Introduction

ObjectLink is an IPC protocol based on the idea to link a local object to a remote object a network connection. Object link allows users to communicate state full objects over a network connection. The protocol is designed to be used with the ObjectAPI specification.

The design is based on several main concepts:

• Properties are reactive and synchronized between the local and remote object
• Operations allows a request/response communication between the local and remote object
• Signals are used to notify the local object about events on the remote object
• Modules are used to group objects and to define a namespace
• The local and remove object a represented by an easy to use object API surface
• The object API surface can be defined by an ObjectAPI document

JSON Notation

Throughout the protocol description, the JSON notation is used. All data types must be valid JSON data types.

For transport several protocols are supported, see list below:

• JSON
• MsgPack
• CBOR

The client and server needs to know their transport and encoding in advance. There is no protocol negotiation designed. This allows us to keep the protocol simple.

Message Types

• Lifecycle
  • --> LINK - link the local object with a remote object
  • <-- INIT - initialized the local object with properties from the remote object
  • --> UNLINK - unlinks a local object from a remote object
• Properties
  • --> SET_PROPERTY - send a property change to a remote object
  • <-- PROPERTY_CHANGE - sends property changes to all linked client objects
• Methods
  • --> INVOKE - invoke a method on a remote object
  • <-- INVOKE_REPLY - reply of an remote invokation
• Signals
  • <-- SIGNAL - send remote events back to all linked client objects
• Errors
  • <-- ERROR - send an error back to the client with the msg type of the original message

Message Formats

Direction	Message	Value	Format
->	LINK	10	[ MsgType, ObjectId ]
<-	INIT	11	[ MsgType, ObjectId, Dict ]
->	UNLINK	12	[ MsgType, ObjectId ]
->	SET_PROPERTY	20	[ MsgType, PropertyId, Value ]
<-	PROPERTY_CHANGE	21	[ MsgType, ObjectId, Value ]
->	INVOKE	30	[ MsgType, RequestID, MethodId, Args ]
<-	INVOKE_REPLY	31	[ MsgType, RequestID, Value ]
<-	SIGNAL	40	[ MsgType, SignalId, Args ]
<-	ERROR	50	[ MsgType, MsgType, RequestID, Error ]

• MsgType: integer value of message type
• ObjectId: a string identifying the resource as module and object name (e.g. "demo.Calc")
• Dict: A JSON dictionary, e.g. { "count": 0}
• Args: A JSON array, e.g. [ 1, 2 ]
• PropertyId: A ObjectID with a property path (e.g. "demo.Calc/count")
• Value: Any valid JSON value including JSON arrays or objects
• MethodId: A ObjectID with a method path (e.g. "demo.Calc/increment")
• RequestId: A unique integer value identifying the request during the connection. Typically a value incremented by one on each request and starting by 1 and then reset to 1 by max value.
• SignalId: A ObjectID with a signal path (e.g. "demo.Calc/shutdown")
• Error: A string describing the error