Architectural quality attributes

Let us now focus on an aspect which forms the main topic for the rest of this book–Architectural Quality Attributes.

In a previous section, we discussed how an architecture balances and optimizes stakeholder requirements. We also saw some examples of contradicting stakeholder requirements, which an architect seeks to balance, by choosing an architecture which does the necessary trade-offs.

The term quality attribute has been used to loosely define some of these aspects that an architecture makes trade-offs for. It is now the time to formally define what an Architectural Quality Attribute is:

"A quality attribute is a measurable and testable property of a system which can be used to evaluate the performance of a system within its prescribed environment with respect to its non-functional aspects"

There are a number of aspects that fit this general definition of an architectural quality attribute. However, for the rest of this book, we will be focusing on the following quality attributes:

• Modifiability
• Testability
• Scalability and performance
• Availability
• Security
• Deployability

Modifiability

Many studies show that about 80% of the cost of a typical software system occurs after the initial development and deployment. This shows how important modifiability is to a system's initial architecture.

Modifiability can be defined as the ease with which changes can be made to a system, and the flexibility with which the system adjusts to the changes. It is an important quality attribute, as almost every software system changes over its lifetime—to fix issues, for adding new features, for performance improvements, and so on.

From an architect's perspective, the interest in modifiability is about the following:

• Difficulty: The ease with which changes can be made to a system
• Cost: In terms of time and resources required to make the changes
• Risks: Any risk associated with making changes to the system

Now, what kind of changes are we talking about here? Is it changes to code, changes to deployment, or changes to the entire architecture?

The answer is: it can be at any level.

From an architecture perspective, these changes can generally be captured at the following three levels:

1. Local: A local change only affects a specific element. The element can be a piece of code such as a function, a class, a module, or a configuration element such as an XML or JSON file. The change does not cascade to any neighboring element or to the rest of the system. Local changes are the easiest to make, and the least risky of all. The changes can usually be quickly validated with local unit tests.
2. Non-local: These changes involve more than one element. The examples are as follows:
   • Modifying a database schema, which then needs to cascade into the model class representing that schema in the application code
   • Adding a new configuration parameter in a JSON file, which then needs to be processed by the parser parsing the file and/or the application(s) using the parameter

   Non-local changes are more difficult to make than local changes, require careful analysis, and wherever possible, integration tests to avoid code regressions.

3. Global: These changes either involve architectural changes from top down, or changes to elements at the global level, which cascade down to a significant part of the software system. The examples are as follows:
   • Changing a system's architecture from RESTful to messaging (SOAP, XML-RPC, and others) based web services
   • Changing a web application controller from Django to an Angular-js based component
   • A performance change requirement which needs all data to be preloaded at the frontend to avoid any inline model API calls for an online news application

   These changes are the riskiest, and also the costliest, in terms of resources, time and money. An architect needs to carefully vet the different scenarios that may arise from the change, and get his/her team to model them via integration tests. Mocks can be very useful in these kinds of large-scale changes.

The following table shows the relationship between Cost and Risk for the different levels of system modifiability:

Level	Cost	Risk
Local	Low	Low
Non-local	Medium	Medium
Global	High	High

Modifiability at the code level is also directly related to its readability:

"The more readable a code is, the more modifiable it is. Modifiability of a code goes down in proportion to its readability."

The modifiability aspect is also related to the maintainability of the code. A code module which has its elements very tightly coupled would yield to modification much less than a module which has a loosely coupled elements—this is the Coupling aspect of modifiability.

Similarly, a class or module which does not define its role and responsibilities clearly would be more difficult to modify than another one which has well-defined responsibility and functionality. This aspect is called Cohesion of a software module.

The following table shows the relation between Cohesion, Coupling, and Modifiability for an imaginary module A. Assume that the coupling is from this module to another module B:

Cohesion	Coupling	Modifiability
Low	High	Low
Low	Low	Medium
High	High	Medium
High	Low	High

It is pretty clear from the preceding table that having higher Cohesion and lower Coupling is the best scenario for the modifiability of a code module.

Other factors that affect modifiability are as follows:

• Size of a module (number of lines of code): Modifiability decreases when size increases.
• Number of team members working on a module: Generally, a module becomes less modifiable when a larger number of team members work on the module due to the complexities in merging and maintaining a uniform code base.
• External third-party dependencies of a module: The larger the number of external third-party dependencies, the more difficult it is to modify the module. This can be thought of as an extension of the coupling aspect of a module.
• Wrong use of the module API: If there are other modules which make use of the private data of a module rather than (correctly) using its public API, it is more difficult to modify the module. It is important to ensure proper usage standards of modules in your organization to avoid such scenarios. This can be thought of as an extreme case of tight Coupling.

Testability

Testability refers to how much a software system is amenable to demonstrating its faults through testing. Testability can also be thought of as how much a software system hides its faults from end users and system integration tests—the more testable a system is, the less it is able to hide its faults.

Testability is also related to how predictable a software system's behavior is. The more predictable a system, the more it allows for repeatable tests, and for developing standard test suites based on a set of input data or criteria. Unpredictable systems are much less amenable to any kind of testing, or, in extreme case, not testable at all.

In software testing, you try to control a system's behavior by, typically, sending it a set of known inputs, and then observing the system for a set of known outputs. Both of these combine to form a testcase. A test suite or test harness, typically, consists of many such test cases.

Test assertions are the techniques that are used to fail a test case when the output of the element under the test does not match the expected output for the given input. These assertions are usually manually coded at specific steps in the test execution stage to check the data values at different steps of the testcase:

Representative flowchart of a simple unit test case for function f('X') = 'Y'

The preceding diagram shows an example of a representative flowchart for a testable function "f" for a sample input "X" with expected output "Y".

In order to recreate the session or state at the time of a failure, the record/playback strategy is often used. This employs specialized software (such as Selenium), which records all user actions that led to a specific fault, and saves it as a testcase. The test is reproduced by replaying the testcase using the same software which tries to simulate the same testcase; this is done by repeating the same set and order of UI actions.

Testability is also related to the complexity of code in a way very similar to modifiability. A system becomes more testable when parts of it can be isolated and made to work independent of the rest of the system. In other words, a system with low coupling is more testable than a system with high coupling.

Another aspect of testability, which is related to the predictability mentioned earlier, is to reduce non-determinism. When writing test suites, we need to isolate the elements that are to be tested from other parts of the system which have a tendency to behave unpredictably so that the tested element's behavior becomes predictable.

An example is a multi-threaded system, which responds to events raised in other parts of the system. The entire system is probably quite unpredictable, and not amenable to repeated testing. Instead, one needs to separate the events subsystem, and possibly, mock its behavior so that those inputs can be controlled, and the subsystem which receives the events becomes predictable and hence, testable.

The following schematic diagram explains the relationship between the testability and predictability of a system to the Coupling and Cohesion between its components:

Relation of testability and predictability of a system to coupling and cohesion

Scalability

Modern-day web applications are all about scaling up. If you are part of any modern-day software organization, it is very likely that you have heard about or worked on an application that is written for the cloud, which is able to scale up elastically on demand.

Scalability of a system is its capacity to accommodate increasing workload on demand while keeping its performance within acceptable limits.

Scalability in the context of a software system, typically, falls into two categories, which are as follows:

• Horizontal scalability: Horizontal scalability implies scaling out/in a software system by adding more computing nodes to it. Advances in cluster computing in the last decade have given rise to the advent of commercial horizontally scalable elastic systems as services on the web. A well-known example is Amazon Web Services. In horizontally scalable systems, typically, data and/or computation is done on units or nodes, which are, usually, virtual machines running on commodity systems known as virtual private servers (VPS). The scalability is achieved "n" times by adding n or more nodes to the system, typically fronted by a load balancer. Scaling out means expanding the scalability by adding more nodes, and scaling in means reducing the scalability by removing existing nodes:
  

  Example deployment architecture showing horizontally scaling a web application server

• Vertical scalability: Vertical scalability involves adding or removing resources from a single node in a system. This is usually done by adding or removing CPUs or RAM (memory) from a single virtual server in a cluster. The former is called scaling up, and the latter, scaling down. Another kind of scaling up is increasing the capacity of an existing software process in the system—typically, by augmenting its processing power. This is usually done by increasing the number of processes or threads available to an application. Some examples are as follows:
  • Increasing the capacity of an Nginx server process by increasing its number of worker processes
  • Increasing the capacity of a PostgreSQL server by increasing its number of maximum connections

Performance

Performance of a system is related to its scalability. Performance of a system can be defined as follows:

"Performance of a computer system is the amount of work accomplished by a system using a given unit of computing resource. Higher the work/unit ratio, higher the performance."

The unit of computing resource to measure performance can be one of the following:

• Response time: How much time a function or any unit of execution takes to execute in terms of real time (user time) and clock time (CPU time).
• Latency: How much time it takes for a system to get its stimulation, and then provide a response. An example is the time it takes for the request-response loop of a web application to complete, measured from the end-user perspective.
• Throughput: The rate at which a system processes its information. A system which has higher performance would usually have a higher throughput, and correspondingly higher scalability. An example is the throughput of an e-commerce website measured as the number of transactions completed per minute.

Performance is closely tied to scalability, especially, vertical scalability. A system that has excellent performance with respect to its memory management would easily scale up vertically by adding more RAM.

Similarly, a system that has multi-threaded workload characteristics and is written optimally for a multicore CPU, would scale up by adding more CPU cores.

Horizontal scalability is thought of as having no direct connection to the performance of a system within its own compute node. However, if a system is written in a way that it doesn't utilize the network effectively, thereby producing network latency issues, it may have a problem scaling horizontally effectively, as the time spent on network latency would offset any gain in scalability obtained by distributing the work.

Some dynamic programming languages such as Python have built-in scalability issues when it comes to scaling up vertically. For example, the Global Interpreter Lock (GIL) of Python (CPython) prevents it from making full use of the available CPU cores for computing by multiple threads.

Availability

Availability refers to the property of readiness of a software system to carry out its operations when the need arises.

Availability of a system is closely related to its reliability. The more reliable a system is, the more available it is.

Another factor which modifies availability is the ability of a system to recover from faults. A system may be very reliable, but if the system is unable to recover either from complete or partial failures of its subsystems, then it may not be able to guarantee availability. This aspect is called recovery.

The availability of a system can be defined as follows:

"Availability of a system is the degree to which the system is in a fully operable state to carry out its functionality when it is called or invoked at random."

Mathematically, this can be expressed as follows:

Availability = MTBF/(MTBF + MTTR)

Take a look at the following terms used in the preceding formula:

• MTBF: Mean time between failures
• MTTR: Mean time to repair

This is often called the mission capable rate of a system.

Techniques for Availability are closely tied to recovery techniques. This is due to the fact that a system can never be 100% available. Instead, one needs to plan for faults and strategies to recover from faults, which directly determines the availability. These techniques can be classified as follows:

• Fault detection: The ability to detect faults and take action helps to avert situations where a system or parts of a system become unavailable completely. Fault detection typically involves steps such as monitoring, heartbeat, and ping/echo messages, which are sent to the nodes in a system, and the response measured to calculate if the nodes are alive, dead, or are in the process of failing.
• Fault recovery: Once a fault is detected, the next step is to prepare the system to recover from the fault and bring it to a state where the system can be considered available. Typical tactics used here include Hot/Warm Spares (Active/Passive redundancy), Rollback, Graceful Degradation, and Retry.
• Fault prevention: This approach uses active methods to anticipate and prevent faults from occurring so that the system does not have a chance to go to recovery.

Availability of a system is closely tied to the consistency of its data via the CAP theorem which places a theoretical limit on the trade-offs a system can make with respect to consistency versus availability in the event of a network partition. The CAP theorem states that a system can choose between being consistent or being available—typically leading to two broad types of systems, namely, CP (consistent and tolerant to network failures) and AP (available and tolerant to network failures).

Availability is also tied to the system's scalability tactics, performance metrics, and its security. For example, a system that is highly horizontally scalable would have a very high availability, since it allows the load balancer to determine inactive nodes and take them out of the configuration pretty quickly.

A system which, instead, tries to scale up may have to monitor its performance metrics carefully. The system may have availability issues even when the node on which the system is fully available if the software processes are squeezed for system resources, such as CPU time or memory. This is where performance measurements become critical, and the system's load factor needs to be monitored and optimized.

With the increasing popularity of web applications and distributed computing, security is also an aspect that affects availability. It is possible for a malicious hacker to launch remote denial of service attacks on your servers, and if the system is not made foolproof against such attacks, it can lead to a condition where the system becomes unavailable or only partially available.

Security

Security, in the software domain, can be defined as the degree of ability of a system to avoid damage to its data and logic from unauthenticated access, while continuing to provide services to other systems and roles that are properly authenticated.

A security crisis or attack occurs when a system is intentionally compromised with a view to gaining illegal access to it in order to compromise its services, copy, or modify its data, or deny access to its legitimate users.

In modern software systems, the users are tied to specific roles which have exclusive rights to different parts of the system. For example, a typical web application with a database may define the following roles:

• user: End user of the system with login and access to his/her private data
• dbadmin: Database administrator, who can view, modify, or delete all database data
• reports: Report admin, who has admin rights only to those parts of database and code that deal with report generation
• admin: Superuser, who has edit rights to the complete system

This way of allocating system control via user roles is called access control. Access control works by associating a user role with certain system privileges, thereby decoupling the actual user login from the rights granted by these privileges.

This principle is the Authorization technique of security.

Another aspect of security is with respect to transactions where each person must validate the actual identity of the other. Public key cryptography, message signing, and so on are common techniques used here. For example, when you sign an e-mail with your GPG or PGP key, you are validating yourself—The sender of this message is really me, Mr. A—to your friend Mr. B on the other side of the e-mail. This principle is the Authentication technique of security.

The other aspects of security are as follows:

• Integrity: These techniques are used to ensure that data or information is not tampered with in anyway on its way to the end user. Examples are message hashing, CRC Checksum, and others.
• Origin: These techniques are used to assure the end receiver that the origin of the data is exactly the same as where it is purporting to be from. Examples of this are SPF, Sender-ID (for e-mail), Public Key Certificates and Chains (for websites using SSL), and others.
• Authenticity: These are the techniques which combine both the Integrity and Origin of a message into one. This ensures that the author of a message cannot deny the contents of the message as well as its origin (himself/herself). This typically uses Digital Certificate Mechanisms.

Deployability

Deployability is one of those quality attributes which is not fundamental to the software. However, in this book, we are interested in this aspect, because it plays a critical role in many aspects of the ecosystem in the Python programming language and its usefulness to the programmer.

Deployability is the degree of ease with which software can be taken from the development to the production environment. It is more of a function of the technical environment, module structures, and programming runtime/languages used in building a system, and has nothing to do with the actual logic or code of the system.

The following are some factors that determine deployability:

• Module structures: If your system has its code organized into well-defined modules/projects which compartmentalize the system into easily deployable subunits, the deployment is much easier. On the other hand, if the code is organized into a monolithic project with a single setup step, it would be hard to deploy the code into a multiple node cluster.
• Production versus development environment: Having a production environment which is very similar to the structure of the development environment makes deployment an easy task. When the environments are similar, the same set of scripts and toolchains that are used by the developers/DevOps team can be used to deploy the system to a development server as well as a production server with minor changes—mostly in the configuration.
• Development ecosystem support: Having a mature tool-chain support for your system runtime, which allows configurations such as dependencies to be automatically established and satisfied, increases deployability. Programming languages such as Python are rich in this kind of support in its development ecosystem, with a rich array of tools available for the DevOps professional to take advantage of.
• Standardized configuration: It is a good idea to keep your configuration structures (files, database tables, and others) the same for both developer and production environments. The actual objects or filenames can be different, but if the configuration structures vary widely across both the environments, deployability decreases, as extra work is required to map the configuration of the environment to its structures.
• Standardized infrastructure: It is a well-known fact that keeping your deployments to a homogeneous or standardized set of infrastructure greatly aids deployability. For example, if you standardize your frontend application to run on 4 GB RAM, Debian-based 64-bit Linux VPS, then it is easy to automate deployment of such nodes—either using a script, or by using elastic compute approaches of providers such as Amazon—and to keep a standard set of scripts across both development and production environments. On the other hand, if your production deployment consists of heterogeneous infrastructure, say, a mix of Windows and Linux servers with varying capacities and resource specifications, the work typically doubles for each type of infrastructure decreasing deployability.
• Use of containers: The user of container software, popularized by the advent of technology such as Docker and Vagrant built on top of Linux containers, has become a recent trend in deploying software on servers. The use of containers allows you to standardize your software, and makes deployability easier by reducing the amount of overhead required to start/stop the nodes, as containers don't come with the overhead of a full virtual machine. This is an interesting trend to watch for.