cygni - replication intro

source/core/replication-introduction.txt

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+========================
+Replication Introduction
+========================
+
+.. default-domain:: mongodb
+
+Replication maintains identical copies of an entire database instance on
+multiple distinct systems. With multiple copies of the data, replication
+makes the deployment resilient with regards to the loss of any
+particular copy or instance. All MongoDB production deployments use
+replication.
+
+Replicated data helps ensure that all changes to a data set survive
+failure. Replication also increases theoretical throughput. With
+replication, if applications do not need to write data they can read
+data from secondary databases, which increases read capacity.
+
+MongoDB provides replication by storing the database in one location,
+called a *primary*, and copying the database to multiple additional
+locations, called *secondaries*. This configuration is called a
+``replica set``. Each replica set has a :term:`primary` and one or more
+:term:`secondaries <secondary>`. The primary is the only member of the
+set that accepts write operations.
+
+Replica Set Architecture
+------------------------
+
+A replica set consists of at least three :program:`mongod` instances: a
+:term:`primary` and either two :term:`secondaries <secondary>` or a
+secondary and an :term:`arbiter`. The primary accepts all writes for the
+set. All members accept reads. Secondaries replicate from the primary by
+asynchronously reading and applying the writes. This keeps the data on
+the primary consistent. The replication process is asynchronous.
+Secondaries replicate operations after the operations are applied to the
+primary.
+
+Failover and Recovery
+---------------------
+
+Replica sets provide automatic failover if the primary is not available.
+In a failover situation, the replica set automatically promotes a
+secondary to become the new primary, without user intervention. When the
+original primary becomes available, it will rejoin the set as a
+secondary.
+
+For More Information
+--------------------
+
+For more information about replica sets, see :doc:`/replication`.