NestJS Guards: 10 Ways I Stopped Them From Crashing My App (Again)

Look, we all use guards in NestJS for auth. It's kinda the point, right? But I screwed up. Big time. I treated them like simple on/off switches. I thought, 'Slap a guard on every route! Security!' Then the AWS t3.medium instances started checking out. One by one. The postgres connection pool hit 500 and choked. Node.js v14.x handled this poorly until we upgraded. Turns out, badly designed guards are like adding concrete shoes to your API.

My first mistake? I was waiting too long to validate anything. The request would hit the controller, then the service, *then* the guard would finally kick in. So, all that processing for nothing if the API key was garbage. The solution? Pre-validation guards. I shoved them right at the entry point to check for basic stuff *before* anything else happened. Saved my bacon, it did.

I wish I could say it was that easy, but it wasn't. Rate limiting? I put it in a guard. Caching? Yep, guard. Feature flags? Guard again. Each one a potential landmine. I'll spare you the gory details of each specific failure, but here's the short version. Always remember: 1. **Short-circuit fast:** Return `false` ASAP if validation fails. 2. **Stateless is your friend:** Avoid session reads. 3. **Context is KING:** Use ExecutionContext to its fullest. Don't redo work. 4. **Memoize, memoize, memoize:** Cache expensive lookups within the request lifecycle. 5. **Async/Await carefully:** Blocking can kill performance. 6. **Avoid N+1 problems**: Batch queries if possible. 7. **Use a dedicated auth service**: Don't bake logic into every guard. 8. **Test, test, test:** I didn't, and it bit me. Hard. 9. **Keep guards small**: They do ONE thing. NOT EVERYTHING.

Guards are powerful, but they're also dangerous. I learned that the hard way. They're not a silver bullet, and they *will* screw you over if you're not careful. Just remember: profile, optimize, and for God's sake, test your guards. Otherwise, you'll be staring at logs at 3 AM, wondering why your app is slower than molasses in January. Trust me, I've been there.