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Replication

  • A replica set in MongoDB is a group of mongod processes that maintain the same data set.
  • Replica set contains several data-bearing nodes but only one is primary (+ optionally one arbiter node).
  • Primary records all changes to its data sets in its operation log (i.e. oplog).
  • The secondaries replicate the primary’s oplog and apply the operations to their data sets (Asynchronously).
  • If the primary is unavailable, an eligible secondary will hold an election to elect itself as the new primary.

What is the difference between replication and sharding?

  • Think of it like a pizza:
    • With replication: you are making a copy of a complete pizza pie on every server.
    • With sharding, you’re sending pizza slices to several different replica sets.
    • Combined together, you have access to the entire pizza pie.
  • Replication and sharding can work together to form something called a sharded cluster, where each shard is replicated in turn to preserve the same high availability.

Sharding:

  • MongoDB supports horizontal scaling through sharding.
  • At the collection level, distributing the collection data across the shards in the cluster.
  • Sharded Cluster consists of:
    • Shard: contains a subset of the sharded data + can be deployed as a replica set.
    • Mongos: acts as a query router providing interface between client apps and the sharded cluster.
    • Config servers: store metadata and configuration settings for the cluster.
  • Once a collection has been sharded, MongoDB provides no method to unshard a sharded collection.
  • Shards are implemented by using clusters which are nothing but a group of MongoDB instances.
  • More details about Shard Key are needed.

Step by Step Sharding Cluster Example:

  • Create Docker network for the cluster: 
    docker network create --driver bridge xnet
  • Run config node: 
    docker run -it --rm --net=xnet -p 27016:27016 \
--hostname config-1 --name config-1 \
mongo:3.4.9 --port 27016 --replSet config --configsvr
  • Init config node: 
    docker exec -it config-1 mongo --port 27016 --eval '
rs.initiate({ _id: "config", members: [
    { _id : 0, host : "config-1:27016" }
]});'
  • Run router node: 
    docker run -it --rm --net=xnet -p 27015:27015 \
--hostname mongos --name mongos \
mongo:3.4.9 mongos \
--port 27015 --configdb config/config-1:27016
  • Run nodes for shard 1 & 2: 
    docker run -it --rm --net=xnet -p 27018:27018 \
--hostname shard-1 --name shard-1 \
mongo:3.4.9 --port 27018 --shardsvr

    docker run -it --rm --net=xnet -p 27019:27019 \
--hostname shard-2 --name shard-2 \
mongo:3.4.9 --port 27019 --shardsvr
  • Configure sharding cluster: 
    docker exec -it mongos mongo --port 27015 --eval '
sh.addShard("shard-1:27018");
sh.addShard("shard-2:27019");
sh.enableSharding("test_db");
sh.shardCollection("test_db.test_collection", {"tag": 1});
sh.status();'
  • Testing by creating a single document and printing some info: 
    docker exec -it mongos mongo --port 27015 test_db --eval '
    db.test_collection.save({tag: "mongo", code: "200"});
    db.test_collection.find({tag: "mongo"}).explain();'

Resources: