cooperative-software-development/chapters/architecture.bd

Once you have a sense of what your design must do (in the form of requirements or other less formal specifications), the next big problem is one of organization. How will you order all of the different data, algorithms, and control implied by your requirements? With a small program of a few hundred lines, you can get away without much organization, but as programs scale, they quickly become impossible to manage alone, let alone with multiple developers. Much of this challenge occurs because requirements _change_, and every time they do, code has to change to accommodate. The more code there is and the more entangled it is, the harder it is to change and more likely you are to break things.
		
This is where *architecture* comes in. Architecture is a way of organizing code, just like building architecture is a way of organizing space. The idea of software architecture has at its foundation a principle of *information hiding*: the less a part of a program knows about other parts of a program, the easier it is to change. The most popular information hiding strategy is *encapsulation*: this is the idea of designing self-contained abstractions with well-defined interfaces that separate different concerns in a program. Programming languages offer encapsulation support through things like *functions* and *classes*, which encapsulate data and functionality together. Another programming language encapsulation method is *scoping*, which hides variables and other names from other parts of program outside a scope. All of these strategies attempt to encourage developers to maximize information hiding and separation of concerns. If you get your encapsulation right, you should be able to easily make changes to a program's behavior without having to change _everything_ about its implementation.
		
When encapsulation strategies fail, one can end up with what some affectionately call a "ball of mud" architecture or "spaghetti code". Ball of mud architectures have no apparent organization, which makes it difficult to comprehend how parts of its implementation interact. A more precise concept that can help explain this disorder is *cross-cutting concerns*, which are things like features and functionality that span multiple different components of a system, or even an entire system. There is some evidence that cross-cutting concerns can lead to difficulties in program comprehension and long-term design degradation<walker12>, all of which reduce productivity and increase the risk of defects. As long-lived systems get harder to change, they can take on _technical debt_, which is the degree to which an implementation is out of sync with a team's understanding of what a product is intended to be. Many developers view such debt as emerging from primarily from poor architectural decisions<ernst15>. Over time, this debt can further result in organizational challenges<khadka14>, making change even more difficult.
		
The preventative solution to this problems is to try to design architecture up front, mitigating the various risks that come from cross-cutting concerns (defects, low modifiability, etc.)<fairbanks10>. A popular method in the 1990's was the [Unified Modeling Language|https://en.wikipedia.org/wiki/Unified_Modeling_Language] (UML), which was a series of notations for expressing the architectural design of a system before implementing it. Recent studies show that UML was generally not used and generally not universal<petre13>. While these formal representations have generally not been adopted, informal, natural language architectural specifications are still widely used. For example, [Google engineers write design specifications|https://www.industrialempathy.com/posts/design-docs-at-google/] to sort through ambiguities, consider alternatives, and clarify the volume of work required. A study of developers' perceptions of the value of documentation also reinforced that many forms of documentation, including code comments, style guides, requirements specifications, installation guides, and API references, are viewed as critical, and are only viewed as less valuable because teams do not adequately maintain them<aghajani20>.
		
More recent developers have investigated ideas of *architectural styles*, which are patterns of interactions and information exchange between encapsulated components. Some common architectural styles include:
		
* *Client/server*, in which data is transacted in response to requests. This is the basis of the Internet and cloud computing<cito15>.
* *Pipe and filter*, in which data is passed from component to component, and transformed and filtered along the way. Command lines, compilers, and machine learned programs are examples of pipe and filter architectures.
* *Model-view-controller (MVC)*, in which data is separated from views of the data and from manipulations of data. Nearly all user interface toolkits use MVC, including popular modern frameworks such as React.
* *Peer to peer (P2P)*, in which components transact data through a distributed standard interface. Examples include Bitcoin, Spotify, and Gnutella.
* *Event-driven*, in which some components "broadcast" events and others "subscribe" to notifications of these events. Examples include most model-view-controller-based user interface frameworks, which have models broadcast change events to subscribers. For example, views may subscribe to models so they may update themselves to render new model state each time it changes.
		
Architectural styles come in all shapes and sizes. Some are smaller design patterns of information sharing<beck96>, whereas others are ubiquitous but specialized patterns such as the architectures required to support undo and cancel in user interfaces<bass03>.
		
One fundamental unit of which an architecture is composed is a *component*. This is basically a word that refers to any abstraction--any code, really--that attempts to _encapsulate_ some well defined functionality or behavior separate from other functionality and behavior. For example, consider the Java class _Math_: it encapsulates a wide range of related mathematical functions. This class has an interface that decide how it can communicate with other components (sending arguments to a math function and getting a return value). Components can be more than classes though: they might be a data structure, a set of functions, a library, an API, or even something like a web service. All of these are abstractions that encapsulate interrelated computation and state for some well-define purpose. 
			
The second fundamental unit of architecture is *connectors*. Connectors are code that transmit information _between_ components. They're brokers that connect components, but do not necessarily have meaningful behaviors or states of their own. Connectors can be things like function calls, web service API calls, events, requests, and so on. None of these mechanisms store state or functionality themselves; instead, they are the things that tie components functionality and state together.
		
Even with carefully selected architectures, systems can still be difficult to put together, leading to *architectural mismatch*<garlan95>. When mismatch occurs, connecting two styles can require dramatic amounts of code to connect, imposing significant risk of defects and cost of maintenance. One common example of mismatches occurs with the ubiquitous use of database schemas with client/server web-applications. A single change in a database schema can often result in dramatic changes in an application, as every line of code that uses that part of the scheme either directly or indirectly must be updated<qiu13>. This kind of mismatch occurs because the component that manages data (the database) and the component that renders data (the user interface) are highly "coupled" with the database schema: the user interface needs to know _a lot_ about the data, its meaning, and its structure in order to render it meaningfully.

The most common approach to dealing with both architectural mismatch and the changing of requirements over time is *refactoring*, which means changing the _architecture_ of an implementation without changing its behavior. Refactoring is something most developers do as part of changing a system<murphyhill09,silva16>. Refactoring code to eliminate mismatch and technical debt can simplify change in the future, saving time<ng06> and preventing future defects<kim12>. However, because refactoring remains challenging, the difficulty of changing an architecture is often used as a rationale for rejecting demands for change from users. For example, Google does not allow one to change their Gmail address, which greatly harms people who have changed their name (such as this author when she came out as a trans woman), forcing them to either live with an address that includes their old name, or abandon their Google account, with no ability to transfer documents or settings. The rationale for this has nothing to do with policy and everything to do with the fact that the original architecture of Gmail treats the email address as a stable, unique identifier for an account. Changing this basic assumption throughout Gmail's implementation would be an immense refactoring task.
		
Research on the actual activity of software architecture is actually somewhat sparse. One of the more recent syntheses of this work is Petre et al.'s book, _Software Design Decoded_<petre16>, which distills many of the practices and skills of software design into a set of succinct ideas. For example, the book states, "_Every design problem has multiple, if not infinite, ways of solving it. Experts strongly prefer simpler solutions over complex ones, for they know that such solutions are easier to understand and change in the future._" And yet, in practice, studies of how projects use APIs often show that developers do the exact opposite, building projects with dependencies on large numbers of sometimes trivial APIs. Some behavior suggests that while software _architects_ like simplicity of implementation, software _developers_ are often choosing whatever is easiest to build, rather than whatever is least risky to maintain over time<abdalkareem17>.