Flipt aims to provide an upgrade process which requires zero downtime with minimal steps. Currently, Flipt supports four relational database backends and uses a migration process to manage their schema. Each new release of Flipt might include updates to database migrations. The release notes on GitHub for each Flipt release should signify whether it does or not. You can find the releases and release notes on GitHub. Before upgrading your Flipt instances to a new version, you may need to run the flipt migrate command against your database. The Flipt binary is self-contained and has all necessary migrations baked in. You should use the version of Flipt you’re attempting to upgrade to.
If you attempt to upgrade to a new version of Flipt before running migrations, then Flipt will fail fast on startup with a message like the following.
migrations pending, please backup your database and run `flipt migrate`

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Upgrade Process

We strongly recommend that you perform a backup of your Flipt database before doing any migrations. While we strive to ensure that Flipt is error free and production ready, we can’t guarantee perfection.
First, run flipt -v to see the version you have installed. You want this to be the version you’re attempting to upgrade to. 
$ flipt -v

 _____ _ _       _
|  ___| (_)_ __ | |_
| |_  | | | '_ \| __|
|  _| | | | |_) | |_
|_|   |_|_| .__/ \__|
          |_|

Version: 1.20.0
Commit: 5badad98844061a15c05bd6b21accde44ea7fcb5
Build Date: 2023-04-11T15:39:19Z
Go Version: go1.20.3
Once you have identified the version you have installed is the target version you are upgrading to, you can run flipt migrate (In this instance I am upgrading to Flipt v1.20.0). 
$ flipt migrate
2023-04-13T15:35:17Z	DEBUG	using driver	{"driver": "sqlite3"}
2023-04-13T15:35:17Z	DEBUG	migrations complete
Now you can update your running instance(s) of Flipt to match the same version. We design our migrations to be compatible with older Flipt server versions, so it’s safe for you to keep running Flipt while the migrations are run and after they have finished.

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Downgrade Process

As previously mentioned, we recommend you backup your database before performing any upgrades. Flipt doesn’t currently come with any built-in downgrade tooling. Our suggestion, if you need to revert to an previous version of Flipt, is to restore your database using a backup from when it was backing the target older version.