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copied!<p>The biggest problem to overcome is lack of will to actually finish the strangling (usually political will from non-technical stakeholders, manifested as lack of budget). If you don't completely kill off the old system, you'll end up in a worse mess because your system now has two ways of doing everything with an awkward interface between the two. Later, another wave of developers will probably decide to strangle what's there, writing yet another strangler application, and again a lack of will might leave the system in an even worse state, with three ways of doing things.</p> <p>If the project is large, run from multiple regions, then you HAVE to get global consensus on what the final state should look like and how everyone is going to cooperate to get there. While the old app is being strangled, it's vital for remote teams to communicate every day, cooperate on the work if possible by doing remote pair programming, and resolve any misunderstandings or disagreements as soon as they arise. Otherwise there's the danger that each regional team will decide to write their own strangler application and they will meet somewhere in the middle and battle it out, leaving the system even worse. </p> <p>Whatever you do, do not do the refactoring/strangling in a different branch from the main stream of development. The merge difficulties will become insurmountable.</p> <p>I've seen critical systems that have suffered both of these fates, and ended up with about four or five "strategic architectural directions" and "future state architectures". One large multi-site project ended up with eight different new persistence mechanisms in its new architecture. Another ended up with two different database schemas, one for the old way of doing things and another for the new way, neither schema was ever removed from the system and there were also multiple class hierarchies that mapped to one or even both of these schemas.</p> <p>Finally, if you're introducing technologies that are new to the team or to support/maintenance staff (e.g. adding reliable async messaging to what is currently a synchronous three-tier client/server architecture) then you have to ensure that there are experienced technical leads on the project who know how to build systems with that technology, and support those systems. And those tech leads have to stick with the project for some time after the old app has been fully strangled. Otherwise, the architecture will degrade as inexperienced developers modify it in ways they know but not in ways that fit with the new architecture.</p>

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