关于Apache表达式中file函数的性能及工作机制咨询

Hey there! I’ve spent some time working with Apache’s file expression function and can share what I’ve learned from hands-on experience and digging into Apache’s internal behavior:

• Does it only load on start-up?
  Nope, it doesn’t preload files during Apache’s startup. The file function is dynamically evaluated based on request context (like request paths or variables), so Apache can’t know which files to load until a matching request comes in.

• Does it load on first access?
  Exactly. The first time a request triggers the configuration section containing the file function, it will read the target file’s content for the first time.

• Does it always re-load if section is (re-)entered?
  This depends on your configuration context. For sections like Location or Directory that get matched on multiple requests, the file function is re-evaluated each time—but this doesn’t always mean re-reading the file from disk (thanks to caching, which I’ll cover next).

• Does it cache the contents?
  Yes, Apache does cache the content fetched by the file function. The cache is tied to the file’s metadata, specifically its modification timestamp.

• Does it re-read on changes only?
  That’s right. Apache checks the file’s modification time each time the file function is evaluated. If the file has been modified since the last cache was stored, it will re-read the updated content; if there’s no change, it uses the cached version instead of hitting the disk again.

I totally feel your pain about the lack of detailed documentation here—Apache’s official docs often skip over these nuanced performance details. If you want to confirm this for your specific setup, you can enable debug logging for mod_expressions and monitor when file read operations are triggered.

备注：内容来源于stack exchange，提问作者luzze