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DISTRIBUTED LOCKING PROTOCOLS – OVERVIEW

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Non-Standard Database Systems Nikolaus Augsten

DISTRIBUTED LOCKING PROTOCOLS – OVERVIEW

Single-Lock Manager Primary Copy Majority Biased

Deadlock Handling Centralized Distributed Distributed Distributed

Availability (*) global SPoF

(central lock manager)

SPoF per data itemQ (primary copy ofQ)

No SPoF

(bn2c+ 1 replicas must be up)

Read: No SPoF

Write: all nreplicas of Q must be up

Bottleneck Yes No No No

Supports Replicas Yes Yes Yes Yes

# of Messages Read: 2 Write: 2 Unlock: 1

Read: 2 Write: 2 Unlock: 1

Read/Write: 2 bn2c+ 1 Unlock: bn2c+ 1

Read: 2 Write: 2n

Unlock: Read: 1, Write: n

Read from Replicas Any Any Any Any

Write to Replicas All All All All

SPoF Single Point of Failure

Q data item to be locked / unlocked n number of replicas for a data itemQ

(*) The availability discussion is limited to lock requests only. Even if a lock is granted, depending on the replication policy it may not be possible to write. For example, the majority protocol grants a write lock if more than half of the replicas are available, but the replication policy may require writes to be executed on all replicas, which requiresall replicas to be available.

Database Research Group Department of Computer Sciences

University of Salzburg

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