How to Slice User Stories Through and Through

When Bill Wake introduced INVEST as a standard for story writing in the emerging Agile software development methodology in 2003 (which stands for Independent, Negotiable, Valuable, Estimable, Small, and Testable), Agile itself was still in its early stages. Even now, in 2024, discussions around “Agile transformation” persist, as some companies are still catching up to modern development practices.

In 2003, Agile’s adaptation varied widely across industries and organizations, with its principles challenging traditional, plan-driven waterfall approaches. Despite the initial skepticism, Agile gained traction due to its emphasis on iterative development, collaboration, and incremental value delivery. For early adopters, the INVEST principle proved valuable in guiding the creation of small, manageable stories.

In this article, we will explore the ways of splitting stories to adhere to the “S” in INVEST – the “small” principle, building on our previous discussion in the article Slice your stories. Then slice them again where we highlighted the benefits of story splitting for better manageability. Today, we will delve into various methods of splitting stories and reflect on the process itself.

“S” – “small”

The concept of “small” in the INVEST criteria emphasizes breaking down stories into manageable pieces that can be completed within a single iteration.

The perception of what constitutes a “small” story may have evolved over time, and the size of a story can be influenced by various factors, including technology complexity, team experience, and project context. In 2003, when INVEST was introduced, the technology landscape was different, and development practices varied.

While it’s true that technology might have been perceived as simpler in some aspects, it’s essential to consider that the complexity of projects can be relative. The goal of breaking down user stories into smaller, more manageable tasks remains relevant regardless of the technological landscape. The intent is to improve agility, reduce risk, and enable faster feedback cycles.

As technology has advanced, development practices and tools have evolved, allowing teams to tackle more complex challenges. However, the principles of Agile, including the idea of keeping user stories small and manageable, continue to be valuable in modern software development. The interpretation of what is considered “small” may vary based on the specific characteristics and requirements of a given project or team.

Vertical vs Horizontal

In summary, while many experienced developers appreciate the benefits of story slicing within an Agile framework, there can be individual preferences and challenges based on factors such as project context and team dynamics. Continuous learning and adaptation are key aspects of the Agile mindset, and editing stories to make them shorter definitely helps.

The two key approaches to slicing user stories are vertical and horizontal techniques. Let’s explore both methods.

Vertical Slicing

Vertical slicing in Agile development involves implementing end-to-end functionality for a specific feature or user story. This approach ensures that each slice represents a complete, usable portion of the overall functionality. Let’s examine a case study to illustrate vertical slicing.

Case Study: E-commerce Checkout System sliced vertically

Consider an e-commerce platform aiming to improve its checkout process. Using vertical slicing, the development team might start by implementing the essential elements of the checkout, such as selecting items, entering shipping details, and processing payments. Each increment provides a fully functional part of the checkout process, allowing for continuous testing and user feedback.

It’s true that the complete process from start to finish may not go live without all its pieces implemented, but by slicing the feature in this vertical way, the team is allowed to develop parts of the process simultaneously and test their pieces assembled together to prepare the complete process for deployment.

Horizontal Slicing

Horizontal story slicing involves breaking down a complex problem into technical layers, aligning with the specialized skills of different team members. This approach results in separate responsibilities for a database team, a UI team, and a server team, each focused on their specific tasks. These slices are framed as user stories but are unable to deliver standalone value to end customers without collaboration and integration across various layers, components, or building blocks of the software system. It’s akin to horizontally slicing a cake, where each layer alone does not offer the complete and satisfying experience users seek in a single bite. This method emphasizes specialization within teams but requires seamless interaction and coordination for a cohesive and valuable end product.

Case Study: E-commerce Checkout System sliced horizontally

In our previous example of an e-commerce checkout process, when sliced horizontally, specialized developers are assigned to UI, API, and database enhancements separately. Incremental improvements are delivered at each layer. These layers may go live independently or, just like in the vertical model, be implemented simultaneously and then gathered together under another parent story, tested as a whole, and launched altogether as a new or improved process.

Vertizontal Slicing

In many cases, a combination of vertical and horizontal slicing proves effective as well. The most important part here is a carefully thought-through and well-documented slicing strategy, clear communication, and a well-crafted testing plan.

Is Slicing Cool in Dev World?

While the majority of opinions on story slicing are positive, and the benefits of slicing are clearly recognized by the Agile community as a valuable practice for delivering value incrementally and maintaining agility in the face of changing project needs, it’s essential to note that the effectiveness of story slicing can vary depending on the specific context, project requirements, and team dynamics. It appears that, overall, teams prefer vertical slicing to horizontal.

Both ways may pose challenges depending on the functionality and its complexity. These challenges may relate to determining the optimal size of slices, maintaining cohesion across increments, and managing dependencies. Understanding these challenges is crucial for Agile teams to effectively implement user stories and deliver incremental value. Let’s explore the main challenges associated with both horizontal and vertical story slicing.

Horizontal Slicing Challenges

1. Shallow Functionality: Horizontal slicing may lead to the delivery of functionality that is broad but shallow. Ensuring that each slice provides meaningful value to the end user while not compromising on depth can be challenging.
2. Cross-Cutting Concerns: When horizontally slicing across features or components, teams may face challenges related to cross-cutting concerns such as security, performance, or usability. Coordinating these concerns across different slices can be complex.
3. User Satisfaction: Users might find it challenging to derive complete value from the system if each slice only addresses a small aspect. Ensuring that the user experience is coherent and satisfactory despite delivering smaller increments is an ongoing challenge.
4. Dependency Management: Managing dependencies between horizontally sliced features can be intricate. Teams need to coordinate the delivery of different slices to ensure they integrate seamlessly and function as intended.
5. Testing Complexity: Many times, when new horizontal slices are launched, they require updates to already live, related slices, resulting in the necessity of their rework and retesting. Even small updates can cause duplicate effort, and extra regression testing may be required as well.

Vertical Slicing Challenges

1. System Integration: Vertical slices often require integration across the entire system, which can be challenging if different parts are developed independently. Ensuring that all slices integrate smoothly and function together cohesively is crucial.
2. Initial Complexity: In some cases, the initial vertical slices might involve implementing foundational aspects of the system, leading to a perception of high complexity without immediately visible user value.
3. Parallel Development: Vertical slicing may lead to parallel development efforts on different parts of the system. Coordinating and synchronizing these parallel efforts to ensure a cohesive outcome can be challenging.
4. Testing Complexity: Comprehensive testing of vertical slices, especially when they span the entire system, can be more complex than testing isolated components. Ensuring thorough testing while maintaining development speed is a balancing act.

Common Challenges

1. Communication and Collaboration: Both horizontal and vertical slicing may require effective communication and collaboration within cross-functional teams. Miscommunication can lead to misaligned slices or confusion regarding priorities.
2. Scope Definition: Determining the appropriate scope for each slice is challenging. Slices should be small enough to provide incremental value but large enough to be independently usable and meaningful.
3. Consistency: Maintaining consistency across slices is vital for a coherent and user-friendly system. Consistency in design, functionality, and user experience can be challenging when slices are developed independently.
4. Estimation: Estimating the effort required for each slice can be challenging, especially when dealing with uncertainty or complexity in certain aspects of the development process.
5. Balancing Detail and Overhead: Some experienced developers may find it challenging to strike the right balance between detailed user stories and avoiding unnecessary overhead in the slicing process.
6. Dependency Management: In complex projects, managing dependencies between slices can be a challenge, requiring careful coordination to maintain a cohesive and functional system.
7. Transitioning from Traditional Approaches: Developers with extensive experience in traditional development methodologies might initially find the shift to Agile practices, including story slicing, to be a significant change.

While horizontal and vertical slicing offer distinct benefits, Agile teams must be mindful of these challenges to successfully implement effective story slicing strategies. Overcoming these challenges often involves continuous communication, collaboration, and a keen understanding of the project’s context and goals.

Choosing between vertical and horizontal slicing depends on project requirements, priorities, and the desired user experience. Vertical slicing ensures that each increment delivers a usable or at least locally fully testable piece of functionality, while horizontal slicing provides a way to split types of development among programmers with specific skills, narrowing down their focus and simplifying testing. Often, a blend of both techniques optimizes development efficiency and user satisfaction.