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Mehwish
Mehwish

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Killing a Silent Performance Bug: Caching Email Templates Instead of Re-Parsing Them on Every Send

#devchallenge #bugsmash

This is a submission for DEV's Summer Bug Smash: Clear the Lineup powered by Sentry.

Project Overview

auth-server is a Go-based authentication service (using the Gin framework) that, among other things, handles sending transactional emails, like email verification and password reset links, to users.

Bug Fix or Performance Improvement

Every time the server sent an email, it read the corresponding HTML template from disk and parsed it from scratch, even though the template content never changes at runtime. This meant unnecessary disk I/O and template-parsing overhead on every single email send, which is wasteful under any real load.

Code

PR: https://github.com/roshankumar0036singh/auth-server/pull/121
Closes Issue: #86

My Improvements

I added a templates cache field to the EmailService struct and moved template parsing to a loadTemplates() call that runs once at startup via NewEmailService. SendEmail now just reads from the in-memory cache instead of hitting disk on every request.

During review, a few important issues came up that pushed the fix further than my first pass:

  • Graceful startup: originally a missing template would crash the server via log.Fatalf. I changed this to log a warning and continue, with ErrTemplateNotFound returned only when that specific template is actually requested.
  • Thread safety: since the cache is shared across concurrent goroutines handling requests, I had to make sure template.Template execution was safe under concurrent access rather than assuming a single cached pointer was automatically safe to reuse everywhere.
  • A tricky merge conflict: a later commit accidentally discarded the template value (t, ok changed to _, ok) while a downstream line still called t.Execute(...), which broke the build. I traced it back through the diff and restored the correct binding.

The result: no repeated disk reads, safer concurrent access, and the server no longer crashes on a missing template file. It degrades gracefully instead.

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