Rest API Get with query parameters proper responses

Question

I have implemented a Rest API with vectored ids:

http://example.com/api/user/1
http://example.com/api/user/2
http://example.com/api/user/1,3
http://example.com/api/user/1,2,3

Rest dictates that if "user/2" doesn't exist to return a 404.

Disregarding a possible solution that involve query parameters, what are the appropriate responses for the following situations?

user/1,2 - user/1 exists and user/2 exists
Response is a 200 with both elements returned in a set

user/1,3 - user/1 exists and user/3 DOES NOT exist
1? Response is 200 with the user 1 in a set?
2? Response is 404 because user/3 can't be returned?

And since we could possibly be dealing with empty sets with 1? above, does this mean
user/3 - user/3 doesn't exist
1? Response is 200 with an empty set?

Answer 1

Make sure that response format is the same for single ID and several IDs (vectored IDs). It's one of REST principals: GET is a "safe method" (nullipotent) and should produce no side-effects.

For endpoints that support vectored IDs in GET request I usually use response format similar to OpenSearch i.e.

GET user/1,3 where user/1 exists and user/3 does not exist returns 200 with

{
  "startIndex": 0,
  "count": 1,
  "itemsPerPage": 1,
  "entry": [
    { /* user/1 */}
  ]
}

GET user/3 where user/3 does not exist returns 200 with

{
  "startIndex": 0,
  "count": 0,
  "itemsPerPage": 0,
  "entry": []
}