ythy
commented
6 years ago - All members of the replica set can accept read operations.
- Secondaries may be non-voting or priority 0.
- A replica set can have up to 50 members but only 7 voting members
- Oplog : operations log
A replica set in MongoDB is a group of mongod processes that maintain the same data set. Replica sets provide redundancy and high availability, and are the basis for all production deployments.
Redundancy and Data Availability
Replication provides redundancy and increases data availability. With multiple copies of data on different database servers, replication provides a level of fault tolerance against the loss of a single database server.
In some cases, replication can provide increased read capacity as clients can send read operations to different servers. Maintaining copies of data in different data centers can increase data locality and availability for distributed applications. You can also maintain additional copies for dedicated purposes, such as disaster recovery, reporting, or backup.
Replication in MongoDB
A replica set is a group of mongod instances that maintain the same data set. A replica set contains several data bearing nodes and optionally one arbiter node. Of the data bearing nodes, one and only one member is deemed the primary node, while the other nodes are deemed secondary nodes.
The primary node receives all write operations. A replica set can have only one primary capable of confirming writes with { w: "majority" } write concern; although in some circumstances, another mongod instance may transiently believe itself to also be primary.
The secondaries replicate the primary’s oplog and apply the operations to their data sets such that the secondaries’ data sets reflect the primary’s data set. If the primary is unavailable, an eligible secondary will hold an election to elect itself the new primary.
Asynchronous Replication
Secondaries apply operations from the primary asynchronously. By applying operations after the primary, sets can continue to function despite the failure of one or more members.
Automatic Failover
When a primary does not communicate with the other members of the set for more than 10 seconds, an eligible secondary will hold an election to elect itself the new primary. The first secondary to hold an election and receive a majority of the members’ votes becomes primary.
Read Operations
By default, clients read from the primary ; however, clients can specify a read preference to send read operations to secondaries. Asynchronous replication to secondaries means that reads from secondaries may return data that does not reflect the state of the data on the primary.