The software development is complex by nature and over the years, I have seen many abstractions for addressing this complexity such as modules, classes, libraries, components, packages, aspects, frameworks and platform, services, etc. Back when I started programming in 80s, I learned about Structured Programming and Top-down Design. Like modern good software practices it encouraged separation of concernsand was modular design. I learned from structured programming that modules should be loosely coupled and highly cohesive. I learned about various forms of coupling such as common, control, data, stamp, etc and how to avoid it. BASIC was my first programming language and I learned a number of languages afterwards, but C became my primary language. In that language, module meant c file that consisted of closely related functions.
I also understood concept of libraries (static), which were sort of modules or composition of modules that offered some useful functionality that I could link at compile/load time. In early 90s, I started learning object oriented programming and design and started using C++. It also highlighted modularity and introduced classes for combining data structure and behavior. It also emphasised encapsulation or information hiding and interfaces. Despite its impurity, I used C++ for many years. It was similar to C and it also used files for modules by defining closely related files in the same file.
In 1995, I also started using Java, which limited one class per file and offered packages for namespace. I heard many people equated modules to classes but I preferred packages for defining modules. Java also offered jar files for packaging libraries. In early days, Java also promoted JavaBeans as a way to add components and runtime wiring.
Around mid 90s to early 2000, I started learning about components, which were hot topics in those days. I learned Catalysis, Iconix and Component Software. A lot of companies were defining what components meant, such as SOM, Micrsoft’s COM, or their distributed counterparts such as DCOM, DSOM, CORBA, etc. Unlike static libraries, which were packaged as part of the application, those components offered binary and runtime pluggability. I used CORBA for many years, then moved to EJBs in early 2000s which offered more object oriened support for components. Historically, compile time components were difficult to use and led to DLL or Jar hell. Recent standars such as OSGI and Java JSR 291 offer nice dependency management and runtime isolation.
In mid 90s, I enrolled for Ph.D and was looking for interesting topics. One of my professor gave me some papers from Xeros Parc about aspect oriented programming. I ended up choosing different topic but liked the idea of aspects. AOP became popular in early 2000s and I have been using them whenever I get chance. Aspects addressed crosscutting concerns and offered a powerful way to modularize concerns like security, monitoring, transactions, caching, etc.
Web Services and Services oriented Architecture became popular in early 2000 and I started using Sun’s toolkit and Weblogic’s generators to convert Java/EJBs to Web Services. Similar to modules, libraries and components they try to sell on partitioning and reusability, though unlike former, the services didn’t have any compile or load time dependencies.
The component based software development then led to frameworks and I started hearing more about frameworks for UIs, eclipse IDE, etc. I found frameworks were different from libraries in several ways. For example, unlike libraries where client code simply invokes some functionality, the frameworks offered scaffolding to built applications and used inversion of control by offering hooks, callbacks or plugins to implment. I have been using a number of frameworks for many years such as Spring, Eclipse, Rails, etc.
For past a few years, more and more companies now offer platforms to build applications. The platforms are sort of like composition of frameworks with an API to build applications. Though, historically platforms were tied with specific operating systems or languages such as Java, .Net, iPhone, JavaME but Web has become the platform, which offers language and operating system independent services. There are unlimited choices for building new mashups and applications (see http://www.programmableweb.com/. Though, some of the web platforms such as Facebook, MySpace, Google App
Engine and Amazon Web Services offer more complete application hosting facilities and in the case of Facebook, MySpace or Twitter they also offer millions of potential users.
The cost of software development has become significantly cheaper due to open source and easily accessible libraries, frameworks and platforms. Though, what I described in this blog is no news to anyone, but I think it’s imporant to think about these differences and choices when building new applications.