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ACID Properties in Database Transactions

ACID Properties In Database Transactions

Learn about the ACID Properties in Database Transactions. Find out what is Atomicity, Consistency, Isolation and Durability with the help of examples.

Database transaction

A database transaction, by definition “is a unit of work performed within a database management system or a similar system, against a database which is treated coherently and reliably, independent of other transactions.” The transaction is generally a change within the database, and it can be any change. The two primary purposes of transactions in the database management system are listed below:

  • There is a need to isolate the programs which are accessing the database concurrently. If this does not happen, the outcomes of these programs can be error-prone. The database transaction provides isolation to these programs which are involved in accessing the database concurrently.
  • As we know, the systems are prone to different kinds of failures like a partial or complete seizure of execution, incomplete operations of the database etc. The transaction helps to provide dependable units of work which have the ability to allow correct recovery from these failures.

This single unit of logical work, a transaction can sometimes be composed of more than one operation. This generally happens in the case of the Database Management System. Here, in a database management system, the transaction is any logical calculation which is done in a consistent mode in the database. One of the examples of this kind of transaction is when you transfer money of $1000 from your bank account to your friend’s bank account. To conclude the transaction, the system first subtracts the $1000 from your account. Then the system adds up $1000 to your friend’s account.

The transaction in a database, by definition, should have the following properties:

  1. Atomicity: The Transaction must either complete in as a whole or have no change at all.
  2. Consistency: The Transaction must obey to the existing constraints in the database.
  3. Isolation: The current Transaction must not influence other transactions.
  4. Durability: This Transaction must get written to persistent storage.

These terms together are referred to as ACID properties and are imperative in database transactions. We will discuss these properties in detail in the latter part of this article.

It is worth noting that the transaction programs can either change or not change the contents of the database. It totally depends on the type of transaction and of course, the database. But one thing is sure about a transaction that is; it will always run as a single unit under any circumstances. If successful, these transactions can change the database from one consistent state to some another consistent state.

A simple example of a transaction

Let us take a reference to the example which we have mentioned above, a bank transfer. So, you have to transfer $1000 to your friend’s account. The transaction will follow the following set of operations:

  1. Read your account balance
  2. Subtract an amount of $1000 from your balance
  3. Write the remaining balance to your account
  4. Read your friend’s account balance
  5. Add the amount of $1000 to his account balance
  6. Write the newly updated balance to his account

Transaction Example

This set of six operations is called a transaction. Now let us show you how we write these set of six operations in DBMS. Suppose you are Linda (Account name: L) and you can to transfer $1000 to Chloe (Account Name: C). Your transaction operations can be written as:

Here, ‘R’ and ‘W’ refer to ‘Read’ and ‘Write’ operations, respectively.

What happens when a transaction fails while executing?

One of the significant problems in DBMS involving transactions is its failure to complete the operation. There can be many reasons behind this, like system crash or power failure. This problem can leave the database in an inconsistent state. Hence, it is considered as a severe issue. Let us look at the set of operations in a transfer of $1000 to your friend’s account:

  1. Read your account balance.
  2. Subtract an amount of $1000 from your balance
  3. Write the remaining balance to your account.
  4. Read your friend’s account balance.
  5. Add the amount of $1000 to his account balance.
  6. Write the newly updated balance to his account.

Let us suppose that the transaction failed after the third operation (that is, after deducting an amount of $1000 from your account). There can be two solutions:

  • Carry on the transaction immediately and automatically after the recovery. In this case, the transaction will ultimately result in permanent changes in the database. That is the $1000 will be deducted from your account and will be added to your friend’s account.
  • In case of transaction failure, there is also a chance to undo all the changes made. That is, the system will restore the amount of $1000 deducted from your account. You will have to transfer the amount again.

These two operations guarantee that we can avoid several potential issues during the transaction. But these are not enough to prevent problems when two transactions are running concurrently. In those case, we need the ACID properties.

The need for concurrency in transactions

We know that the database is a shared resource by more than one individual or institutions. For example, the banking database is accessed by many institutions like railways, airways, third party merchants etc. concurrently. Hence managing this concurrent access is crucial to smooth execution of the transaction process and operation of DBMS. Not managing concurrent access properly may lead to system crashes, slow or deadlock of performance.

Different states of Transactions

  1. Active state: This is the state where the system executes the transaction process in a database. Read and write operations are performed in this state.
  2. Partially committed: This state comes when a transaction is just concluded.
  3. Committed state: Transaction being committed to a state means that the execution of the transaction process is already completed successfully. The changes made by the transaction are recorded in the database permanently.
  4. Failed state: One defines a transaction as ‘failed’ when any one of the checks or the operations in the transaction fails, which results in aborting the transaction while it is still active.
  5. Terminated state: The Transaction is ‘terminated’ when a particular ‘failed’ transaction cannot be restarted because of various circumstances.

ACID Properties

What is ACID?

Computer Science defines ACID (atomicity, consistency, isolation and durability) as “a set of properties of database transactions which are intended to guarantee validity even in the events of errors, power failures etc.” Hence the language of database and DBMS defines a transaction as a set of operations which satisfy the ACID properties.  

It was Andreas Reuter and Theo Harder who made the term ACID popular, and used it as an acronym for the properties of the database, viz atomicity, consistency, isolation and durability. They upgraded the works of Jim Gray, the first man to introduce the concept of atomicity, consistency and durability. He, at that time, left out isolation among these essential properties.  

Familiarization with ACID

As discussed earlier, the term ACID is an acronym for four essential properties for database transaction, namely, atomicity, consistency, isolation and durability. Together, these four properties guarantee the successful and safe execution of the transaction process in any database. Let us discuss these properties in detail.

Atomicity

As the name suggests, atom means indivisible. We are well aware of the fact that the transactions are composed of multiple ‘indivisible’ operations. In such cases, the property of atomicity guarantees that these operations remain indivisible. Further, it also makes sure that either these operations are completed successfully, or they simply fail. It makes sure that the process of the transaction will only go forward if each of these ‘indivisible’ operations is successful. Otherwise, the process of the transaction will not take place. And no changes in the database will be made. It goes by the motto: “All occurs, or nothing occurs”. Any atomic system must guarantee atomicity in all the possible situation, even in case of system crashing or power failure. This prevents any partial change in a database which can create more significant headache to the user or the system operators.

To talk about an example, let us move back to the $1000 transfer in the above cases.

  1. Read your account balance
  2. Subtract an amount of $1000 from your balance
  3. Write the remaining balance to your account
  4. Read your friend’s account balance
  5. Add the amount of $1000 to his account balance
  6. Write the newly updated balance to his account

These are the six indivisible operations which take place while making a transaction of $1000 from your bank account to your friend’s bank account. The property of atomicity ensures each these six processes take place and that too in order. In case of any failure midst the transaction process, i.e. any of the operation fail to be concluded, the whole process will rollback. This prevents an uncomfortable situation of money being deducted from your account or money being added to your friend’s account without complete process.

Consistency

This property makes sure that the change brought by the database is only from one valid state to another. It makes sure that the data changes made in the database are correct according to all the predefined rules like constraints, cascades triggers and any competition similar to them. With this, the user and the operator do not have to worry about corruption by a third party transaction. It is noteworthy that consistency does not guarantee that the transaction is correct.

Isolation

We know former that most of the transactions are often executed concurrently. This means that multiple transactions are taking place at the same time. The property of isolation shows that one transaction process does not affect another and it leaves the database at the same state which would have been obtained even when the transactions were executed sequentially. To explain it in a better way we can say that suppose two transaction processes are going on simultaneously T1 & T2. While T1 is accessing the data of the database; T2 cannot access any data from the same database. This makes sure the database is not accessed by two transaction process simultaneously and hence reduces chances of error or system crash. At the same time, if these two processes occur simultaneously, the chances of error also increase significantly.

Durability

The property of durability guarantees that once per transaction has been committed, it will remain committed even in the case of system failure. Basically, this means that the transaction is carried out in non-volatile memory.

This property guarantees a permanent survival of the change. Let us take an example of ticket booking of a particular flight from Washington D.C. to Los Angeles.  Now suppose Daniel books a ticket in that specific flight for a date, let’s say June 20, 2020. Once the booking is concluded, the given seat will remain booked even if the system of that airline company crashes. This is ensured because of the durability property of transaction. This state of durability is achieved by flushing the complete transaction log records into non-volatile storage before acknowledging the commitment.

This property of durability is achieved by various DBMSs by first writing transactions into a transaction log which can be recreated into the system just before any failure. Confirmation of the successful conclusion of the transaction comes only after the data changes enters the log.

I hope you found this guide useful. If so, do share it with others who are willing to learn about the different topics that we publish here on our blog. If you have any questions related to this article, feel free to ask us in the comments section.

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