Friday 22 August 2008

Binary Object REst Distributed (BORED) system - Part 2 - Blueprint

The discussions on Steve vinoski's blog regarding REST, RPC, ORBs, etc highlighted that there's usually a very high level model that a distributed system is built upon. In the case of RPC it is the notion that a procedure call can be made to look local. In REST/Hypermedia it is a distributed document model with loose coupling through hyperlinks. Finally, in the case of ORBs it is an Object Request Broker; an object based remote procedure call. In all these cases the model helps define many of the constraints of the system. These restrains often permeate into every aspect of the system.

The BORED system has the rather interesting task of trying to combine the REST/Hypermedia constraints with that of an object orientated system. To do this, a high level model needs to be defined to use as the blueprint.

The BORED blueprint in this case is quite simple:
client --[request]--> Server --> Container --> Object Receiver | Object

client <--[response] -- Server <-- Container <-- Object Receiver | Object

The idea behind BORED is that a message is being delivered directly to an Object Receiver via a server and container. The message can be any data. It is up to the Object Receiver to decide how to process the message received. The Container and Server are there as conduits for the message to be delivered, however, they do not directly respond to the message. The conduits can add security constraints on who can interact with the target object and manage the life cycle of creating and destroying the target object. The container itself could also be the Object Receiver, however, to keep the model simple these types of adaptations won't be discussed.

The Object Receiver is able to respond directly to the message by returning the object data (as in a document or image). This should allow a hypermedia solution to be developed that has simple file based Object Receiver. Alternatively, the Object Receiver may process the message and call a method as is done in a traditional RPC or ORB. Interactions could involve any one of the following:
Object Receiver -------> Document/File

Object Receiver -------> Object Instance with public methods

Object Receiver -------> Data Collection

Object Receiver -------> Proxy Interface

Object Receiver -------> Etc...

An important note here regarding RPC and BORED. BORED is designed to support a RPC mechanism, however, it is not locked into a single mechanism. Different types of skeletons could be built into the Object Receiver. The initial mechanism will likely use a Remote Message Call mechanism, however, it is up to the Object Receiver to define the meta data and interfaces associated with it.

I have used the name “Remote Message Call” to describe the BORED call method; this is to separate it from the traditional Remote Procedure Call (RPC). A Remote Procedure Call is the language centric view that maps a set of parameters of a method on a server to an equivalent local call on a client. This is a simplistic view of RPC. In BORED there is a message centric view of RPC. That is, the remote call is defined by the data contained in the request object and the data returned in the response. The message data in the request and response forms the contract between client and server. This message data in the request or response can be bound to a language based method call on the client and server, however, it is not a requirement. As BORED is based on describing the request/response data, not the remote method call this is not a traditional RPC mechanism. The distinction is important and ensures that some of the issues of RPC do not get embedded into BORED.

Interestingly, the BORED/RMC model reflects message queuing semantics more than it does RPC, REST or ORB semantics. The fundamental idea is that the data is contained in an envelope and delivered directly to an end point. The difference is that the BORED model is designed for synchronous request/reply semantics, where as message queuing is uni-directional.

As an example of how BORED semantics differ from HTTP, we can look at a HTTP GET request. The HTTP protocol uses the verb GET before specifying the location of the document to be retrieved. This creates a model where the server is performing the GET operation on the document requested. The BORED system will include a GET verb inside the message and deliver it to the Object Receiver. By moving the GET verb into the message it is the “Object Receiver” processing the verb instead of the server. The intention of this is to localise the requested data to the object to which it is being delivered.

One thing I should point out at this point; the model is already making trade-offs. The most obvious to the REST aware folk will be that by encapsulating the message in an envelope and by allowing language orientated mechanisms in the object receiver, BORED removes the Uniform Interface constraint of REST at the protocol level. The Uniform Interface constraint can be catered for, however, in BORED it is not a constraint of the protocol and must be defined in the message data structure. This is an area that still needs to be explored to work out how to combine the two seemingly opposed constraints. This will be expanded further in future posts when the message data structure is explored.

In the on-going REST debate, Tim Bray provided a good description of the trade-offs found in REST versus other systems. These are good things to keep in mind while designing BORED. It reminded me that I had'nt described the layered approach used in BORED.

In the BORED model there's at least three different layers. Defining layers in a protocol ensures that concepts of each layer does not infect other layers. The following compares the traditional OSI 7-layer model with HTTP and BORED.

applicationapplication (browser/client)object receiver(client)
presentationmime (presentation)message (DATA)
sessiontransfer (HTTP)transfer (BORED)
transporttransport (TCP/IP)transport (TCP/IP)

I've had a few conversations where people viewed the OSI 7-layer model is seen as out dated and not very useful in today’s protocol developments. This may be true, however I still find it useful as a backdrop to understanding the layers of different protocols. By modelling a protocol stack using this type of layering provides another view of the protocol.

In the HTTP model the HTTP transfer protocol is easily recognisable as fitting the OSI session layer. It sets up the structure of the conversation between client and server. The data returned by a GET request specifies the mime-type which sets the presentation format for the response. The uniform interface specified by HTTP spreads across both the session and application layers. This unclear distinction of which layer the GET belongs is one example of how having a layered stack model can ensure each layers purpose is well understood.

In the BORED model, the transfer part of the protocol will define the request/response semantics and setup the basis for communications. The transfer layer will also provide the security and location of where the message will be delivered. The message data layer defines the presentation layer of the model. The message data should provide all the data required by the Object Receiver to perform its request.

A good analogy is a physical envelope. The transfer layer is the envelope which has the address, any routing information, the sender and any security information. The message layer is the paper that is put into envelope. The paper can contain any sort of information that the recipient can process. The Object Receiver layer is the actual data contained on the paper and directs the Object Receiver to perform an action. By ensuring the each layer is self contained, the whole system will be more flexible and easier to work with.

This post outlines the model for the BORED system. It has constrained BORED to request/response semantics directed to an Object Receiver. It has outlined the Remote Message Call (RMC) semantics used to create a solid distinction between it and Remote Procedure Calls (RPC). Finally, it outlined the layers in the protocol stack so that each layer can be analysed and its purpose described independently. In the next post I'll do a first cut of the logical elements of the protocol.

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