事先申明，此篇文章只是自己看到的各式文章中，擷取個人覺得不錯的部分。
Log after, not before
在執行後才 log，可以表明之前的操作是成功的。如果 REST 失敗，將不會看到此日誌，而是會出現異常
// don't do that
log.info("Making request to REST API")
restClient.makeRequest()
// do that
restClient.makeRequest()
log.info("Made request to REST API")
Separate parameters and messages
分離錯誤訊息和參數，不要將參數內容放到錯誤訊息之中。
// don't do that
restClient.makeRequest()
log.info("Made request to {} on REST API.", url)
// do that
restClient.makeRequest()
log.info("Made request to REST API. [url={}]", url)
Distinguish between WARNING and ERROR
Log 機制通常都會有明確的 level，分別是 ERROR 和 WARNING。但是通常程式上常常會混用。覺得的確是滿需要去注意的部分。
如果今天操作是可以執行，但是會有一些 issues 那就是 - WARNING
今天操作是不能執行的，則是很明確的是 - ERROR
try {
restClient.makeRequest()
log.info("Made request to REST API. [url={}]", url)
} catch(e: UnauthorizedException) {
log.warn("Request to REST API was rejected because user is unauthorized. [url={}, result={}]", url, result)
} catch(e: Exception) {
log.error("Request to REST API failed. [url={}, exception={}]", url, exception)
}
INFO is for business, DEBUG for technology
INFO 的內容應該要像是一本書，告訴你發生了什麼，而不需要如何發生。所以建議 INFO 應該是要記錄商業相關的 log，DEBUG 才比較是技術方面的資訊。
INFO | User registered for newsletter. [user="Thomas", email="thomas@tuhrig.de"]
INFO | Newsletter send to user. [user="Thomas"]
INFO | User unsubscribed from newsletter. [user="Thomas", email="thomas@tuhrig.de"]
DEBUG | Saved user to newsletter list. [user="Thomas", email="thomas@tuhrig.de"]
DEBUG | Send welcome mail. [user="Thomas", email="thomas@tuhrig.de"]
INFO | User registered for newsletter. [user="Thomas", email="thomas@tuhrig.de"]
DEBUG | Started cron job to send newsletter of the day. [subscribers=24332]
INFO | Newsletter send to user. [user="Thomas"]
INFO | User unsubscribed from newsletter. [user="Thomas", email="thomas@tuhrig.de"]