What is a good shard key?

What is a good shard key?

The Perfect Shard Key All queries would be uniformly distributed across all of the shards in the cluster. All operations would only target the shards of interest: an update or delete would never be sent to a shard which didn’t own the data being modified.

Can you change the shard key after a collection is Sharded?

Starting in MongoDB 5.0, you can reshard a collection by changing a document’s shard key. Starting in MongoDB 4.4, you can refine a shard key by adding a suffix field or fields to the existing shard key. In MongoDB 4.2 and earlier, the choice of shard key cannot be changed after sharding.

Where is shard key in MongoDB?

As outlined in the Sharding Administration Docs, you can use db. printShardingStatus() to see this information. For sharded collections it will print the key pattern. You can login to any “mongos” instance (or config server instance) on the cluster and query against the collections collection.

What is sharding hash?

Hashed sharding uses either a single field hashed index or a compound hashed index (New in 4.4) as the shard key to partition data across your cluster. Compound hashed index compute the hash value of a single field in the compound index; this value is used along with the other fields in the index as your shard key.

How do you change shard keys?

The steps if we want to change our shard key are as follows:

1. Export all data from MongoDB.
2. Drop the original sharded collection.
3. Configure sharding with the new key.
4. Presplit the new shard key range.
5. Restore our data back into MongoDB.

Does shard key have to be unique?

Although you can have a unique compound index where the shard key is a prefix, if using unique parameter, the collection must have a unique index that is on the shard key. You cannot specify a unique constraint on a hashed index.

Is MongoDB key value?

Is MongoDB a key-value database? MongoDB is a document database, which means that it stores data in the form of “objects” which have properties which can be changed, added to, deleted, and queried against.

Why is hash consistent?

Consistent hashing solves the problem of rehashing by providing a distribution scheme which does not directly depend on the number of servers. In consistent hashing when a server is removed or added then the only key from that server are relocated. …

What is MongoDB hash value?

MongoDB hashed indexes truncate floating point numbers to 64-bit integers before hashing. For example, a hashed index would store the same value for a field that held a value of 2.3 , 2.2 , and 2.9 .

Why is the _ ID field not part of the shard key?

Because the _id key is not part of the shard key, the collection could have a document with _id value 1 in shard A and another document with _id value 1 in shard B. If the _id field is not the shard key nor the prefix of the shard key, MongoDB expects applications to enforce the uniqueness of the _id values across the shards.

Is the shard key field immutable in MongoDB?

Starting in MongoDB 4.2, you can update a document’s shard key value unless the shard key field is the immutable _id field. Before MongoDB 4.2, a document’s shard key field value is immutable. For details on updating the shard key, see Change a Document’s Shard Key Value. For more information on the sharding method, see sh.shardCollection ().

What to do if you drop an index on a shard key?

If the collection is not empty, you must create the index first before using sh.shardCollection (). If you drop the last valid index for the shard key, recover by recreating an index on just the shard key. MongoDB can enforce a uniqueness constraint on a ranged shard key index.

Which is the best way to use sharding?

The best you can hope for is to create a cluster (which has nothing to do with sharding fundamentally, by the way) or go for a managed solution like Amazon’s RDS or Google’s Cloud SQL, which become prohibitively expensive as your data grows.