view docs/instability.rst @ 288:d64f8468aaf9

merge back with stable tag

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The instability Principle
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An intrinsic contradiction
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XXX starts by talking about getting ride of changeset.

DVCS bring two new major concepts to the Version Control Scene:

    * History is organized as a robust DAG,
    * History can be rewritten.


However, the two concepts are in contradiction:

To achieve a robust history, three key elements are gathered in *changeset*:

    * Full snapshot of the versioned content,
    * Reference to the previous full snapshot used to build the new one,
    * A description of the change who lead from the old content to the new old.

All three elements are to compute a *unique* hash that identify the changeset
(with various other metadata). This identification is a key part of DVCS design.

This is a very useful property because Changing B parent means changing B
content too.  This require the creation of **another** changeset which is a good
semantic.

::

  Schema base,  A, B and B'

To avoid duplication, the older changeset is usually discarded from accessible
history. I'm calling them *obsolete* changesets.


But rewriting a changeset with children does not changes children parent! And
because children of the rewritten changeset still **depends** on the older
"dead" version of the changeset with can not get ride of this dead version.

::

  Schema base,  A and A' and B.

I'm calling those children **unstable** because they are based one a dead
changeset and prevent people to get ride of it.

This instability is an **unavoidable consequence** of the strict dependency of
changeset.  History Rewriting history alway  need to take it in account and
provide a way to rewrite the descendant on the new changeset to avoid
coexistence of the old and new version of a rewritten changeset..


Everybody is working around the issue
------------------------------------------------

I'm not claiming that rewriting history is impossible. People are successfully
doing for years. However they all need to work around *instability*. Several
work around strategy exists.


Rewriting all at once
``````````````````````````

The simplest way to avoid instability is to ensure rewriting operation always
ends in a stable situation. This is achieve by rewriting all impacted changeset
at the same time.

Rewriting all descendants at the same time than the rewritted of a changeset.

::

  Schema!

Several Mercurial commands apply it: rebase, collapse, histedit.  Mercurial also
refuse to amend changeset with descendant.  The git branch design enforce such
approach in git too.


However, DVCS are **Distributed**. This means that you do not control what
happen outside your repository. Once a changeset have been exchanged *outside*,
there is no way to be sure it does not have descendants somewhere else.
Therefore **if you rewrite changeset that exists elsewhere, you can't eradicate
the risk of instability.**

Do not rewrite exchanged changeset
```````````````````````````````````

To work around the issue above, mercurial introduced phases that prevent you to
rewrite shared changeset and ensure other can't pull certain changeset from you.
But this is a very frustrating limitation that prevent you to efficiently share,
review and collaborate on mutable changeset.

Git world use another approach to prevent instability.  By convention only a
single developper works on a changeset contained in a named branch. But once
again this is a huge blocker for collaborating. Moreover clueless people
**will** mess up social convention soon or later.


Loose the DAG robustness
````````````````````````````

The other approach use in Mercurial is to keep the mutable part of the history
outside the DVCS constraint. This is the MQ approach of sticking a quilt queue
over Mercurial.

This allow much more flexible workflow but two major feature are lost in the
process:

:Graceful merge: MQ use plain-patch to store changeset content and patch have
                 trouble to apply in changing context. Applying your queue
                 becomes very painful when context changes.

:easy branching: A quilt queue is by definition a linear queue. Increasing risk
                 of conflict

It is possible to collaborate over versioned mq! But you are going ahead a lot
of troubles.

.. Ignore conflicts
.. ```````````````````````````````````
.. 
.. Another ignored issue is conflicting rewritting of the same changeset. If a
.. changeset is rewritten two times we have two newer version, duplicated history
.. complicate to merge.
.. 
.. Mercurial work around by
.. 
.. The "One set of mutable changset == One developper" mantra is also a way to work
.. around conflicting rewritting of changeset. If two different people are able to
.. 
.. The git branch model allow to overwrite changeset version by another one. But it
.. does not care about divergent version. It is the equilent of "common ftp" source
.. management for changeset.

Facing The Danger Once And For All
------------------------------------------------

Above we saw that, the more effort you put to avoid instability, the more option
you deny. And even most restrictive work flow can't guarantee that instability
will never show up!

Obsolete marker can handle the job
```````````````````````````````````

It is time to provide a full featured solution to deal with instability and to
stop working around the issue! This is why I developing a new feature for
mercurial called "Obsolete markers". Obsolete markers have two key properties:


* Any "old" changeset we want to get ride of is **explicitly** marked as "obsolete"
  by history rewriting operation.

  By explicitly marking the obsolete part of the history, we will be able to
  easily detect instability situation.

* Relations between old and new version of changesets are tracked by obsolete
  markers.

  By Storing a meta-history of changeset evolution we are able to easily resolve
  instability and edition conflict [#]_ .

.. [#] edition conflict is another major obstable to collaboration. See the
       section dedicated to obsolete marker for details.

Improves robustness == improves simplicity
````````````````````````````````````````````````

This proposal should **first** be seen as a safety measure.

It allow to detect instability as soon as possible

::

    $ hg pull
    added 3 changeset
    +2 unstable changeset
    (do you want "hg stabilize" ?)
    working directory parent is obsolete!
    $ hg push
    outgoing unstable changesets
    (use "hg stabilize" or force the push)

And should not not encourage people to create unstability

::

    $ hg up 42
    $ hg commit --amend
    changeset have descendant.
    $ hg commit --amend -f
    +5 unstable changeset

    $ hg rebase -D --rev 40::44
    rebasing already obsolete changeset 42:AAA will conflict with newer version 48:BBB

While allowing powerful feature
````````````````````````````````````````````````


* Help to automatically solve instability.

* "kill" changeset remotely.

* track resulting changeset when submitting patch//pull request.

* Focus on what you do:

  I do not like the "all at once" model of history rewriting. I'm comfortable
  with unstability and obsolete marker offer all the tool to safely create and
  handle unstability locally.