A deck on how Transactions work in Apache Geode (incubating).

1. Transactions
Swapnil Bawaskar
(Incubating)

2. Agenda
• Semantics
• Implementation
– Replicated Regions
– Partitioned Region
– From Client
• Failure Handling
• Road Map
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3. Semantics
• Repeatable Read
– Thread sees own changes
– Other threads do not until commit is called
• Optimistic
– No Entry level locks, Readers not blocked
– Conflict Detection
• NOT Persistent (yet)
– Not a problem if you have at-least one member up
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4. API
BASIC
CacheTransactionManager mgr = cache.getCacheTransactionManager();
mgr.begin();
region1.put("K1", "V1");
region1.put("K2", "V2");
region2.put("K2", "V2");
mgr.commit();
SUSPEND/RESUME
TransactionId txId = mgr.suspend();
… other non-transactional work
mgr.resume(txId);
mgr.tryResume(…);
SINGLE ENTRY
region.putIfAbsent(K, V);
region.replace(K, V, V);
region.remove(K, V);
OTHER
mgr.addListener(new MyTransactionListener());
mgr.setWriter(new TransactionWriter());
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5. Implementation
• Isolation through ThreadLocal
– Copy existing
reference in
ThreadLocal
– Perform conflict
check under lock
at commit time
Region
Thread
TXState
Entry
Reference
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6. At Commit
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7. Notes
• copy-on-read must be set to true
• D-lock grantor single point of contention
• Non-transactional threads may see
intermediate state (non atomic)
– Reads should be done within a transaction
– Set system property Dgemfire.detectReadConflicts=true
• Prone to ABA problem
• Faster than doing individual operations
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8. Failure Scenarios
• Replica Fails
– No problem, It will do a GII from other members
• Coordinator Fails
– Replicas gossip to arrive at the outcome of the
transaction
• If no member has “Apply Commit” message, some
members missing commit set, Abort transaction
• If at-least one member has “Apply Commit” message,
all members have commit set, apply transaction
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9. PR HostPR Host
Thread
TX State
Partitioned Region
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PR Host

10. PR HostPR Host
Thread
TX State
PR Host
Partitioned Region
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11. PR HostPR Host
Thread
TX State
PR Host
Partitioned Region
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12. Partitioned Region
• TX State on member with Primary copy (TX HOST)
• Only one TX Host per transaction
– First operation in TX establishes the host
– All subsequent operations (even for Replicate Regions)
sent to the same host
– Throws TransactionDataNotColocatedException if TX Host is
not primary
• D-lock service is striped
– TX Locks are local, no messaging
– No single point of contention in the system
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13. Data Colocation
• Inspiration
“For scalability, applications should manipulate single
collection of data that lives on one JVM”
- Pat Helland (Life Beyond Distributed Transactions)
- Custom Partitioning
- Within one Partitioned Region
- E.g. All trades in January
- Data Colocation
- Between Two or more Partitioned Regions
- All Orders of a Customer
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14. Failure Scenarios
• Failures before Commit
– TX Host Crashes (TransactionDataNodeHasDeparted)
– On Re-balance (TransactionDataRebalanced)
• Entire transaction should be re-tried
• Failures after Commit
– TX Host Crashes
» Replicas would have applied all or none changes
» Consistent but Outcome unknown TransactionInDoubt
– Replica Crashes, Succeeds
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15. • All operations sent to the server
• If necessary, server delegates to primary
• HA supported when delegate fails
Client
Delegate TX Host
TX State
Client Initiated
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16. Feature Interaction
• Eviction/Expiration
– Entry is reference counted
– Entry kept around if reference count > 0
• OQL
– Does not honor Repeatable Read
• Persistence
– gemfire.ALLOW_PERSISTENT_TRANSACTIONS=true
– No delineation on Disk
– Works as long as one replica survives
• Functions
– Can begin, commit, suspend/resume transactions
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17. Handling Failure
• Types of Exception
– CommitConflictException
– TransactionDataNodeHasDepartedException
– TransactionDataNotColocatedException
– TransactionDataRebalancedException
– TransactionInDoubtException
• Catch TransactionException and retry in loop
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18. JTA
• Enlists as a Synchronization with external JTA
Managers
– Last to prepare, first to Commit
• Has a JCA adapter for Last Resource Commit
with Weblogic.
• Has an implementation of JTA Manager
– not production grade
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19. Road Map
• Distributed Transactions (GEODE-16)
• XA Data Source
• Persistent Transactions
• OQL/Query Engine support
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20. Questions?
http://geode.incubator.apache.org/
(sbawaskar at apache dot org)