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Mon, 2008-Jun-02

The 4+1 Development and Deployment Views

This is part four of my amateur coverage of the . I left my coverage last time with the logical and process views complete. The logical view is a system's engineering coverage of the system's functional architecture. My Process View closed out most of the design aspects of the system. We now have a set of software components and functional interfaces which we may further refine.

The Development View and Deployment (Physical) view close out main set of views by moving software components out of their final assembled forms and into other contexts. The Development View captures components "in the factory". In other words, it shows build-time relationships between components. The Deployment View captures components being deployed to the operational system. If we were continue clockwise around the views through Development and Deployment we would naturally reach the Process View again. In other words, components once built and deployed are assembled into processes.

The Development View

The Development View is where I like to travel to in telling my architectural story, after the Process View. We have seen the final assembly of processes, now let's go back to where it all began. If your architecture has a strong distributed object focus this view will be relatively simple, but it does serve an important function.

The Development View is where we make a break from whole Logical Functions, and narrow ourselves down to their core for deployment. Let's follow our example through:

Development Packages

The packages view isn't very interesting. It would perhaps be more so if I had included actual database software or other basic components to get the job done. Those would very likely appear at this level, alongside packages we discovered in the Process View. There are no build-time relationships in this set, only run-time relationships.

Sales (Development View)

The Development View for Sales includes all the components that made up the Process View diagram of the same name. These components are everything that is required to put together the Logical View Sales function.

Something has changed, though. This time around our packages are in the diagram itself. We can see a Sales package, and it clearly doesn't contain all of the software components involved. When we deploy "Sales" it will be this core set of components (only one in this case) that will be deployed. The Sales Manager component will be deployed as part of General Ledger. The Web Browser will be deployed as part of its own package.

Inventory (Development View)

Inventory shows multiple components within a functional core. Inventory contains both its client- and server- side components.

Shares (Development View)

Shares shows that some of the run-time relationships found in the Process View are linked together at build-time. The Portfolio Manager controller component depends on and links together all of the other constituent parts.

Tax Reporting (Development View) Tax Reporting (Development View) Tax Reporting (Development View)

The remainder of these diagrams are fairly boring. However, they do ask us to consider what libraries or other components we might have missed in our Process View analysis. This will be important contributors to the final set of software components and may significantly influence planning and estimation on a large monolithic project.

The Deployment (Physical) View

The Deployment View is about the final installation of components into the operational system. It is theoretically concerned both with the mapping of software components and processes to physical devices. However, again mine didn't quite turn out that way. I ended up focusing on packaging. Here is my top-level diagram:

Deployment (Physical) Packages

This image shows the physical structure of final Accounts system. We have a Stock Handling workstation with a Scanning device attached. It contains the thick inventory client.

On the right we see a single Accounts server on which we plan to deploy the entire server workload. A generic client machine carries a Web Browser to render the various HMIs.

We are limited in what we can do with a UML deployment diagram like this. We obviously can't dive too far into the nature of the hardware components in what is really a software diagram. We can't draw the racks or get too far into the detail of the network architecture. These facets seem to fit outside of the 4+1 views.

Sales (Deployment View)

The Deployment View for Sales is significantly slimmed down from previous logical-aligned views. For the first time we see the non-core components such as the Web Browser stripped away and absent. What is left is a pure server-side component, so we put it in a package of that name and deploy it onto the Accounts Server class of machine.

Inventory (Deployment View)

Inventory is more interesting. We have a client and server side to this functional core. They must be deployed separately. Even at this late stage we can see potential holes in our design: Did we think about the client/server split? Can we see components that need extreme bandwidth or latency settings, and should be co-located?

Shares (Deployment View)

I haven't shown all the dependent components for Portfolio Manager under the Shares function core. I assume they are all included at build time and do not require special attention when it comes to this level of packaging.

Tax Reporting (Deployment View) General Ledger (Deployment View) Web Browser (Deployment View)

Again, the rest of these diagrams are pretty boring due to lack of intelligent detail.


That concludes the main four views. I hope this coverage has been useful, and expect to see a number of "no, this is the way it is done!" posts in response. My approach has not been entirely kosher, and is not based on a broad enough base of experience. However, it does what I set out for it to do. It identified a set of software components and functional interfaces. It aligned the set through multiple views to the end user's functional requirements. It examined the set from multiple angles to see how they would play out:

It did not decompose components or interfaces any more than necessary, hopefully making the necessary high level decisions without boxing individual component authors in unnecessarily.

This approach is suitable for architectures of size, both REST and non-REST. Again, it might surprise some that REST has not specifically appeared at this level.

The way I view it is that this is an approach to selecting components to build and identifying the demands on interfaces between them. Each component will still be built according to best practice and will conform to appropriate standards. If I were to build a circuit breaker for the electric power industry, or an impulse fan for environmental control, I would supply digital inputs and outputs of a standard type for connections to RTUs, PLCs, or other field devices. If I am to build a software component, I will supply URLs to allow it to be connected freely and generally also. I will follow industry best practice for loose coupling, maximum scalability, and maximum evolvability. These views told me what to build. REST tells me how to build it.

To some extent I want REST advocates to see this other side of things. Whether you do this formally or informally and by what ever technique: There is a process of deciding what to build and what interfaces are required in software engineering. There is an architecture role that is about the specifics of what is needed in a particular system. REST is a technique that informs this level of architecture, but is subservient to it. I sometimes wonder if this is the main reason for disconnect between the REST and SOA camps. One is talking about which components to build, while the other is talking about how to connect them together. Little wonder that we can't agree on how to do "it".

I plan to round out this series with a discussion of the Scenarios View, and about other supporting documentation that might be needed to support your architectural decisions.

Just a reminder: You can find the StarUML model I used in this example here.