Saturday, April 30, 2016

Basic of Transaction

Have you ever though about how below classic example works:
           You swipe your credit card in a shop and money gets debited from account but overall billing fails. In that case your money gets credited back to your account instantly.

If you can already relate this to one of the key property of Database called Transaction, then you already have some head start for this topic. If not then no need to worry, I will take you from very basic of Transactions.

Background

Transactions are one of the most fundamental key concept for any database including PostgreSQL. It enables multiple step actions to be grouped together in such a way that either all step executions are accepted or all are rejected. If all of step executions are success then as a group whole actions will be considered as success. If any of the step execution fails in between, then it will make sure to nullify the effect of all other previous successful steps. 
Consider an example of bank fund transfer, where in there is table consisting of customer name, their balance etc. There is a customer 'A' who wants to transfer 1000Rs to customer 'B'. In terms of SQL it will look like:

            UPDATE accounts SET balance = balance - 1000 where cust_name='A';
            UPDATE accounts SET balance = balance + 1000 where cust_name='B';

There are two steps involved in overall fund transfer activity and as per the above definition of transaction, either both of them should succeed i.e. once 1000Rs debited from A's account it must be credited to B's account or there should not be even debit from A's account. Also there should not be case where debit from A's account failed but B's account got credited with 1000Rs, in which case bank will not be happy.
Just explained property of relation is called as "Atomicity", there are other 3 additional properties of each transaction explained below:

Properties

There are four mandatory properties of each transaction:

Atomicity: 

This property make sure that all changes to data are performed as if they are a single operation. That is, all the changes are performed, or none of them. This is already explained above with example.

Consistency: 

This property ensures that database is in consistent state when transaction state and when it ends i.e. it just bring the database from one valid state to another. In above example balance data will remain in consistent state when transaction ends.

Isolation: 

This property insures that the intermediate state of a transaction is invisible to other concurrent running transactions. As a result, transactions that run concurrently appear to be serialized. In above example, if there is any concurrent transaction executing the below SQL command:

               SELECT sum(balance) FROM account;

Then it should always show the same amount i.e. it should not be able to see intermediate stage where 1000Rs has been debited from A's account but not credited to B's account.

Durability: 

This property related to action after transaction has completed and it ensures that once a transaction completes changes to data persists and are not undone even in event of system failure. In above example, once 1000Rs debited from A's account, even on system restart changed balance will be shown for A's account.

Major Commands

Below are the major commands related to transaction:

1. START TRANSACTION / BEGIN: This command start a new transaction. 
2. COMMIT: This command ends already running transaction.
3. ROLLBACK: This command is used to undo all changes done by the statements running in the current transaction. Some depending on your business logic you might need to execute this command.

In order to execute multiple statements in a single transaction, then before writing any statement, first command-1 needs to be executed and then after all statements are executed, command -2 or command -3 should be executed.
e.g..
START TRANSACTION
SQL-STMT-1
SQL-STMT-2
..
..
SQL-STMT-3
COMMIT/ROLLBACK.

Note: If an explicit START TRANSACTION/BEGIN command is not given before a SQL-statement then explicitly BEGIN command will be executed for that SQL-statement and once this statement finishes, an explicit COMMIT command will be executed.

I hope by now you understand how swiping credit card example works. Here debiting of money from your card, generation of bill all happens in the same transaction. So even if first step of debiting money success but final billing failed then whole transaction will be rollbacked and hence debited money will be credited back.

Do visit my next post to see how transaction works in PostgreSQL from code flow perspective.

Any comment/query/suggestion welcome.



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