[thomaslangston]

The general idea I've seen (for both SQL and NoSQL databases) is two apps never should write to the same database to ensure separation of concerns. Some API layer instead handles all write operations.

Disclaimer: MongoDB employee. All opinions are my own.

[almostdeadguy]

That's generally a good practice (though not always, many people do blue-green deployments, for instance), but it's rarely an assumption you want to make. Lots of deployment blunders can happen that render assumptions like that incorrect.

Even if your team executes perfectly and never runs into this, the biggest problem IMO is that you can't really enforce most of your guarantees w/ any degree of confidence w/o a typed schema. Even if you work within a typed language that perfectly validates all the invariants of your application before storing anything, the second you need to perform work that does not strictly funnel data through your application (i.e. an update query), you are effectively gambling on whether or not those invariants will hold. This kind of "read-modify-write" flow of data doesn't really perform well (or even hold validity) for a lot of common use cases, so in reality you need your database to ensure these things for you.

Also the two deployed apps problem is just a special case of two people interacting w/ a database who aren't working under the same assumptions as to what invariants should hold. That can happen in single code bases, even with a lot of care taken.

[chousuke]

You only need a separate API layer if the database can't enforce constraints properly.

The database schema (along with stored procedures, views etc.) is an API and database engines are designed to have multiple concurrent writers. Multiple applications and users needing access to the same data is largely why databases exist in the first place.

[int_19h]

It doesn't matter in this case - all that matters is that they share the same data store, regardless of how concurrent access to it is organized. The problem here isn't concurrency, but the semantics of data stored - if one app can change it such that the other app can later retrieve data that it considers invalid according to its business constraints, what is the other app supposed to do?