Tuesday, December 10, 2019

Historical - Make User Delayed

This post was shared on code.google.com many years ago but code.google has been shutdown. It describes work done by my team at Google. I am interested in the history of technology and with some spare time have been enable to republish it.

I added support to rate limit DBMS accounts that were too busy. It wasn't successful in production for the obvious reason that it just shifts the convoy from the database to the app server -- the problem still exists. The better solution is to fix the application or improve DBMS capacity but that takes time.

This describes SQL commands added to rate limit queries per account and per client IP.

Per account rate limiting

Per-account query delays use new SQL commands to set a query delay for an account. The delay is the number of milliseconds to sleep before running a SQL statement for the account. These values are transient and all reset to zero delay on server restart. The values are set by the command MAKE USER 'user' DELAYED 100 where the literals user and 100 are the account and number of milliseconds to sleep. There is no delay when the value is 0. The values are displayed by the command SHOW DELAYED USER.

MySQL had a feature to limit the number of queries per hour for an account. This is done by setting the _user.max_questions_ column for the account. We have changed this to be the max queries per minute so that when an account reaches the limit, it doesn't have to wait for an hour for the reset.

These don't change the behavior for existing connections. There must be a reconnect to get the new values.

Per client IP rate limiting

Per-client rate limiting is done by the command MAKE CLIENT 'IP-address' DELAYED 100 where the literal IP-address is the exact match for the client IP that should be delayed and 100 is the number of milliseconds to delay each statement. The delays are displayed by the command SHOW DELAYED CLIENT.

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