On 23 Jul 2009, at 21:32, Mircea Markus wrote:
Hi,
Right now we support DLD in the following scenarios: local caches,
symmetric tx that create deadlocks over two replicated caches
(symmetric
in the sense that A replicates to B and B replicates to A at the same
time).
In the case of distribution (replication as well, especially async
one)
there might still be a deadlock if both A and B replicate to C, but
they
replicate transactions that would result in deadlocks, so deadlock
would
result in C.
Let's take an example, A wants to replicate Tx1 that affects (key1,
key2) in this sequence. B wants to replicate Tx2 that affects (key2,
key1) in this sequence. While replicating on C, Tx1 and Tx2 would
result
in a deadlock (classic scenario).
Now a simple way of solving this (if I'm not missing something!!) is
to
order the keys in the replicated transaction based on some criterion
(e.g. lexicographic) and apply them in the same sequence:
Tx1(key1,key2)
and Tx2(key1, key2). This will avoid deadlocks -> increase throughput.
Imposing Comparable on keys is too intrusive. And re: Galder's
suggestion on hashcodes, hashcodes may not be the same on different
JVM impls/OSs for JDK classes (e.g., String).
Now, at the core of this approach is the fact that the order of
operations in the transaction is not relevant - which does not stand
in
current implementation. E.g. let's say we have a tx that does
(remove(key), put(key, 'otherVal')). If we change the order result is
totally different - not good! A way to avoid this (and to also reduce
the amount of replication, by compacting changes by key) is to keep
the
modifications in a Map<key,Modification>. For each key we keep only
the
last value set within it, so if we modify the same key 1000 times
within
a tx we only replicate last modification vs current approach where
1000
operations are replicated. If a key is deleted, we still keep it in
the
map, with a marker for deletion. This way we only replicate the
delta of
modifications, and the order of operations is no longer relevant, so
that we can leverage previously described deadlock detection.
Isn't this expensive though? We spoke about compacting for JBC 3, and
the reason why we did not do this is because traversing through a
modification list each time a new mod is encountered was considered
unnecessarily expensive especially since there as no guarantee as to
whether modifications to the same key is a very common thing in the
same transaction.
Thinking about it again, it should be an O(1) problem though - if you
maintain a hash map of keys -> Modifications as you say. Ah, except
where you have PutMapCommands and ClearCommands in the modification
list... then you still need to traverse the mod list and test for
whether a modification affects a given key.
The advantages of this would be higher throughput by reducing the
chance
of 'asymmetric' deadlocks (A,B encounter a deadlock while
replicating on
C) and possible reduction in the size of the transaction (if it has
multiple operations on the same key). The drawback is mainly the fact
that some additional computation needs to be made to manage the map
content (not a lot I reckon, instead of a List.add we'll do an
map.get()
+ map.put(), for each modification within the transaction).
Wdyt?
Cheers,
Mircea
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Manik Surtani
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