What is the meaning of []json.Rawmessage - arrays

Question

What is the meaning of []json.Rawmessage. Its within this structure here:
type Request struct {
Jsonrpc string `json:"jsonrpc"`
Method string `json:"method"`
Params []json.RawMessage `json:"params"`
ID interface{} `json:"id"`
}
I know its a slice of type json. I do not understand what the .RawMessage is referring to. I tried looking it up in both golang tour and my golang book. Also ultimately I know Params is type []json.Rawmessage being bundled into another type called Request
Futhermore:
What is going on with these segments json:"jsonrpc". Is a string literal somehow being attached to var? Again this is not in golang tour or my golang book. Thanks for your time.

Answer 1

[] is defining a slice
json is the package import name (from your import statement above)
RawMessage is the type within the package. In this case for a []byte type.
json:"params" is a field tag. The json package reads it with reflection and determines what name to use for json.

Answer 2

Most of the time time you need to look in to the package doc rather than some book and the online tour.
The json:"jsonrpc" is a struct tag.
For this specific case the json.Marshal function will read that struct tag and return the JSON field name with the given value. For more about "encoding/json" struct tags usage: https://golang.org/pkg/encoding/json/#Marshal
For RawMessage, you can read more about it here https://golang.org/pkg/encoding/json/#RawMessage
type RawMessage []byte
Normally I use it when having a "generic" kind of JSON object that don't need to be process right the way (maybe I just send them to another service, or I will unmarshall it later depending on conditions). For example (I will use your Request struct):
jsonString := `[
{
"id": 123,
"method": "getSomething",
"params": [{"A": 1, "B": 2}]
}
{
"id": 123,
"method": "getSomethingElse",
"params": [{"C": 1, "D": 2}]
}
]`
With this processing code:
var requests []Request
json.Unmarshal([]byte(jsonString), &requests)
// if no error, you will have 2 items in requests
// requests[0].Params[0] is equal to []byte(`{"A": 1, "B": 2}`)
// requests[1].Params[0] is equal to []byte(`{"C": 1, "D": 2}`)
for _, req := range requests {
if req.Method == "getSomething" {
justProxyThisRequestToAnotherService(req)
} else if req.Method == "getSomethingElse" {
var params []map[string]int
json.Unmarshal(req.Params, &params)
// then do something with params
}
}