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Chapter 6. Concurrency control

Chapter 6. Concurrency control. Databases and Applications with Relaxed ACID Properties. •. Often Deadlock prevention is impossible:. Example: In e-commerce products may be put in the basket in any order. •. Pessimistic concurrency control:.

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Chapter 6. Concurrency control

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  1. Chapter 6. Concurrency control Databases and Applications with Relaxed ACID Properties •

  2. Often Deadlock prevention is impossible: Example:In e-commerce products may be put in the basket in any order. •

  3. Pessimistic concurrency control: • Locking is a pessimistic concurrency control, because access for a new transaction to data is blocked if there is a chance for conflict with old accesses. Therefore, there are no conflicts when a transaction is committed. (What is a conflict between transactions?) On the other hand, transactions may have waited for non existing conflicts or expierensed deadlock. •

  4. What is read-write conflicts? What is write-write conflicts?

  5. Optimistic concurrency control • Concurrency control is optimistic if access to committed data always is allowed. Therefore, there are no access to updated data before it is committed. • A transaction can only commit if it has no conflicts at the time of commit.(Backward validation). •

  6. The anomalies of SQL’s isolation levels

  7. Different lock durations: • Long duration locks = All locks are obtain before global commit and released after global commit • Short duration locks = All locks are obtained and released in local transactions executed both before and after global commit.Short duration locks does not lock data accross a dialog with the user or when there is a risk for deadlock.

  8. Long-lived transactions Long-lived transactions are transactions where only short duration locking are acceptable. • workflows are normally long-lived • in cooperative work there is a need for user notification which is a countermeasure to prevent anomalies. • applications such as cooperative CAD need user involvement in conflict resolution •

  9. Hotspots: A hotspot is a record normally updated by many concurrent transactions. In hotspots, pessimistic concurrency control is generally better than optimistic concurrency control (O’Neil, 1986). However, a hotspot is best managed by using short duration locking around the hotspot. Examples: • In e-commerce best-seller products may be hot spots • The generator of transaction sequense numbers.

  10. Methods to increase concurrency

  11. ERP system: Exercise What concurrency control method would you recommend in e-commerce systems?

  12. End of session Thank you !!!

  13. Evaluering af concurrency control metoderne: Design af DBMS/transaktioner så en rette information haves til rette tid til rette person/system på rette sted i rette kvalitet (consistency) til de mindste omkostninger.

  14. Short duration locking: Ved kort låsning låses der ikke på tværs af en brugerdialog/subtransaktioner samt ved risiko for deadlock og andre problemer, som gør låsetiden uforudsigelig. I denne situation må man bruge modforholdsregler mod de mulige anomalies samt kompensering på tværs af opdaterende subtransaktioner i stedet for roll back retablering. Eksempler: E-handel samt lagerstyring generelt.

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