Pattern of the Month: Single Piece Flow

The genius of Lean and Agile practice is to translate end-user demand into an even and consistent flow across the entire development and delivery process. In Lean and Agile terminology this concept is known as pull.

As you'd expect, pull ultimately starts with consumer demand for a product or service; however, to enable smooth flow, at each station where work is done, the number of items that can be handled at any one time must be subject to a Work In Progress (WIP) limit. Anything below the WIP limit implies a potential for accommodating more work. It is this "pull signal" which draws work on from the previous station in the value chain.

The theoretical WIP limit for achieving optimum pull is exactly one. This is known as single piece flow (SPF) and it has the clear advantage of reducing lead time, depreciation of stock-on-hand, and the cost of delay on each item to the absolute minimum. SPF requires cross-functional team members, all of whom can swarm on a single work item to progress it.

In fact, in such cases, it can be argued that a WIP limit greater than one must mean a push system. SPF can be very difficult to achieve and yet the potential rewards are indisputable. With only one item on hand at any one time, there will be no opportunity for work to pool in the team's engineering process and very little chance for technical debt and waste to accumulate. The delivery of value can be maximized.

Single piece flow implies that any impediments must be resolved on the spot since it would be unacceptable to bring other work forward and progress it instead. This means that as soon as an impediment is noticed a stop-the-line emergency should be declared and the entire team must swarm to resolve it. No work can be "pushed" or otherwise brought forward since that would break the WIP limit.

Single Piece Flow

Intent

Minimize stock-on-hand to one item to deliver value more quickly and reduce waste

Proverbs

• It’s best to do one thing at a time

Also Known As

• One-Piece Flow

Motivation

In theory, the most efficient way to work is one item at a time. This minimizes the costs associated with maintaining inventory (stock on hand) and also brings value to the market more quickly, thereby allowing a return on investment to be leveraged sooner.

Structure

A development team accepts and actions only one backlog item at a time. The Work In Progress is limited to one.

Applicability

Single Piece Flow should be considered in situations where the value stream needs to be optimized as far as possible. Note that for Single-Piece Flow to be viable, it must be possible for multiple developers to work on one item simultaneously and without causing each other impediment.

Consequences

Single Piece Flow, when applied in the right situation, can result in a faster time to market, an earlier return on investment, reduced inventory costs, and rapid feedback cycles. However, it can also lead to waste if developers are allowed to get in each other's way.

Implementation

Single piece flow can be implemented for Lean Kanban teams of small size, where developers will not cause each other impediment by working on the same item. It is rarely implemented in Scrum development teams since they are expected to formulate their plan for the delivery of a Sprint Backlog, the release of value being deferred until the end of a Sprint and not before. 

Further Reading

Anti-Pattern of the Month: WaterScrumFall