Serializability in DBMS | Conflict Serializability

Schedules in DBMS-

 

Before you go through this article, make sure that you have gone through the previous article on Schedules in DBMS.

 

We have discussed-

  • A schedule is the order in which the operations of multiple transactions appear for execution.
  • Serial schedules are always consistent.
  • Non-serial schedules are not always consistent.

 

In DBMS, schedules may be classified as-

 

 

In this article, we will discuss about Serializability in DBMS.

 

Serializability in DBMS-

 

  • Some non-serial schedules may lead to inconsistency of the database.
  • Serializability is a concept that helps to identify which non-serial schedules are correct and will maintain the consistency of the database.

 

Serializable Schedules-

 

If a given non-serial schedule of ‘n’ transactions is equivalent to some serial schedule of ‘n’ transactions, then it is called as a serializable schedule.

 

Characteristics-

 

Serializable schedules behave exactly same as serial schedules.

Thus, serializable schedules are always-

 

Serial Schedules Vs Serializable Schedules-

 

Serial SchedulesSerializable Schedules
No concurrency is allowed.

Thus, all the transactions necessarily execute serially one after the other.

Concurrency is allowed.

Thus, multiple transactions can execute concurrently.

Serial schedules lead to less resource utilization and CPU throughput.Serializable schedules improve both resource utilization and CPU throughput.
Serial Schedules are less efficient as compared to serializable schedules.

(due to above reason)

Serializable Schedules are always better than serial schedules.

(due to above reason)

 

Types of Serializability-

 

Serializability is mainly of two types-

 

 

  1. Conflict Serializability
  2. View Serializability

 

In this article, we will discuss about Conflict Serializability.

Learn about View Serializability.

 

Conflict Serializability-

 

If a given non-serial schedule can be converted into a serial schedule by swapping its non-conflicting operations, then it is called as a conflict serializable schedule.

 

Conflicting Operations-

 

Two operations are called as conflicting operations if all the following conditions hold true for them-

  • Both the operations belong to different transactions
  • Both the operations are on the same data item
  • At least one of the two operations is a write operation

 

Example-

 

Consider the following schedule-

 

 

In this schedule,

  • W1 (A) and R2 (A) are called as conflicting operations.
  • This is because all the above conditions hold true for them.

 

Checking Whether a Schedule is Conflict Serializable Or Not-

 

Follow the following steps to check whether a given non-serial schedule is conflict serializable or not-

 

Step-01:

 

Find and list all the conflicting operations.

 

Step-02:

 

Start creating a precedence graph by drawing one node for each transaction.

 

Step-03:

 

  • Draw an edge for each conflict pair such that if Xi (V) and Yj (V) forms a conflict pair then draw an edge from Ti to Tj.
  • This ensures that Ti gets executed before Tj.

 

Step-04:

 

  • Check if there is any cycle formed in the graph.
  • If there is no cycle found, then the schedule is conflict serializable otherwise not.

 

NOTE-

 

 

Next Article- Practice Problems On Conflict Serializability

 

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Summary
Serializability in DBMS | Conflict Serializability
Article Name
Serializability in DBMS | Conflict Serializability
Description
Serializability in DBMS identifies the non-serial schedules that will maintain the database consistency. Conflict Serializability & View Serializability are the two types of Serializability in DBMS.
Author
Publisher Name
Gate Vidyalay
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