What is the benefit of using a "document-oriented DBMS"?

I must be missing something, because everything I've seen so far suggests that it isn't any more interesting than a single table for storing blobs and a second table for tags that apply to it.

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Now I certainly can see some benefit to that from a design pattern, but why would I want to use a "document-oriented DBMS" instead of just building it using a traditional database like SQL Server, Oracle, or Postgres?

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I enjoyed listening to the floss weekly episode about CouchDB. Lots of reasoning and ideas there.

Prior to listening, most of the stuff I read about this topic triggered not much insight (for me). Listening to people talking and reasoning about why&where you want to use document-oriented DBs helped me a lot to really get the concepts, reasoning, pros and cons. Now all the articles and statements (IMHO) suddenly make a lot more sense.

Your mileage may vary, but this helped me a lot.

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I presume you are meaning products such as Couch DB or Tokyo Cabinet (rather than ECM products like Documentum). I think the attraction for many developers is familiarity.

Firstly, the conceptual model (in most cases) is key-value pairs, like a configuration file. As most frameworks seem to require a lot of configuration-wrangling, front-end/middle tier developers are comfortable with that way of working working. Secondly, these tools offer interfaces in developer-friendly languages like Java, Python, etc.

Whereas, traditional RDBMS products require thinking in a different fashion - relationally. And they require learning not just a weird language, SQL, but a new way of programming: set-based rather than procedural. If you rehearse the arguments for putting business logic in the middle tier rather stored procedures in the database, well a lot of them apply to No SQL as well.

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why would I want to use a "document-oriented DBMS" instead of just building it using a traditional database like SQL Server, Oracle, or Postgres?

Historically document-oriented DBMS do not allow to span ACID transactions across documents. They have poor support for Foreign Keys. The trade ought to allow for more accessible operations, maintenance and scaling.