When can you call operation idempotent?

Question

The definition of idempotent in wikipedia is:Idempotence is the property of certain operations in mathematics and computer science, that can be applied multiple times without changing the result beyond the initial application.

The problem is: I have REST API PUT call, which updates properties of domain aggregate. Also, it fires event for each property, which was updated. Now if we have two exact the same PUT calls one after the other to our backend:

First PUT call updates the properties of aggregate and fires lets say 5 events.
Second PUT call updates the properties of aggregate, but do not fire any event, because the properties of aggregate did not change (first PUT call updated the values of aggregate properties).

The question is: is this operation idempotent?

The kind of operations referred to in the Wikipedia article usually don't have side effects. — Thomas, Sep 01 '16 at 10:28

score 1 · Answer 1 · edited May 23 '17 at 12:22

1

This question and its answers explain what an idempotent operation is. In short: repeated calls do not change the outcome.

So from your description of this operation, it seems it qualifies as idempotent.

edited May 23 '17 at 12:22

Community

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answered Sep 01 '16 at 10:30

DarkDust

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score 0 · Answer 2 · answered Sep 01 '16 at 10:29

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Yes, it is: no matter how many times you send the same PUT request, it leaves your system (your aggregate) in the same state.

answered Sep 01 '16 at 10:29

Alexander Guz

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score 0 · Accepted Answer · answered Sep 01 '16 at 10:32

Yes and No. It is idempotent from the data point of view. The data in the database won't change no matter how many times you execute the call. But it is not idempotent in the sense that some logging or other events might occur that would change the "Entropy" of the system :)

score 0 · Answer 4 · answered Sep 01 '16 at 10:37

It depends on your definition. Since you have side-effects (fiering off some events if there is a difference), multiple calls might cause more side-effects than desired. However, the state of the application, ignoring the side-effects, with no concurrency, will be idempotent. Remember that REST calls are made over an asynchronous network, so this is a distributed system.

If you have two concurrent processes, they may fire a different number of side-effects. For example:

a = 2
a = 3
a = 2 
a = 3

will fire twice as many events as

a = 2
a = 2 
a = 3
a = 3

which might cause some trouble.

When can you call operation idempotent?

4 Answers4