Class KetchLeader
- java.lang.Object
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- org.eclipse.jgit.internal.ketch.KetchLeader
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public abstract class KetchLeader extends Object
A leader managing consensus across remote followers.A leader instance starts up in
KetchLeader.State.CANDIDATE
and tries to begin a new term by sending anElectionRound
to all replicas. Its term starts if a majority of replicas have accepted this leader instance for the term.Once elected by a majority the instance enters
KetchLeader.State.LEADER
and runs proposals offered toqueueProposal(Proposal)
. This continues until the leader is timed out for inactivity, or is deposed by a competing leader gaining its own majority.Once timed out or deposed this
KetchLeader
instance should be discarded, and a new instance takes over.Each leader instance coordinates a group of
KetchReplica
s. Replica instances are owned by the leader instance and must be discarded when the leader is discarded.In Ketch all push requests are issued through the leader. The steps are as follows (see
KetchPreReceive
for an example):- Create a
Proposal
with theReceiveCommand
s that represent the push. - Invoke
queueProposal(Proposal)
on the leader instance. - Wait for consensus with
Proposal.await()
. - To examine the status of the push, check
Proposal.getCommands()
, looking atCommand.getResult()
.
The leader gains consensus by first pushing the needed objects and a
RefTree
representing the desired target repository state to therefs/txn/accepted
branch on each of the replicas. Once a majority has succeeded, the leader commits the state by either pushing therefs/txn/accepted
value torefs/txn/committed
(for Ketch-aware replicas) or by pushing updates torefs/heads/master
, etc. for stock Git replicas.Internally, the actual transport to replicas is performed on background threads via the
KetchSystem
's executor service. For performance, theKetchLeader
,KetchReplica
andProposal
objects share some state, and may invoke each other's methods on different threads. This access is protected by the leader'slock
object. Care must be taken to prevent concurrent access by correctly obtaining the leader's lock. - Create a
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Nested Class Summary
Nested Classes Modifier and Type Class Description static class
KetchLeader.State
Current state of the leader instance.
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Constructor Summary
Constructors Modifier Constructor Description protected
KetchLeader(KetchSystem system)
Construct a leader for a Ketch instance.
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Method Summary
All Methods Instance Methods Abstract Methods Concrete Methods Modifier and Type Method Description protected abstract Repository
openRepository()
Get an instance of the repository for use by a leader thread.void
queueProposal(Proposal proposal)
Queue a reference update proposal for consensus.void
setReplicas(Collection<KetchReplica> replicas)
Configure the replicas used by this Ketch instance.void
shutdown()
Gracefully shutdown this leader and cancel outstanding operations.LeaderSnapshot
snapshot()
Snapshot this leaderString
toString()
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Constructor Detail
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KetchLeader
protected KetchLeader(KetchSystem system)
Construct a leader for a Ketch instance.- Parameters:
system
- Ketch system configuration the leader must adhere to.
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Method Detail
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setReplicas
public void setReplicas(Collection<KetchReplica> replicas)
Configure the replicas used by this Ketch instance.Replicas should be configured once at creation before any proposals are executed. Once elections happen, reconfiguration is a complicated concept that is not currently supported.
- Parameters:
replicas
- members participating with the same repository.
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openRepository
protected abstract Repository openRepository() throws IOException
Get an instance of the repository for use by a leader thread.The caller will close the repository.
- Returns:
- opened repository for use by the leader thread.
- Throws:
IOException
- cannot reopen the repository for the leader.
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queueProposal
public void queueProposal(Proposal proposal) throws InterruptedException, IOException
Queue a reference update proposal for consensus.This method does not wait for consensus to be reached. The proposal is checked to look for risks of conflicts, and then submitted into the queue for distribution as soon as possible.
Callers must use
Proposal.await()
to see if the proposal is done.- Parameters:
proposal
- the proposed reference updates to queue for consideration. Once execution is complete the individual reference result fields will be populated with the outcome.- Throws:
InterruptedException
- current thread was interrupted. The proposal may have been aborted if it was not yet queued for execution.IOException
- unrecoverable error preventing proposals from being attempted by this leader.
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snapshot
public LeaderSnapshot snapshot()
Snapshot this leader- Returns:
- snapshot of this leader
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shutdown
public void shutdown()
Gracefully shutdown this leader and cancel outstanding operations.
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