What are ACID properties

Hi DBAs & Developers,

I want to understand ACID properties through sql queries. Can anybody help me out ??

I have seen real world example everywhere but if I want to test it then how do I use sql queries to understand practically on my PC??

Regards,
Yashwant Vishwakarma | SQLOcean.com


June 19th, 2015 7:10am

Please take a look at these links:

ACID properties

The ACID Properties of Statements & Transactions

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June 19th, 2015 7:15am

Atomicity  -- Either all or no work is performed.

Consistency -- A transaction must leave data in a consistent state.

Isolation  -- Each transaction is independent of all other transactions. That means each transaction will read data that was committed prior to beginning of the other transactions or after the end of the other transactions.

Durability  -- After transaction is committed, the data is in a persistent state, regardless of the circumstances. SQL Server records the transaction in the transaction log, and marks it as being committed. If the transaction is not committed, then SQL Server will roll all data changes back.

For more info CLICK HERE

Thanks


June 19th, 2015 7:22am

Hi SequelMate,

Can you explain ACID in below sql statements:

BEGIN TRANSACTION
    INSERT purchase (
       item, 
       customer, 
       price, 
       item_count, 
       total)
    VALUES (
       'ring', 
       'ms. jones', 
       1000, 
       1, 
       1000)

    IF @@ERROR <> 0
       BEGIN
           RAISERROR('error occured while recording purchase', 16, 1)
           ROLLBACK
       END

    UPDATE 	ring_inventory 
    SET       current_count = current_count - 1
    WHERE 	price = 1000

    IF @@ERROR <> 0
       BEGIN
           RAISERROR('error occured while adjusting inventory', 16, 1)
           ROLLBACK
       END

    UPDATE balance
    SET      balance = balance + 1000
    WHERE customer = 'ms. jones'

    IF @@ERROR <> 0
       BEGIN
           RAISERROR('error occured while adjusting balance', 16, 1)
           ROLLBACK
       END

COMMIT TRANSACTION

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June 19th, 2015 7:28am

Hi Saeid,

I simply wants some good examples step by step sql statements from which I can easily understand??

Regards,
Yashwant Vishwakarma | SQLOcean.com

June 19th, 2015 7:32am

Yashwant,

I would suggest you try to read as much as possible atomicity means either whole transaction would comit or every thing would be rolled back like

begin transaction--transaction start update t1 set a=2 where b=3 update t1 set a1=2 where b1=3 update t1 set a2=2 where b2=3 update t1 set a3=2 where b3=3 Rollback--

--commit

If you run above code even if you ran update statement but rillback would undo all changes because of transaction. If you commit it all changes would be there
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June 19th, 2015 7:32am

Hello - See these example with explanation:

Atomicity
Definition: The atomicity property identifies that the transaction is atomic. An atomic transaction is either fully completed, or is not begun at all. An example of an atomic transaction is updating an employee record by updating the salary, in between the the process, the system goes down so when it comes back again you should see the original value for the salary for that employee

Example:

-- Start the Transaction in First Query Session BEGIN TRANSACTION UPDATE Employee SET Salary = 20000 WHERE EmpID = 1 -- In other query session, simulate failure -- By shutdown no wait SHUTDOWN WITH NOWAIT

-- Now again start the SQL Server Service and Query, you will realize no updates happened

SELECT * FROM Employee


Consistency
A transaction enforces consistency in the system state by ensuring that at the end of any transaction the system is in a valid state. If the transaction completes successfully, then all changes to the system will have been properly made, and the system will be in a valid state. If any error occurs in a transaction, then any changes already made will be automatically rolled back. This will return the system to its state before the transaction was started. Since the system was in a consistent state when the transaction was started, it will once again be in a consistent state.

Example:

-- First Query Session
BEGIN TRANSACTION
UPDATE Employee SET Salary = 20000 WHERE EmpID = 1

-- Inserting entry into TransactionRecords Table about the 
-- Do not execute this

INSERT INTO TransactionRecords SELECT 1, 20000, '2015-06-19'

-- Second Query Session
SHUTDOWN WITH NOWAIT

In the above example, we updated the salary & the next thing to be done was making an entry in TransactionRecords table about salary update but before that we simulated the failure by issuing shutdown, so when services are restored again we should old consistent value for the salary instead of 20,000

Isolation
When a transaction runs in isolation, it appears to be the only action that the system is carrying out at one time. If there are two transactions that are both performing the same function and are running at the same time, transaction isolation will ensure that each transaction thinks it has exclusive use of the system. 

-- First Query Session
BEGIN TRANSACTION
UPDATE Employee SET Salary = 20000 WHERE EmpID = 1

-- Second Query Session
BEGIN TRANSACTION
-- This will be prohibited (BLOCKED) because other session is making change to this data
UPDATE Employee SET Salary = 30000 WHERE EmpID = 1

-- However this will be allowed because this transaction is updating different row
UPDATE Employee SET Salary = 20000 WHERE EmpID = 2

Durability
A transaction is durable in that once it has been successfully completed, all of the changes it made to the system are permanent. There are safeguards that will prevent the loss of information, even in the case of system failure. By logging the steps that the transaction performs, the state of the system can be recreated even if the hardware itself has failed. 

Example: First example can be considered for this explanation

Hope this helps !

June 19th, 2015 7:44am

Hi Shanky,

Thanks for your explanation.

The way you explained atomicity is very simple easy to grasp for me. Its just like that what I want.

In the same way can you explain consistency, isolation & durability also, so that I can perform a practical on my PC.

Regards,

Yashwant Vishwakarma | SQLOcean.com

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June 19th, 2015 7:46am

Hi Manu,

Thank You for your nice explanation. I was seeking for this answer only.

Many Thanks Again :-)

Wishing you a bright future always :)

Regards,
Yashwant Vishwakarma | SQLOcean.com

June 19th, 2015 8:29am

In the same way can you explain consistency, isolation & durability also, so that I can perform a practical on my PC.

Not all these are easily demonstrated by tests. Consistency means that the transaction takes the database from one consistent state to another. As it happens, all constraints in SQL Server are evaluated directly, and the same is true for triggers, so you cannot easily see inconsistent states in a transaction.

But take this example:

CREATE TABLE Products (ProductID int NOT NULL PRIMARY KEY,
                       --other columns)

CREATE TABLE OrderDetails (...
                           ProductID int NOT NULL
                              REFERENCES Products ON UPDATE CASCARE,
                           ..)

You do

UPDATE Products
SET    ProductID = 133
WHERE  ProductID = 9887

When the statement has completed you will see that entries with ProductIF in OrderDetails have been updated with the new ProductID. But if you put a lock on the OrderDetails table and peek on the two tables with NOLOCK, you may find product 9887 in one table, and 133 in the other. That is the database is not consistent. And this is alright, because you are inside a transaction. Once the transaction has committed, the database is again consistent.

Here I told you to use NOLOCK to peek at the data, and this brings us to Isolation. Transactions are isolated, meaning that two transactions do not see each others intermediate states. In a relational database this is typically implmented by locking or versioning. That is, if you attempt to read a row that another transaction has updated, you are either blocked until that transaction has committed, or you see an older version of that row. The exception is when you use the NOLOCK hint, which permits you to see uncommitted data.

Finally, durability, this means that data is durable. Once the data has been committed, you can turn off the power to the computer, plug the disk into a different computer. The data is still there.

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June 19th, 2015 8:44am

Hi Manu,

>. An atomic transaction is either fully completed, or is not begun at all.

This is not fully correct although I understand your intention but words are incorrect. A atomic transaction is one where transaction either completes or is left to position where it was before the beginning of transaction like what i showed in my update statement. Its incorrect to say "not begun at all"

Yashwant

Isolation and consistency has been explained clearly by Manu and Erland I will give some points on Durability. Durability means the changes which have been committed should be available to user at any time when SQL Server is queries. To make sure durability is achieved SQL Server first writes changes into Transaction log buffer updates the data page in memory and then fires a commit of transaction either manually or explicitly. Please note that changes are still persisted on disk it is still in memory later checkpoint processes flushes it to disk and changes are saved to disk.

In above scenario after changes are written to log the transaction is safe so even if power is out SQL Server can read from trn log and get the state of transaction.

I strongly suggest you to get copy of SQL Server 2012 internals and troubleshooting The very first chapter will tell you all

June 19th, 2015 10:13am

Nice explanation. Thanks :)
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June 19th, 2015 11:18am

Atomicity  -- Either all or no work is performed.

Consistency -- A transaction must leave data in a consistent state.

Isolation  -- Each transaction is independent of all other transactions. That means each transaction will read data that was committed prior to beginning of the other transactions or after the end of the other transactions.

Durability  -- After transaction is committed, the data is in a persistent state, regardless of the circumstances. SQL Server records the transaction in the transaction log, and marks it as being committed. If the transaction is not committed, then SQL Server will roll all data changes back.

For more info CLICK HERE

Thanks


  • Edited by SequelMate Friday, June 19, 2015 11:22 AM
June 19th, 2015 11:20am

Hi Shanky,

Thank you :) for your nice & clear cut explanation.

I am feeling blessed with very nice Answers by You, Manu & Erland.

Now I am able to understand ACID , I will also read SQL Server 2012 internals and troubleshooting recommended by you.

Thanks again :) , Keep Smiling n Shining always :)

Regards,
Yashwant Vishwakarma | SQLOcean.com


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June 22nd, 2015 1:47am

Hi Erland,

Thank you so much :), its really such a nice explanation.

I am feeling blessed with all clear & simple answers by You, Manu & Shanky.

In my all threads , this thread is super awesome in which I got really good answers & easy to understand.

Hats Off to All DBAs which put their efforts here in this thread to explain me very well.

Keep Smiling n Shining:) 

Regards,
Yashwant Vishwakarma | SQLOcean.com

June 22nd, 2015 1:57am

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