Type of fault

Failure within a transaction

Failure within a transaction

Some can be found through the transaction program itself ( See the example of transfer transaction below )
Some are unexpected , That cannot be handled by a transactional program .
for example , Bank transfer transactions , This transaction transfers an amount from one account a to another account b .

 BEGIN TRANSACTION
	       Read the balance of account a BALANCE;
       BALANCE=BALANCE-AMOUNT;    /*AMOUNT  Is the transfer amount */
       IF(BALANCE < 0 ) THEN
             {
     Print ‘ Insufficient amount , Can't transfer ’;
                                         /* Internal transactions may cause transactions to be rolled back */
              ROLLBACK;                        /* Undo the changes just made , Resume business */ 
             }
       ELSE
             {
     Read the balance of account b BALANCE1;
               BALANCE1=BALANCE1+AMOUNT;
                Write back to BALANCE1;
               COMMIT;}

The two update operations included in this example are either completed or not done at all . Otherwise, the database will be in an inconsistent state , For example, only the balance of account a is reduced, but the balance of account B is not increased .
In this procedure, if the balance of account a is insufficient , The application can discover and roll transactions back , Undo the changes made , Restore the database to the correct state .
More failures within the transaction are unexpected , It can't be handled by an application .
Operation overflow
A concurrent transaction has a deadlock and is selected to undo the transaction
Termination for violation of certain integrity restrictions, etc
in the future , Transaction failure only refers to this kind of unexpected failure

Transaction failure means

The transaction did not reach the expected end point (COMMIT Or explicitly ROLLBACK)
The database may be in an incorrect state .

Recovery of transaction failure ： Transaction undo （UNDO）

Force rollback （ROLLBACK） This transaction
Undo any changes made to the database by the transaction , Make the transaction seem to have not started at all

System failure

System failure

It's called soft fault , It refers to any event that causes the system to stop working , bring
The system needs to restart .
The normal operation of the whole system was suddenly destroyed
All running transactions terminate abnormally
Do not destroy the database
All information in the database buffer in memory is lost

Common causes of system failures

Specific types of hardware errors （ Such as CPU fault ）
Operating system failure
Database management system code error
System power failure

Recovery of system failure

In the event of a system failure , The results of some unfinished transactions may have been sent to the physical database , This may cause the database to be in an incorrect state .
Recovery strategy ： When the system restarts , The recovery program rolls back all abnormally terminated transactions , Forcibly withdraw （UNDO） All outstanding transactions
In the event of a system failure , Some completed transactions may be partially or even completely left in the buffer , Not yet written back to the physical database on disk , The system failure makes some or all of the modifications of these transactions to the database lost
Recovery strategy ： When the system restarts , The recovery program needs to be redone （REDO） All committed transactions

Media failure

Media failure

It's called hard fault , Refers to external memory failure
Disk damage
Head collision
Instantaneous strong magnetic field interference
Media failure destroys the database or part of the database , And affect all transactions that are accessing this data
Medium failure is much less likely than the first two types of failure , But it's much more destructive

Computer virus

Computer virus

An artificial failure or destruction , It's a computer program developed by some pranksters
It can reproduce and spread , Cause harm to computer system, including database

Types of computer viruses

The only small viruses are 20 Orders , Less than 50B
A large virus is like an operating system , It consists of tens of thousands of instructions

The harm of computer virus

Some viruses spread quickly , Once you invade the system, destroy the system immediately
Some viruses have a long incubation period , Computers don't start to get sick until a few days or months after infection
Some viruses infect all the programs and data of the system
Some are only interested in certain specific programs and data
Computer viruses have become a major threat to computer systems , Naturally, it is also the main threat to the database system
Once the database is damaged, we still need to use recovery technology to recover the database

Breakdown summary

Various faults , There are two possibilities for the impact on the database

One is that the database itself is destroyed
Second, the database has not been damaged , But the data may not be correct , This is caused by the abnormal termination of the transaction .

recovery

Basic principles of recovery operation ： redundancy

Use the redundant data stored elsewhere in the system to reconstruct the damaged or incorrect part of the data in the database

Recovery implementation technology ： complex

A large database product , The code of the recovery subsystem accounts for 10% above
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