MongoDB

No. If you don't call getLastError (aka "Safe Mode") the server does exactly the same behavior as if you had. The getLastError call simply lets one get confirmation that the write operation was successfully committed. Of course, often you will want that confirmation, but the safety of the write and its durability is independent.

A secondary is a node/member which applies operations from the current primary. This is done by tailing the replication oplog (local.oplog.rs).

Replication from primary to secondary is asynchronous, however the secondary will try to stay as close to current as possible (often this is just a few milliseconds on a LAN).

This is a node/member which is currently the primary and processes all writes for the replica set. In a replica set, on a failover event, a different member can become primary.

It may take 10-30 seconds for the primary to be declared down by the other members and a new primary elected. During this window of time, the cluster is down for "primary" operations – that is, writes and strong consistent reads. However, you may execute eventually consistent queries to secondaries at any time (in slaveOk mode), including during this window.

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.

MongoDB does aggressive preallocation of reserved space to avoid file system fragmentation.

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_old.)

Yes, you remove the attribute and then re-save() the object.

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.

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.

Yes

MongoDB includes a database profiler which shows performance characteristics of each operation against the database. Using the profiler you can find queries (and write operations) which are slower than they should be; use this information, for example, to determine when an index is needed.

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Journaling activates the memory mapped file additional. This will further suppress the size of the database is a 32 bit version. Therefore, journaling is now the 32 bit system is disabled by default.

MongoDB includes a display the operating property database each database analyzer. Through this analyzer you can find the slower than expected query (or write); using this information, for example, to determine whether there is a need to add index.

Both MySQL and MongoDB are free open source database. MySQL and MongoDB have many fundamental differences including data representation (data representation), query, relationship, business, design and definition of schema, the standard (normalization), speed and performance. Through the comparison of MySQL and MongoDB, we are in fact more relational and non relational database.

　　MongoDB and CouchDB are document oriented database. MongoDB and CouchDB are the most typical representative of the open source NoSQL database. They have nothing in common other than are stored in the document outside. MongoDB and CouchDB, the data model interface, object storage and replication methods have many different.

For example: MongoDB, Cassandra, CouchDB, Hypertable, Redis, Riak, Neo4j, HBASE, Couchbase, MemcacheDB, RevenDB and Voldemort are the examples of NoSQL databases.

Yes