changeset 192:3b95da26a544

quick start: Included and partly reworked the mail from Martin Geisler for understanding Mercurial.

--- a/text/quick_start.txt	Thu Jun 04 23:11:09 2009 +0200
+++ b/text/quick_start.txt	Thu Jun 04 23:46:59 2009 +0200
@@ -30,4 +30,89 @@
 
 == Part 2: Understanding Mercurial ==
 
-... compile the great mails from M. Geisler ...
\ No newline at end of file
+Let's look at some of the basic concepts of Mercurial to get a better understanding of its internals: 
+
+* Like in Subversion, history consists of a number of commits. They're
+  called changesets in Mercurial.
+
+* Subversion requires a strict linear ordering of the commits and
+  gives nice linear revision numbers to them. So revision N has only
+  one child revision, rN+1.
+
+  This is simple, but it requires a central server to make sure that
+  everybody agrees on the revision numbers.
+
+* Mercurial generalizes this by letting each changeset have multiple
+  children. If I work alone and make commits I'll make
+
+    C1 --> C2 --> C3
+
+  by making three commits. The commit C3 with no children is a "head".
+  It is also the newest changeset in the repository -- called "tip".
+
+  If I shared C1 with you and you started your work from that, your
+  commits will build a repository like this:
+
+    C1 --> C2' --> C3'
+
+  Here C3' is a head in your repository and I don't know anything
+  about C2' and C3' yet.
+
+* If I pull from you, or you push to me, the two repositories are
+  compared. By default, all missing changesets are transferred. This
+  is all there is to push/pull: compare two graphs of changesets and
+  transfer the missing ones.
+
+  After a pull from you my repository will look like this:
+
+         /-> C2 --> C3
+    C1 -<
+         \-> C2' --> C3'
+
+  Here C1 has two child changesets, and the repository has two heads
+  since the development has diverged.
+
+  The changeset C3' will be the new tip since it is the newest
+  changeset in the repository. Note that tip is always a head, but a
+  head need not be the tip.
+
+* Having two heads suggest that someone should merge them -- otherwise
+  the changes from one will never be combined with the changed made in
+  the other head.
+
+  When merging with 'hg merge' the task is to figure out the canonical
+  way to combine the changesets. If the changes do not overlap this is
+  usually trivial, otherwise you have to do a three-way merge. The
+  merge must be committed and this creates a changeset which explains
+  to the world how you think the two heads should be combined:
+
+         /-> C2 --> C3   -\
+    C1 -<                  >-> M
+         \-> C2' --> C3' -/
+
+  Note that the merge changeset M has two parents.
+
+  If you do not merge C3 and C3' and try to push you get the 'new
+  remote head' message and push aborts. It aborts since it is a little
+  "impolite" to leave the job of merging to someone else -- he who
+  created the two heads by pulling in some code should also normally
+  do the merging.  
+
+
+In all this it helped my understanding a lot to think in terms of the changeset graph. Just remember
+that:
+
+  * "hg commit" adds a new node. The parent changesets of the new node
+    is given by "hg parents"
+
+  * "hg push" and "hg pull" transfer nodes in the graph between two
+    repositories.
+
+  * "hg update" updates the working copy to reflect a given node in
+    the history graph. This also changes the parent changeset of the
+    next commit, see "hg parents".
+
+
+A final note: If you want to quickly look things up, you can use one of the [Mercurial cheatsheets](http://www.selenic.com/mercurial/wiki/index.cgi/QuickReferenceCardsAndCheatSheets). 
+
+*compiled from a great Mail by Martin Geisler*