Also check out Markus Gattol's excellent FAQ on his website. What's a "namespace"?MongoDB stores BSON objects in collections. The concatenation of the database name and the collection name (with a period in between) is called a namespace. For example, acme.users is a namespace, where acme is the database name, and users is the collection name. Note that periods can occur in collection names, so a name such as acme.blog.posts is legal too (in that case blog.posts is the collection name. How do I copy all objects from one database collection to another?See below. The code below may be ran server-side for high performance with the eval() method. db.myoriginal.find().forEach( function(x){db.mycopy.save(x)} );
If you remove an object attribute is it deleted from the store?Yes, you remove the attribute and then re-save() the object. Are null values allowed?For members of an object, yes. You cannot add null to a database collection though as null isn't an object. You can add {}, though. Does an update fsync to disk immediately?No, writes to disk are lazy by default. A write may hit disk a couple of seconds later. For example, if the database receives a thousand increments to an object within one second, it will only be flushed to disk once. (Note fsync options are available though both at the command line and via getLastError.) How do I do transactions/locking?MongoDB does not use traditional locking or complex transactions with rollback, as it is designed to be lightweight and fast and predictable in its performance. It can be thought of as analogous to the MySQL MyISAM autocommit model. By keeping transaction support extremely simple, performance is enhanced, especially in a system that may run across many servers. The system provides alternative models for atomically making updates that are sufficient for many common use cases. See the wiki page Atomics Operations for detailed information. How do I do equivalent of SELECT count * and GROUP BY?See aggregation. What are so many "Connection Accepted" messages logged?If you see a tremendous number of connection accepted messages in the mongod log, that means clients are repeatedly connecting and disconnected. This works, but is inefficient. With CGI this is normal. If you find the speed acceptable for your purposes, run mongod with --quiet to suppress these messages in the log. If you need better performance, which to a solution where connections are pooled -- such as an Apache module. What RAID should I use?We recommend not using RAID-5, but rather, RAID-10 or the like. Both will work of course. Can I run on Amazon EBS? Any issues?Works fine in our experience; more information here. Why are my data files so large?MongoDB does aggressive preallocation of reserved space to avoid file system fragmentation. This is configurable. More info here. |
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