/spec/v0.2.0.md

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layout: default
title: Semantic Versioning 0.2.0
---

Semantic Versioning 0.2.0
=========================

In the world of software management there exists a dread place called
"dependency hell." The bigger your system grows and the more packages you
integrate into your software, the more likely you are to find yourself, one
day, in this pit of despair.

In systems with many dependencies, releasing new package versions can quickly
become a nightmare. If the dependency specifications are too tight, you are in
danger of version lock (the inability to upgrade a package without having to
release new versions of every dependent package). If dependencies are
specified too loosely, you will inevitably be bitten by version promiscuity
(assuming compatibility with more future versions than is reasonable).
Dependency hell is where you are when version lock and/or version promiscuity
prevent you from easily and safely moving your project forward.

As a solution to this problem, I propose a simple set of rules and
requirements that dictate how version numbers are assigned and incremented.
For this system to work, you first need to declare a public API. This may
consist of documentation or be enforced by the code itself. Regardless, it is
important that this API be clear and precise. Once you identify your public
API, you communicate changes to it with specific increments to your version
number. Consider a version format of X.Y.Z (Major.Minor.Patch). Bug fixes not
affecting the API increment the patch version, backwards compatible API
additions/changes increment the minor version, and backwards incompatible API
changes increment the major version.

I call this system "Semantic Versioning." Under this scheme, version numbers
and the way they change convey meaning about the underlying code and what has
been modified from one version to the next.


Semantic Versioning Specification (SemVer)
------------------------------------------

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119.

1. Software using Semantic Versioning MUST declare a public API. This API
could be declared in the code itself or exist strictly in documentation.
However it is done, it should be precise and comprehensive.

1. A normal version number MUST take the form X.Y.Z where X, Y, and Z are
integers. X is the major version, Y is the minor version, and Z is the patch
version. Each element MUST increase numerically. For instance: 1.9.0 < 1.10.0
< 1.11.0.

1. A special version number MAY be denoted by appending an arbitrary string
immediately following the patch version. The string MUST be comprised of only
alphanumerics plus dash [0-9A-Za-z-] and MUST begin with an alpha character
[A-Za-z]. Special versions satisfy but have a lower precedence than the
associated normal version. Precedence SHOULD be determined by lexicographic
ASCII sort order. For instance: 1.0.0beta1 < 1.0.0beta2 < 1.0.0.

1. Once a versioned package has been released, the contents of that version
MUST NOT be modified. Any modifications must be released as a new version.

1. Major version zero (0.y.z) is for initial development. Anything may change
at any time. The public API should not be considered stable.

1. Version 1.0.0 defines the public API. The way in which the version number
is incremented is now dependent on this public API and how it changes.

1. Patch version Z (x.y.Z | x > 0) MUST be incremented if only backwards
compatible bug fixes are introduced. A bug fix is defined as an internal
change that fixes incorrect behavior.

1. Minor version Y (x.Y.z | x > 0) MUST be incremented if new, backwards
compatible functionality is introduced to the public API. It MAY be
incremented if substantial new functionality or improvements are introduced
within the private code. It MAY include patch level changes.

1. Major version X (X.y.z | X > 0) MUST be incremented if any backwards
incompatible changes are introduced to the public API. It MAY include minor
and patch level changes.


Tagging Specification (SemVerTag)
---------------------------------

This sub-specification SHOULD be used if you use a version control system
(Git, Mercurial, SVN, etc) to store your code. Using this system allows
automated tools to inspect your package and determine SemVer compliance and
released versions.

1. When tagging releases in a version control system, the tag for a version
MUST be "vX.Y.Z" e.g. "v3.1.0".

1. The first revision that introduces SemVer compliance SHOULD be tagged
"semver". This allows pre-existing projects to assume compliance at any
arbitrary point and for automated tools to discover this fact.


Why Use Semantic Versioning?
----------------------------

This is not a new or revolutionary idea. In fact, you probably do something
close to this already. The problem is that "close" isn't good enough. Without
compliance to some sort of formal specification, version numbers are
essentially useless for dependency management. By giving a name and clear
definition to the above ideas, it becomes easy to communicate your intentions
to the users of your software. Once these intentions are clear, flexible (but
not too flexible) dependency specifications can finally be made.

A simple example will demonstrate how Semantic Versioning can make dependency
hell a thing of the past. Consider a library called "Firetruck." It requires a
Semantically Versioned package named "Ladder." At the time that Firetruck is
created, Ladder is at version 3.1.0. Since Firetruck uses some functionality
that was first introduced in 3.1.0, you can safely specify the Ladder
dependency as greater than or equal to 3.1.0 but less than 4.0.0. Now, when
Ladder version 3.1.1 and 3.2.0 become available, you can release them to your
package management system and know that they will be compatible with existing
dependent software.

As a responsible developer you will, of course, want to verify that any
package upgrades function as advertised. The real world is a messy place;
there's nothing we can do about that but be vigilant. What you can do is let
Semantic Versioning provide you with a sane way to release and upgrade
packages without having to roll new versions of dependent packages, saving you
time and hassle.

If all of this sounds desirable, all you need to do to start using Semantic
Versioning is to declare that you are doing so and then follow the rules. Link
to this website from your README so others know the rules and can benefit from
them.


FAQ
---

### How do I know when to release 1.0.0?

If your software is being used in production, it should probably already be
1.0.0. If you have a stable API on which users have come to depend, you should
be 1.0.0. If you're worrying a lot about backwards compatibility, you should
probably already be 1.0.0.

### Doesn't this discourage rapid development and fast iteration?

Major version zero is all about rapid development. If you're changing the API
every day you should either still be in version 0.x.x or on a separate
development branch working on the next major version.

### If even the tiniest backwards incompatible changes to the public API require a major version bump, won't I end up at version 42.0.0 very rapidly?

This is a question of responsible development and foresight. Incompatible
changes should not be introduced lightly to software that has a lot of
dependent code. The cost that must be incurred to upgrade can be significant.
Having to bump major versions to release incompatible changes means you'll
think through the impact of your changes, and evaluate the cost/benefit ratio
involved.

### Documenting the entire public API is too much work!

It is your responsibility as a professional developer to properly document
software that is intended for use by others. Managing software complexity is a
hugely important part of keeping a project efficient, and that's hard to do if
nobody knows how to use your software, or what methods are safe to call. In
the long run, Semantic Versioning, and the insistence on a well defined public
API can keep everyone and everything running smoothly.

### What do I do if I accidentally release a backwards incompatible change as a minor version?

As soon as you realize that you've broken the Semantic Versioning spec, fix
the problem and release a new minor version that corrects the problem and
restores backwards compatibility. Remember, it is unacceptable to modify
versioned releases, even under this circumstance. If it's appropriate,
document the offending version and inform your users of the problem so that
they are aware of the offending version.

### What should I do if I update my own dependencies without changing the public API?

That would be considered compatible since it does not affect the public API.
Software that explicitly depends on the same dependencies as your package
should have their own dependency specifications and the author will notice any
conflicts. Determining whether the change is a patch level or minor level
modification depends on whether you updated your dependencies in order to fix
a bug or introduce new functionality. I would usually expect additional code
for the latter instance, in which case it's obviously a minor level increment.


About
-----

The Semantic Versioning specification is authored by [Tom Preston-Werner](http://tom.preston-werner.com), inventor of Gravatars and cofounder of GitHub.

If you'd like to leave feedback, please [open an issue on GitHub](https://github.com/mojombo/semver.org/issues).