Minimising interruption.
WHAT CAUSES DOWNTIME
Files being replaced while requests arrive Database changes incompatible with running code Caches rebuilding Workers restarting
WHAT REDUCES IT
Preparing the new version completely, then switching to it Keeping the previous version available
HOW THAT WORKS
Deploy to a new directory Prepare it fully Switch a link to point at it Keep previous versions for reverting
WHY THAT HELPS
The switch is instantaneous, and reverting is another switch.
WHAT TO BE CAREFUL WITH
Shared directories: uploads, logs, configuration.
Those must persist across versions rather than being replaced.
WHAT TO LINK RATHER THAN COPY
Uploaded files Environment configuration
WHAT REQUIRES MORE CARE
Database changes.
WHY
The old and new code may both run briefly.
WHAT TO DO ABOUT IT
Make schema changes compatible with both versions.
Add columns before using them. Remove them in a later release.
WHAT TO ACCEPT
For a small application, brief maintenance downtime is frequently acceptable.
