MarkMerge
Monotone makes use of the MarkMerge (also known as *merge) algorithm. The emails reproduced below document the algorithm. Further information can be found at revctrl.org.
Initial markmerge proposal
From: Nathaniel Smith <njs <at> pobox.com>
Subject: [cdvdevel] more merging stuff (bit long...)
Newsgroups: gmane.comp.versioncontrol.codeville.devel, gmane.comp.versioncontrol.monotone.devel
Date: 20050806 09:08:09 GMT
I set myself a toy problem a few days ago: is there a really, truly,
right way to merge two heads of an arbitrary DAG, when the object
being merged is as simple as possible: a single scalar value?
I assume that I'm given a graph, and each node in the graph has a
value, and no other annotation; I can add annotations, but they have
to be derived from the values and topology. Oh, and I assume that no
revision has more than 2 parents; probably things can be generalized
to the case of indegree 3 or higher, but it seems like a reasonable
restriction...
So, anyway, here's what I came up with. Perhaps you all can tell me
if it makes sense.
User model

Since the goal was to be "really, truly, right", I had to figure out
what exactly that meant... basically, what I'm calling a "user model"
 a formal definition of how the user thinks about merging, to give
an operational definition of "should conflict" and "should clean
merge". My rules are these:
1) whenever a user explicitly sets the value, they express a claim
that their setting is superior to the old setting
2) whenever a user chooses to commit a new revision, they implicitly
affirm the validity of the decisions that led to that revision's
parents
Corollary of (1) and (2): whenever a user explicitly sets the
value, they express that they consider their new setting to be
superior to _all_ old settings
3) A "conflict" should occur if, and only if, the settings on each
side of the merge express parallel claims.
This in itself is not an algorithm, or anything close to it; the hope
is that it's a good description of what people actually want out of a
merge algorithm, expressed clearly enough that we can create an
algorithm that fits these desiderata.
Algorithm

I'll use slightly novel notation. Lower case letters represent values
that scalar the scalar takes. Upper case letters represent nodes in
the graph.
Now, here's an algorithm, that is supposed to just be a transcription
of the above rules, one step more formal:
First, we need to know where users actively expressed an intention.
Intention is defined by (1), above. We use * to mark where this
occurred:
i) a* graph roots are always marked
a
ii)  no mark, value was not set
a
a
iii)  b != a, so b node marked
b*
a b
iv) \ /
c*
c is totally new, so marked
a a
\ /
c*
a b we're marking places where users expressed
v) \ / intention; so b should be marked iff this
b? was a conflict (!)
a a for now I'm not specialcasing the coincidental
vi) \ / clean merge case, so let's consider this to be
a? a subclass of (v).
That's all the cases possible. So, suppose we go through and
annotate our graph with *s, using the above rules; we have a graph
with some *s peppered through it, each * representing one point that
a user took action.
Now, a merge algorithm per se: Let's use *(A) to mean the unique
nearest marked ancestor of node A. Suppose we want to merge A and
B. There are exactly 3 cases:
 *(A) is an ancestor of B, but not vice versa: B wins.
 *(B) is an ancestor of A, but not vice versa: A wins.
 *(A) is _not_ an ancestor of B, and vice versa: conflict,
escalate to user
Very intuitive, right? If B supercedes the intention that led to A,
then B should win, and viceversa; if not, the user has expressed
two conflicting intentions, and that, by definition, is a conflict.
This lets us clarify what we mean by "was a conflict" in case (v)
above. When we have a merge of a and b that gives b, we simple
calculate *(a); if it is an ancestor of 'b', then we're done, but if
it isn't, then we mark the merge node. (Subtle point: this is
actually not _quite_ the same as detecting whether merging 'a' and
'b' would have given a conflict; if we somehow managed to get a
point in the graph that would have clean merged to 'a', but in fact
was merged to 'b', then this algorithm will still mark the merge
node.) For cases where the two parents differ, you have to do this
using the losing one; for cases where the two parents are the same,
you should check both, because it could have been a clean merge two
different ways. If *(a1) = *(a2), i.e., both sides have the same
nearest marked ancestor, consider that a clean merge.
That's all.
Examples

Of course, I haven't shown you this is welldefined or anything, but
to draw out the suspense a little, have some worked examples (like
most places in this document, I draw graphs with two leaves and assume
that those are being merged):
graph:
a*
/ \
a b*
result: *(a) is an ancestor of b, but *(b) is not an ancestor of a;
clean merge with result 'b'.
graph:
a*
/ \
b* c*
result: *(b) = b is not an ancestor of c, and *(c) = c is not an
ancestor of c; conflict.
graph:
a*
/ \
b* c* < these are both marked, by (iii)
\ /
 X 
/ \
b* c* < which means these were conflicts, and thus marked
result: the two leaves are both marked, and thus generate a conflict,
as above.
Right, enough of that. Math time.
Math

Theorem: In a graph marked following the above rules, every node N
will have a unique least marked ancestor M, and the values of M and N
will be the same.
Proof: By downwards induction on the graph structure. The base case
are graph roots, which by (i) are always marked, so the statement is
trivially true. Proceeding by cases, (iii) and (iv) are trivially
true, since they produce nodes that are themselves marked. (ii) is
almost as simple; in a graph 'a' > 'a', the child obviously
inherits the parent's unique least marked ancestor, which by
inductive hypothesis exists. The interesting case is (v) and (vi):
a b
\ /
b
If the child is marked, then again the statement is trivial; so
suppose it is not. By definition, this only occurs when *(a) is an
ancestor of 'b'. But, by assumption, 'b' has a unique nearest
ancestor, whose value is 'b'. Therefore, *(a) is also an ancestor
of *(b). If we're in the weird edge case (vi) where a = b, then
these may be the same ancestor, which is fine. Otherwise, the fact
that a != b, and that *(a)'s value = a's value, *(b)'s value = b's
value, implies that *(a) is a strict ancestor of *(b). Either way,
the child has a unique least marked ancestor, and it is the same
ULMA as its samevalued parent, so the ULMA also has the right
value. QED.
Corollary: *(N) is a welldefined function.
Corollary: The three cases mentioned in the merge algorithm are the
only possible cases. In particular, it cannot be that *(A) is an
ancestor of B and *(B) is an ancestor of A simultaneously, unless
the two values being merged are identical (and why are you running
your merge algorithm then?). Or in other words: ambiguous clean
merge does not exist.
Proof: Suppose *(A) is an ancestor of B, and *(B) is an ancestor of A.
*(B) is unique, so *(A) must also be an ancestor of *(B).
Similarly, *(B) must be an ancestor of *(A). Therefore:
*(A) = *(B)
We also have:
value(*(A)) = value(A)
value(*(B)) = value(B)
which implies
value(A) = value(B). QED.
Therefore, the above algorithm is welldefined in all possible cases.
We can prove another somewhat interesting fact:
Theorem: If A and B would merge cleanly with A winning, then any
descendent D of A will also merge cleanly with B, with D winning.
Proof: *(B) is an ancestor of A, and A is an ancestor of D, so *(B) is
an ancestor of D.
I suspect that this is enough to show that clean merges are order
invariant, but I don't have a proof together ATM.
Not sure what other properties would be interesting to prove; any
suggestions? It'd be nice to have some sort of proof about "once a
conflict is resolved, you don't have to resolve it again"  which is
the problem that makes ambiguous clean merge so bad  but I'm not
sure how to state such a property formally. Something about it being
possible to fully converge a graph by resolving a finite number of
conflicts or something, perhaps?
Funky cases

There are two funky cases I know of.
Coincidental clean merge:

a
/ \
b* b*
Two people independently made the same change. When we're talking
about textual changes, some people argue this should give a conflict
(reasoning that perhaps the same line _should_ be inserted twice). In
our context that argument doesn't even apply, because these are just
scalars; so obviously this should be a clean merge. Currently, the
only way this algorithm has to handle this is to treat it as an
"automatically resolved conflict"  there's a real conflict here, but
the VCS, acting as an agent for the user, may decide to just go ahead
and resolve it, because it knows perfectly well what the user will do.
In this interpretation, everything works fine, all the above stuff
applies; it's somewhat dissatisfying, though, because it's a violation
of the user model  the user has not necessarily looked at this
merge, but we put the * of userassertion on the result anyway. Not a
showstopper, I guess...
It's quite possible that the above stuff could be generalized to allow
nonunique least marked ancestors, that could only arise in exactly
this case.
I'm not actually sure what the right semantics would be, though. If
we're merging:

a
/ \
b b
\ / \
b c
Should that be a clean merge? 'b' was set twice, and only one of
these settings was overridden; is that good enough?
Do you still have the same opinion if the graph is:

a

b
/ \
c b
 / \
b b c
\ /
b
? Here the reason for the second setting of 'b' was that a change
away from it was reverted; to make it extra cringeinducing, I threw
in that change being reverted was another change to 'c'... (this may
just be an example of how any merge algorithm has some particular case
you can construct where it will get something wrong, because it
doesn't _actually_ know how to read the users's minds).
Supporting these cases may irresistably lead back to ambiguous clean,
as well:

a
/ \
b* c*
/ \ / \
c* X b*
\ / \ /
c b
The other funky case is this thing (any clever name suggestions?):
a
/ \
b* c*
\ / \
c* d*
Merging here will give a conflict, with my algorithm; 3way merge
would resolve it cleanly. Polling people on #monotone and #revctrl,
the consensus seems to be that they agree with 3way merge, but giving
a conflict is really not _that_ bad. (It also seems to cause some
funky effects with darcsmerge; see zooko's comments on #revctrl and
darcsusers.)
This is really a problem with the user model, rather than the
algorithm. Apparently people do not interpret the act of resolving
the b/c merge to be "setting" the result; They seem to interpret it as
"selecting" the result of 'c'; the 'c' in the result is in some sense
the "same" 'c' as in the parent. The difference between "setting" and
"selecting" is the universe of possible options; if you see
a b
\ /
c
then you figure that the person doing the merge was picking from all
possible resolution values; when you see
a b
\ /
b
you figure that the user was just picking between the two options
given by the parents. My user model is too simple to take this into
account. It's not a huge extension to the model to do so; it's quite
possible that an algorithm could be devised that gave a clean merge
here, perhaps by separately tracking each node's nearest marked
ancestor and the original source of its value as two separate things.
Relation to other work

This algorithm is very close to the traditional codevillemerge
approach to this problem; the primary algorithmic difference is the
marking of conflict resolutions as being "changes". The more
important new stuff here, I think, are the user model and the proofs.
Traditionally, merge algorithms are evaluated by coming up with some
set of examples, eyeballing them to make some guess as to what the
"correct" answer was, comparing that to the algorithm's output, and
then arguing with people whose intuitions were different.
Fundamentally, merging is about deterministically guessing the user's
intent in situations where the user has not expressed any intent.
Humans are very good at guessing intent; we have big chunks of squishy
hardware designed to form sophisticated models of others intents, and
it's completely impossible for a VCS to try and duplicate that in
full. My suggestion here, with my "user model", is to seriously and
explicitly study this part of the problem. There are complicated
tradeoffs between accuracy (correctly modeling intention),
conservatism (avoiding incorrectly modeling intention), and
implementability (describing the user's thought processes exactly
isn't so useful if you can't apply it in practice). It's hard to make
an informed judgement when we don't have a name for the thing we're
trying to optimize, and hard to evaluate an algorithm when we can't
even say what it's supposed to be doing.
I suspect the benefit of the proofs is obvious to anyone who has spent
much time banging their head against this problem; until a few days
ago I was skeptical there _was_ a way to design a merge algorithm that
didn't run into problems like ambiguous clean merge.
I'm still skeptical, of course, until people read this; merging is
like crypto, you can't trust anything until everyone's tried to break
it... so let's say I'm cautiously optimistic . If this holds up,
I'm quite happy; between the user model and the proofs, I'm far more
confident that this does something sensible in all cases and has no
lurking edge cases than I have been in any previous algorithm. The
few problem cases I know of display a pleasing conservatism  perhaps
more cautious than they need to be, but even if they do cause an
occasional unnecessary conflict, once the conflict is resolved it
should stay resolved.
So... do your worst!
 Nathaniel

So let us espouse a less contested notion of truth and falsehood, even
if it is philosophically debatable (if we listen to philosophers, we
must debate everything, and there would be no end to the discussion).
 Serendipities, Umberto Eco
Replies and further discussion concerning this email can be found in the monotonedevel archives
Improvements to *merge
From: Nathaniel Smith <njs@...>
Subject: improvements to *merge
Newsgroups: gmane.comp.versioncontrol.revctrl, gmane.comp.versioncontrol.monotone.devel
Date: 20050830 09:21:18 GMT
This is a revised version of *merge:
http://thread.gmane.org/gmane.comp.versioncontrol.monotone.devel/4297
that properly handles accidental clean merges. It does not improve
any of the other parts, just the handling of accidental clean merges.
It shows a way to relax the uniqueness of the *() operator, while
still preserving the basic results from the above email. For clarity,
I'll say 'unique*merge' to refer to the algorithm given above, and
'multi*merge' to refer to this one.
This work is totally due to Timothy Brownawell <tbrownaw@...>.
All I did was polish up the proofs and write it up. He has a more
complex version at:
http://article.gmane.org/gmane.comp.versioncontrol.monotone.devel/4496
that also attempts to avoid the conflict with:
a
/ \
b* c*
\ / \
c* d*
and has some convergence in it, but the analysis for that is not done.
So:
User model

We keep exactly the same user model as unique*merge:
1) whenever a user explicitly sets the value, they express a claim
that their setting is superior to the old setting
2) whenever a user chooses to commit a new revision, they implicitly
affirm the validity of the decisions that led to that revision's
parents
Corollary of (1) and (2): whenever a user explicitly sets the
value, they express that they consider their new setting to be
superior to _all_ old settings
3) A "conflict" should occur if, and only if, the settings on each
side of the merge express parallel claims.
The difference is that unique*merge does not _quite_ fulfill this
model, because in real life your algorithm will automatically resolve
coincidental clean merge cases without asking for user input; but
unique* is not smart enough to take this into account when inferring
user intentions.
Algorithm

We start by marking the graph of previous revisions. For each node in
the graph, we either mark it (denoted by a *), or do not. A mark
indicates our inference that a human expressed an intention at this
node.
i) a* graph roots are always marked
a1
ii)  no mark, value was not set
a2
a
iii)  b != a, so 'b' node marked
b*
a b
iv) \ /
c*
'c' is totally new, so marked
a1 a2
\ /
c*
a b1 we're marking places where users expressed
v) \ / intention; so 'b' should be marked iff this
b2? was a conflict
a1 a2 'a' matches parents, and so is not marked
vi) \ / (alternatively, we can say this is a special
a3 case of (v), that is never a conflict)
Case (vi) is the only one that differs from unique* merge. However,
because of it, we must use a new definition of *():
Definition: By *(A), we mean we set of minimal marked ancestors of A.
"Minimal" here is used in the mathematical sense of a node in a graph
that has no descendents in that graph.
Algorithm: Given two nodes to merge, A and B, we consider four cases:
a) value(A) = value(B): return the shared value
b) *(A) > B: return value(B)
c) *(B) > A: return value(A)
d) else: conflict; escalate to user
Where "*(A) > B" means "all elements of the set *(A) are nonstrict
ancestors of the revision B". The right way to read this is as "try
(a) first, and then if that fails try (b), (c), (d) simultaneously".
Note that except for the addition of rule (a), this is a strict
generalization of the unique* algorithm; if *(A) and *(B) are
singleelement sets, then this performs _exactly_ the same
computations as the unique* algorithm.
Now we can say what we mean by "was a conflict" in case (v) above:
given a > b2, b1 > b2, we leave b2 unmarked if and only if
*(a) > b1.
Examples

1.
a1*
/ \
a2 b*
result: *(a2) = {a1}, a1 > b, so b wins.
2.
a*
/ \
b* c*
result: *(b) = {b}, *(c) = {c}, neither *(b) > c nor *(c) > b, so
conflict.
3.
a*
/ \
b1* b2*
\ / \
b3 c1*
result: *(b3) = {b1, b2}; b2 > c1, but b1 is not > c, so c does not
win. *(c1) = {c1}, which is not > b3. conflict.
note: this demonstrates that this algorithm does _not_ do convergence.
Instead, it takes the conservative position that for one node to
silently beat another, the winning node must preempt _all_ the
intentions that created the losing node. While it's easy to come up
with justso stories where this is the correct thing to do (e.g., b1
and b2 each contain some other changes that independently require 'a'
to become 'b'; c1 will have fixed up b2's changes, but not b1's), this
doesn't actually mean much. Whether this is good or bad behavior a
somewhat unresolved question, that may ultimately be answered by which
merge algorithms turn out to be more tractable...
4.
a*
/ \
b1* b2*
\ /
 X 
/ \
b3 c*
result: *(b3) = {b1, b2} > c. *(c) = {c}, which is not > b3. c wins
cleanly.
5.
a*
/ \
b1* c1*
/ \ / \
c2* X b2*
\ / \ /
c3 b3
result: *(c3) = {c1, c2}; c1 > b3 but c2 is not > b3, so b3 does not
win. likewise, *(b3) = {b1, b2}; b1 > c3 but b2 is not > c3, so c3
does not win either. conflict.
6.
a*
/ \
b1* c1*
/ \ / \
c2* X b2*
\ / \ /
c3 b3
\ /
 X 
/ \
c4* b4*
(this was my best effort to trigger an ambiguous clean merge with this
algorithm; it fails pitifully:)
result: *(c4) = {c4}, *(b4) = {b4}, obvious conflict.
Math

The interesting thing about this algorithm is that all the unique*
proofs still go through, in a generalized form. The key one that
makes *merge tractable is:
Theorem: In a graph marked by the above rules, given a node N, all
nodes in *(N) will have the same value as N.
Proof: By induction. We consider the cases (i)(vi) above. (i)
through (iv) are trivially true. (v) is interesting. b2 is marked
when *(a) not > b1. b2 being marked makes that case trivial, so
suppose *(a) > b1. All elements of *(a) are marked, and are
ancestors of b1; therefore, by the definition of *() and "minimal",
they are also all ancestors of things in *(b1). Thus no element of
*(a) can be a minimal marked ancestor of b2.
(vi) is also trivial, because *(a3) = *(a1) union *(a2). QED.
We also have to do a bit of extra work because of the sets:
Corollary 1: If *(A) > B, and any element R of *(B) is R > A, then
value(A) = value(B).
Proof: Let such an R be given. R > A, and R marked, imply that there
is some element S of *(A) such that R > S.
On the other hand, *(A) > B implies that S > B. By similar reasoning
to the above, this means that there is some element T of *(B) such
that S > T. So, recapping, we have:
nodes: R > S > T
from: *(B) *(A) *(B)
*(B) is a set of minimal nodes, yet we have R > T and R and T both in
*(B). This implies that R = T. R > S > R implies that S = R,
because we are in a DAG. Thus
value(A) = value(S) = value(R) = value(B)
QED.
Corollary 2: If *(A) > B and *(B) > A, then not only does value(A) =
value(B), but *(A) = *(B).
Proof: By above, each element of *(B) is equal to some element of
*(A), and viceversa.
This is good, because it means our algorithm is welldefined. The
only time when options (b) and (c) (in the algorithm) can
simultaneously be true, is when the two values being merged are
identical to start with. I.e., no somewhat anomalous "4th case" of
ambiguous clean merge.
Actually, this deserves some more discussion. With *() returning a
set, there are some more subtle "partial ambiguous clean" cases to
think about  should we be worrying about cases where some, but not
all, of the marked ancestors are preempted? This is possible, as in
example 5 above:
a*
/ \
b1* c1*
/ \ / \
c2* X b2*
\ / \ /
c3 b3
A hypothetical (convergence supporting?) algorithm that said A beats B
if _any_ elements of *(A) are > B would give an ambiguous clean merge
on this case. (Maybe that wouldn't be so bad, so long as we marked
the result, but I'm in no way prepared to do any sort of sufficient
analysis right now...)
The nastiest case of this is where *(A) > B, but some elements of *(B)
are > A  so we silently make B win, but it's really not _quite_
clear that's a good idea, since A also beat B sometimes  and we're
ignoring those user's intentions.
This is the nice thing about Corollary 1 (and why I didn't just
collapse it into Corollary 2)  it assures us that the only time this
_weak_ form of ambiguous clean can happen is when A and B are already
identical. This _can_ happen, for what it's worth:
a*
/\
/  \
/  \
/  \
b1* b2* d*
\ /\ /
 \ / \/
 X b3*
 / \ /
/ b4
b5
Here *(b5) = {b3, b2}, *(b6) = {b2, b4}. If we ignore for a moment
that b4 and b5 have the same value, this is a merge that b4 would win
and b5 would lose, even though one of b4's ancestors, i.e. b1, is
preempted by b5. However, it can _only_ happen if we ignore that
they have the same value...
The one other thing we proved about unique* merge also still applies;
the proof goes through wordforword:
Theorem: If A and B would merge cleanly with A winning, then any
descendent D of A will also merge cleanly with B, with D winning.
Proof: *(B) > A, and A > D, so *(B) > D.
Discussion

This algorithm resolves one of the two basic problems I observed for
unique* merge  coincidental clean merges are now handled, well,
cleanly, and the user model is fully implemented. However, we still
do not handle the unnamed case (you guys totally let me down when I
requested names for this case last time):
a
/ \
b* c*
\ / \
c* d*
which still gives a conflict. We also, of course, continue to not
support more exotic features like convergence or implicit rollback.
Not the most exciting thing in the world. OTOH, it does strictly
increase the complexity of algorithms that are tractable to formal
analysis.
Comments and feedback appreciated.
 Nathaniel

"The problem...is that sets have a very limited range of
activities  they can't carry pianos, for example, nor drink
beer."
Replies and further discussion concerning this email can be found in the monotonedevel archives.
More on "markmerge"
From: Timothy Brownawell <tbrownaw@...>
Subject: more on "markmerge"
Newsgroups: gmane.comp.versioncontrol.revctrl, gmane.comp.versioncontrol.monotone.devel
Prerequisite:
http://thread.gmane.org/gmane.comp.versioncontrol.monotone.devel/4297
A user can make 2 types of merge decisions:
(1): One parent is better than the other (represented by *)
(2): Both parents are wrong (represented by ^)
Since there are 2 types of merge decisions, it would be bad to treat all
merge decisions the same. Also, in the case of merge(a, a) = a, it is
possible for there to be multiple least decision ancestors.
=====
Define: ^(A) is the set of ancestors of A that it gets its value from
(found by setting N=A and iterating N = *(N) until there is no change)
*(A) is the set of least ancestors of A in which the user made a
decision
note that erase_ancestors(^(A)) = ^(A),
and erase_ancestors(*(A)) = *(A)
=====
& is intersection,  is union
*(A) has the same properties as before, except that it is not a single
ancestor, but a set. This set can acquire more than one member only in
the case of
Aa Ba
\ /
Ca
, where *(A) and *(B) are different; *(C) will be
erase_ancestors(*(A)  *(B))
The ancestory corollary becomes:
any ancestor C of A with value(C) != value(A) will be an ancestor of at
least one member of *(A)
When merging A and B:
# if one side knows of _all_ places that the other side was chosen, it
wins
(1)
set X = erase_ancestors(*(A)  *(B))
if X & *(B) = {}, A wins
if X & *(A) = {}, B wins
else, X contains members of both *(A) and *(B)
# if one side knows of _all_ places that the other side originated, it
wins
(2)
set Y = erase_ancestors(*(A)  ^(B))
set Z = erase_ancestors(*(B)  ^(A))
if Y & ^(B) = {} and Z & ^(A) = {}, conflict
if Y & ^(B) = {}, A wins
if Z & ^(A) = {}, B wins
# if one side knows of _any_ places that the other side originated, it
wins
(3)
if Y & ^(B) != ^(B) and Z & ^(A) != ^(A), conflict
if Y & ^(B) != ^(B), A wins
if Z & ^(A) != ^(A), B wins
# else, nobody knows anything
(4) conflict
(3) is convergence, and can be safely left out if unwanted
====
"Funky cases"
Coincidental clean does not exist; a mark is only needed when there is
user intervention.

a
/ \
b b
\ / \
b c
and the example after it will resolve cleanly iff (3) is included.

a
/ \
b* c*
/ \ / \
c* X b*
\ / \ /
c b
will be a conflict.
a
/ \
b* c*
\ / \
c* d*
This ("the other funky case") is handled by (2), and resolves cleanly.
Tim
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