CONCURRENCY CONTROL

CONCURRENCY CONTROL

1. Problem Definition

   • Objective
     • Maximize transaction throughput(work perform) while preventing interference among multiple users
     • Throughput: number of transactions processed per unit time eg >1million transaction per second
   • Constraint
     • No interference: result same as serial/sequential effect
     • Interference occurs on commonly manipulated data known as hot spots

2. Concurrency Control Problem

   • Lost Update Problem e.g:slide no.11&12
     • Preventing Lost Update Problem Using 2PL e.g:slide no.28
   • Uncommitted Dependency Problem e.g:slide no.13&14
     • Preventing Uncommitted Using 2PL e.g:slide no.29

3. Concurrency Control Tools

   • Locking
     • Obtain lock before accessing an item
     • Wait if a conflicting lock is held
       • Shared lock (S lock): conflicts with exclusive locks
       • Exclusive lock (X lock): conflicts with all other kinds of locks
     • Locking Conflict e.g:slide no.22
     • Locking Granularity e.g:slide no 23
   • Deadlock
     • Deadlock(Mutual Waiting) e.g:slide no.24
     • Deadlock Resolution
       • Detection
         • Overhead is reasonable for deadlocks among 2 or 3 transactions
       • Timeout
         • Waiting limit
         • Can abort transactions that are not deadlocked
         • Timeout interval is difficult to determine