diff docs/instability.rst @ 243:2f68c708e10b

fix typo in file name

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/docs/instability.rst	Fri May 11 11:52:15 2012 +0200
@@ -0,0 +1,218 @@
+
+-----------------------------------
+The instability Principle
+-----------------------------------
+
+
+
+An intrinsic contradiction
+-----------------------------------
+
+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.
+
+