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# Support, Compatibility, Deprecations, and Removal

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## Introduction

This document describes the strategy regarding supported Releases, compatibility, deprecations, and removal of functionality.

#### Change

This document and the strategy it describes was introduced in Erlang/OTP 21.

## Supported Releases

In general, bugs are only fixed on the latest release, and new features are introduced in the upcoming release that is under development. However, when we, for internal reasons, fix bugs on older releases, these will be available and announced as well.

Pull requests are only accepted on the `maint` and the `master` branches in our git repository. The `maint` branch contains changes planned for the next maintenance patch package on the latest OTP release and the `master` branch contain changes planned for the upcoming OTP release.

## Compatibility

We strive to remain as compatible as possible, even in cases where we give no compatibility guarantees.

Different parts of the system will be handled differently regarding compatibility. The following items describe how different parts of the system are handled.

- Erlang Distribution - Erlang nodes can communicate across at least two preceding and two subsequent releases.

- Compiled BEAM Code, NIF Libraries, and Drivers - Compiled code can be loaded on at least two subsequent releases. To achive the highest possible performance for Erlang code, ensure it is compiled using the same release as the one it will be deployed on.

  Loading on previous releases is not supported.

- APIs - Compatible between releases.

- Compiler Warnings - New warnings may be issued between releases.

- Command Line Arguments - Incompatible changes may occur between releases.

- OTP Build Procedures - Incompatible changes may occur between releases.

Under certain circumstances incompatible changes might be introduced even in parts of the system that should be compatible between releases. Things that might trigger incompatible changes like this are:

- Security Issues - It might be necessary to introduce incompatible changes in order to solve a security issue. This kind of incompatibility might occur in a patch.

- Bug Fixes - We will not be bug-compatible. A bug fix might introduce incompatible changes. This kind of incompatibility might occur in a patch.

- Severe Previous Design Issues - Some parts of OTP were designed a very long time ago and did not necessarily take today's computing environments into account. Consequently, the ramifications of these design choices can be quite significant, impacting performance, scalability, and more. If we determine that these consequences are too substantial, we may implement incompatible changes. Such changes are never introduced in a patch, but in the subsequent release.

Peripheral, trace, and debug functionality is at greater risk of being changed in an incompatible way than functionality in the language itself and core libraries used during operation.

There is a page in the documentation regarding incompatibilities:

- Upcoming Potential Incompatibilities - lists all upcoming potential incompatibilities.

## Deprecation

Deprecation of functionality occurs when newer, preferred alternatives are introduced. The deprecation does not imply future removal of the functionality unless an upcoming removal is explicitly stated in the deprecation notice.

Deprecated functionality will be documented as deprecated and highlighted in a release note as early possible. If appropriate, the compiler will issue warnings when the deprecated functionality is used.

There is a page in the documentation regarding deprecations:

- Deprecations - lists all deprecated functionality.

## Removal

It can become necessary to remove legacy solutions. In such instances, they will be gradually phased out over a sufficient period to allow users to adjust. Before functionality is removed, it will be deprecated for at least one release, with an explicit announcement about the upcoming removal.

Peripheral, trace, and debug functionality is at greater risk of removal than functionality in the language itself and core libraries used during operation.

There are two pages in the documentation regarding removal:

- Scheduled for Removal - lists all functionality that is schedule for removal in upcoming releases.

- Removed Functionality - lists functionality that has been removed.

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