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ReplicaSet.md

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Client for Replica Sets

IMPORTANT: ReplicaSet support is not included in current V5 driver!

Introduction

The driver described above is enough in a MongoDB standalone server configuration where there only one server can execute the operations. This job can get much more complex when the configuration is a MongoDB Replica Set. In this case, a group of servers maintain the same data set providing redundancy and high availability access.

The following figure shows a Replica Set configuration composed by 3 servers (a.k.a. members). The Primary server is the only member in the replica set that receives write operations. However, all members of the replica set can accept read operations (see Read Preference).

ReplicaSetFigure

The replica set can have at most one primary. If the current primary becomes unavailable, an election determines the new primary.

MongoClient

To help in this kind of scenarios, the MongoClient monitors the Replica Set status to provide the instance of Mongo that your application requires to perform an operation.

You can create a client and make it start monitoring as follows:

client := MongoClient withUrls: urlsOfSomeReplicaSetMembers.
client start.

After some milliseconds, it should be ready to, for example, receive write operations such as:

client primaryMongoDo: [ :mongo |
	((mongo
		databaseNamed: 'test')
		getCollection: 'pilots')
		add: { 'name' -> 'Fangio' } asDictionary ].

Until more documentation is available, you have these options to learn about this client:

  • Example. Evaluate and browse this code: MongoClientExample openInWindows.

  • Test suites. Browse the class hierarchy of MongoClientTest where you can see diverse tests, setUps, and tearDowns.

  • Visual Monitor. You can check this repository, which watches the events announced by a MongoClient to help to better understand them via visualizations.