Mastering MongoDB 4.x - Second Edition

Choosing the correct shard key

After the previous section, it's now self-evident that we need to take the choice of our shard key into consideration as it is a decision that we have to stick with.

A great shard key has three characteristics:

• High cardinality
• Low frequency
• Nonmonotonic changes in value

We will go over the definitions of these three properties first to understand what they mean:

• High cardinality: It means that the shard key must have as many distinct values as possible. A Boolean can take only the values of true/false, and so it is a bad shard key choice. A 64-bit long value field that can take any value from −(2^63) to 2^63−1 is a good shard key choice, in terms of cardinality.

• Low frequency: It directly relates to the ...