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    <p>Fundamentally, I difference between a MMDB and a OODB is that the MMDB has the expectation that all of its data is based in RAM, but persisted to disk at some point. Whereas an OODB is more conventional in that there's no expectation of the entire DB fitting in to RAM.</p> <p>The MMDB can leverage this by giving up on the concept that the persisted data doesn't necessarily have to "match" the in RAM data.</p> <p>The way anything with persistence is going to work, is that it has to write the data to disk on update in some fashion.</p> <p>Almost all DBs use some kind of log for this. These logs are basically "raw" pages of data, or perhaps individual transactions, appended to a file. When the file gets "too big", a new file is started.</p> <p>Once the logs are properly consolidated in to the main store, the logs are discarded (or reused).</p> <p>Now, a crude, in RAM DB can exist simply by appending transactions to a log file, and when it's restarted, it just loads the log in to RAM. So, in essence, the log file IS the database.</p> <p>The downside of this technique is the longer and more transactions you have, the bigger your log/DB is, and thus the longer the DB startup time. But, ideally, you can also "snapshot" the current state, which eliminates all of the logs up to date, and effectively compresses them.</p> <p>In this manner, all the routine operations of the DB have to manage is appending pages to logs, rather than updating other disk pages, index pages, etc. Since, ideally, most systems don't need to "Start up" that often, perhaps start up time is less of an issue.</p> <p>So, in this way, a MMDB can be faster than an OODB who has a different contract with the disk, maintaining logs and disk pages. In this way, an OODB can be slower even if the entire DB fits in to RAM and is properly cached, simply because you incur disk operations outside of the log operations during normal operations, vs a MMDB where these operations happen as a "maintenance" task, which can be scheduled during down time and/or quiet time.</p> <p>As to whether either of these systems can meet you actual performance needs, I can't say.</p>
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    1. COthanks for a very good explanation for 2 different technologies, Will. Did you ever use any OODBMS or MMBDBMS? If so, which ones? How did you like them compared to a traditional RDBMS?
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    2. CONo, I've not used either in any "legitimate" way, and even then, none of my projects have had your bandwdith requirements, so even if I had, the experience may not have been valid.
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