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Effects of patterns on client code

From: David Booth <dbooth@w3.org>
Date: Tue, 01 Jul 2003 18:07:07 -0400
Message-ID: <1057787708.IAA22192@phantom.w3.org>
To: public-ws-desc-meps@w3.org

How is the client affected if we adopt p2d or p2 instead of p2e[1]?  This 
question has come up a number of times.  Umit and I have argued that the 
client code would be written differently if the pattern itself does not 
indicate that the response goes back to the original requester.  That's 
what I tried to illustrate in Use Case UC1[2].  Here is another attempt to 
illustrate why.

To make this explanation more concrete, I'll use pseudo-code.  However, it 
should be understood that I am not trying to imply that this is the only 
way that a client could implement it.  Rather, this is merely offered as an 
example of *one* logical way that someone might want to implement this.

First, let's assume that we wish to have a clean separation of concerns in 
the client code, between:

         - application-specific code that is independent of the specific
           bindings that are used; and

         - binding-specific code that is auto-generated.

In order to allow the application-specific code to be written independently 
of the bindings, we wish to define standard interfaces that the client 
programmer can use to send and receive messages according to the patterns 
that we adopt.  Each of the binding-specific implementations for a 
particular pattern would implement the common interface defined for that 
pattern.

For example, for a simple input-only pattern (p1a), the interface could 
define a sendMessage function:

         void sendMessage(ServiceAddress s, Message m);

Each of various binding styles (HTTP, SOAP over HTTP, etc.) would implement 
this function.  Or for a classic request-response pattern (p2e), the 
interface could define a sendAndReceiveMessage function:

         Message sendAndReceiveMessage(ServiceAddress s, Message m);

and each binding style would implement this function.

With that context in mind, here are examples of what these interfaces might 
look like for pattern variations p2, p2a1, p2b, p2c, p2d and p2e.

############################################################################
#############################  p2 family  ##################################
############################################################################

###########################
interface i_p2
{
// For message 1:
void multicastMessage(ServiceAddressList ss, Message m);

// For message 2 (either output or ofault), we need to be ready to
// receive a message at any time, so we'll need to use some kind of
// event-driven style.  One possibility is to register a callback
// function that would be called when a message is received.  The
// user might write a MessageReceiverCallback function f like:
//         void receiveMessage(ServiceAddress s, Message m);
// and register it to be called later using:
void registerMessageReceiverCallback(MessageReceiverCallback f);
}

###########################
interface i_p2a1
{
// For message 1:
void sendMessage(ServiceAddress s, Message m);

// For message 2 (either output or ofault), we again need to be ready to
// receive a message at any time, so we might have:
void registerMessageReceiverCallback(MessageReceiverCallback f);
}

###########################
interface i_p2b
{
// For A to send message 1 and receive message 2 (either output or ofault):
Message sendAndReceiveMessage(ServiceAddress s, Message m);

// For B1 ... Bn to receive an output message asynchronously:
void registerMessageReceiverCallback(MessageReceiverCallback f);
}

###########################
interface i_p2c
{
// Message 1: In this pattern the ofault is optional, so when we send
// message 1, we can't block waiting for a possibly-non-existing ofault
// to come back.  So instead, we'll handle the ofault separately.
void sendMessage(ServiceAddress s, Message m);

// Message 2 (either ofault or output) again require an event-driven style
// such as:
void registerMessageReceiverCallback(MessageReceiverCallback f);
}

###########################
interface i_p2d
{
// The interface for p2d is exactly the same as the interface for p2c.
// Message 1:
void sendMessage(ServiceAddress s, Message m);

// Message 2 (either ofault or output) again require an event-driven style:
void registerMessageReceiverCallback(MessageReceiverCallback f);
}

###########################
interface i_p2e
{
// Message 1 and 2 (either output or ofault):
Message sendAndReceiveMessage(ServiceAddress s, Message m);
}

############################################################################
#############################  Conclusions  ################################
############################################################################

The main thing to notice is that the interface can be much simpler for p2e 
-- classic request-response -- because the reply is expected and known to 
come back to the requester.  In comparison, p2d needs to have some kind of 
event-driven interface, because output messages from the service may come 
asynchronously, at any time.

It's also worth comparing these functions with the functions that would be 
needed for the other patterns.  To summarize, here are the functions needed 
for the various patterns:

p1:
         void multicastMessage(ServiceAddressList ss, Message m);

p1a:
         void sendMessage(ServiceAddress s, Message m);

p1b, p2c, p2d, p2d1, p4c, p4d, p4e, p6a, p6b, p7a, p7b, p7c, p8a, p8b, p8c, 
p8d:
         void sendMessage(ServiceAddress s, Message m);
         void registerMessageReceiverCallback(MessageReceiverCallback f);

p2, p2a, p2a1, p2a2, p2umci-7, p4, p4a, p6, p7, p8:
         void multicastMessage(ServiceAddressList ss, Message m);
         void registerMessageReceiverCallback(MessageReceiverCallback f);

p2b:
         Message sendAndReceiveMessage(ServiceAddress s, Message m);
         void registerMessageReceiverCallback(MessageReceiverCallback f);

p2e:
         Message sendAndReceiveMessage(ServiceAddress s, Message m);

p3:
         [Skipped, as I'm not sure what would be needed for this one.]

p5, p5a:
         void registerMessageReceiverCallback(MessageReceiverCallback f);


In other words, other than p3 (which has the "same channel" requirement),
these patterns use only 4 interface functions:

         void multicastMessage(ServiceAddressList ss, Message m);
         void sendMessage(ServiceAddress s, Message m);
         Message sendAndReceiveMessage(ServiceAddress s, Message m);
         void registerMessageReceiverCallback(MessageReceiverCallback f);


References
1. 
http://dev.w3.org/cvsweb/~checkout~/2002/ws/desc/wsdl12/meps-vs-iops/meps-vs-iops_clean.htm 

2. 
http://dev.w3.org/cvsweb/~checkout~/2002/ws/desc/wsdl12/meps-vs-iops/meps-vs-iops_clean.htm#uc1

-- 
David Booth
W3C Fellow / Hewlett-Packard
Telephone: +1.617.253.1273
Received on Tuesday, 1 July 2003 18:07:11 GMT

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