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Understanding Revision Numbers
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==============================
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All revisions in the mainline of a branch will have a simple increasing
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While Bazaar is similar to other VCS tools in many ways, there are
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some important differences that are not necessarily obvious at first
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glance. This section attempts
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to explain some of the things users need to know in order to "grok" Bazaar,
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i.e. to deeply understand it.
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Note: It isn't necessary to fully understand this section to use Bazaar.
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You may wish to skim this section now and come back to it at a later time.
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Understanding revision numbers
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------------------------------
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All revisions in the mainline of a branch have a simple increasing
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integer. (First commit gets 1, 10th commit gets 10, etc.) This makes them
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fairly natural to use when you want to say "grab the 10th revision from my
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branch", or "fixed in revision 3050".
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For revisions which have been merged into a branch, a dotted notation is
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used (eg, 3112.1.5). Dotted revision numbers have three numbers. The first
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For revisions which have been merged into a branch, a dotted notation is used
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(e.g., 3112.1.5). Dotted revision numbers have three numbers [#]_. The first
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number indicates what mainline revision change is derived from. The second
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number is the branch counter. There can be many branches derived from the
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same revision, so they all get a unique number. The third number is the
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number of revisions since the branch started. For example, 3112.1.5 is the
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first branch from revision 3112, the fifth revision on that branch.
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Revisions are numbered in a stable way, such that if two branches have the
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same revision in their mainline, all revisions in the ancestry of that
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revision will have the same revision numbers. (So if Alice and Bob's
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.. [#] Versions prior to bzr 1.2 used a slightly different algorithm.
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Some nested branches would get extra numbers (such as 1.1.1.1.1)
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rather than the simpler 3-number system.
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Hierarchical history is good
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----------------------------
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Imagine a project with multiple developers contributing changes where
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many changes consist of a series of commits. To give a concrete example,
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consider the case where:
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* The tip of the project's trunk is revision 100.
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* Mary makes 3 changes to deliver feature X.
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* Bill makes 4 changes to deliver feature Y.
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If the developers are working in parallel and using a traditional
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centralized VCS approach, the project history will most likely be linear
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with Mary's changes and Bill's changes interleaved. It might look like this::
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107: Add documentation for Y
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106: Fix bug found in testing Y
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105: Fix bug found in testing X
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103: Add documentation for X
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102: Add code and tests for X
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Many teams use this approach because their tools make branching and merging
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difficult. As a consequence, developers update from and commit to the trunk
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frequently, minimizing integration pain by spreading it over every commit.
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If you wish, you can use Bazaar exactly like this. Bazaar does offer other
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ways though that you ought to consider.
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An alternative approach encouraged by distributed VCS tools is to create
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feature branches and to integrate those when they are ready. In this case,
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Mary's feature branch would look like this::
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103: Fix bug found in testing X
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102: Add documentation for X
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101: Add code and tests for X
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And Bill's would look like this::
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104: Add documentation for Y
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103: Fix bug found in testing Y
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If the features were independent and you wanted to keep linear history,
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the changes could be pushed back into the trunk in batches. (Technically,
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there are several ways of doing that but that's beyond the scope of
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this discussion.) The resulting history might look like this::
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107: Fix bug found in testing X
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106: Add documentation for X
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105: Add code and tests for X
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104: Add documentation for Y
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103: Fix bug found in testing Y
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While this takes a bit more effort to achieve, it has some advantages over
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having revisions randomly intermixed. Better still though, branches can
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be merged together forming a non-linear history. The result might look
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100.2.3: Fix bug found in testing X
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100.2.2: Add documentation for X
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100.2.1: Add code and tests for X
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100.1.4: Add documentation for Y
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100.1.3: Fix bug found in testing Y
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100.1.2: Add code for Y
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100.1.1: Add tests for Y
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Or more likely this::
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100.2.2: Add documentation
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100.2.1: Add code and tests
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100.1.4: Add documentation
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100.1.3: Fix bug found in testing
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This is considered good for many reasons:
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* It makes it easier to understand the history of a project.
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Related changes are clustered together and clearly partitioned.
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* You can easily collapse history to see just the commits on the mainline
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of a branch. When viewing the trunk history like this, you only see
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high level commits (instead of a large number of commits uninteresting
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* If required, it makes backing out a feature much easier.
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* Continuous integration tools can be used to ensure that
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all tests still pass before committing a merge to the mainline.
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(In many cases, it isn't appropriate to trigger CI tools after
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every single commit as some tests will fail during development.
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In fact, adding the tests first - TDD style - will guarantee it!)
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In summary, the important points are:
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*Organize your work using branches.*
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*Integrate changes using merge.*
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*Ordered revision numbers and hierarchy make history easier to follow.*
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Each branch has its own view of history
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---------------------------------------
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As explained above, Bazaar makes the distinction between:
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* mainline revisions, i.e. ones you committed in your branch, and
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* merged revisions, i.e. ones added as ancestors by committing a merge.
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Each branch effectively has its own view of history, i.e. different
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branches can give the same revision a different "local" revision number.
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Mainline revisions always get allocated single number revision numbers
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while merged revisions always get allocated dotted revision numbers.
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To extend the example above, here's what the revision history of
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Mary's branch would look like had she decided to merge the project
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trunk into her branch after completing her changes::
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100.2.1: Merge feature Y
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100.1.4: Add documentation
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100.1.3: Fix bug found in testing
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103: Fix bug found in testing X
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102: Add documentation for X
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101: Add code and tests for X
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Once again, it's easy for Mary to look at just *her* top level of history
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to see the steps she has taken to develop this change. In this context,
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merging the trunk (and resolving any conflicts caused by doing that) is
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just one step as far as the history of this branch is concerned.
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It's important to remember that Bazaar is not changing history here, nor
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is it changing the global revision identifiers. You can always use the
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latter if you really want to. In fact, you can use the branch specific
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revision numbers when communicating *as long as* you provide the branch
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URL as context. (In many Bazaar projects, developers imply the central
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trunk branch if they exchange a revision number without a branch URL.)
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Merges do not change revision numbers in a branch, though they do
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allocate local revision numbers to newly merged revisions. The only time
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Bazaar will change revision numbers in a branch is when you explicitly
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ask it to mirror another branch.
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Note: Revisions are numbered in a stable way: if two branches have
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the same revision in their mainline, all revisions in the ancestry of that
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revision will have the same revision numbers. For example, if Alice and Bob's
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branches agree on revision 10, they will agree on all revisions before
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that.) Future merges will not change revision numbers. However doing
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``bzr pull`` can change revision numbers, because it changes the
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Versions prior to bzr 1.2 used a slightly different algorithm. Some nested
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branches would get extra numbers (such as 1.1.1.1.1) rather than the
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simpler 3-number system.
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In general, if you follow the advice given earlier - organise
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your work in branches and use merge to collaborate - you'll find Bazaar
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generally does what you expect.
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In coming chapters, we examine various ways of using Bazaar beginning with
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the simplest: using Bazaar for personal projects.
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