Interviewer: How does MySQL implement ACID?

In the interview, the interviewer only needs to ask about the ACID of MySQL, and then he can immediately recite the eight-part essay (some people may not be able to answer it yet). What's even more disgusting is that some interviewers don't follow the routine and continue to ask, how does MySQL implement ACID?

I’m confused. To be honest, this question can persuade 95% of people.

Today, this article mainly discusses the implementation principle of ACID under the MySQL InnoDB engine. It does not elaborate too much on basic knowledge such as what a transaction is and the meaning of isolation level.

ACID

As a relational database, MySQL uses the most common InnoDB engine to ensure ACID.

• （Atomicity）Atomicity: Transaction is the smallest execution unit and does not allow splitting. Atomicity ensures that actions are either fully completed or have no effect at all;
• (Consistency) Consistency: The data remains consistent before and after executing the transaction;
• (Isolation) Isolation: When accessing the database concurrently, a transaction will not be interfered by other transactions.
• (Durability) Durability: After a transaction is committed. Changes to data in the database are persistent, even if the database fails.

Isolation

Let’s talk about isolation first. The first is the four isolation levels.

##Read UncommittedWhen a transaction has not been submitted, the changes it makes can be seen by other transactionsRead submissionAfter a transaction is submitted, the changes it makes can only be seen by other transactions. Will be seen by other transactions

Isolation Level	Description
Repeatable Reading	In a transaction, the result of reading the same data is always the same, regardless of whether other transactions operate on the data and whether the transaction is committed. InnoDB default level.
Serialization	Transactions are executed serially. Each read requires a table-level shared lock. Reading and writing will block each other. The isolation level is the highest, sacrificing the system. Concurrency.

Different isolation levels are designed to solve different problems. That is, dirty reads, phantom reads, and non-repeatable reads.

隔离级别	脏读	不可重复读	幻读
Read uncommitted	can appear	can appear	can appear
Read submission	Not allowed	Can occur	Can occur
Repeatable read	Not allowed	Not allowed	Can appear
Serialization	Not allowed	Not allowed	Not allowed

So different isolation levels, how is isolation achieved, and why can different things not interfere with each other? The answer is Locks and MVCC.

Lock

Let’s talk about locks first. How many locks does MySQL have?

Granularity

In terms of granularity, it is table lock, page lock, and row lock. Table locks include intentional shared locks, intentional exclusive locks, self-increasing locks, etc. Row locks are implemented by each engine at the engine level. But not all engines support row locks. For example, the MyISAM engine does not support row locks.

Types of row locks

In InnoDB transactions, row locks are implemented by locking index entries on the index. This means that InnoDB uses row-level locks only when data is retrieved through index conditions, otherwise table locks are used. Row-level locks are also divided into two types: shared locks and exclusive locks, as well as intention shared locks and intention exclusive locks that need to be obtained before locking.

• Shared lock: read lock, other transactions are allowed to add S lock, other transactions are not allowed to add X lock, that is, other transactions can only read but not write. select...lock in share mode Lock.
• Exclusive lock: write lock, no other transactions are allowed to add S lock or X lock. insert, update, delete, for updateLock.

Row locks are added when needed, but they are not released immediately when they are no longer needed, but until the end of the transaction. Only then released. This is the two-phase lock protocol.

Implementation algorithm of row lock

Record Lock

The lock on a single row record will always lock the index record.

Gap Lock

Gap lock, think about the reason for phantom reading. In fact, row lock can only lock rows, but when inserting a new record, what needs to be updated is the "" between records. gap". So add gap lock to solve phantom reading.

Next-Key Lock

Gap Lock Record Lock, left open and closed.

Locks and Isolation

A general introduction to the lower lock can be seen. With locks, when a transaction is writing data, other transactions cannot obtain the write lock and cannot write data. This ensures the isolation between transactions to a certain extent. But as mentioned earlier, after adding a write lock, why can other transactions also read data? Isn’t it that the read lock cannot be obtained? ?

MVCC

As mentioned earlier, with the lock, the current transaction cannot modify the data without a write lock, but it can still be read, and when reading, even if the row of data has been modified and submitted by other transactions, You can still read the same value repeatedly. This is MVCC, multi-version concurrency control, Multi-Version Concurrency Control.

Version chain

The storage format of row records in Innodb has some additional fields: DATA_TRX_ID and DATA_ROLL_PTR.

• DATA_TRX_ID: Data row version number. Used to identify the transaction ID that recently modified the record of this row.
• DATA_ROLL_PTR: Pointer to the rollback segment of the row. All old versions recorded in this line are organized in the form of a linked list in undo log.

#undo log: Record the log before the data is modified, which will be discussed in detail later.

##ReadView

is created at the beginning of each SQL and has several important attributes:

• trx_ids: Collection of active (uncommitted) transaction version numbers in the current system.
• low_limit_id: "The current system's maximum transaction version number 1" when creating the current read view.
• up_limit_id: "The system is in active transactionminimum version number" when creating the current read view
• creator_trx_id: Create the transaction version number of the current read view;

Start query

Now start the query, a select comes over, and a row of data is found.

• DATA_TRX_ID 39c5fed9120ad32d2105b40b3d59495e= low_limit_id：

  Indicates that the data is generated after the current read view is created, and the data is not displayed .

• 
  
• What to do if it is not displayed? Find the historical version from the undo log according to
  DATA_ROLL_PTR , if not found, it will be empty.
• ##up_limit_id <low_limit_id: It depends on the isolation level.

RR level phantom reading

With locks and MVCC, transaction isolation Sex is resolved. Let me expand here. Does the default RR level solve phantom reading? Phantom reading usually targets INSERT, and non-repeatability targets UPDATE.

##begin beginselect * from dept

Thing 1	Thing 2
-	insert into dept(name) values("A")
-	commit
update dept set name="B"
commit

We expected it to be

id  name
1   A
2   B

actually it was

id  name
1   B
2   B

In fact, the isolation level of MySQL repeatable read does not completely solve the problem of phantom reading, but solves it. Phantom reading problem when reading data. There is still a phantom read problem for modification operations, which means that MVCC is not thorough in solving phantom reads.

Atomicity

Let’s talk about atomicity. As mentioned earlier, undo log rolls back the log. Isolation MVCC actually relies on it to achieve, as does atomicity. The key to achieving atomicity is to be able to undo all successfully executed SQL statements when the transaction is rolled back.

When a transaction modifies the database, InnoDB will generate the corresponding undo log; if the transaction execution fails or rollback is called, causing the transaction to be rolled back, the information in the undo log can be used to roll back the data to How it looked before modification. Undo log is a logical log, which records information related to SQL execution. When a rollback occurs, InnoDB will do the opposite of the previous work based on the contents of the undo log:

• For each insert, delete will be executed during rollback;
• For each delete, insert will be executed during rollback;
• For each update, an opposite update will be executed during rollback to change the data back.

Take the update operation as an example: when a transaction executes an update, the undo log generated will contain the primary key of the modified row (so as to know which rows have been modified) and which columns have been modified. , the values ​​of these columns before and after modification and other information, you can use this information to restore the data to the state before the update during rollback.

Persistence

Innnodb has many logs, and persistence relies on redo log.

How to run a SQL update statement

Persistence is definitely related to writing. WAL technology is often mentioned in MySQL. The full name of WAL is Write-Ahead Logging. Its key point is to write first. log, and then write to disk. Just like doing business in a small shop, there is a pink board and an account book. When guests come, write on the pink board first, and then write down the account book when you are not busy.

redo log

redo log is this pink board. When a record needs to be updated, the InnoDB engine will first write the record to the redo log (and update the memory). At this time, the update is completed. At the appropriate time, this operation record is updated to the disk, and this update is often done when the system is relatively idle, just like what the shopkeeper does after closing.

redo log has two characteristics:

• Fixed size, cyclic writing
• crash-safe

There are two stages for redo log: commit and prepare If you do not use "two-phase commit", the state of the database may be inconsistent with the state of the library restored using its log. Well, let's go here first and look at the other one.

Buffer Pool

InnoDB also provides a cache. The Buffer Pool contains the mapping of some data pages on the disk as a buffer for accessing the database:

• When reading data, it will be read from the Buffer Pool first. If it is not in the Buffer Pool, it will be read from the disk and then put into the Buffer Pool;
• When writing data to the database, it will be written to the Buffer Pool first, and the modified data in the Buffer Pool will be regularly refreshed to the disk.

The use of Buffer Pool greatly improves the efficiency of reading and writing data, but it also brings new problems: if MySQL goes down, the modified data in the Buffer Pool has not been refreshed at this time. to the disk will result in data loss, and the durability of the transaction cannot be guaranteed.

So I added redo log. When the data is modified, in addition to modifying the data in the Buffer Pool, the operation will also be recorded in the redo log;

When the transaction is submitted, the fsync interface will be called to flush the redo log.

If MySQL goes down, you can read the data in the redo log and restore the database when restarting.

The redo log uses WAL (Write-ahead logging, write-ahead log). All modifications are first written to the log and then updated to the Buffer Pool, ensuring that the data will not be lost due to MySQL downtime, thus Durability requirements are met. And there are two advantages to doing this:

• Brushing dirty pages is random IO, redo log sequential IO
• Brushing dirty pages is based on Page, and the entire modification on a Page must be written ;The redo log only contains what really needs to be written, reducing invalid IO.

binlog

Speaking of this, you may wonder that there is also a bin log that is also used for write operations and is used for data recovery. What is the difference?

• Level: redo log is unique to the innoDB engine, and the server layer is called binlog (archive log)
• Content: redolog is a physical log, recording "in "What modifications have been made on a certain data page"; binlog is a logical log, which is the original logic of the statement, such as "Add 1 to the c field of the row with ID=2"
• Write : redolog writes in a loop and has many writing opportunities, binlog is appended and written when the transaction is committed

binlog and redo log

For the statement update T set c=c 1 where ID=2;

1. The executor first looks for the engine to get the line ID=2. ID is the primary key and can be found directly using tree search. If the data page where the row with ID = 2 is located is in the memory, it will be returned directly to the executor; otherwise, it needs to be read into the memory from the disk first and then returned.
2. The executor gets the row data given by the engine, adds 1, N 1 to this value, gets a new row of data, and then calls the engine interface to write this new row of data.
3. The engine updates this new row of data into the memory and records the update operation into the redo log. At this time, the redo log is in the prepare state. Then inform the executor that the execution is completed and the transaction can be submitted at any time.
4. The executor generates the binlog of this operation and writes the binlog to disk.
5. The executor calls the engine's commit transaction interface, and the engine changes the redo log just written to the commit state, and the update is completed

Why write redo log first?

• Redo first and then bin: binlog is lost, one update is missing, and it is still 0 after recovery.
• First bin and then redo: There is one more transaction, and it is 1 after recovery.

Consistency

Consistency is the ultimate goal pursued by the transaction. The atomicity and durability mentioned in the previous question Security and isolation are actually to ensure the consistency of the database state. Of course, the above are all guarantees at the database level, and the implementation of consistency also requires guarantees at the application level.

That is, your business. For example, the purchase operation only deducts the user's balance without reducing the inventory. It is definitely impossible to ensure that the status is consistent.

Summary

We are all familiar with MySQL, and we also know what ACID is, but how is MySQL’s ACID implemented?

Sometimes, just like you know that there are undo log and redo log, but you may not know why there are them. When you know the purpose of the design, it will become clearer.

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