November 2020
Beginner to intermediate
250 pages
7h 41m
English
Content preview from 97 Things Every SRE Should Know
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Chapter 14. The Role of Cardinality
Charity Majors and Liz Fong-Jones
In the context of databases, cardinality refers to the uniqueness of data values contained in a set. Low cardinality means that a column has a lot of duplicate values in its set. High cardinality means that the column contains a large percentage of completely unique values. A column containing a single value will always be the lowest possible cardinality. A column containing unique IDs will always be the highest possible cardinality.
For example, if you had a collection of a hundred million user records, you can assume that userID numbers will have the highest possible cardinality. First name and last name will be high cardinality, though lower than userID because some names repeat. A field like gender would be fairly low cardinality, given the nonbinary but finite choices it could have. A field like species would be the lowest possible cardinality, presuming all of your users are humans.
Cardinality matters for observability, because high-cardinality information is the most useful data for debugging or understanding a system. Consider the usefulness of sorting by fields like user IDs, shopping cart IDs, request IDs, or myriad other IDs such as instances, container, build number, spans, and so forth. ...
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