Using continuous integration represents an (almost) mandatory requirement for a software project to be agile. And without CI there can be no continuous delivery/deployment.
But CI can face scalability challenges, see How can continuous integration scale for very large projects/teams?.
Splitting work on multiple smaller pre-integration branches would allow work on those branches to be agile, but then merging of those pre-integration branches needs to be handled. And those would be massive, unpredictable merges, rather difficult to plan and incorporate into an Agile framework. Until the merge is actually performed one can't really tell if it'll take 2 hours, 2 days or 2 entire sprints to complete.
So how can Agile methodologies still be applied to massive, monolithic software projects with 100s/1000s of developers?
