What do you mean by schedule in the context of concurrent execution of transactions in RDBMS? What is serializable schedule? Discuss the various types of serializability with a suitable example.

Introduction to Schedule in RDBMS

A schedule in the context of a Relational Database Management System (RDBMS) refers to the chronological order in which instructions (read, write, commit, abort) from transactions are executed. A transaction is a single logical unit of work that accesses and possibly modifies the contents of a database. Transactions access the database through a sequence of read and write operations. The concurrent execution of transactions leads to a schedule.

For example, consider two transactions T1 and T2. T1 reads an item X, writes an item X, reads an item Y, and writes an item Y. T2 reads an item Y, writes an item Y, reads an item X, and writes an item X. The schedule of these transactions can be represented as T1: R(X), W(X), R(Y), W(Y) and T2: R(Y), W(Y), R(X), W(X).

Serializable Schedule

A serializable schedule is a schedule that is equivalent to some serial execution of the transactions. Serializability is the classical concurrency scheme. It ensures that a schedule for executing concurrent transactions is equivalent to one that executes the transactions serially in some order. It is important in maintaining consistency in the database.

For example, consider the transactions T1 and T2 mentioned above. A serializable schedule would be one where either T1 is executed followed by T2 or T2 is executed followed by T1.

Types of Serializability

Conflict Serializability

Conflict serializability is a type of serializability where a schedule is conflict equivalent to a serial schedule. Two schedules are conflict equivalent if: for any pair of conflicting operations in the schedules, the order of their occurrence is the same.

For example, consider the transactions T1: R(X), W(X) and T2: R(X), W(X). A conflict serializable schedule would be one where either T1's operations are executed followed by T2's operations or vice versa.

View Serializability

View serializability is a type of serializability where a schedule is view equivalent to a serial schedule. Two schedules are view equivalent if: the same set of transactions reads the initial value of every item, for each item, the same transaction updates the item last in both schedules, and for each item, the set of transactions that read the updated value of the item is the same in both schedules.

For example, consider the transactions T1: R(X), W(X) and T2: R(X), W(X). A view serializable schedule would be one where either T1's operations are executed followed by T2's operations or vice versa, and the same set of transactions reads and writes the values of X.

Comparison of Conflict and View Serializability

In conclusion, a schedule and serializable schedule play a crucial role in the concurrent execution of transactions in an RDBMS. While conflict serializability is easier to implement, view serializability offers more flexibility. The choice between the two depends on the specific requirements of the database system.

Summary

A schedule in the context of a Relational Database Management System (RDBMS) refers to the chronological order in which instructions from transactions are executed. A serializable schedule is a schedule that is equivalent to some serial execution of the transactions. There are two types of serializability: conflict serializability and view serializability.

Analogy

Imagine a group of people waiting in line to buy movie tickets. Each person is a transaction, and the order in which they buy tickets is the schedule. A serializable schedule is one where the order of ticket purchases is the same as if each person bought their ticket individually, one after the other.