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What are the ACID properties?

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The abbreviation ACID (Atomicity, Consistency, Isolation, Durability) is a term from database theory and describes rules and procedures for database transactions. If the specifications of ACID are adhered to, the data in a system is reliable and consistent.

What are the A-C-I-D basic principles?

Classical relational databases fulfill the four ACID properties. These state that the most important requirement for a database is to maintain the truthfulness and meaningfulness of the data. In many cases, data stores are seen as a “single point of truth”, thus it would be fatal if erroneous information is stored and passed on. The four properties include the following points:

  • Atomicity (A): Data transactions, e.g. the entry of a new data record or the deletion of an old one, should either be executed completely or not at all. For other users, the transaction is only visible when it is completely executed.
  • Consistency (C): This property is satisfied when each data transaction moves the database from a consistent state to a consistent state.
  • Isolation (I): When multiple transactions occur simultaneously, the final state must be the same as if the transactions occurred separately. That is, the database should pass the stress test. In other words, it should not result in incorrect database transactions due to overload.
  • Durability (D): The data within the database must only change as a result of a transaction and must not be changeable by external influences. For example, a software update must not inadvertently cause data to change or possibly be deleted.
Das Diagramm zeigt die Grundeigenschaften von ACID.
ACID Basic Principles

The Basic Principles on the Example

The most common example to illustrate the components of ACID are bank transfers, where money is transferred from one account to another. The goal, of course, is to ensure that all transfers are correct and that all customers have the amount of money in their account that they are entitled to.

Assume that a transfer from account A to account B takes place. Atomicity describes that transactions are either executed completely or fail completely. For our example, this means that if account A is debited with the amount of money and then there is a system failure, the money is simply credited back to account A. If this did not happen, we would have destroyed money and the system would be in a false state.

For consistency, after each transaction, it must be determined that the database is still in a consistent state, for example, that it does not contain any conflicting data. Suppose our example bank maintains a table with all accounts and the current balance amounts. In this table, the account number is a primary key so that each account number may occur only once in the database. If, after an incorrect database transaction, there may be two records for one account number, there is an inconsistency and the transaction must be reversed.

Isolation states that several transactions running in parallel must not lead to different results than if the transactions had taken place individually and one after the other. Thus, if a bank has to process 100 transfers simultaneously during peaks, it must be ensured that the balances of the affected accounts are just as high as if the transfers had taken place one after the other.

Finally, for durability, the bank must be able to guarantee that the consistent data inventory is not impaired by external influences. This includes, for example, power failures, system crashes, or software updates.

What are the Benefits of ACID?

In application, databases that comply with ACID principles offer many advantages. These include:

  • ACID makes it possible for several people to work on a database without any concerns.
  • Database users and developers can assume that the database is error-free and do not have to deal with troubleshooting.
  • Manual debugging is no longer necessary because no errors occur.

Do NoSQL Databases fulfill the ACID Properties?

NoSQL solutions generally cannot comply with the ACID properties, although there are exceptions, such as graph databases, which comply with all the concepts. NoSQL databases are in many cases distributed across multiple devices and servers. This allows much larger amounts of data to be processed and stored simultaneously, which is a key requirement for these systems. However, this means that they do not fulfill the property of consistency.

Suppose we have implemented a NoSQL database on two physical servers, one located in Germany and the other in the USA. The databases contain the account balances and transactions of German and American customers. The German accounts are stored in Germany and the American accounts are stored on the American server.

It may now happen that a German customer makes a transfer to an American account. Then both data stores are changed and are inconsistent during this processing period. For example, it may happen that we start a database query while the processing in Germany has already been completed, but the processing in the USA has not yet been completed. In this time window, the “Inconsistency window”, the data in the database is not correct and is inconsistent. This would not happen in a relational database.

This is what you should take with you

  • ACID (Atomicity, Consistency, Isolation, Durability) is a term from database theory and describes rules and procedures for database transactions.
  • Relational databases fulfill these properties and are therefore consistent at all times. NoSQL databases, on the other hand, are to a large extent not ACID compliant.
  • Compliance with the principles ensures that databases have an error-free database at all times and that concurrent accesses are possible without any concerns.

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