Fixing production bugs on local environment using a proxy

January 31, 2021

Say you have an app that reads json from an API, renders some UI. And you have a specific API endpoint that crashes the UI on production. This was exactly the case couple days ago in our project, there are several ways to fix this, and I chose to use a proxy to fix it on my local environment.

I saved the API response that crashes the UI to a file on my computer. Installed Proxyman (other alternatives as Charles or mitmproxy are good too). Routed my browser traffic through the proxy. Setup a rule in proxy so that when I go to localhost/api/aResourceThatWorksFine it would capture it. Then, instead of asking my local api server, it would return the response I saved. The file contains the response from production/api/theResourceThatCrashes. The term for this kind of rule in Proxyman is Map Local. This way, I was able to investigate the issue with little hassle. Without for example moving the production database to my local environment.

Couple gotchas were that if your server sends a Date header. You need to remove it from the response you saved to the file proxy reads. Since it will be the same and behind every time proxy returns it, browsers don’t like it. Other header you might need to change is Access-Control-Allow-Origin. Make sure it has your localhost uri as value or just set it to *.