This document describes an extension to the W3C automotive group's W3C Vehicle Information Service (VISS) that supports Remote Procedure Calls (RPC).
It intends to be backward compatible with the existing VISS specification.
A stripped-down reference implementation is provided to enable protocol explorations by trial.
A VISS client can invoke a function on VISS server via a RPC call, transmitted as JSON over the same websocket as the VISS signal protocol.
An RPC call will always generate a JSON reply, sent back from server to client, containing information if the function was successfully invoked or not. A return value can also optionally be included.
Below is sequence diagram combining signal subscriptions, RPC calls, and signal deliveries into a single call flow.
Fig. 1 - Standard call flow
The steps executed are as follows.
Signal subscribption will bfe done through the subscribeRequest and
subscribeSuccessResponse schemas specified in the
VISS Spec
The JSON payload of a VISS RPC call has the following elements:
| Element | Type | Value | Description |
|---|---|---|---|
action |
string | "call" |
Specifies that this is a remote function call request |
requestId |
string | sender-set | A sender-assigned transaction ID that will be sent back with the function call request reply |
function |
string | sender-set | The function that we want to execute |
arguments |
object list | sender-set | A list of arguments to supply with the function |
Each JSON object in the argument list has the following elements
NOTE: Do we need to support nested structs?
| Element | Type | Value | Description |
|---|---|---|---|
type |
string | "int8" |
Specifies a signed 8 bit integer argument |
"uint8" |
Specifies an unsigned 8 bit integer argument | ||
"int16" |
Specifies a signed 16 bit integer argument | ||
"uint16" |
Specifies an unsigned 16 bit integer argument | ||
"int32" |
Specifies a signed 32 bit integer argument | ||
"uint32" |
Specifies an unsigned 32 bit integer argument | ||
"bool" |
Specifies a boolean argument | ||
"float" |
Specifies an floating point argument | ||
"double" |
Specifies is an double (floating point) argument | ||
"string" |
Specifies dynamic length string argument | ||
size |
integer | sender-set | The number of values provided in the value element array. |
value |
See below | sender-set | A list of values to be forwarded as a single (array) argument to the remote function. |
The value element varies its format depending on the type and size elements:
| Type | Size | Format | Min | Max | Example | Description |
|---|---|---|---|---|---|---|
| int8 | 1 | string | "-128" |
"-127" |
"-42" |
Signed 8 bit integerinteger |
| int8 | >1 | string list | "-128" |
"-127" |
["-42", "42"] |
Array of signed 8 bit integers |
| uint8 | 1 | string | "0" |
"255" |
"237" |
Unsigned 8 bit integer |
| uint8 | >1 | string list | "0" |
"255" |
["-42", "42"] |
Array of unsigned 8 bit integers |
| int16 | 1 | string | "-32768" |
"32767" |
"-4711" |
Signed 16 bit integerinteger |
| int16 | >1 | string list | "-32768" |
"32767" |
["-4711","4711"] |
Array of signed 16 bit integers |
| uint16 | 1 | string | "0" |
"65535" |
"47111" |
Unsigned 16 bit integer |
| uint16 | >1 | string list | "0" |
"65535" |
[ "1", "47111"] |
Array of unsigned 16 bit integers |
| int32 | 1 | string | "-2147483648" |
"2147483647" |
"-1234567" |
Signed 32 bit integerinteger |
| int32 | >1 | string list | "-2147483648" |
"2147483647" |
["-1234567", "1234567"] |
Array of signed 32 bit integers |
| uint32 | 1 | string | "0" |
"4294967295" |
"32101234567" |
Unsigned 32 bit integer |
| uint32 | >1 | string list | "0" |
"4294967295" |
["0", "32101234567"] |
Array of unsigned 32 bit integers |
| bool | 1 | string | "0" |
"1" |
"1" |
Boolean |
| bool | >1 | string list | "0" |
"1" |
["0", "1"] |
Array of boolean |
| float | 1 | string | N/A | N/A | "123.321" |
32 bit floating point |
| float | >1 | string list | N/A | N/A | ["-123.123", "123.123"] |
Array of 32 floating points |
| double | 1 | string | N/A | N/A | "-321.321" |
64 bit floating point |
| double | >1 | string list | N/A | N/A | ["-321.321", "321.321"] |
Array of 64 floating points |
The server responds to the remote function call message with a reply indicating if the remote function was invoked or not.
| Element | Type | Value | Description |
|---|---|---|---|
action |
string | "reply" |
Specifices that this is a reply message |
requestId |
string | sender-set | Set to the same value as provided with the original "call" message |
reply |
object | See below | Reply payload sent back by the invoked function. None if error or no reply |
error |
object | See below | Only provided if function invocation fails Error description |
The reply object is an JSON object with the same format as arguments in a RPC call.
The error object has the string members number, reason, and message as specified
in the VISS Spec.
If the call is successful, no error element is provided with the server reply.
The following combinations can be returned in code, reason, and message
number |
reason |
message |
|---|---|---|
"503" |
"unknown_function" |
The function specified in the call request function element is not available |
"400" |
"missing_argument" |
One or more elements in the function request request table were not provided |
"400" |
"unknown_type" |
One or more arguments in the arguments object list were if an unknown type |
"406" |
"protocol_mismatch" |
The VISS server does not support the protocol version. |
Signal delivery will be done through the subscriptionNotification schema specified
in the VISS Spec.
Signals can be subscribed to outside the use case of remote function calls as specified by the current specification, as shown in fig 3.
Fig. 3 - External signal delivery
The call flow for signal subscription and delivery is idenical to that of the remote function call. The only difference is the VISS server may deliver a signal to the VISS client on its own in iniative, for example due to an external event suc as the volume dial being turned by the driver.
sudo apt install python3-websockets
git clone git@github.com:PDXostc/viss-rpc.git
or
git clone https://github.com/PDXostc/viss-rpc.git
cd viss-rpc
./viss_rpc_server.py
The server will wait for inbound calls and, sporadically, publish signals to subscribers.
./viss_rpc_client.py -S ws://localhost:8088 -s Vehicle.DriveTrain.FuelSystem.Level -c 'print_name_and_age string:1:Bob int32:42'
The client will subscribe to Vehicle.DriveTrain.FuelSystem.Level and
then invoke print_name_and_age with a string and an int as an
argument.
The server will pick up the call and send back a pre-define reply.
The output will provide all JSON calls, both for signals and RPCs.