Mongodb - Cheat Sheet (Page 4 of 4) in pdf

More information can be found from the MongoDB Tutorial

MongoDB - Cheat Sheet

Version 1.0 / 27

December 2012 / Page 4 of 4

Replica Sets

Type

Allowed to

Can become

Description

vote?

Primary?

Regular

Yes

This is the most typical kind of node. It can act as a primary or

secondary node

Arbiter

Yes

Arbiter nodes are only there for voting purposes. They can be

used to ensure that there is a certain amount of nodes in a replica

set even though there are not that many physical servers.

Delayed

Yes

Often used as a disaster recovery node. The data stored here is

usually a few hours behind the real working data.

Hidden

Often used for analytics in the replica set.

Sharding



Every document has to define a shard-key.



The value of the shard-key is immutable.



The shard-key must be part of an index and it must be the first field in that index.



There can be no unique index unless the shard-key is part of it and is then the first field.



Reads done without specifying the shard-key will lead to requests to all the different shards.



The shard-key must offer sufficient cardinality to be able to utilize all shards.

Durability of Writes



w – This tells the driver to wait for the write to be acknowledged. It also ensures no indexes are

violated. Nevertheless the data can still be lost as it is not necessarily already persisted to disc.



j – This stands for

journal

-mode. It tells the driver to wait until the journal has been committed to

disk. Once this has happened it is quite sure that the write will be persistent unless there are any

disc-failures.

w=0 j=0

This is “fire and forget”.

w=1 j=0

Waits for an acknowledgement that the write was received and no indexes have been

In the context of replica sets the value for the w-parameter now means the

amount of nodes that have acknowledged a write. There is a useful short

violated. Data can still be lost.

notation to ensure write was done to a majority of nodes by using

w=1 j=1

The most save configuration by waiting for the write to the journal to be completed.

w=’majority’. For the journal-parameter the value of one is still the best that

w=0 j=1

Basically the same as above.

can be done. It means the data is written to the journal of the primary node.

Mongodb - Cheat Sheet Page 4

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