3 IC Protocol
The purpose of this chapter is to explain the bits and bytes of the IC protocol, which is a composition of the Erlang distribution protocol and the Erlang/OTP gen_server protocol. If you do not intend to replace the Erlang distribution protocol, or replace the gen_server protocol, skip over this chapter.
3.1 Introduction
The IDL Compiler (IC) transforms Interface Definition Language (IDL) specifications files to interface code for Erlang, C, and Java. The Erlang language mapping is described in the Orber documentation, while the other mappings are described in the IC documentation (they are of course in accordance with the CORBA C and Java language mapping specifications, with some restrictions).
The most important parts of an IDL specification are the operation declarations. An operation defines what information a client provides to a server, and what information (if any) the client gets back from the server. We consider IDL operations and language mappings in section 2.
What we here call the IC protocol, is the description of messages exchanged between IC end-points (client and servers). It is valid for all IC back-ends, except the 'erl_plain' and 'erl_corba' back-ends. The IC protocol is in turn embedded into the Erlang gen_server protocol, which is described below. Finally, the gen_server protocol is embedded in the Erlang distribution protocol. Pertinent parts of that protocol is described further below.
3.2 Language mappings and IDL operations
IDL Operations
An IDL operation is declared as follows:
[oneway] RetType Op(in IType1 I1, in IType2 I2, ..., in ITypeN IN,
out OType1 O1, out OType2 O2, ..., out OTypeM OM)
N, M = 0, 1, 2, ... (2.1.1)`Op' is the operation name, RetType is the return type, and ITypei, i = 1, 2, ..., N, and OTypej, j = 1, 2, ..., M, are the `in' types and `out' types, respectively. The values I1, I2, ..., IN are provided by the caller, and the value of RetType, and the values O1, O2, ..., OM, are provided as results to the caller.
The types can be any basic types or derived types declared in the IDL specification of which the operation declaration is a part.
If the RetType has the special name `void' there is no return value (but there might still be result values O1, 02, ..., OM).
The `in' and `out' parameters can be declared in any order, but for clarity we have listed all `in' parameters before the `out' parameters in the declaration above.
If the keyword `oneway' is present, the operation is a cast, i.e. there is no confirmation of the operation, and consequently there must be no result values: RetType must be equal to `void', and M = 0 must hold.
Otherwise the operation is a call, i.e. it is confirmed (or else an exception is raised).
Note carefully that an operation declared without `oneway' is always a call, even if RetType is `void' and M = 0.
Language Mappings
There are several CORBA Language Mapping specifications. These are about mapping interfaces to various programming languages. IC supports the CORBA C and Java mapping specifications, and the Erlang language mapping specified in the Orber documentation.
Excerpt from "6.4 Basic OMG IDL Types" in the Orber User's Guide:
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Functions with return type void will return the atom ok.
Excerpt from "6.13 Invocations of Operations" in the Orber User's Guide:
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A function call will invoke an operation. The first parameter of the function should be the object reference and then all in and inout parameters follow in the same order as specified in the IDL specification. The result will be a return value unless the function has inout or out parameters specified; in which case, a tuple of the return value, followed by the parameters will be returned.
Hence the function that is mapped from an IDL operation to Erlang always have a return value (an Erlang function always has). That fact has influenced the IC protocol, in that there is always a return value (which is 'ok' if the return type was declared 'void').
3.3 IC Protocol
Given the operation declaration (2.1.1) the IC protocol maps to messages as follows, defined in terms of Erlang terms.
Call (Request/Reply, i.e. not oneway)
request: Op atom() N = 0
{Op, I1, I2, ..., IN} tuple() N > 0
(3.1.1)
reply: Ret M = 0
{Ret, O1, O2, ..., OM} M > 0
(3.1.2)Notice: Even if the RetType of the operation Op is declared to be 'void', a return value 'ok' is returned in the reply message. That return value is of no significance, and is therefore ignored (note however that a C server back-end returns the atom 'void' instead of 'ok').
Cast (oneway)
notification: Op atom() N = 0
{Op, I1, I2, ..., IN} tuple() N > 0
(3.2.1)(There is of course no return message).
3.4 Gen_server Protocol
Most of the IC generated code deals with encoding and decoding the gen_server protocol.
Call
request: {'$gen_call', {self(), Ref}, Request} (4.1.1)
reply: {Ref, Reply} (4.1.2)where Request and Reply are the messages defined in the previous chapter.
Cast
notification: {'$gen_cast', Notification} (4.2.1) where Notification is the message defined in the previous chapter.
3.5 Erlang Distribution Protocol
Messages (of interest here) between Erlang nodes are of the form:
Len(4), Type(1), CtrlBin(N), MsgBin(M) (5.1)
Type is equal to 112 = PASS_THROUGH.
CtrlBin and MsgBin are Erlang terms in binary form (as if created by term_to_binary/1), whence for each of them the first byte is equal to 131 = VERSION_MAGIC.
CtrlBin (of interest here) contains the SEND and REG_SEND control messages, which are binary forms of the Erlang terms
{2, Cookie, ToPid} , (5.2) and
{6, FromPid, Cookie, ToName} , (5.3) respectively.
The CtrlBin(N) message is read and written by erl_interface code (C), j_interface code (Java), or the Erlang distribution implementation, which are invoked from IC generated code.
The MsgBin(N) is the "real" message, i.e. of the form described in the previous section.