TRANSACTION Isolation Levels in SQL Server

SQL Server 2005 has some unique features to deal with the Transaction system in the database world. It has some unique sets to take care of every possibility of transactions or types of transaction. Technically, it will give us discrete ways to isolate the transactions from occurrence of deadlocks or crashes.

Before going deeper to the Isolation level that SQL Server provides to distinguish types of transaction, let’s have a look on the definition of the TRANSACTION. What does transaction means in real world and in a database scenario?

Transaction: When you give something to me, and I take it; then it’s a transaction. When you withdraw money from an ATM machine, and you receive the money; then it is also a kind of transaction. Here, what I am trying to reflect is a simple question that, Is the transaction above is valid or consistent. What if I deny accepting that you haven’t given me anything, may be you have given to someone else instead of me? What if after withdrawing of money from your account, your account balance still shows the same amount as before. (Oh! For this case you have to be lucky enough ). And what will happen if you and your partner are withdrawing all your money from the joint account at the same time from different ATMs.

So there must be some methodology to keep track of all these things and to manage them perfectly even in such natural disaster conditions, the database and the information regarding any transaction must be in a consistent form.

To achieve the above thought in the database system, we have Locking mechanism. It acts like this, suppose there is a room and that is electronically locked and only the person who knows the password can enter, provided the room is empty or he has to wait till the room is evacuated by the other person. But here we have a little controversy, like the person who is waiting outside may have some different task than the person who is already inside. And it may be possible that both of the tasks may not interfere to each other or may interfere slightly that may be manageable. So at the end of the discussion, we may conclude that the security system must provide different types of security code or passwords to the corresponding person. Let’s have a deeper look on this.

Suppose you are doing a transaction for withdrawing money from the ATM machine and at the same time the bank manager is doing a routine checkup of your transaction which is totally a different operation and suppose at the same time the bank teller is checking your account for the remaining balance. All these operations are different but accessing the same entity or resource and that is your account information that is kept inside the database. Out of these operations only you are doing write operation in the database as you are withdrawing money and the remaining balance has to be updated in the database. So a proper security mechanism must be implemented here to ensure non-conflict or smooth going of these operations. Here the security can be ensured by putting locks (and of course the type of locks) for each type of operations, which means you are isolating the resources from other transactions that may hamper its consistency. Here comes the role of Isolation levels.

The Isolation levels are categorized depending on the type of locks it uses for a particular level. At lower level of isolation more users can access the same resource without any confliction, but they may face concurrency related issues such as dirty-reads and data inaccuracy (described below). At higher Isolation level, these types of issues can be eliminated but here only a limited no. of users can access the resource.

Let’s have a look on Locks and type of Locks. Locks can be treated as a policy which will prevent you or a process to perform any action (that may conflict with other actions) on an object or resource if that object or resource is already occupied by any other process or user. It’s something like you are going to propose someone who is already with someone else. But situation matters (may be you are lucky enough for this). Like it depends on what you are going to do and on what the other person is doing. So for such type of situations, we have types of locks.

Types of Locks:

• Shared Locks(S): This lock is useful when you are doing some read operations and no manipulations like write operations (update/delete/insert). This is compatible with other shared locks, update locks and Intent shared locks. It can prevent users from performing dirty reads (described below).
• Exclusive Locks(X): These locks are big possessive types. They are not compatible with any other locks. Like these locks will not work if any other locks are already there with the resource neither it will let other locks to be created on the resource until it finishes its job. This lock used for data-modification operations, such as INSERT, UPDATE or DELETE.
• Update Locks (U): This can be treated as a mixture and perfect collaboration of the above two locks (Shared and Exclusive). Let’s take an example. You are going to perform an update operation on a table at row number 23. So here you are doing two types of operation, one is searching the record 23 which can be achieved by implementing shared lock and the other is updating the record after it has found which will be achieved by Exclusive lock. So, here the shared lock transforms to exclusive lock when it finds the target or else it will be remain as shared lock only. This prevents deadlocks to a great extent. This lock is compatible with Intent shared and shared locks.
• Intent locks (also called as Demand Locks): These are used to establish a lock hierarchy. Here it will protect placing a shared (S) lock or exclusive (X) lock on a resource lower in the lock hierarchy. For example, suppose you are performing a read operation on a piece of data with shared lock. At the same time another user wants to modify data with exclusive lock, but the shared lock is compatible with other shared locks as a result any number of shared locks can be obtained on a piece of data and hence the user with exclusive has to wait indefinitely till the completion of all shared lock operations. So to avoid this type of starving situation, Intent locks are very useful. Here if the second user comes with Intent Exclusive lock, then no other transaction can grab a shared lock. Here it can claim the use of exclusive lock after the first transaction completes.

There are basically three types of Intent Locks that are most popular:

a) Intent Shared Lock(IS)
b) Intent exclusive (IX)
c) Shared with intent exclusive (SIX)

To get more information on Intent Locks, refer the link below:
http://msdn.microsoft.com/en-us/library/aa213039(SQL.80).aspx

• Schema Locks: These locks protect the schema of the database. This deals with the DDL (Data Definition Language) commands like adding or dropping column information for a table, rename table, drop table, blocking any DDL operation during the execution of the query. There are two types of Schema Locks:

a) Schema modification (Sch-M): This lock is applied only when the SQL Server engine is modifying the structure of the schema like adding or dropping the columns of a table. During this period if any other transaction tries to access that object then that will be denied or delayed.

b) Schema stability (Sch-S): This indicates a query using this table being compiled. Here it will not block any transactional locks like shared locks or exclusive locks to perform any operation on the data. But if the query is in running condition, it will prevent execution of any DDL commands on that table.

Bulk Update Locks: This lock is useful while performing BULK operation on the TABLE like BULK INSERT. It will prevent any other types of normal T-SQL operations to be executed on the table except BULK processing of the data.

Now let us explore some buzzwords in Isolation Level:

Lost updates: It generally occurs when more than one transaction tries to update any specific record at a time i.e. when one update is successfully written to the database, but accidently a second update from different transaction overwrites the previous update information. This is called Lost Updates.

Non-repeatable reads (also called Inconsistent analysis): Dealing with inconsistent data i.e. suppose you read one value from a table and started working on it but meanwhile some other process modifies the value in the source resulting a false output in your transaction, then it is called Non-repeatable reads. Let’s have a more practical example, suppose before withdrawing money from your account, you always perform a balance check and you find 90$ as a balance in your account. Then you perform withdraw operation and try to withdraw 60$ from your account but meanwhile the bank manager debits 50$ from your account as a penalty of minimum balance (100$), as a result you have only 40$ in your account now. So your transaction either fails as the demanded amount (60$) is not there in your account or it may show (-20$) (which is quite impossible as of banking constraints ). More simply we can say Non-repeatable reads take place if a transaction is able to read the same row several times and gets a different value for each time.

Repeatable Reads: This specifies that transactions cannot read data that has been modified by other transactions but not yet committed and if the current transaction is reading some data then no other transactions can modify that data until the current transaction completes.

Phantom reads: Don’t be afraid, we are not talking about ghosts or phantom in opera. Here Phantom means unexpected or unrealistic. It occurs basically when two identical queries are executed, and the set of rows returned by the second query is different from the first. Let’s have a simple example; suppose your banking policy got changed and according to that the minimum balance should be 150$ instead of 100$ for each account type, anyways this is not a big deal for a data base administrator. He will perform an update statement for each account type where the minimum balance is less than 150$ and updates the value to 150$. But unfortunately when the manager checks the database, he got one record with minimum balance less than 150$ in the same table. The DBA got surprised, how come this is possible as he performed the update statement on the whole table.

This is called Phantom read. The occurrence of Phantom reads are very rare as it needs proper circumstances and timing for such type of events as in the above example, someone may have inserted one new record with the minimum balance less than 150$ at the very same time when the DBA executed the UPDATE statement. And as it is a new record, it didn’t interfere with the UPDATE transaction and executed successfully. This type of Phantom reads can be avoided using higher level of isolation i.e. SERIALIZABLE (described below).

Dirty reads: This is one of the types of Non-repeatable Reads. This happens when a process tries to read a piece of data while some other process is performing some update operations on that piece of data and is not completed yet.

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Now coming to the root point of the article i.e. the Isolation levels; we have basically five types of Isolation level in SQL Server 2005. Each one is described below:

Here we consider a simple example for all the below cases. The data shown in the table is taken by assumption and is only used for example purpose; the data given may or may not be right as per real scenario. The table information is given below:

Database Name: OLAP

Table Name: dbo.car_info

Table Column Information:

Column_name	Type
Car_Sl_No	int
CarCompany	varchar
CarBodyType	varchar
CarName	varchar
EngineType	varchar

Table Data:

Car_Sl_No	CarCompany	CarBodyType	CarName	EngineType
1	Maruti	small	Maruti-800	petrol
2	Honda	sedan	City	petrol
3	Maruti	small	Maruti-800	petrol
4	Maruti	small	Waganor Duo	petrol
5	Honda	sedan	City	petrol
6	TATA	small	indica	diesel
7	Mahindra	SUV	Scorpio	diesel
8	TATA	SUV	Sumo	diesel
9	Maruti	sedan	SX4	petrol
10	Maruti	sedan	Swift-Dzire	diesel
11	TATA	small	Nano	petrol

Assumption: Here in all our examples, two different transactions can be considered as done by two different users. For testing, you can achieve this by two separate Query windows or two separate instances for SQL Server Management Studio (SSMS). But you have to be careful enough to run the queries for both the connections simultaneously or immediately.

1. READ UNCOMMITTED Isolation Level: This is very useful in case you need higher concurrency in the transactions. Here one transaction can access the data that has been modified by the second transaction even if the second transaction is not committed.

Syntax:

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

Example: Suppose the User1 is trying to update the EngineType from ‘petrol’ to ‘diesel’ for Car_Sl_No with value 2. And at the same time User2 is trying to read the data for the Car_Sl_No with value 2. Under normal condition or default setting, User2 cannot read the data from that row. But if the User2 sets the transaction isolation level to ‘Read Uncommitted’, then it is possible to read that row with updated information even if the transaction is not committed by User1.

For User1:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
BEGIN TRAN
UPDATE [OLAP].[dbo].[car_info]
   SET [EngineType] = 'diesel'
 WHERE Car_Sl_No = 2

Here, note that the transaction is still running, as there is no commit statement in the above code. Under default condition, the query ran by User2 will keep executing till the User1 commits the transaction.

For User2:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
--Above statment is used to read the updated value even if the transation is not committed.
SELECT [Car_Sl_No]
      ,[CarCompany]
      ,[CarBodyType]
      ,[CarName]
      ,[EngineType]
  FROM [OLAP].[dbo].[car_info]
where Car_Sl_No = 2

As in the above code, we set the transaction isolation level to ‘Read Uncommitted’; User2 can access that record with updated data.

Output:

Although it increases the concurrency of the transactions but did you notice the disadvantage behind this. What if User1 ROLLBACK his transaction or if somehow the management studio of User1 crashed or hanged (As the transaction is not committed yet, it will rollback itself, resulting false or inconsistent value to User2).

Limitations:

• Dirty-reads
• Lost Updates
• Phantom reads
• Non-repeatable reads

Advantages:

• Higher Concurrency

In SSIS (SQL Server Integration Service): To achieve the above norm in SSIS, select the task or container on which you want to set the isolation level. Then go to Properties, and set the property named ‘IsolationLevel’ to “ReadUncommitted”.

The benefit here is that more than one task can access the same table simultaneously in case of parallel execution of the package.

2. READ COMMITTED Isolation Level: This is the default level set in SQL Server 2005 and the immediate higher level of ‘READ UNCOMMITTED Isolation Level’. It prevents transactions to read data if some other transaction is doing some update operation on the data as a result eliminates Dirty Reads. It prevents reading of uncommitted data. But is affected with other demerits like ‘Lost Updates’.

Syntax:

SET TRANSACTION ISOLATION LEVEL READ COMMITTED

Example: Considering our previous example, let the EngineType for Car_Sl_No with value 2 is NULL and User1 is trying to update the EngineType to ‘petrol’, but at the same time User2 started a new transaction checked the value as Null and starts updating the record to ‘diesel’ before the transaction is committed by User1. As a result User1 lost its updated value, it is overwritten by User2.

For User1:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
BEGIN TRAN
 
DECLARE @EngineType varchar(20)
SELECT @EngineType = [EngineType] FROM [OLAP].[dbo].[car_info] where Car_Sl_No = 2
--The below waitfor statement is used for other opearations that User1 is doing for this transaction.
WAITFOR DELAY '00:00:10' --For acheiving real time Concurrency in this example
IF @EngineType IS NULL
BEGIN
UPDATE [OLAP].[dbo].[car_info]
   SET [EngineType] = 'petrol'
 WHERE Car_Sl_No = 2
END
ELSE
BEGIN
 Print 'Record is already updated'
END
 
COMMIT TRAN

For User2:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
BEGIN TRAN
 
DECLARE @EngineType varchar(20)
SELECT @EngineType = [EngineType] FROM [OLAP].[dbo].[car_info] where Car_Sl_No = 2
--Here waitfor statement is same for User2 also
WAITFOR DELAY '00:00:10' --For acheiving real time Concurrency in this example
IF @EngineType IS NULL
BEGIN
UPDATE [OLAP].[dbo].[car_info]
   SET [EngineType] = 'diesel'
 WHERE Car_Sl_No = 2
END
ELSE
BEGIN
 Print 'Record is already updated'
END
 
COMMIT TRAN

Here both the users successfully updated the value, but the value updated by User2 persists and User1 lost its updated value.

Output: The final output for the record is

Limitations:

• Lower Concurrency than ReadUncommitted
• Lost Updates

Advantage:

• Eliminates Dirty Reads

In SSIS (SQL Server Integration Service): Select the task or container on which you want to set the isolation level. Then go to Properties, and set the property named ‘IsolationLevel’ to “ReadCommitted”.

3. REPEATABLE READ Isolation Level: It is the next higher level than the previous isolation level and the main point here is it does not release the shared lock once the transaction starts for reading data. In simple terms, a transaction cannot read data if it has been modified by other transaction but not yet committed. Also no other transactions can modify data if that data has been read by the current transaction until the current transaction completes. Here in this isolation level, the concurrency rate is very low. As a result, eliminates ‘Lost updates’, non-repeatable reads, etc. But still has a big problem and that is called ‘Phantom read’. Let’s have an example to elaborate this.

Syntax:

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ

Example: Suppose the manager of a showroom declares to transfer all the cars manufactured by Honda Company to another showroom and to maintain a proper record for this operation. We need to add one more column called ‘TransferredSatus’ to indicate whether that car is transferred or not. Here, the DBA will check for the presence of any Honda Company cars in the record that are not yet transferred by checking the value of the column ‘TransferredSatus’. If he found some, then corresponding transfer operations will be performed and the record will be updated to ‘1’ (i.e. transferred). Here by using ‘Repeatable Read’ isolation level, we can eliminate ‘Lost Update’, ‘dirty reads’ and ‘non-repeatable reads’. But what if at the time of updating the database, someone else from the inventory system inserts one record about the new Honda Company car that just arrived to the showroom. Let’s see the effect.

For User1:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
BEGIN TRAN
 
 --check the existance Honda company cars
 Declare @Car_Sl_No int
 Declare TransferingCarsCursor CURSOR FOR 
 Select Car_Sl_No from dbo.car_info where CarCompany = 'Honda' and TransferredSatus = 0
 
 OPEN TransferingCarsCursor
 
 FETCH NEXT FROM TransferingCarsCursor 
 INTO @Car_Sl_No
 WHILE @@FETCH_STATUS = 0
 BEGIN
  ----------------------------------
  ------Car transfering operations--
  ----------------------------------
 FETCH NEXT FROM TransferingCarsCursor 
  INTO @Car_Sl_No
 END 
 CLOSE TransferingCarsCursor
 DEALLOCATE TransferingCarsCursor
 
 WAITFOR DELAY '00:00:10' --For acheiving real time Concurrency in this example
 -- This is the time when the other user inserts new record about new Honda car.
 
 Update dbo.car_info
  set TransferredSatus = 1 where CarCompany = 'Honda' and TransferredSatus = 0
 
COMMIT TRAN

Here it found only 2 records from Honda Company.

For User2:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
BEGIN TRAN
INSERT INTO [OLAP].[dbo].[car_info]
           ([CarCompany]
           ,[CarBodyType]
           ,[CarName]
           ,[EngineType]
           ,[TransferredSatus])
     VALUES
           ('Honda','sedan','Civic GX','petrol',0)
 
COMMIT TRAN

But in between the execution of the transaction by User1, User2 inserts one new record about the new Honda Car. Assume the record is inserted before the Update statement of User1, as a result instead of updating only 2 records; User1 updates the new record as well along with the earlier records, showing wrong information in the chart. This is called ‘Phantom Read’. Even ‘Repeatable Read’ isolation mode can’t resolve this problem. For this, you need to implement higher isolation level i.e. SERIALIZABLE.

Output for User1:

(3 row(s) affected)

Limitations:

• Lower Concurrency
• Phantom Reads

Advantage:

• Eliminates Dirty Reads
• Eliminates Lost Updates
• Eliminates Non-Repeatable Reads

In SSIS (SQL Server Integration Service): Select the task or container on which you want to set the isolation level. Then go to Properties, and set the property named ‘IsolationLevel’ to “RepeatableRead”.

4. SERIALIZABLE Isolation Level: It is highest level in Isolation levels as a result the concurrency rate is low. But it eliminates all issues related to concurrency like dirty read, non repeatable reads, lost updates and even phantom reads. According to this Isolation Level:

1. Statements cannot read data if other transactions are performing update operations on the data and is not committed yet.
2. Also no other transactions can perform any update operations until the current transaction completes its read operations.
3. And the important point here is that it is performing a Range Lock based on the filters used to get the data from the table i.e. it locks not only the current records but also the new records that are falling under the current filter condition. In simple language, no other transactions can insert new rows that are falling under the current filter condition until the transaction completes.

Considering our previous example, we will set the isolation level to Serializable.

Syntax:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE

For User1:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
BEGIN TRAN
 
 --check the existance Honda company cars
 Declare @Car_Sl_No int
 Declare TransferingCarsCursor CURSOR FOR 
 Select Car_Sl_No from dbo.car_info where CarCompany = 'Honda' and TransferredSatus = 0
 
 OPEN TransferingCarsCursor
 
 FETCH NEXT FROM TransferingCarsCursor 
 INTO @Car_Sl_No
 WHILE @@FETCH_STATUS = 0
 BEGIN
  ----------------------------------
  ------Car transfering operations--
  ----------------------------------
 FETCH NEXT FROM TransferingCarsCursor 
  INTO @Car_Sl_No
 END 
 CLOSE TransferingCarsCursor
 DEALLOCATE TransferingCarsCursor
 
 WAITFOR DELAY '00:00:10' --For acheiving real time Concurrency in this example
 -- This is the time when the other user inserts new record about new Honda car.
 
 Update dbo.car_info
  set TransferredSatus = 1 where CarCompany = 'Honda' and TransferredSatus = 0
 
COMMIT TRAN

For User2:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
BEGIN TRAN
INSERT INTO [OLAP].[dbo].[car_info]
           ([CarCompany]
           ,[CarBodyType]
           ,[CarName]
           ,[EngineType]
           ,[TransferredSatus])
     VALUES
           ('Honda','sedan','Civic GX','petrol',0)
 
COMMIT TRAN

Output for User1:

(2 row(s) affected)

Here User2 transaction will wait till the User1 transaction completed avoiding ‘Phantom reads’.

Limitations:

• Lower Concurrency

Advantage:

• Eliminates Dirty Reads
• Eliminates Lost Updates
• Eliminates Non-Repeatable Reads
• Eliminates Phantom Reads

In SSIS (SQL Server Integration Service): Select the task or container on which you want to set the isolation level. Then go to Properties, and set the property named ‘IsolationLevel’ to “Serializable”.

5. SNAPSHOT Isolation Level: It specifies that the data accessed by any transaction is consistent and valid for that particular transaction and the data will be same throughout the whole transaction. It implements Row Versioning to isolate data for each transaction i.e. it will keep separate version of each modified row in the transaction in the tempdb database totally dedicated to that transaction. Any update of data in the original row will not affect the current transaction.

The ALLOW_SNAPSHOT_ISOLATION database option must be set to ON before you can start a transaction that uses the SNAPSHOT isolation level. It is by default kept as OFF because of performance issues.

To enable SNAPSHOT isolation level, use the below alter database command.

ALTER DATABASE OLAP SET ALLOW_SNAPSHOT_ISOLATION ON

We will consider a small example to illustrate the above condition.

Syntax:

SET TRANSACTION ISOLATION LEVEL SNAPSHOT

Example: We will try to insert a new record in the [car_info] table by User1 and at the same time we will try to fetch the records by User2.

For User1:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
BEGIN TRAN
INSERT INTO [OLAP].[dbo].[car_info]
           ([CarCompany]
           ,[CarBodyType]
           ,[CarName]
           ,[EngineType]
           ,[TransferredSatus])
     VALUES
           ('Honda','sedan','Civic Hybrid','petrol',0)

Note: The above transaction is not committed yet.

For User2:

[ Copy to Clipboard ] | [ View Source ]

USE OLAP
Go
SET TRANSACTION ISOLATION LEVEL SNAPSHOT
BEGIN TRAN
Select * from dbo.car_info where CarCompany = 'Honda' 
COMMIT TRAN

Output for User1:

(1 row(s) affected)

Output for User2:

One record is successfully inserted by User1, but a consisted version of the previous data is kept in Version store (in tempdb) before the starting of the transaction. So User2 is accessing the data from the version store and is unable to show the newly inserted record.

Now commit the transaction for User1 by “COMMIT TRAN” command, and again run the transaction for User2, the output will be as below:

You can check the version store for the current transaction along with other information regarding the current transaction by running the below DMVs (Dynamic Management Views) before committing User1 transaction.

select * from sys.dm_tran_active_snapshot_database_transactions

Output:

Limitations:

• Low performance due to versioning in tempdb

Advantage:

• Eliminates Dirty Reads
• Eliminates Lost Updates
• Eliminates Non-Repeatable Reads
• Allows multiple updates by versioning

In SSIS (SQL Server Integration Service): Select the task or container on which you want to set the isolation level. Then go to Properties, and set the property named ‘IsolationLevel’ to “Snapshot”.

Other Isolation Levels in SSIS:

• Chaos Isolation Level: Behaves the same way as ReadUncommitted, with additional features as stated below:

1. It permits viewing uncommitted changes by other transactions.
2. It checks any other uncompleted update transactions with higher restrictive isolation levels to ensure not to raise any conflicts i.e. any pending changes from more highly isolated transactions cannot be overwritten.
3. Rollback is not supported in this Isolation level.

If you want to perform read operations over the data once per transaction, then go for the Chaos isolation level.

In SSIS (SQL Server Integration Service): Select the task or container on which you want to set the isolation level. Then go to Properties, and set the property named ‘IsolationLevel’ to “Chaos”.

• Unspecified Isolation Level: When the Isolation level of any transaction cannot be determined, then it comes under ‘Unspecified Isolation Level’ i.e. a different isolation level than the ones above are used. For example performing custom transaction operation like ODBCtransaction, if the transaction level does not set by the user then it will execute according to the isolation level associated by the ODBC driver.

In SSIS (SQL Server Integration Service): Select the task or container on which you want to set the isolation level. Then go to Properties, and set the property named ‘IsolationLevel’ to “Unspecified”.

Optimistic Vs Pessimistic:

Optimistic concurrency: Here SQL Server assumes that the occurrence of resource conflicts between different transactions are very rare but not impossible. So it allows transactions to execute without locking any resources. Only in case of any modifications in the data, it will check for any conflicts, if it finds any then it will perform the locking operations accordingly. In simple terms, we are assuming that every transaction will carry on without any problem except some exceptional cases.

Pessimistic Concurrency: Here it will lock resources irrespective of the type of transaction to ensure successful completion of transaction without deadlocks. Here, we are assuming that the conflicts are likely and some major steps have to be taken to avoid those conflicts.

Let’s have an example on this:

Suppose in a car showroom, a customer wants to go for a test drive, but before the manager say something, it has to be clear that the car is empty and is ready for driving. What if another customer is already requested for the test drive for the same car? If the manager allows both of them to drive the car simultaneously, considering mutual understanding between the customers then we call it as an Optimistic concurrency. But if the manager wants to be sure about non-conflicts of the customer, then he allows the customers for test driving one-by-one. This is what we call as Pessimistic Concurrency.

Reference:

MSDN Books Online
http://msdn.microsoft.com/en-us/library/ms173763.aspx