--- a/docs/unstability.rst Wed May 09 16:41:38 2012 +0200
+++ b/docs/unstability.rst Wed May 09 18:52:57 2012 +0200
@@ -1,5 +1,6 @@
+
-----------------------------------
-The Unstability Principle
+The instability Principle
-----------------------------------
@@ -7,14 +8,15 @@
An intrinsic contradiction
-----------------------------------
+XXX starts by talking about getting ride of changeset.
DVCS bring two new major concepts to the Version Control Scene:
- * Organisation of the history with robust DAG,
- * Mutation of history.
+ * History is organized as a robust DAG,
+ * History can be rewritten.
-However, the two concepts opposes them self:
+However, the two concepts are in contradiction:
To achieve a robust history, three key elements are gathered in *changeset*:
@@ -22,33 +24,34 @@
* 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 used generate a *unique* hash that identify the changeset
+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'
-The old changeset is usually discarded
-t in DVCS history.
+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.
-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.
-
-This is a very useful property because Changing B parent means changing B
-content too. This require the creation of **another** changeset.
-
-I'll qualify those children as **unstable** because they are based one a dead
+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
-changese. History Rewriting history alway need to take it in account and
+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..
@@ -56,73 +59,158 @@
Everybody is working around the issue
------------------------------------------------
-I'm not claiming that rewriting history is impossible. People are
-
+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.
-stable situation to stable situation
+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.**
-Distributed means that you do not control what happen outside your repository:
-
+Do not rewrite exchanged changeset
+```````````````````````````````````
-* phase.
-* overwrite.
+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.
-
-Boiler Plate
+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 is
+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.
-mq -- quilt
+This allow much more flexible workflow but two major feature are lost in the
+process:
-Conflict too much conflict
+: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.
-Linear
-
+:easy branching: A quilt queue is by definition a linear queue. Increasing risk
+ of conflict
-Deny a lot of option
-````````````````````````````
-
-
+It is possible to collaborate over versioned mq! But you are going ahead a lot
+of troubles.
-[] rewrite
-
-[] exchange
-
-[] collaborate
-
+.. 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
------------------------------------------------
-The more effort you put to avoid instability, the more option you deny. And even
-most restrictive work flow can't garantee that instability will never show up!
+
+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 marker".
+mercurial called "Obsolete markers". Obsolete markers have two key properties:
-* Any changeset is we want to get ride of is **explicitly** marked as "obsolete"
- by history rewritting operation..
+* Any "old" changeset we want to get ride of is **explicitly** marked as "obsolete"
+ by history rewriting operation.
-* Relations between old and new version of changesets are tracked by Obsolete
+ 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 explicitly marking the obsolete part of the history, we will be able to
-easily detect appearance of unstability. By Storing a meta-history of changeset
-evolution we are able to easily resolve instability and edition conflict.
+ 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.
-No instability is still a bad situation.
-No instability is still a bad situation that should be avoided.