[cljacoby]

I also used RDS Blue/Green deployment to apply a MySQL major engine version upgrade from 5.7 to 8.0. With respect to downtime it worked fantastically, I think we measured 13 seconds of observable downtime from the API.

However we did learn the hard way that RDS Blue/Green cannot be used to apply arbitrary changes. In our case, we discovered RDS Blue/Green can only be used to move up engine versions, not down.

We discovered on MySQL 8.0 one of our stored procedures had very occasional failures, and considered the option of using RDS Blue/Green again to move back down to 5.7. Turns out that's not an option.

[0xbadcafebee]

Database changes are typically one-way. If your new change includes creating or modifying a table, such that there are new additional columns, and you populate those with data, then downgrading would destroy the changed columns and the data in them. Hence you can't downgrade once you upgrade or you'd potentially be breaking things. To downgrade safely you'd need to backup or snapshot the old database, and then restore your database back to the backup/snapshot, but that's not blue/green.

[fulafel]

DB schema migration script frameworks (at least in Python, Ruby & Java lands) do typically support both upgrade and downgrade directions. People skip implementing and testing the downgrade side if the development model doesn't need it but the problem of what happens to the data is controlled by what you put in the "down" migration script.

I'd guess if you can't throw the data away, you won't do a down migration, you'll do an up migration that changes the db to save that data in your preferred way before undoing or reworking the previous schema change.

[Mavvie]

I wonder if that could be because MySQL 8's replication is backwards compatible but MySQL 5.7's isn't forwards compatible. If so, it makes sense that you're only able to move forward.