How does the sharded cluster balancer work in MongoDB?

How does the sharded cluster balancer work in MongoDB?

To address uneven chunk distribution for a sharded collection, the balancer migrates chunks from shards with more chunks to shards with a fewer number of chunks. The balancer migrates the chunks until there is an even distribution of chunks for the collection across the shards. For details about chunk migration, see Chunk Migration Procedure.

How is data distributed in a MongoDB cluster?

MongoDB partitions data in a sharded collection into ranges based on the values of the shard key. Then, MongoDB distributes these chunks to shards. The shard key determines the distribution of chunks to shards. This can affect the performance of write operations in the cluster.

What happens when you remove a shard from MongoDB?

Removing a shard from a cluster creates a similar imbalance, since chunks residing on that shard must be redistributed throughout the cluster. While MongoDB begins draining a removed shard immediately, it can take some time before the cluster balances. Do not shutdown the servers associated to the removed shard during this process.

How many chunk migrations can MongoDB do at a time?

Changed in version 3.4: Starting in MongoDB 3.4, MongoDB can perform parallel chunk migrations. Observing the restriction that a shard can participate in at most one migration at a time, for a sharded cluster with n shards, MongoDB can perform at most n/2 (rounded down) simultaneous chunk migrations.

What’s the latest version of the MongoDB balancer?

Changed in version 3.4: The balancer process has moved from the mongos instances to the primary member of the config server replica set. This page describes common administrative procedures related to balancing. For an introduction to balancing, see Sharded Cluster Balancer. For lower level information on balancing, see Cluster Balancer.

Which is better sharding or replication in MongoDB?

In replication, read performance can be enhanced by adding more slave/secondary servers, whereas, in sharding, both read and write performance will be enhanced by adding more shards nodes. The Sharded cluster doesn’t support unique indexing across the shards until the unique index is prefixed with full shard key.

How to disable balancing on a MongoDB collection?

When you disable balancing on a collection, MongoDB will not interrupt in progress migrations. To disable balancing on a collection, connect to a mongos with the mongo shell and call the sh.disableBalancing () method. The sh.disableBalancing () method accepts as its parameter the full namespace of the collection.

Can a client connect to more than one shard in Mongo?

Clients should never connect to a single shard in order to perform read or write operations. You can connect to a mongos the same way you connect to a mongod, such as via the mongo shell or a MongoDB driver. MongoDB supports two sharding strategies for distributing data across sharded clusters.

Why are my shard key fields missing in MongoDB?

Starting in version 4.4, documents in sharded collections can be missing the shard key fields. Missing shard key fields are treated as having null values when distributing the documents across shards but not when routing queries. For more information, see Missing Shard Key.