A teddy bear, drawn in chalk and dressed in a t-shirt and jeans.
I always just ask my bear if it'll work. That's not the exercise, though.

I have an exercise you can do to tell if your Big Rewrite software project will work out. It’s a simple one, but a good one. But first, a story.

Back in 2013 I worked for a company called OnLive. They brought me on as part of a project called “Valhalla” to rewrite a big chunk of their existing system in Ruby. The idea was that the old tech-debt-laden system would be rewritten into clean, modern Ruby without carrying forward all the existing junk that had accumulated over time.

They were using some uncommon combinations of technologies that I knew well. My own personal plan was to be a high-skill expert in unusual technology, save the team some effort and missteps, and thus be hired on for a lot of money (that part worked.)

You’ve heard Big Rewrite stories before: Chad Fowler tells you to be really careful about it, while Joel Spolsky outright tells you to never do it.

The project failed. I’m sure you’re shocked. With a little work we got it to fail quickly and cheaply, with a minimum of disruption. As big rewrites go, that is a happy ending.

Alex was a gangling red-haired Russian who worked on Valhalla as well. He was an entertaining engineer who would do anything if it seemed fun. His grasp of boring business value and boring tech was shaky. A few months after Valhalla had failed, we were eating OnLive-provided stir-fry lunch and I was fidgeting with a notebook. We were talking about some smaller improvements to the old system. Not surprisingly, Alex was entirely in favour of just rewriting everything again (but “doing it right.”)

This wasn’t as popular with me, a couple of the other Valhalla engineers. Don, the quiet Director of Engineering was watching, unmoving, as he often did. Don had been an engineer long enough to know the right answer, then a manager long enough to let us figure it out on our own.

Alex said, “look, we want this done. Instead of doing in little pieces, the fastest way is direct.” He gulped his black coffee.

I said, “first off, you’re right. That’s absolutely true.” The unspoken “and more fun” hung in the air.

“So, look,” I said. “We know how many people we need for a skeleton crew on the old product. We just did that for Valhalla.” Alex nodded. “Say we scope this new thing at eight months of work.” He nodded. Eight months was definitely too short — and exactly how long Alex thought it would take.

“Now say this thing runs over a little bit.” Somehow these projects always get just slightly too little time, unless they get far too little time.

“Something comes up. We lose a few engineers to firefight on the old product.” Alex fidgeted with his coffee. This had happened, and then happened again. It was how Valhalla died, in fact, leaving us with egg on our face. “And now it runs over a little more.”

“Nobody wants to join — or rejoin — a project that looks like it’s dying.” Don raised an eyebrow. Alex looked away and took a big bite of stir-fry to avoid answering. This, too, had been Valhalla’s fate.

Let’s Talk About Your Good Reasons

We’ll get back to Alex and Don. But first, let’s say you’re considering a “Big Rewrite”-style project.

I think you’re doing it for good reasons.

A stick figure types at a keyboard as the sun goes up (or down) in the window behind.
Sun rise, sun set, the app’s not any good yet…

For instance, I’ll bet the old codebase isn’t well-divided into modules. I’ll bet you have two or three subsystems that need separating off because they hardly ever change and you don’t want to deploy them constantly. You have two modules that are so horribly entwined that nearly everything about them feels like it should be somewhere else. You have a giant “misc” model (the User model, right?) and then also a giant “misc” library. A huge number of methods are full of weird little exceptions and annoyances. The testing is dubious-quality and incomplete.

These are all real problems.

You’d like to cut through all the noise and see the better design that’s hiding in your system. So: if you have good reasons, and you have a good goal, that means that’s the right answer.


Maybe not. Let’s talk about that exercise.

Step One is… Step One

If you were going to start a big rewrite, you’d want to sit down and do some design, yes? Even if you’re so big on Agile that you normally prefer emergent design, in this case I’d highly recommend it. This is a case where you already have a lot of domain experts (your current team) who are dissatisfied with the current design.

Also: you know this is a hard design problem because you messed it up last time.

Sit down and start figuring out what a good design would look like. I’d recommend you involve the whole team, everybody who would be doing the rewrite. You can draw the diagrams first if you like, but the team should look over them and give feedback.

If this takes a few days, that’s actually a few days well spent. Seriously, do this.

After all, you want to see that better design hidden inside your codebase.

What’s the Exercise?

A chalk figure sprawled on the floor reading books.
But which part tells me if it’ll work?

Most of the magic “tell you if your rewrite will work” exercise was that design specification. Let’s say you’ve finished your diagram, including getting feedback from your fellow rewriters.

Now you’re going to look at it and start thinking about all the ways your current system doesn’t measure up, module by module. I’d recommend writing them down as well. If there’s not a perfect module-to-module correspondence (and there shouldn’t be) then just group them roughly. That’s okay. The important thing is to find out where the current system doesn’t measure up, and what each corresponding part of the new system would look like.

This step should be quicker if your new design is good. You should understand it well enough to see where your current version is wrong. If you can’t then you don’t know your new version well enough — add more detail to it and try again.

Back to Alex

A pear-cut purple gemstone stands, with its point upwards, on the ground.
Your design in its gem-like beauty.
Photo credit: Conscious Design on Unsplash

“So if the project starts to stall,” I said, “then any code we wrote, we lose. It’s not ready to deploy. It’s not done. And who would want to come back to the wreckage of a dead project later?”

“That’s not a project to get people a good reputation!” burst out Alex. He was still sensitive about Valhalla. Fair.

“True. So: let’s think about a different way. What if we start from the old code, and build in pieces?” Alex frowned. He didn’t much like the old code either.

I tried to sweeten my response a bit: “you’re right that it’s harder. It’s more steps, and more work to get done. So let’s say in seven or eight months we haven’t gotten as far — maybe only three quarters of the way to where we wanted.”

Alex slumped, staring into his cold coffee. Who wants three quarters of a success?

“Ah,” I said, “but look. It’s like climbing a cliff, but going from ledge to ledge and stopping in between. If we get three quarters of the way through a Big Rewrite then we fail, we go home and all that effort was wasted. But if we build incrementally we get to keep all that progress.”

Alex looked resigned. This wasn’t the fun way he wanted. But Don was smiling, just a bit. Don always played his cards close to the vest, but he liked that explanation.

What to Do With That Exercise

I said the exercise was putting together your design specification. The title of this post says the exercise will tell you if your rewrite will work or fail.

But how does that work?

First, let me tell you a few things. You, right now in this very moment, haven’t actually put together that design specification yet. That’s because you’re just reading this blog post.

But once you do put together that design, magic is going to happen.

Look at that rough correspondence, that “diff,” between your existing components and your new components. I have another prediction: those new modules are going to look, very roughly, like your current ones. You and/or your team have a lot of experience with your existing structure and you’re going to produce a new structure that’s approximately the same shape.


I mean, it would be a shame if you had done all that work and hadn’t learned anything useful, right?

But Will the Rewrite Work?

There are very few times a rewrite actually makes business sense. Again, read Chad Fowler or Joel Spolsky for details about why.

So here are a few times that you should rewrite.

One: you simply can’t bear the old code any more. You’re going to throw it away no matter what the test says. A business won’t usually do this, but you may not be a business. Or you may have sole authority to choose.

Two: everything is so bad that even for an external team it would be faster and easier to build from scratch. This is almost never true. But if you’re inheriting a sloppy prototype from outside contractors, or if an abandoned codebase is very bad and you have nobody that worked on the original, you may already be operating from nearly nothing. If old code stopped working long ago, you don’t know for sure how it worked or even if it worked. In those cases, sure, you rewrite.

But what about a codebase for a working business, where many of the people who wrote it still work there? How about then?

First, take that diff you made between the old design and the new one. Each chunk of the diff is one smaller project you could divide the Big Rewrite into. You could alter or rewrite just a few of modules from your codebase at once and you’d get just one part of the software working much better in a shorter period of time.

You could plan on a “divided-up rewrite,” basically, where you replace one part of the code at once. For an example of treating this as the plan for your architecture I like Chad Fowler’s RubyConf keynote about ‘Legacy’ in software.

If you consider your diff component-by-component, a 100% Big Rewrite almost never makes sense. Five smaller partial-rewrites is better than one huge one. Ten smaller rewrites is better yet. If you have pieces you can separate then you can prioritise and rewrite the worst parts first. And any progress you make, you get to keep.

This is the same general idea as sprints in Agile, dividing up large tasks into smaller tasks. A rewrite, like any other large task, is much more predictable and manageable if you divide it up into smaller pieces.

But What if There’s Nothing Similar in the Diff?

A person sleeps with an ignored fishing pole at their feet and a hat over their face.
Eh, design will take care of itself.

There’s an easy way to foil my advice here if you want to. You can intentionally make the new system share nothing with the old system’s structure. It’s hard to do that by accident, but you could do it on purpose.

If the entire structure is totally different, you’ve thrown away nearly everything you learned from the old system.

Which means your design is going to be as bad as a new team starting from nothing. Because that’s what you are.

If you can’t come up with a good design for the new system, rewriting won’t fix your problems. You’ll just get a new bad system instead of the old one.

You’ll write the new system and rapidly discover that your code still isn’t divided into good components. When you try to add new ones, it will still be a mess. When you try to change a feature you’ll have to modify it in five or ten different places, just like last time.

Until you can see what the new architecture should be, you’re wandering in the dark. And once you can see what it should be, you can write a plan to get there in steps. If you can’t, that’s a very bad sign.

And so a rewrite is normally a way to be lazy — to not work out the new architecture and just hope it shows up already perfect. How well did that work for you the first time?

PS: Microservices

It used to be more popular to do pointless rewrites. These days, we understand that it’s a bad idea.

It’s now more popular to rewrite your monolith into microservices. Soon, we will understand that it’s a bad idea.

It’s not that microservices are inherently bad. It’s not that rewriting is inherently bad. It’s that if something looks like a lazy way to not think through your design, there will be developers that use it that way.

One way to remove that temptation is to require you to think through your design first, before you decide the right way to get there.