!CHAPTER Document Keys 

Users can define their own keys for documents they save. The document key will
be saved along with a document in the *_key* attribute. Users can pick key
values as required, provided that the values conform to the following
restrictions:

* The key must be at least 1 byte and at most 254 bytes long. Empty keys are 
  disallowed when specified (though it may be valid to completely omit the
  *_key* attribute from a document)
* It must consist of the letters a-z (lower or upper case), the digits 0-9,
  the underscore (`_`), dash (`-`), colon (`:`), dot (.) or at-mark (`@`) characters 
  only
* Any other characters, especially multi-byte sequences, whitespace or punctuation 
  characters cannot be used inside key values
* The key must be unique within the collection it is used

Keys are case-sensitive, i.e. *myKey* and *MyKEY* are considered to be
different keys.

Specifying a document key is optional when creating new documents. If no
document key is specified by the user, ArangoDB will create the document key
itself as each document is required to have a key.

There are no guarantees about the format and pattern of auto-generated document
keys other than the above restrictions. Clients should therefore treat
auto-generated document keys as opaque values and not rely on their format.