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How does sharding work in a MongoDB cluster?
MongoDB supports horizontal scaling through sharding. A MongoDB sharded cluster consists of the following components: shard: Each shard contains a subset of the sharded data. Each shard can be deployed as a replica set. mongos: The mongos acts as a query router, providing an interface between client applications and the sharded cluster.
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.
How are read and write workloads distributed in MongoDB?
MongoDB distributes the read and write workload across the shards in the sharded cluster, allowing each shard to process a subset of cluster operations. Both read and write workloads can be scaled horizontally across the cluster by adding more shards.
How does the shard key work in MongoDB?
MongoDB uses the shard key to distribute the collection’s documents across shards by assigning a range of values to a shard. Shard keys are based on fields inside each document. The values in those fields will decide on which shard the document will reside, according to the shard ranges and amount of chunks.
When to create a supporting Index in MongoDB?
When sharding an empty collection, MongoDB creates the supporting index if the collection does not already have an appropriate index for the specified shard key. See Shard Key Indexes.
Do you have to have a shard key in MongoDB?
To shard a non-empty collection, the collection must have an index that starts with the shard key. For empty collections, MongoDB creates the index if the collection does not already have an appropriate index for the specified shard key. See Shard Key Indexes.
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.
How do I create a replica in MongoDB?
For each member, start a mongod instance with the following settings: Set replication.replSetName option to the replica set name. If your application connects to more than one replica set, each set must have a distinct name. Set net.bindIp option to the hostname/ip or a comma-delimited list of hostnames/ips.
How many data centers does MongoDB need to be distributed?
If possible, distribute members across at least three data centers. For config server replica sets (CSRS), the best practice is to distribute across three (or more depending on the number of members) centers.
How to enable the balancer in MongoDB 4.2?
To enable the balancer from a driver, use the balancerStart command against the admin database, as in the following: Starting in MongoDB 4.2, sh.startBalancer () also enables auto-splitting for the sharded cluster. If MongoDB migrates a chunk during a backup, you can end with an inconsistent snapshot of your sharded cluster.
When to use CSRs in a MongoDB cluster?
As of MongoDB 3.4, config servers must be deployed as a replica set (CSRS). In a production cluster, ensure that data is redundant and that your systems are highly available. Consider the following for a production sharded cluster deployment:
Can a rolling index be built on a sharded cluster?
Rolling Index Builds on Sharded Clusters ¶ Index builds can impact sharded cluster performance. By default, MongoDB 4.4 and later build indexes simultaneously on all data-bearing replica set members. Index builds on sharded clusters occur only on those shards which contain data for the collection being indexed.
Can you run MongoDB on a single machine?
Yes, the hardware you described makes sense if you are going to run multiple shards on a single machine. A single MongoDB on that powerful machine will leave the machine mostly idle. A single mongod process cannot use that much RAM, I/O, or CPU. You will want to “core shard” the host.