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Importance_4
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The Version class processes string versions into comparable values. A version string should normally be a series of numbers separated by periods. Each part (digits separated by periods) is considered its own number, and these are used for sorting. So for instance, 3.10 sorts higher than 3.2 because ten is greater than two.

If any part contains letters (currently only a-z are supported) then that version is considered prerelease. Versions with a prerelease part in the Nth part sort less than versions with N-1 parts. Prerelease parts are sorted alphabetically using the normal Ruby string sorting rules. If a prerelease part contains both letters and numbers, it will be broken into multiple parts to provide expected sort behavior (1.0.a10 becomes 1.0.a.10, and is greater than 1.0.a9).

Prereleases sort between real releases (newest to oldest):

  1. 1.0

  2. 1.0.b1

  3. 1.0.a.2

  4. 0.9

How Software Changes

Users expect to be able to specify a version constraint that gives them some reasonable expectation that new versions of a library will work with their software if the version constraint is true, and not work with their software if the version constraint is false. In other words, the perfect system will accept all compatible versions of the library and reject all incompatible versions.

Libraries change in 3 ways (well, more than 3, but stay focused here!).

  1. The change may be an implementation detail only and have no effect on

the client software.
  1. The change may add new features, but do so in a way that client software

written to an earlier version is still compatible.
  1. The change may change the public interface of the library in such a way

that old software is no longer compatible.

Some examples are appropriate at this point. Suppose I have a Stack class that supports a push and a pop method.

Examples of Category 1 changes:

  • Switch from an array based implementation to a linked-list based

implementation.
  • Provide an automatic (and transparent) backing store for large stacks.

Examples of Category 2 changes might be:

  • Add a depth method to return the current depth of the stack.

  • Add a top method that returns the current top of stack (without

changing the stack).
  • Change push so that it returns the item pushed (previously it

had no usable return value).

Examples of Category 3 changes might be:

  • Changes pop so that it no longer returns a value (you must use

<tt>top</tt> to get the top of the stack).
  • Rename the methods to push_item and pop_item.

RubyGems Rational Versioning

  • Versions shall be represented by three non-negative integers, separated

by periods (e.g. 3.1.4).  The first integers is the "major" version
number, the second integer is the "minor" version number, and the third
integer is the "build" number.
  • A category 1 change (implementation detail) will increment the build

number.
  • A category 2 change (backwards compatible) will increment the minor

version number and reset the build number.
  • A category 3 change (incompatible) will increment the major build number

and reset the minor and build numbers.
  • Any “public” release of a gem should have a different version. Normally

that means incrementing the build number.  This means a developer can
generate builds all day long for himself, but as soon as he/she makes a
public release, the version must be updated.

Examples

Let’s work through a project lifecycle using our Stack example from above.

Version 0.0.1

The initial Stack class is release.

Version 0.0.2

Switched to a linked=list implementation because it is

cooler.

Version 0.1.0

Added a depth method.

Version 1.0.0

Added top and made pop return nil

(<tt>pop</tt> used to return the  old top item).

Version 1.1.0

push now returns the value pushed (it used it

return nil).

Version 1.1.1

Fixed a bug in the linked list implementation.

Version 1.1.2

Fixed a bug introduced in the last fix.

Client A needs a stack with basic push/pop capability. He writes to the original interface (no top), so his version constraint looks like:

gem 'stack', '~> 0.0'

Essentially, any version is OK with Client A. An incompatible change to the library will cause him grief, but he is willing to take the chance (we call Client A optimistic).

Client B is just like Client A except for two things: (1) He uses the depth method and (2) he is worried about future incompatibilities, so he writes his version constraint like this:

gem 'stack', '~> 0.1'

The depth method was introduced in version 0.1.0, so that version or anything later is fine, as long as the version stays below version 1.0 where incompatibilities are introduced. We call Client B pessimistic because he is worried about incompatible future changes (it is OK to be pessimistic!).

Preventing Version Catastrophe:

From: http://blog.zenspider.com/2008/10/rubygems-howto-preventing-cata.html

Let’s say you’re depending on the fnord gem version 2.y.z. If you specify your dependency as “>= 2.0.0” then, you’re good, right? What happens if fnord 3.0 comes out and it isn’t backwards compatible with 2.y.z? Your stuff will break as a result of using “>=”. The better route is to specify your dependency with a “spermy” version specifier. They’re a tad confusing, so here is how the dependency specifiers work:

Specification From  ... To (exclusive)
">= 3.0"      3.0   ... &infin;
"~> 3.0"      3.0   ... 4.0
"~> 3.0.0"    3.0.0 ... 3.1
"~> 3.5"      3.5   ... 4.0
"~> 3.5.0"    3.5.0 ... 3.6

Constants

ANCHORED_VERSION_PATTERN = /\A\s*(#{VERSION_PATTERN})*\s*\z/

VERSION_PATTERN = '[0-9]+(\.[0-9a-zA-Z]+)*'

Attributes

[R] to_s

A string representation of this Version.

[R] version

A string representation of this Version.

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