Add comments for the json interface

Signed-off-by: Stephen Sun <[email protected]>

Author: stephenxs

common/json.h

@@ -12,10 +12,49 @@ namespace swss {
 class JSon
 {
 private:
+    /*
+     * el_count represents the number of elements in each object,
+     * which means each object have exactly 2 elements: the data and the operator
+     */
     static const int el_count = 2;
 public:
     static std::string buildJson(const std::vector<FieldValueTuple> &fv);
     static void readJson(const std::string &json, std::vector<FieldValueTuple> &fv);
+    /*
+       bool loadJsonFromFile(std::ifstream &fs, std::vector<KeyOpFieldsValuesTuple> &db_items);
+
+       parse the json file and construct a vector of KeyOpFieldsValuesTuple as the result
+       the json file should be a list objects with each consisting of a data field and an operator field
+         - the data should be a dictionary
+         - the operator field should be a string, "SET" or "DEL"
+       an example:
+       [
+           {
+               "QOS_TABLE:TC_TO_QUEUE_MAP_TABLE:AZURE": {
+                   "5": "1",
+                   "6": "1"
+               },
+               "OP": "SET"
+           },
+           {
+               "QOS_TABLE:DSCP_TO_TC_MAP_TABLE:AZURE": {
+                   "7":"5",
+                   "6":"5",
+                   "3":"3",
+                   "8":"7",
+                   "9":"8"
+               },
+               "OP": "SET"
+           }
+       ]
+      
+       parameters:
+         fs: the input ifstream representing the json file
+         fv: the output vector
+       return: boolean
+         True: the input json file has been succefully parsed
+         False: there are some errors found
+     */
     static bool loadJsonFromFile(std::ifstream &fs, std::vector<KeyOpFieldsValuesTuple> &db_items);
 };