Error Handling in Mongo Drivers

If an error occurs on a query (or getMore operation), Mongo returns an error object instead of user data.

The error object has a first field guaranteed to have the reserved key $err. For example:

{ $err : "some error message" }

The $err value can be of any type but is usually a string.

Drivers typically check for this return code explicitly and take action rather than returning the object to the user. The query results flags include a set bit when $err is returned.

/* db response format

Query or GetMore: // see struct QueryResult
  int resultFlags;
  int64 cursorID;
  int startingFrom;
  int nReturned;
  list of marshalled JSObjects;

*/

struct QueryResult : public MsgData {
  enum {
    ResultFlag_CursorNotFound = 1, /* returned, with zero results, when getMore is called but the cursor id is not valid at the server. */
    ResultFlag_ErrSet = 2          /* { $err : ... } is being returned */
  };
  ...
};

Error Handling in Mongo Drivers

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