"A" in ACID - Atomicity

In this short essay, we dive deep and understand the "A" of ACID - Atomicity.

In this quick read, we will take a detailed look into Atomicity, understand its importance, and learn about implementing it at various levels.

What is atomicity?

A single database transaction often contains multiple statements to be executed on the database. In Relational Databases, these are usually multiple SQL statements, while in the case of non-Relational Databases, these could be multiple database commands.

Atomicity in ACID mandates that each transaction should be treated as a single unit of execution, which means either all the statements/commands of that transaction are executed, or none of them are.

At the end of the successful transaction or after a failure while applying the transaction, the database should never be in a state where only a subset of statements/commands is applied.

An atomic system thus guarantees atomicity in every situation, including successful completion of transactions or after power failures, errors, and crashes.

Atomicity ACID

A great example of seeing why it is critical to have atomicity is Money Transfers.

Imagine transferring money from bank account A to B. The transaction involves subtracting balance from A and adding balance to B. If any of these changes are partially applied to the database, it will lead to money either not debited or credited, depending on when it failed.

How is atomicity implemented?

Atomicity in Databases

Most databases implement Atomicty using logging; the engine logs all the changes and notes when the transaction started and finished. Depending on the final state of the transactions, the changes are either applied or dropped.

Atomicity can also be implemented by keeping a copy of the data before starting the transaction and using it during rollbacks.

Atomicity in File Systems

At the file system level, atomicity is attained by atomically opening and locking the file using system calls: open and flock. We can choose to lock the file in either Shared or Exclusive mode.

Atomicity at Hardware Level

At the hardware level, atomicity is implemented through instructions such as Test-and-set, Fetch-and-add, Compare-and-swap.

Atomicity in Business Logic

The construct of atomicity can be implemented at a high-level language or business logic by burrowing the concept of atomic instructions; for example, you can use compare and swap to update the value of a variable shared across threads concurrently.

Atomicity is not just restricted to Databases; it is a notion that can be applied to any system out there.

✨ Next up is "C" in ACID - Consistency. Stay tuned.


Arpit Bhayani

Arpit's Newsletter

CS newsletter for the curious engineers

❤️ by 38000+ readers

If you like what you read subscribe you can always subscribe to my newsletter and get the post delivered straight to your inbox. I write essays on various engineering topics and share it through my weekly newsletter.

Other essays that you might like

ACID - Durability

335 reads 2021-07-19

Durability seems to be a taken-for-granted requirement, but to be honest, it is the most important one. Let's deep dive ...

ACID - Isolation

301 reads 2021-07-05

Isolation is the ability of the database to concurrently process multiple transactions in a way that changes made in one...

ACID - Consistency

387 reads 2021-07-02

In the context of databases, Consistency is Correctness, which means that under no circumstance will the data lose its c...

Image Steganography

304 reads 2020-01-17

Steganography has been around since at least 440 BCE but with the rise of computers, the techniques have evolved to hand...

Be a better engineer

A set of courses designed to make you a better engineer and excel at your career; no-fluff, pure engineering.

Paid Courses

System Design for Beginners

A masterclass that helps early engineers and product managers become great at designing scalable systems.

132+ learners

Details →

System Design Masterclass

A masterclass that helps you become great at designing scalable, fault-tolerant, and highly available systems.

1000+ learners

Details →

Redis Internals

Learn internals of Redis by re-implementing some of the core features in Golang.

98+ learners

Details →

Free Courses

Designing Microservices

A free playlist to help you understand Microservices and their high-level patterns in depth.

823+ learners

Details →

GitHub Outage Dissections

A free playlist to help you learn core engineering from outages that happened at GitHub.

651+ learners

Details →

Hash Table Internals

A free playlist to help you understand the internal workings and construction of Hash Tables.

1027+ learners

Details →

BitTorrent Internals

A free playlist to help you understand the algorithms and strategies that power P2P networks and BitTorrent.

692+ learners

Details →