Synchronization

Physical Clocks Solar Physical Clocks Cesium Clocks International Atomic Time Universal Coordinate Time (UTC) Clock Synchronization Algorithms –One machine is connected to WWV –No machine is receiving WWV time

Election Algorithms Many distributed systems require one process to act as coordinator Assume: –All processes are the same –Each process has a process number –Each process knows the process number of every other process

The Bully Algorithm When a process P notices the coordinator is no longer responding it initiates and election –P sends ELECTION to every process with a higher number –If no one responds P wins and becomes coordinator –If one of the higher-numbered processes responds then it takes over and P is done

The bully algorithm

Ring Algorithm The process noticing that the coordinator is not functioning builds an ELECTION message containing its process number and sends it its successor When the message gets back to the process that started it, its type is changed to COORDINATOR and sent around the ring The process with the largest number is elected.

Ring Algorithm

Atomic Transactions A transaction is either committed completely or nothing is done (all-or- nothing) Sample tape based application

Atomic Transaction System consists of some number of independent processes Each process can fail at random Assume communication errors are handled by underlying software

Storage types Volatile RAM memory Disk storage Stable storage –Implemented using two or more disks –In case a failure, the second disk is used for recovery

Stable storage

Transaction primitives BEGIN TRASACTION END TRANSACTION ABORT READ WRITE

Example Transaction

Properties of Transactions Serializability Concurrent transactions do not interfere with each other Atomicity To the outside world, the transaction happens indivisibly Permanence Once a transaction commits, the changes are permanent

Nested Transactions The transactions can be contain sub- transactions Either all sub-transactions should commit or abort Each sub-transaction is given a private copy of data

Implementation Private Workspace –The copying is expensive –Optimizations for read and write are available No copy if no write is going on Only copy the block list of files and update modified blocks

Private Workspace

Writeahead Log A list called an intension list or LOG is created with TransID, BlockNo, OldVal, NewVal fields After the log has been successfully written, the changes are made to the file In case of ABORT, the operation are reversed from last to first

Writeahead LOG

Two Phase Commit Send each subordinate a message A subordinate responds when it is ready to commit If all subordinates are ready then commit else abort

Two Phase Commit

Concurrency Control Locking –A process can lock a file before a transaction –Smaller piece of data can be lock for efficiency (Granularity of locking) –Two phase locking (growing and shrinking) can be used to avoid deadlocks and cascaded aborts

Optimistic concurrency Control Perform any transaction without considering other processes When committing the transaction, check if the files have been changed or not It is DeadLock Free In case of failure the transaction should re- run

Time stamp Assign a time stamp to each transaction at the moment it begins On committing check the orders –If orders true then commit –Else abort

Time stamp

Questions?