Apache Jackrabbit Oak offers better horizontal scalability and concurrency than its predecessor, Apache Jackrabbit 2. The downside of which is increased chances of conflicts between concurrent updates. In this session I demonstrate how to deal with such conflicts by taking advantage of Oak's underlying consistency model. I will show how to build functionality like counting, voting, rating, negotiating, bidding, etc. common to collaborative applications. Such functionality traditionally requires some form of global consensus (e.g. locking, atomic commit protocols, ...). I will show how with Oak it is often possible to avoid conflicts all together by choosing the right content model. For cases where this is not possible I will discuss the mechanisms that Oak provides to deal with conflicts while they occur and after the fact.

1. APACHE SLING&FRIENDSTECHMEETUP
BERLIN, 28-30SEPTEMBER 2015
ConflicthandlingwithOak
Michael Dürig, Adobe Research

2. Agenda
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 Why?
 What?
 How?
 Conclusion
https://www.flickr.com/photos/35168673@N03/3792471453/

3. All you need toknow about JCR
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 Hierarchical database
 Oak implementsJCR
 Pluginsfor customisation

4. adaptTo()2015 4
Why?

5. Collaboration causesconflicts
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 Collaborative applications
 Internetscale applications
 Scalability
 Weak consistency

6. adaptTo()2015 6
What?

7. Whatis a conflict?
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 Conflictsemantics
 Back-end
 Application
 Incompatible, concurrent updates

8. Resource inJCR
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 Identifiedby path
 No conflicts between
 Nodes and properties
 Items with different names

9. Incompatibleupdates
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Change Add Remove
Remove NA
Add NA
Change

10. Saving changes
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 Rebase
 Handleconflicts
 Built-in
 Custom
 Persist or fail
 Run commit hooks HEAD

11. adaptTo()2015 11
How?

12. Conflicthandling strategies
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Lock Retry Resolve Avoid
Pessimistic Optimistic Proactive Anticipatory
Consensus Bruteforce Resolving Contentionfree
Wasteful Wasteful Economical Economical
Serial Parallel
Transparent Nottransparent Transparent Nottransparent
Constrained Constrained

13. Atomiccounter
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 Use cases
 Rating
 Booking
 Buildingblocks
 Up/down votes
 Average

14. Naïve implementation
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void increment(long delta) {
while (true) {
Property counter = node.getProperty("count");
long count = counter.getLong();
counter.setValue(count + delta);
try {
counter.getSession().save();
break;
} catch (RepositoryException ignore) { }
}
}

15. Avoiding conflicts
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 Datastructure
 Avoid application conflicts
 Leverage back-end

16. Share nothing
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 Counterper process
 Sumof counters
 Accumulatevia commithook

17. Usage
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void increment(long delta) {
session.getNode("/counter")
.setProperty("oak:increment", delta);
session.save();
}
long get() {
session.getNode("/counter")
.getProperty("oak:counter").getLong();
}

18. Accumulate
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public class AtomicCountEditor extends DefaultEditor {
@Override
public void propertyAdded(PropertyState after) {
...
}
@Override
public void leave(NodeState before, NodeState after) {
...
}
}

19. Accumulate
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public class AtomicCountEditor extends DefaultEditor {
private final NodeBuilder builder;
private long total;
public AtomicCountEditor(NodeBuilder builder) {
this.builder = builder;
total = builder.getProperty("oak:counter").getValue(LONG);
}
...
}

20. Accumulate
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public void propertyAdded(PropertyState after) {
if ("oak:increment".equals(after.getName()) {
total += after.getValue(LONG);
builder.removeProperty("oak:increment");
}
}
public void leave(NodeState before, NodeState after) {
builder.setProperty("oak:counter", total);
}

21. Demo
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https://www.flickr.com/photos/ruiwen/3260095534/

22. From here...
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 Bidding
 Maximum instead of addition
 Shopping
 Set union insteadof addition
 Signal
 Logical or insteadof addition

23. Resolvingconflicts
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 Conflicthandler
 Close to source
 Noretry

24. Multi-valueregister
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 Store conflictingvalues
 Materialiseconflict
 Delegate resolution
 Additional round trip

25. Usage
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session1.getNode("/mv").setProperty("value", 1);
session2.getNode("/mv").setProperty("value", 2);
session1.save();
session2.save();
session3.getProperty("/mv/value"); // Multi valued [1, 2]

26. Conflicthandler
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public interface PartialConflictHandler {
Resolution addExistingProperty(...);
Resolution changeDeletedProperty(...);
Resolution changeChangedProperty(...);
Resolution deleteDeletedProperty(...);
Resolution deleteChangedProperty(...);
Resolution addExistingNode(...);
Resolution changeDeletedNode(...);
Resolution deleteChangedNode(...);
Resolution deleteDeletedNode(...);
}
enum Resolution {
OURS,
THEIRS,
MERGED
}

27. Conflicthandler
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@Override
public Resolution addExistingProperty(
NodeBuilder parent,
PropertyState ours,
PropertyState theirs) {
parent.setProperty(
ours.getName(),
union(getValues(ours), getValues(theirs)), LONGS);
return Resolution.MERGED;
}
@Override
public Resolution changeChangedProperty(
NodeBuilder parent,
PropertyState ours,
PropertyState theirs) {
parent.setProperty(
ours.getName(),
union(getValues(ours), getValues(theirs)), LONGS);
return Resolution.MERGED;
}

28. Conflicthandler
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@Override
public Resolution changeDeletedProperty(...) {
return Resolution.OURS;
}
@Override
public Resolution deleteChangedProperty(...) {
return Resolution.THEIRS;
}
@Override
public Resolution deleteDeletedProperty(...) {
return Resolution.MERGED;
}

29. Demo
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https://www.flickr.com/photos/ruiwen/3260095534/

30. adaptTo()2015 30
Conclusion

31. Conclusion
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 Avoid conflicts
 Private copy
 Accumulate
 Resolve conflicts
 Custom handler
 Prevent retry

32. Thank you
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 https://github.com/mduerig/oak-crdt