Let’s dig into A Design Methodology For Reliable Software Systems published by Barbara Liskov in 1972.
The focus of this paper is on how to make reliable software systems and what techniques can help us achieve that. Reliability here implies that a system works as expected under a given set of conditions.
The unfortunate fact is that the standard approach to building
systems, involving extensive debugging, has not proved successful in producing reliable software, and there is no reason to suppose it ever will. Although improvements in debugging techniques may lead to the detection of more errors, this does not imply that all errors will be found. There certainly is no guarantee of this implicit in debugging: as Dijkstra said, “Program testing can be used to show the presence of bugs, but never to show their absence.”
To be confident that our system works correctly, we need testing that meets the following conditions:
- We can generate a minimal set of relevant test cases
- All test cases in the set can be generated
The solutions to these problems do not lie in the domain of debugging, which has no control over the sources of the problems. Instead, since it is the system design which determines how many test cases there are and how easily they can be identified, the problems can be solved most effectively during the design process: The need for exhaustive testing must influence the design.
The paper further argues that reliability is a major issue with complex systems. It goes on to define complex systems as follows:
- The system has many states and it’s difficult to organize program logic to handle all of them correctly
- It requires several people working together in a coordinated manner
Criteria for a Good Design:
To tame the design of a complex system, we need to use modularization and divide the program into several modules (sub programs, later on referred to as partitions in the paper to avoid overloading the term “modules”) which can be compiled separately but are connected to other modules.
The connections are defined by Parnas as follows:
The connections between modules are the assumptions which the modules make about each other.
Although the idea of modularity sounds like a great tool for building large complex software systems, it can introduce additional complexity if not done right.
The success of modularity depends directly on how well modules are chosen.
Some common issues are:
- A module does too many things
- A common function is distributed among many different modules
- A module behaves unexpectedly with common data
The next question that arises: What is good modularity?
We use two techniques to answer that: levels of abstraction to tackle the inherent complexity of the system and structured programming to represent the design in software.
Levels of abstraction:
Levels of abstraction…provide a conceptual framework for achieving a clear and logical design for a system. The entire system is conceived as a hierarchy of levels, the lowest levels being those closest to the machine.
A group of related functions make up a level of abstraction. Each level can have the following two types of functions:
- External: These functions can be called by functions in other levels
- Internal: These functions do a common task within the level and cannot be called by other functions in a different level
Levels of abstraction are governed by the following two rules:
- Each level has exclusive control over some kind of resource
- Lower levels aren’t aware of higher levels and can’t reference them in any way. However, higher levels can ask lower levels to perform an action or for info.
A structured program defines the way control passes among various partitions in a system. It is defined by the following two rules:
- The program is developed in a top down fashion and divided into levels (the notion of levels here is different from that of levels of abstraction because the first rule isn’t satisfied)
- Only the following control structures can be used: concatenation, selection
of the next statement based on the testing of a condition,
and iteration. Jumping using
Back to the question that was posed earlier: how do we define good modularity?
In a modular system that is also reliable, the connections between partitions are limited as follows:
- They need to follow the rules imposed by levels of abstraction and the structured programming
- Passing of data between partitions should be done using explicit arguments passed to external functions of another partition
- Partitions should be logically independent — the functions within a partition should support its own abstraction only
The next question that arises after we’ve figured out how to defined good modularity is — how do we achieve it in our design?
The traditional technique for modularization is to analyze the execution-time flow of the system and organize the system structure around each major sequential task. This technique leads to a structure which has very simple connections in control, but the connections in data tend to be complex.
Partitions supports abstractions that a system designer finds helpful when thinking about the system.
Abstractions are introduced in order to make what the system is doing clearer and more understandable; an abstraction is a conceptual simplification because it expresses what is being done without specifying how it is done.
The paper then presents some guidelines for identifying different types of abstractions while designing a system:
- Abstraction of resources: for every hardware resource on the system, we can map characteristics of the abstract resource to the underlying resource
- Abstract characteristics of data: how it is stored
- Simplification via limiting information the partition needs to know or has access to
- Simplification via generalization by identifying functions that perform a common task. Such functions can be grouped together in one partition. “The existence of such a group simplifies other partitions, which need only appeal to the functions of the lower partition rather than perform the tasks themselves.”
- System maintenance and modification: functions performing a task whose definition is prone to change in the future should be part of independent partitions. For example, functions which deal with connecting to a particular kind of storage back end so that if a different back end is used in the future, only functions in that partition will be affected.
Now that we have some idea about how we can achieve good modularity while designing our system, how do we proceed with it?
The first phase is to identify a set of abstractions that represent the eventual behavior of the system in a general way. The next phase “establishes the data connections between the partitions and describes the flow of control among the partitions”.
The second phase occurs concurrently with the first; as abstractions
are proposed, their utility and practicality are immediately investigated.
A partition has been adequately investigated when
its connections with the rest of the system are known
and when the designers are confident that they understand
exactly what its effect on the system will be.
The next question one would ask is:** how do we identify when the design is finished**?
- All major abstractions have been identified and been linked to a partition. The system resources have been divided among the various partitions and their positions in the hierarchy have been defined
- The interfaces and flow of control among the partitions is clearly defined. The test cases for each partition have been identified
- A basic user guide for the system can be written