Bottleneck
The concept comes from lean manufacturing and is central to Eliyahu Goldratt's Theory of Constraints: the throughput of the system as a whole is determined by its weakest constraint, not by the sum of its parts.
Common bottlenecks in agile teams
- Code review: the team produces code quickly, but few people are available to review it. WIP rises, but pull requests pile up unmerged.
- QA and testing: development moves faster than validation capacity. Stories pile up "in testing" at the end of the sprint.
- Product Owner approval: the team finishes work but cannot consider it "done" without approval from a single person who has many other responsibilities.
- External dependencies: the team's work depends on an API, service, or decision from another team or department that operates at a different cadence.
How to identify them
The kanban board makes bottlenecks visible: when a column piles up cards while the downstream columns are empty, there is a bottleneck at that stage. A high cycle time at a specific stage is also a sign.
WIP limits are the response mechanism: when a column reaches its limit, the team stops adding new work and resolves the bottleneck before continuing.
Common mistake
Optimizing stages that are not the bottleneck. If the bottleneck is QA, making the development team work faster does not solve the problem: it only increases the backlog in QA. Goldratt is explicit: any improvement made anywhere other than at the bottleneck is an illusion of improvement. Optimization work must always begin by identifying and resolving the current constraint.
See also
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