Underlying WiredTiger files are key/value stores, that is, entries in the file are paired keys and values. These keys and values can be accessed directly, and a file cursor will only return the data from the single file, without reference to any other related files or data. For example, if an index were lost or to become inconsistent with its primary, a file cursor could read from the index without problems (even though some of the returned keys might not exist in the primary).

In the case of a row-store file, keys and values are both variable-length byte strings, up to (4GB - 512B) in length.

In the case of a column-store file, keys are 64-bit record numbers, where the first record in the file is record number 1. Values are either variable-length byte strings, up to (4GB - 512B) in length, or bit fields, where a bit field is 1 to 8 bits.