What is the maximum number of characters that can be used to define the key_name of a datastore entity?
Is it bad to have very long key_names?
For example: Lets say we use key_names of a 170 characters, which is the length of a Twitter message 140 plus 10 numeric characters for latitude and 10 for longtitude and 10 for a timestamp.
(Reasoning of such a key_name: So by using such a key_name we can easily and quickly be sure of no duplicate postings, since the same message should not come from the same place and time more than once.)
There's no hard maximum - the maximum length of a key name is the maximum length of a key, less some overhead, and keys can get pretty long.
It is bad to have very long key names, however: Apart from storing and retrieving it, every index entry contains the key name it's referring to, so longer key names mean higher indexing overhead. If you want to ensure uniqueness over a large text, your best option is to make the key name the MD5 or SHA1 sum of the input, which ensures both uniqueness and a short(-ish) key name.
actually, key names are limited to 500 characters just like string property values. see e.g. Key.to_path(), which calls ValidateString():
which defaults max_len to _MAX_STRING_LENGTH, which is 500:
来源：https://stackoverflow.com/questions/2557632/how-long-max-characters-can-a-datastore-entity-key-name-be-is-it-bad-to-haver