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Node health checks

To serve as a validator, both CL and EL need to be up to date with the network. There are a couple of techniques of how to check that.
All this health check data should lead to a monitoring tool of your choice.

Internal health checks

Most of the nodes have exposed health checks APIs, that return HTTP 5xx if the node is not syncing properly, and HTTP 200 OK if everything is okay. That is the most simple and most basic version of the health check.

Timestamp Checks

One more strategy is based on the timestamp of the latest block.
For EL that is a response to eth_getBlockByNumber("latest", false).
It has a field called timestamp. By knowing the timestamp of the block and the block production rate (1 block per 12 seconds), it is possible to see how "old" is the current block of the node.
Since sometimes the block proposals could be missed, it doesn't make sense to keep this threshold too tight, but if it is > 5 minutes old, it makes sense to mark the node as "unhealthy" and notify your monitoring system.

Source Of Truth Checks (Forks)

Finally, the case when the blocks are being synced, but you are on the wrong fork. Detection of that could happen on the EL very easily, by using the block hash returned from eth_getBlockByNumber("latest", false).
You can compare these hashes across the nodes and also across the external sources of truth.
The information in the Scaled Node Operators section has been written and reviewed by GatewayFM, a leading large scale Ethereum staking infrastructure provider.