This is an original planning guide. It is not a vendor manual, an official standard, or legal advice. Example addresses use documentation ranges. Review every command before you paste it.
What this hour is for
The planner writes three steps for a change such as a NOT NULL column: add it nullable and deploy writers that fill it, backfill in batches, then set NOT NULL and remove the old path. Avoid one ALTER that rewrites a large table during peak traffic. The command builder writes gh-ost and pt-online-schema-change with execute left false and a dry-run flag.
Do not remove the dry-run flag during the reading. Do not run either command. Read the chunk size before any later window.
192.0.2.10, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32 are documentation. example.com is a documentation name. A sample password is replaced. It is not a password you keep.
The change you can finish
Apply nothing while you are writing the note. When the lab window opens, it is one change on the database named in the ticket. Read the draft, then the database, then decide.
Rollback is the backup, the file, or the snapshot you saved first. You do not invent a rollback after the only copy is gone.
- Name the change.
- Read expand, backfill, contract.
- Leave the commands as dry runs.
- Do not run them.
- Stop.
Mistakes that outlive the statement
Zero-Downtime Schema Changes goes wrong in a small set of ways. The list is here so the ticket can forbid them before the window.
- A dry-run flag removed during the reading.
- A large ALTER started at peak.
- A command run from the page.
Decisions to write down
One decision for Zero-Downtime Schema Changes: The three steps are written. The commands are still dry runs. Nothing was run. And the table is yours.
Checks before you call it done
Confirm this before you call the change done: The change is written. The three steps were read. Dry-run is still set. Nothing was run. And the table is yours. If one of those fails, stop.
# Expand, backfill, contract.
# gh-ost --execute=false and pt-online-schema-change --dry-run.
# Do not run them during the reading.Before you leave the database
Zero-Downtime Schema Changes ends when the database in the prompt is the database in the ticket. If it is not, you are on the wrong database. Stop.
The draft stays a draft until a person has read the names in it. A generator does not log into the database. You delete a line that names a host, a database, or an account you did not write down.
192.0.2.10, 198.51.100.0/24, 203.0.113.0/24, and 2001:db8::/32 are documentation. example.com is a documentation name. They belong in the note. They are not values you leave on a database because a draft included them.
prompt-for-this is a reminder that a secret is missing. It is not a password, a key, or a token you save in the ticket or in git. A sample password in a draft is replaced. It is not a password you keep.
One change is the window. If two things move and the result is wrong, you will not know which one did it.
If the same step fails twice, stop for the day. Restore the copy you took first.
A second person reads the database name and the change aloud. If they cannot find them in the ticket, the note is not done.
This page is a planning guide. It is not a vendor manual, not an official standard, and not permission to touch a database you do not administer. You do not disable authentication, a firewall, or TLS to make a check pass.
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