Transactions

STUDY
PLAY

Terms in this set (...)

Advantages of Interleaving
The performance of the system is better if the CPU is working on several transactions, since disk storage is slower. User should not have to wait for larger transactions to compute first.
Transaction
A sequence of R,W operations that must be processed as a unit or not at all
Consistent State
-Satisfies the integrity constrains described by the schema. - Satisfies other constrains required by the system that might not be explicitly stated as integrity constraints
NAME THE ACID
Atomicity, Consitency, Isolation, Durability
Atomicity
A transaction must be either 1: executed entirely or 2: not executed at all. It should not be possible to execute one part of a transaction and not another.
Consistency
If we assume thaat the database is in a consisten state before the transaction is executed, then the database must remain in a consisten state after the transaction has committed.
Isolation
Transactions executed concurrently must not interact. A transaction connot exchange messages with another transaction. Each transaction must feel as if it is the only transaction being executed at a time. Interaction between transactions negates that condition.
Durability
The effects of a transaction become permanent after it has
committed.
Atomicity responsible
The DBMS must make sure that a given transaction is executed "all or
nothing". If there are problems while executing a transaction, and the transaction cannot
be entirely executed, it must be aborted and its effects rolled-back
Consistency responsible
The programmer is responsible for writing transactions in such a way
that after their execution, the database remains in a consistent state. Notice that not only
those constraints given in the schema determine consistency (example: a transaction
that is supposed to transfer balance from one account to another, but that does not
correctly compute the balance to be written on one of the accounts, would still satisfy
integrity constraints).
Isolation responsible
The DBMS will provide the programmer with the choice of several levels of
isolation (when to make changes accessible to other transactions). The concurrency
control of the DBMS will enforce the desired level of isolation. Depending on the level of
isolation, different types of problems (anomalies) might occur. The programmer must
not allow transactions to exchange messages
Durability responsible
The DBMS must make sure that the effects of a transaction are permanent
even in the case of system failure.
Schedule
A schedule is a description of order in which interleaved transactions are executed.
Interleaving
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Key
K is a candidate key of R <=> K is a minimal subset of R such that K->R
Superkey
Y is a superkey of R <=> K subset of Y and K is a candidate key of R
ISO: Read uncommitted
No S-locks are used Allows all of the anomalies described before
Read commited
S-lock is needed to Read and released after each Read. Allows unrepeatable reads
Repeatable Read
S-lock is needed to read and released after the XACT has commited. Allows phantom reads