meaning a `config` from one package should not be directly used by another.

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

```go
// configure returns an appropriately configured config.
func configure([]Option) config {
    // Set default values for config.
    config := config{/* […] */}
    for _, option := range options {
        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 `configure` 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)
}
```

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) {
    c.Bool = bool(o)
}

// 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) {
    c.Bool = bool(o)
}

// 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) {
    c.MyType = o.MyType
}

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

#### Instantiation

Using this configuration pattern to configure instantiation with a `New*`
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)
}

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

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

type weightOption float64
func (o weightOption) ApplyDog(c *config)  { c.Weight = float64(o) }
func (o weightOption) ApplyBird(c *config) { c.Weight = float64(o) }
func WithWeight(w float64) Option          { return weightOption(w) }

type furColorOption string
func (o furColorOption) ApplyDog(c *config) { c.Color = string(o) }
func WithFurColor(c string) DogOption       { return furColorOption(c) }

type maxAltitudeOption float64
func (o maxAltitudeOption) ApplyBird(c *config) { c.MaxAltitude = float64(o) }
func WithMaxAltitude(a float64) BirdOption      { return maxAltitudeOption(a) }

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

## Approvers and Maintainers

Approvers:

- [Liz Fong-Jones](https://github.com/lizthegrey), Honeycomb
- [Evan Torrie](https://github.com/evantorrie), Verizon Media
- [Josh MacDonald](https://github.com/jmacd), LightStep

Maintainers:

- [Anthony Mirabella](https://github.com/Aneurysm9), Centene
- [Tyler Yahn](https://github.com/MrAlias), New Relic

### Become an Approver or a Maintainer

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