This document discusses various algorithms for achieving mutual exclusion in distributed systems. It begins by defining mutual exclusion and describing how it is achieved in centralized systems using techniques like semaphores. It then presents several algorithms for distributed mutual exclusion, including a token ring algorithm, the Ricart-Agrawala algorithm using message passing and logical clocks, Lamport's algorithm also using message passing and timestamps, and a decentralized algorithm based on distributed hash tables. It analyzes the characteristics and tradeoffs of each approach.

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Mutual Exclusion*
Distributed Systems
Prepared By:
Er. Shikha Manrai

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Mutual Exclusion?
• A condition in which there is a set of
processes, only one of which is able to
access a given resource or perform a given
function at any time

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Centralized Systems
Mutual exclusion via:
– Test & set
– Semaphores
– Messages
– Monitors

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Distributed Mutual Exclusion
• Assume there is agreement on how a resource
is identified
– Pass identifier with requests
• Create an algorithm to allow a process to
obtain exclusive access to a resource

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Distributed Mutual Exclusion
• Centralized Algorithm
• Token Ring Algorithm
• Distributed Algorithm
• Decentralized Algorithm

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Centralized algorithm
• Mimic single processor system
• One process elected as coordinator
P
C
request(R)
grant(R)
1. Request resource
2. Wait for response
3. Receive grant
4. access resource
5. Release resource release(R)

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Centralized algorithm
If another process claimed resource:
– Coordinator does not reply until release
– Maintain queue
• Service requests in FIFO order
P0
C
request(R)
grant(R)
release(R) P1
P2
request(R)
Queue
P1
request(R)
P2
grant(R)

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Centralized algorithm
Benefits
• Fair
– All requests processed in order
• Easy to implement, understand, verify
Problems
• Process cannot distinguish being blocked from
a dead coordinator
• Centralized server can be a bottleneck

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Token Ring algorithm
Assume known group of processes
– Some ordering can be imposed on group
– Construct logical ring in software
– Process communicates with neighbor
P0
P1
P2
P3
P4
P5

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Token Ring algorithm
• Initialization
– Process 0 gets token for resource R
• Token circulates around ring
– From Pi to P(i+1)mod N
• When process acquires token
– Checks to see if it needs to enter critical section
– If no, send token to neighbor
– If yes, access resource
• Hold token until done
P0
P1
P2
P3
P4
P5
token(R)

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Token Ring algorithm
• Only one process at a time has token
– Mutual exclusion guaranteed
• Order well-defined
– Starvation cannot occur
• If token is lost (e.g. process died)
– It will have to be regenerated
• Does not guarantee FIFO order

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Ricart & Agrawala algorithm
• Distributed algorithm using reliable multicast
and logical clocks
• Process wants to enter critical section:
– Compose message containing:
• Identifier (machine ID, process ID)
• Name of resource
• Timestamp (totally-ordered Lamport)
– Send request to all processes in group
– Wait until everyone gives permission
– Enter critical section / use resource

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Ricart & Agrawala algorithm
• When process receives request:
– If receiver not interested:
• Send OK to sender
– If receiver is in critical section
• Do not reply; add request to queue
– If receiver just sent a request as well:
• Compare timestamps: received & sent messages
• Earliest wins
• If receiver is loser, send OK
• If receiver is winner, do not reply, queue
• When done with critical section
– Send OK to all queued requests

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Ricart & Agrawala algorithm
• N points of failure
• A lot of messaging traffic
• Demonstrates that a fully distributed
algorithm is possible

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Lamport’s Mutual Exclusion
Each process maintains request queue
– Contains mutual exclusion requests
Requesting critical section:
– Process Pi sends request(i, Ti) to all nodes
– Places request on its own queue
– When a process Pj receives
a request, it returns a timestamped ack
Lamport time

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Lamport’s Mutual Exclusion
Entering critical section (accessing resource):
– Pi received a message (ack or release) from every
other process with a timestamp larger than Ti
– Pi’s request has the earliest timestamp in its queue
Difference from Ricart-Agrawala:
– Everyone responds … always - no hold-back
– Process decides to go based on whether its
request is the earliest in its queue

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Lamport’s Mutual Exclusion
Releasing critical section:
– Remove request from its own queue
– Send a timestamped release message
– When a process receives a release message
• Removes request for that process from its queue
• This may cause its own entry have the earliest timestamp in
the queue, enabling it to access the critical section

18. Characteristics of Decentralized
Algorithms
 No machine has complete information about the system state
 Machines make decisions based only on local information
 Failure of one machine does not ruin the algorithm
 Three is no implicit assumption that a global clock exists

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Decentralized Algorithm
• Based on the Distributed Hash Table (DHT)
system structure previously introduced
– Peer-to-peer
– Object names are hashed to find the successor
node that will store them
• Here, we assume that n replicas of each
object are stored

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Placing the Replicas
• The resource is known by a unique name:
rname
– Replicas: rname-0, rname-I, …, rname-(n-1)
– rname-i is stored at succ(rname-i), where names
and site names are hashed as before
– If a process knows the name of the resource it
wishes to access, it also can generate the hash
keys that are used to locate all the replicas

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The Decentralized Algorithm
• Every replica has a coordinator that controls
access to it (the coordinator is the node that
stores it)
• For a process to use the resource it must
receive permission from m > n/2 coordinators
• This guarantees exclusive access as long as a
coordinator only grants access to one process
at a time

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The Decentralized Algorithm
• The coordinator notifies the requester when
it has been denied access as well as when it is
granted
– Requester must “count the votes”, and decide
whether or not overall permission has been granted
or denied
• If a process (requester) gets fewer than m
votes it will wait for a random time and then
ask again

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Analysis
• If a resource is in high demand, multiple
requests will be generated
• It’s possible that processes will wait a long
time to get permission
• Deadlock?
• Resource usage drops

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Analysis
• More robust than the central coordinator
approach and the distributed approaches. If
one coordinator goes down others are
available.
– If a coordinator fails and resets then it will not
remember having granted access to one requestor,
and may then give access to another. According to
the authors, it is highly unlikely that this will lead
to a violation of mutual exclusion. (See the text
for a probabilistic argument.)