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Comment by sp332 | Hacker News Reader

Not really. It just means that it's indivisible (the original meaning of "atom"). Either it succeeds or fails, you never have to worry about it being half-finished. This includes actions which are so small they are literally indivisible, or actions which roll back to the original state if they fail.

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Not just about it only completing or failing, but another observer in the system should never be able to find it in the half-way state. To everyone but the implementer of the atomic operation, it has no half-way states.

In the database terminology that's usually called "Isolation" to keep the concept separate from the rather restricted definition of atomicity.

Got it. Now it makes more sense to me. Now I know people tend to talk about atomicity when it comes to low-level-ish things. But say I create some sort of a web service with a bunch of business logic. Does it worth to follow this principle in that case? For instance, client sends an API request (let's say "Add user to friends"), is it even possible to apply atomicity for these type of things?

Edit: Thanks everyone for taking time to explain it to me.

Yes. To take a larger example action, creating a user could fail to be atomic if, say, it stored the username in a separate table from the user object, wrote the username first, then referenced it from the user object, but didn't roll back the username insert if the user object insert failed.

Likewise, for the seemingly simpler example if establishing a friend relationship, you may be tracking that relationship in both directions, in which case one could fail and the other succeed.

The business logic of your web application should resolve around database calls. Popular databases should already guarantee these atomicity properties for you through transactions.

Database guarantees are great until your data doesn't fit in a single database; it's good to examine what your database is providing for you, and to contemplate the costs for providing it in the database level.

> The business logic of your web application should resolve around database calls

Might, not should. All web apps are not just front ends to a single database where transactions are useful and once you leave the realm of a single database into a more distributed type system then transactions are no longer an option.

Yes, depending on how you store the relationships, its possible that when a adds b to its list of friends, a could be added to b's list of 'friendofs'. If they're stored separately, the edit may not be atomic. If it's stored in a single place (queryable from either direction), it's generally going to be atomic, unless you're doing something way outside the norm.

Atomicity can be important at any level. For example, assume that adding a friend involves two edges in a graph, one edge in each direction. Now assume that there is some other piece of code that does some analysis or processing of friend relationships. This piece of code might rely on there always being two edges, and crash or give erroneous results if not.

Thus, the adding of the two edges must be atomic (when observed from the rest of the system).

(The example is a bit contrived, but hopefully gets the idea across.)

Here are some real-world examples of what can go wrong if your API requests aren't atomic http://josipfranjkovic.blogspot.jp/2015/04/race-conditions-o...

Yes. But in this case, the concept you are probably looking for is "transactional". Transactions are atomic, but the difference is that they can fail (the state is then rolled back), and they can be retried at a later point.

(A bit late, but hoping that you see this:)

If possible you should probably go for an even stronger property, namely Idempotence[1]. (This can be relatively easy if you can force clients to provide some sort of unique token for every operation.)

It's usually makes this even easier to reason about for clients since they can just retry anything while knowing that it doesn't matter if they retry an already "applied" operation.

[1] https://en.wikipedia.org/wiki/Idempotence

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