Categories
Development

Replacing legacy systems: Proxy to learn and control

TL;DR – Create a custom proxy to make sure you know how the old system behaves and can transition off it in a controlled manner.

A couple of weeks back I held a talk at JavaZone about replacing legacy systems (Norwegian only, sorry). One of the techniques I talked about that seemed to spark an interest was creating a custom proxy to take control and figure out what the old system does.

Not a dedicated proxy like Nginx or Apache, a custom proxy in the language and technology you are comfortable with. It can ease the replacement and transition a lot.

For us that was KTor Server+Client.

What?

When you have a system with a Webapp, Mobileapp and API like this:

You can take control of the system by introducing a proxy like this:

Simple as that. πŸ˜‰ HTTP request in, HTTP request out, receive response and send response back to the source. A bit of fiddling around, but you essentially copy everything forward and back. Headers, Method, response codes and of course body.

The figure above also shows how we used the proxy to double check some data results in the DB before switching out the legacy system (the dotted lines).

Why?

By creating a custom proxy you can monitor, log, analyze and even compare results. All in your preferred development language.

When you gain insights into how your system is working, you can manage which parts controls and receives each call.

Wan’t to switch over based on user-id? That’s a simple conditional statement in your proxy code.

Switch gradually based on endpoints? Even easier, just change which system is the recipient for your call.

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How?

We used KTor Server+Client because it made it easier to know that coroutines were handled correctly but you could also use OKHttp as a client if you wanted.

Be careful with big payloads though, stream the results as a default, and only load the body into memory if you know the size and actually need to analyze or modify (don’t) it.

I don’t have the proxy as an example myself but this doesn’t look like a bad starting point: https://github.com/ranbims/ktor-server-proxy/blob/main/src/main/kotlin/org/ranbi/HttpProxy.kt

No HTTP?

Yeah, that’s harder. But still just streams of bytes back and forth. Even Cobol Copybooks has a format that can be parsed. πŸ™‚

Happy migrating πŸ˜‰

What’s your best techniques for migrating off legacy systems?

Categories
Architecture Development Tech Web

Full stack development with KTor and HTMX ❀️

Sometimes it feels like I have been yak shaving for the last 20 years. Well, not entirely. But things like Rules Engines, SOA, App Servers and debugging Hibernate comes to mind. πŸ˜‰

On the back end, things have gotten better over time. But on the front end it, feels a bit like it is history repeating. While it is impressive what you can do in React. React and all the accompanying tools is too complex for many normal web applications.

So when I discovered HTMX it really tickled my interest to see how I could combine this into an efficient development flow. Back to basics. Hypermedia and HTML. πŸ™‚

And I think I have a pretty decent starting point in the Github repo linked below. πŸ™‚

A full stack environment with Kotlin, KTor and HTMX. Everything loaded, compiled and packaged with Gradle. Compile in IDEA to see the changes full stack.

The following repo contains a running example (just do ./gradlew run to test it) and a semi-detailed Readme: https://github.com/anderssv/kotlin-htmx/blob/main/Readme.md

You can try the “application” here (initial load might be a little slow as it is on free fly.io which suspends inactive processes): https://kotlin-htmx.fly.dev/

This holds real promise, and I hope I get to work with this for real in production soon. πŸ™‚

Thoughts? πŸ™‚

After doing this I discovered that there is a similar example linked directly from the HTMX site. Nice one πŸ™‚ https://github.com/Rattlyy/htmx-ktor/tree/master


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