The Strangler is an extremely effective technique for phasing out legacy systems over time. Instead of spending months getting a new system up to parity with the current system so that clients can use it, you place The Strangler between the original system and the clients.

The Strangler passes any request that the new system can’t handle on to the legacy system. Over time, the new system handles more and more, the legacy system does less and less. Most importantly, your clients won’t have to do any migration work and won’t notice a thing as the legacy system fades away.

A common objection to setting up a Strangler is that it Yet Another Thing that your overloaded team needs to build. Write a request proxy on top of rewriting the original requests! Who has time?

Except, AWS customers already have a fully featured Strangler on their shelf. The Elastic Load Balancer (ELB) is a tool that takes incoming requests and forwards them on to another server.

The only requirement is that your clients access your application through a DNS name.

With an afternoon’s worth of effort you can set up a Strangler for your legacy application.

You no longer need to get the new system up to feature parity for clients to start using it! Instead, new features get routed to the new server, while old ones stay with the legacy system. When you do have time or a business reason to replace an existing feature the release is nothing more than a config change.

Getting a new system up to parity with the legacy system is a long process with little business value. The Strangler lets your new system leverage the legacy system, and you don’t even have to let your clients know about the migration. The Strangler is your Best Alternative to a Total Rewrite!

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