Recoverable schedules in database transaction

Recoverable schedules in database transaction

Recoverable schedules

A schedule is recoverable if every transaction in that schedule commits only after all the transactions whose changes they read have committed.

A schedule where, for each pair of transactions T_i and T_j, if T_j reads a data item previously written by T_i, then the commit operation of T_i precedes the commit operation of T_j.

For example, assume a schedule S with two transactions T₁, and T₂. If T₂ consumes a data item written by T₁ then T₁ must commit before T₂. If this happens then the schedule S is said to be recoverable schedule.

If a schedule should be recoverable, then there shouldn’t be any dirty reads [reading of uncommitted data].

Example:

Is the following transaction is recoverable or not?

Instruction	T1	T2
1 2 3 4 5 6	R(x) W(y) Commit	R(x) W(y) Commit

This schedule is recoverable because there are no dirty reads. Here, T₁ commits before T₂ but T₂ is not consuming any data that is written by T₁. Hence, this schedule is recoverable. Also, the order of commit is not an issue as neither T₁ nor T₂ has consumed data written by the other transactions. 

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