Blocking of concurrency control

What is blockade

Blockade is business T On a data object （ Such as table 、 Records, etc. ） Before the operation , Make a request to the system first , Lock it
After lock transaction T It has certain control over the data object , In the transaction T Before releasing its lock , Other transactions cannot update this data object .
Blocking is a very important technology to realize concurrency control

Basic blockade types

What kind of control a transaction has after locking a data object is determined by the type of blocking .
Basic blockade types

Exclusive lock （Exclusive Locks, Short for X lock ）

Exclusive lock is also called write lock
If business T For data objects A add X lock , Only T Read and modify A, Nothing else can be done to A Add any type of lock , until T Release A The lock on the
Make sure other things are in T Release A The lock on can't be read or modified before A

Shared lock （Share Locks, Short for S lock ）

Shared locks are also called read locks
If business T For data objects A add S lock , The transaction T You can read A But it can't be modified A, Other things can only be done to A Add S lock , Instead of X lock , until T Release A Upper S lock
Make sure other transactions are readable A, But in T Release A Upper S You can't do it before you lock it A Make any changes

Compatibility matrix of lock

Y=Yes, Compatible requests
N=No, Incompatible requests
In the compatibility matrix of locks ：
The leftmost column represents transactions T1 The type of lock on the acquired data object , The horizontal line indicates that it is not locked .
The top line represents another transaction T2 Blocking request for the same data object .
T2 Whether the blocking request can be satisfied is used in the matrix Y and N Express
Y Presentation transaction T2 The blockade requirements and T1 Lock held , The blocking request can satisfy
N Express T2 Your blocking request is related to T1 Lock already held conflicts ,T2 's request was rejected
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