Mysql 45 study notes (twenty-five) MYSQL guarantees high availability

One. Eventual consistency

• As long as main library performs an updateall binlogs generated will be sent to standby and executed correctly
• The standby database can reach the same state as the main database

2. Primary and secondary delay

1. Synchronization delay

• Main library A executes a transaction and writes it to binlog. At this time, we record this moment as T1;
• Pass it to the standby database B, we record the moment when the standby database B receives the binlog as T2;
• Standby B executes this transaction, and we record this moment as T3

Main-standby delay: The difference between the time when the same transaction is completed in the standby database and the time when the main database is completed----T3-T1

2.show salve status command

The standby database executes the show slave status command, and the returned result shows seconds_behind_master, indicating how many seconds the current standby database is delayed

3.seconds_behind_master calculation method

• The binlog of each transaction contains a aTime field for records the time of writing on the master library;
• Standby libraryRemove the current executing transactionThe value of the time field, calculates its difference from the current system time

Three. The source of the main and standby delays

1. The machine performance of the standby database is worse than that of the main database, which may lead to the delay of the active and standby databases

2. Statistical analysis and query on the standby database consumes a lot of CPU resources              

• One master and multiple slaves: multiple connections and slave libraries share the pressure of reading
• binlog is output to external systems, such as Hadoop systems, so that external systems can provide statistical query capabilities

3. Big business

Four. Reliability priority strategy

• Determine the current second_behind_master of standby database B, if it is less than 5s, continue, otherwise try again
• Change the main library A to read-only state and set readonly to true
• Determine the value of second_behind_master of standby database B until the value becomes 0 (Maintain reliability---need to be small enough, because it is unavailable at this time
strong>)
• Change the standby database B to readable and writable state, that is, set readonly to false;
• Switch the business request to the standby database B

V. Availability priority strategy

• Change the standby database B to readable and writable state in step 4, that is, set readonly to false;
• Step 5 Switch the business request to the standby database B
• Directly execute unequal data synchronization and consistency

Disadvantages:

Inconsistent data

• On the premise of data reliability, the availability of the MYAQL high-availability system depends on the delay of the active and standby systems

• The smaller the delay time, the shorter the recovery time of the faulty service in the main database, the higher the availability