Two stage locking protocol for concurrency control

The database management system generally adopts the method of two-stage lock protocol to realize the serializability of concurrent scheduling , So as to ensure the correctness of scheduling

Two stage lock protocol

It means that all transactions must be divided into two phases to lock and unlock data items
Reading any data 、 Before writing operations , The first thing a transaction needs to do is get a block on that data
After releasing a blockade , The transaction no longer applies for and obtains any other blockages

“ Two paragraphs ” The meaning of lock

The transaction is divided into two stages
The first stage is to get the blockade , Also known as the extension phase
Transactions can apply for any type of lock on any data item , But you can't release any locks
The second stage is to release the blockade , Also known as the contraction phase
A transaction can release any type of lock on any data item , But you can't apply for any more locks

example

Business Ti Comply with the two-stage lock protocol , The blocking sequence is ：
Slock A Slock B Xlock C Unlock B Unlock A Unlock C;
|← Expansion stage →| |← Contraction stage →|
Business Tj Failure to comply with the two-stage lock agreement , The blocking sequence is ：
Slock A Unlock A Slock B Xlock C Unlock C Unlock B;

The scheduling of graph follows the two-stage locking protocol , Therefore, it must be a feasible scheduling .
How to validate ？
It is a sufficient condition for serializable scheduling that transactions obey two-stage locking protocol , It's not necessary .
If both concurrent transactions follow the two-stage locking protocol , Any concurrent scheduling strategy for these transactions is serializable
If a schedule of concurrent transactions is serializable , Not all transactions are subject to the two-stage locking protocol

Two stage locking protocol and one-time blocking method to prevent deadlock

One time blocking requires every transaction to lock all data to be used at one time , Otherwise, we can't carry on , Therefore, the one-time blocking law abides by the two-stage locking agreement
However, the two-stage locking protocol does not require the transaction to lock all data to be used at one time , Therefore, transactions complying with the two-stage locking protocol may deadlock

[ example ] A transaction that abides by the two-stage lock protocol has a deadlock

Welcome to join me for wechat exchange and discussion （ Please note csdn Add ）