Where to handle errors on database in mongoDB?

Question

When user is registering on website, e-mail needs to be provided which is unique. I've made unique index on schema's email attribute, so if I try to save the document in database, error with code 11000 will be returned. My question is, regarding to business layer and data layer, should I just pass the document to database and catch/check error codes which it returns or should I check if the user with that e-mail exists before? I've being told that data integrity should be checked before passing it to the database by the business layer, but I don't see the reason why should I do that since I believe that mongo would be much faster raising the exception itself since it has that index provided. The only disadvantage I see in error code checking is that error codes might change (but I could abstract them) and the syntax might be changed.

I think it is fine to try the insertion and handle the duplicate key error. This saves you the cost of a read request that would have to be paid on every insertion. If you want to avoid hitting the DB to check you would have to cache existing user email addresses in the business layer - which may or may not be feasible depending on # of users and the resources you have available. — James Wahlin
– James Wahlin, Commented Jul 29, 2014 at 19:53

Bob Kuhar · Accepted Answer · 2014-07-29 20:02:45Z

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There is the practical matter of speed and the fragility of "check-then-set" systems. If you try and check if an email exists before you write the document keyed on email, there is a chance that between the time you check and the time you right the conditions of the unique index are met and your write fails anyhow. This is a classic race condition. Further, it takes 2 queries to do check-then-set but only 1 query to do the insert and handle the failure. In my application I am having success with just letting the failure occur and reacting to the result.

answered Jul 29, 2014 at 20:02

Bob Kuhar

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Sammaye · Accepted Answer · 2014-07-29 20:00:34Z

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As @JamesWahlin says, it is the difference between dong this all in one or causing mixed results (along with the index check) from potential race conditions by adding the extra client read.

Definitely rely on the response of only insert from MongoDB here.

answered Jul 29, 2014 at 20:00

Sammaye

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