Michael Sproul 60449849e2 Document database migrations (#3203 )

## Proposed Changes

Add documentation for the `lighthouse db migate` command, which users will be able to use to downgrade from Lighthouse v2.3.0 on non-merge networks (mainnet & Prater).

I think it's important to get this into the live instance of the book so we can link to it from the v2.3.0 release notes.

2022-05-23 03:52:32 +00:00

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Database Migrations

Lighthouse uses a versioned database schema to allow its database design to evolve over time.

Since beacon chain genesis in December 2020 there have been several database upgrades that have been applied automatically and in a backwards compatible way.

However, backwards compatibility does not imply the ability to downgrade to a prior version of Lighthouse after upgrading. To facilitate smooth downgrades, Lighthouse v2.3.0 and above includes a command for applying database downgrades.

Everything on this page applies to the Lighthouse beacon node, not to the validator client or the slasher.

How to apply a database downgrade

To apply a downgrade you need to use the lighthouse db migrate command with the correct parameters.

Make sure you have a copy of the latest version of Lighthouse. This will be the version that knows about the latest schema change, and has the ability to revert it.
Work out the schema version you would like to downgrade to by checking the Lighthouse release notes. E.g. if you want to downgrade from v2.3.0, which upgraded the version from v8 to v9, then you'll want to downgrade to v8 in order to run v2.2.x or earlier.
Ensure that downgrading is feasible. Not all schema upgrades can be reverted, and some of them are time-sensitive. The release notes will state whether a downgrade is available and whether any caveats apply to it.
Work out the parameters for Running lighthouse db correctly, including your Lighthouse user, your datadir and your network flag.
After stopping the beacon node, run the migrate command with the --to parameter set to the schema version you would like to downgrade to.

sudo -u "$LH_USER" lighthouse db migrate --to "$VERSION" --datadir "$LH_DATADIR" --network "$NET"

For example if you want to downgrade to Lighthouse v2.1 or v2.2 from v2.3 and you followed Somer Esat's guide, you would run:

sudo -u lighthousebeacon lighthouse db migrate --to 8 --datadir /var/lib/lighthouse --network mainnet

Where lighthouse is Lighthouse v2.3.0+. After the downgrade succeeds you can then replace your global lighthouse binary with the older version and start your node again.

How to apply a database upgrade

Database upgrades happen automatically upon installing a new version of Lighthouse. We will highlight in the release notes when a database upgrade is included, and make note of the schema versions involved (e.g. v2.3.0 includes an upgrade from v8 to v9).

They can also be applied using the --to parameter to lighthouse db migrate. See the section on downgrades above.

How to check the schema version

To check the schema version of a running Lighthouse instance you can use the HTTP API:

curl "http://localhost:5052/lighthouse/database/info"

{
  "schema_version": 8,
  "config": {
    "slots_per_restore_point": 8192,
    "slots_per_restore_point_set_explicitly": true,
    "block_cache_size": 5,
    "compact_on_init": false,
    "compact_on_prune": true
  }
}

The schema_version key indicates that this database is using schema version 8.

Alternatively, you can check the schema version with the lighthouse db command.

sudo -u lighthousebeacon lighthouse db version --datadir /var/lib/lighthouse --network mainnet

See the section on Running lighthouse db correctly for details.

How to run `lighthouse db` correctly

Several conditions need to be met in order to run lighthouse db:

The beacon node must be stopped (not running). If you are using systemd a command like sudo systemctl stop lighthousebeacon will accomplish this.
The command must run as the user that owns the beacon node database. If you are using systemd then your beacon node might run as a user called lighthousebeacon.
The --datadir flag must be set to the location of the Lighthouse data directory.
The --network flag must be set to the correct network, e.g. mainnet, prater or ropsten.

The general form for a lighthouse db command is:

sudo -u "$LH_USER" lighthouse db version --datadir "$LH_DATADIR" --network "$NET"

If you followed Somer Esat's guide for mainnet:

sudo systemctl stop lighthousebeacon

sudo -u lighthousebeacon lighthouse db version --datadir /var/lib/lighthouse --network mainnet

If you followed the CoinCashew guide for mainnet:

sudo systemctl stop beacon-chain

lighthouse db version --network mainnet

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