We have very little logging (at any level) in our upgrade process which makes debugging upgrade issues in production quite difficult. At a minimum we should have logs for the major steps of the process, with a log when each step starts, completes successfully, or fails:

• Pre-upgrade cleanup
• Calculation of source URI for binary download
• Download of artifact and hash and signature files (this is partially logged right now)
• Hash and signature verification of the artifact
• Unpacking of the artifact
  • Location of the artifact on disk and destination of unpack
  • We should add debug-level logs for each file that gets extracted including the path from the archive and the destination path on the OS
  • Final hash that is calculated
• Copying of the action store
• Change of the sym link
• Bookmarking of when the upgrade completed
• Invoking of the upgrade watcher
• Removal of the old downloads directory

Most of this code is contained in this function: https://github.com/elastic/elastic-agent/blob/b27a90780fdd0566c0575efe5bccc98289513b7f/internal/pkg/agent/application/upgrade/upgrade.go#110