Code ownership is the concept of who is responsible for the oversight, maintenance, and updates to a particular block of code. Some places have a collective code ownership policy, others require specific people to sign off on changes to a given section of code that they are the expert for.
Code ownership deals with how strong the bond between a developer and a given part of code is. This can range from strong, to week, to collective.
In a strong ownership model, specific developers are responsible for all the changes to a given module and only they are allowed to make changes to that model. Someone else desires changes, they must talk to the owner of that code and have them make the code (or send them a patch to incorporate).
In the weak model, ownership is still assigned to a given developer, but the developer is expected to keep an eye on changes and do code reviews of code in it rather than be responsible for all changes.
The collective model of code ownership is that there is no individual who owns the changes over oversight to any given model and anyone can make changes anywhere. This model is often found in agile shops and part of the extreme programming principles (see Extreme Programming Code Ownership).
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