What's a vertical slice? Think of a delicious layered cake. Say we have 3 layers and you slice a piece. A thin slice of each layer produces one solid piece. Think of the cake layers as architectural pieces: database layer, services layer, the user interface layer, etc.. Each of these layers are as important as the last to get to the final goal: a solid, functional piece of software.
In any project, especially a green field project, building in vertical slices is critical to success. A solid vertical slice proves two things:
1.) Proven architecture
2.) Produces a complete, functional, shippable piece of working software that works from end to end.
The risk of not having a Vertical SliceNot having a vertical slice presents an unproven architecture and much unneeded risk. Not having a vertical slice poses the possibility that the entire database is built with no user interface or service layer, or the opposite, a great looking UI and service layer but the data model is a mess and not scalable. Either way, much rework is involved for further scalability. Having a proven architecture by developing in vertical slices allows you the ability to continue working and not end up with a fantastic UI but a non-scalable database model.
Don't forget ETL (Extract Transform Load)When developing a vertical slice, consider the notion of having to importing existing data. Does this change the implementation? Can the data model be simplified or restructured to suite an ETL more effectively? While I'm not saying to develop an application for the sole purpose of an ETL, the ETL needs to be considered in the vertical slice.
But individuals know one layer...WRONG! One characteristic of a good agile team are generalized specialists. Individuals who may specialize in one layer or area but also able to build up the layers. Ideally, team members' skills should complement each other's in order to create an effective synergy. Have team members pair often to help facility the synergy and allow team members to grow from one another. This type of cross functional team mentality allows vertical slices to be built effectively. Thus coming to value added software quicker.
Identifying the vertical slice and developing a story via a vertical slice rather than by architectural layers proves the architecture and produces a piece of quality, shippable software faster.