The mapping a document had to be cast against could not be fetched.

Casting is what turns a date field into a DateTime, a date_range into a Date.Range, and back. Without the mapping, Dowser.Codec has nothing to dispatch on and every value passes through as the raw JSON term — so the type a caller gets back would depend on whether a _mapping request happened to succeed. That request is a call like any other, most likely to fail exactly when the cluster is overloaded, which is when code is least able to cope with a %{"gte" => ..., "lte" => ...} where it expects a Date.Range.

So a failed fetch raises this by default, and the request fails with it rather than quietly returning uncast values. :index is the index whose mapping was wanted and :reason what the fetch returned.

See Dowser.Codec's :mapping_failure option for the other policies (:warn, :ignore), and Dowser.MappingCacher for mappings that are never fetched and so can never fail.