I have not used ES before and am having trouble finding any guidelines
on what constitutes a valid document ID (that follows the type in
index and get operations).
The examples in the guide use numbers while the auto generated IDs are
base 64 UUIDs based on a 128 bit number.
Are the document IDS restricted to the string representation of
numbers or are they arbitrary strings?
If strings, what characters are allowed (eg I expect a '/' is not
allowed as it is the delimiter for URIs, any other) ?
Any other restrictions on what cannot be used for document IDS, such
as reserved IDs?