Concurrency www.thoughts-on-java.org Databases try to isolate - - PowerPoint PPT Presentation

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Mar 17, 2023 153 likes •238 views

Concurrency www.thoughts-on-java.org Databases try to isolate concurrent transactions Concurrency Mechanism and isolation levels are database specific Hibernate inherits concurrency control from the database Uses database features

SLIDE 1

Concurrency

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SLIDE 2

Concurrency

Databases try to isolate concurrent transactions
Mechanism and isolation levels are database specific
Hibernate inherits concurrency control from the database
Uses database features to lock specific records

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SLIDE 3

Isolution Issues

Lost update
2 transactions update record without isolation
Rollback of transaction 2 removes changes of transaction 1
Dirty read
Transaction 1 reads uncommitted changes of transaction 2

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SLIDE 4

Isolation Issues

Unrepeatable read
Transaction reads a record twice and gets different states
Special case: Last commit wins
Transaction 2 overwrites changes of transaction 1
Phantom read
2nd execution of query returns data which wasn‘t visible

before

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SLIDE 5

Isolation Levels

Defined by ANSI SQL standard
Read uncommitted isolation
Allows dirty reads but not lost updates
Read committed isolation
Allows unrepeatable reads but not dirty reads
JPA default isolation level

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SLIDE 6

Isolation Levels

Repeatable read isolation
Allows phantom reads but not unrepeatable or dirty reads
Serializable isolation
Emulates serial execution of transactions

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SLIDE 7

Locking

Optimistic locking
Detects concurrent modifications during commit
Requires no additional database locks
Pessimistic locking
Locks records in the database to prevent concurrent

modifications