# Software development process

In software engineering, a software development process or software development life cycle (SDLC) is a process of planning and managing software development. It typically involves dividing software development work into smaller, parallel, or sequential steps or sub-processes to improve design and/or product management. The methodology may include the pre-definition of specific deliverables and artifacts that are created and completed by a project team to develop or maintain an application.
Most modern development processes can be vaguely described as agile. Other methodologies include waterfall, prototyping, iterative and incremental development, spiral development, rapid application development, and extreme programming.
A life-cycle "model" is sometimes considered a more general term for a category of methodologies and a software development "process" is a particular instance as adopted by a specific organization. For example, many specific software development processes fit the spiral life-cycle model. The field is often considered a subset of the systems development life cycle.

## History

The software development methodology framework did not emerge until the 1960s. According to Elliott (2004), the systems development life cycle can be considered to be the oldest formalized methodology framework for building information systems. The main idea of the software development life cycle has been "to pursue the development of information systems in a very deliberate, structured and methodical way, requiring each stage of the life cycle––from the inception of the idea to delivery of the final system––to be carried out rigidly and sequentially" within the context of the framework being applied. The main target of this methodology framework in the 1960s was "to develop large scale functional business systems in an age of large scale business conglomerates. Information systems activities revolved around heavy data processing and number crunching routines."
Requirements gathering and analysis: The first phase of the custom software development process involves understanding the client's requirements and objectives. This stage typically involves engaging in thorough discussions and conducting interviews with stakeholders to identify the desired features, functionalities, and overall scope of the software. The development team works closely with the client to analyze existing systems and workflows, determine technical feasibility, and define project milestones.
Planning and design: Once the requirements are understood, the custom software development team proceeds to create a comprehensive project plan. This plan outlines the development roadmap, including timelines, resource allocation, and deliverables. The software architecture and design are also established during this phase. User interface (UI) and user experience (UX) design elements are considered to ensure the software's usability, intuitiveness, and visual appeal.
Development: With the planning and design in place, the development team begins the coding process. This phase involves writing, testing, and debugging the software code. Agile methodologies, such as scrum or kanban, are often employed to promote flexibility, collaboration, and iterative development. Regular communication between the development team and the client ensures transparency and enables quick feedback and adjustments.
Testing and quality assurance: To ensure the software's reliability, performance, and security, rigorous testing and quality assurance (QA) processes are carried out. Different testing techniques, including unit testing, integration testing, system testing, and user acceptance testing, are employed to identify and rectify any issues or bugs. QA activities aim to validate the software against the predefined requirements, ensuring that it functions as intended.
Deployment and implementation: Once the software passes the testing phase, it is ready for deployment and implementation. The development team assists the client in setting up the software environment, migrating data if necessary, and configuring the system. User training and documentation are also provided to ensure a smooth transition and enable users to maximize the software's potential.
Maintenance and support: After the software is deployed, ongoing maintenance and support become crucial to address any issues, enhance performance, and incorporate future enhancements. Regular updates, bug fixes, and security patches are released to keep the software up-to-date and secure. This phase also involves providing technical support to end users and addressing their queries or concerns. Methodologies, processes, and frameworks range from specific prescriptive steps that can be used directly by an organization in day-to-day work, to flexible frameworks that an organization uses to generate a custom set of steps tailored to the needs of a specific project or group. In some cases, a "sponsor" or "maintenance" organization distributes an official set of documents that describe the process. Specific examples include:

### 1970s

* Structured programming since 1969
* Cap Gemini SDM, originally from PANDATA, the first English translation was published in 1974. SDM stands for System Development Methodology

### 1980s

* Structured systems analysis and design method (SSADM) from 1980 onwards
* Information Requirement Analysis/Soft systems methodology

### 1990s

* Object-oriented programming (OOP) developed in the early 1960s and became a dominant programming approach during the mid-1990s
* Rapid application development (RAD), since 1991
* Dynamic systems development method (DSDM), since 1994
* Scrum, since 1995
* Team software process, since 1998
* Rational Unified Process (RUP), maintained by IBM since 1998
* Extreme programming, since 1999

### 2000s

* Agile Unified Process (AUP) maintained since 2005 by Scott Ambler
* Disciplined agile delivery (DAD) Supersedes AUP

### 2010s

* Scaled Agile Framework (SAFe)
* Large-Scale Scrum (LeSS)
* DevOps
Since DSDM in 1994, all of the methodologies on the above list except RUP have been agile methodologies - yet many organizations, especially governments, still use pre-agile processes (often waterfall or similar). Software process and software quality are closely interrelated; some unexpected facets and effects have been observed in practice.
Among these, another software development process has been established in open source. The adoption of these best practices known and established processes within the confines of a company is called inner source.

## Prototyping

Software prototyping is about creating prototypes, i.e. incomplete versions of the software program being developed.
The basic principles are:
* Prototyping is not a standalone, complete development methodology, but rather an approach to try out particular features in the context of a full methodology (such as incremental, spiral, or rapid application development (RAD)).
* Attempts to reduce inherent project risk by breaking a project into smaller segments and providing more ease of change during the development process.
* The client is involved throughout the development process, which increases the likelihood of client acceptance of the final implementation.
* While some prototypes are developed with the expectation that they will be discarded, it is possible in some cases to evolve from prototype to working system.
A basic understanding of the fundamental business problem is necessary to avoid solving the wrong problems, but this is true for all software methodologies.

## Methodologies
### Agile development

"Agile software development" refers to a group of software development frameworks based on iterative development, where requirements and solutions evolve via collaboration between self-organizing cross-functional teams. The term was coined in the year 2001 when the Agile Manifesto was formulated.
Agile software development uses iterative development as a basis but advocates a lighter and more people-centric viewpoint than traditional approaches. Agile processes fundamentally incorporate iteration and the continuous feedback that it provides to successively refine and deliver a software system.
The Agile model also includes the following software development processes:
* Dynamic systems development method (DSDM)
* Kanban
* Scrum
* Lean software development

### Continuous integration

Continuous integration is the practice of merging all developer working copies to a shared mainline several times a day. Grady Booch first named and proposed CI in his 1991 method, although he did not advocate integrating several times a day. Extreme programming (XP) adopted the concept of CI and did advocate integrating more than once per day – perhaps as many as tens of times per day.

### Incremental development

Various methods are acceptable for combining linear and iterative systems development methodologies, with the primary objective of each being to reduce inherent project risk by breaking a project into smaller segments and providing more ease-of-change during the development process.
There are three main variants of incremental development:
* A series of mini-waterfalls are performed, where all phases of the waterfall are completed for a small part of a system, before proceeding to the next increment, or
* Overall requirements are defined before proceeding to evolutionary, mini-waterfall development of individual increments of a system, or
* The initial software concept, requirements analysis, and design of architecture and system core are defined via waterfall, followed by incremental implementation, which culminates in installing the final version, a working system.

### Rapid application development

Rapid application development (RAD) is a software development methodology, which favors iterative development and the rapid construction of prototypes instead of large amounts of up-front planning. The "planning" of software developed using RAD is interleaved with writing the software itself. The lack of extensive pre-planning generally allows software to be written much faster and makes it easier to change requirements.
The rapid development process starts with the development of preliminary data models and business process models using structured techniques. In the next stage, requirements are verified using prototyping, eventually to refine the data and process models. These stages are repeated iteratively; further development results in "a combined business requirements and technical design statement to be used for constructing new systems".
The term was first used to describe a software development process introduced by James Martin in 1991. According to Whitten (2003), it is a merger of various structured techniques, especially data-driven information technology engineering, with prototyping techniques to accelerate software systems development.
The basic principles of rapid application development are:
* Key objective is for fast development and delivery of a high-quality system at a relatively low investment cost.
* Attempts to reduce inherent project risk by breaking a project into smaller segments and providing more ease of change during the development process.
* Aims to produce high-quality systems quickly, primarily via iterative Prototyping (at any stage of development), active user involvement, and computerized development tools. These tools may include graphical user interface (GUI) builders, Computer Aided Software Engineering (CASE) tools, Database Management Systems (DBMS), fourth-generation programming languages, code generators, and object-oriented techniques.
* Key emphasis is on fulfilling the business need, while technological or engineering excellence is of lesser importance.
* Project control involves prioritizing development and defining delivery deadlines or “timeboxes”. If the project starts to slip, the emphasis is on reducing requirements to fit the timebox, not on increasing the deadline.
* Generally includes joint application design (JAD), where users are intensely involved in system design, via consensus building in either structured workshops, or electronically facilitated interaction.
* Active user involvement is imperative.
* Iteratively produces production software, as opposed to a throwaway prototype.
* Produces documentation necessary to facilitate future development and maintenance.
* Standard systems analysis and design methods can be fitted into this framework.

### Waterfall development

The waterfall model is a sequential development approach, in which development is seen as flowing steadily downwards (like a waterfall) through several phases, typically:
* Requirements analysis resulting in a software requirements specification
* Software design
* Implementation
* Testing
* Integration, if there are multiple subsystems
* Deployment (or Installation)
* Maintenance
The first formal description of the method is often cited as an article published by Winston W. Royce in 1970, although Royce did not use the term "waterfall" in this article. Royce presented this model as an example of a flawed, non-working model.
The basic principles are:
* The Project is divided into sequential phases, with some overlap and splashback acceptable between phases.
* Emphasis is on planning, time schedules, target dates, budgets, and implementation of an entire system at one time.
* Tight control is maintained over the life of the project via extensive written documentation, formal reviews, and approval/signoff by the user and information technology management occurring at the end of most phases before beginning the next phase. Written documentation is an explicit deliverable of each phase.
The waterfall model is a traditional engineering approach applied to software engineering. A strict waterfall approach discourages revisiting and revising any prior phase once it is complete. This "inflexibility" in a pure waterfall model has been a source of criticism by supporters of other more "flexible" models. It has been widely blamed for several large-scale government projects running over budget, over time and sometimes failing to deliver on requirements due to the big design up front approach. Except when contractually required, the waterfall model has been largely superseded by more flexible and versatile methodologies developed specifically for software development. See Criticism of waterfall model.

### Spiral development

In 1988, Barry Boehm published a formal software system development "spiral model," which combines some key aspects of the waterfall model and rapid prototyping methodologies, in an effort to combine advantages of top-down and bottom-up concepts. It provided emphasis on a key area many felt had been neglected by other methodologies: deliberate iterative risk analysis, particularly suited to large-scale complex systems.
The basic principles are:
* Focus is on risk assessment and on minimizing project risk by breaking a project into smaller segments and providing more ease-of-change during the development process, as well as providing the opportunity to evaluate risks and weigh consideration of project continuation throughout the life cycle.
* "Each cycle involves a progression through the same sequence of steps, for each part of the product and for each of its levels of elaboration, from an overall concept-of-operation document down to the coding of each individual program."
* Each trip around the spiral traverses four basic quadrants: (1) determine objectives, alternatives, and constraints of the iteration, and (2) evaluate alternatives; Identify and resolve risks; (3) develop and verify deliverables from the iteration; and (4) plan the next iteration.
* Begin each cycle with an identification of stakeholders and their "win conditions", and end each cycle with review and commitment.

### Shape Up

Shape Up is a software development approach introduced by Basecamp in 2018. It is a set of principles and techniques that Basecamp developed internally to overcome the problem of projects dragging on with no clear end. Its primary target audience is remote teams. Shape Up has no estimation and velocity tracking, backlogs, or sprints, unlike waterfall, agile, or scrum. Instead, those concepts are replaced with appetite, betting, and cycles. As of 2022, besides Basecamp, notable organizations that have adopted Shape Up include UserVoice and Block.

### Advanced methodologies

Other high-level software project methodologies include:
* Behavior-driven development and business process management.
* Chaos model - The main rule always resolves the most important issue first.
* Incremental funding methodology - an iterative approach
* Lightweight methodology - a general term for methods that only have a few rules and practices
* Structured systems analysis and design method - a specific version of waterfall
* Slow programming, as part of the larger Slow Movement, emphasizes careful and gradual work without (or minimal) time pressures. Slow programming aims to avoid bugs and overly quick release schedules.
* V-Model (software development) - an extension of the waterfall model
* Unified Process (UP) is an iterative software development methodology framework, based on Unified Modeling Language (UML). UP organizes the development of software into four phases, each consisting of one or more executable iterations of the software at that stage of development: inception, elaboration, construction, and guidelines.

## Process meta-models

Some "process models" are abstract descriptions for evaluating, comparing, and improving the specific process adopted by an organization.
* ISO/IEC 12207 is the international standard describing the method to select, implement, and monitor the life cycle for software.
* The Capability Maturity Model Integration (CMMI) is one of the leading models and is based on best practices. Independent assessments grade organizations on how well they follow their defined processes, not on the quality of those processes or the software produced. CMMI has replaced CMM.
* ISO 9000 describes standards for a formally organized process to manufacture a product and the methods of managing and monitoring progress. Although the standard was originally created for the manufacturing sector, ISO 9000 standards have been applied to software development as well. Like CMMI, certification with ISO 9000 does not guarantee the quality of the end result, only that formalized business processes have been followed.
* ISO/IEC 15504 Information technology—Process assessment is also known as Software Process Improvement Capability Determination (SPICE), is a "framework for the assessment of software processes". This standard is aimed at setting out a clear model for process comparison. SPICE is used much like CMMI. It models processes to manage, control, guide, and monitor software development. This model is then used to measure what a development organization or project team actually does during software development. This information is analyzed to identify weaknesses and drive improvement. It also identifies strengths that can be continued or integrated into common practice for that organization or team.
* ISO/IEC 24744 Software Engineering—Metamodel for Development Methodologies, is a power type-based metamodel for software development methodologies.
* Soft systems methodology - a general method for improving management processes.
* Method engineering - a general method for improving information system processes.