lighthouse/book/src/redundancy.md

Redundancy

There are three places in Lighthouse where redundancy is notable:

1. ✅ GOOD: Using a redundant beacon node in lighthouse vc --beacon-nodes
2. ❌ NOT SUPPORTED: Using a redundant execution node in lighthouse bn --execution-endpoint
3. ☠️ BAD: Running redundant lighthouse vc instances with overlapping keypairs.

I mention (3) since it is unsafe and should not be confused with the other two uses of redundancy. Running the same validator keypair in more than one validator client (Lighthouse, or otherwise) will eventually lead to slashing. See Slashing Protection for more information.

From this paragraph, this document will only refer to the first two items (1, 2). We never recommend that users implement redundancy for validator keypairs.

Redundant Beacon Nodes

The Lighthouse validator client can be configured to use multiple redundant beacon nodes.

The lighthouse vc --beacon-nodes flag allows one or more comma-separated values:

1. lighthouse vc --beacon-nodes http://localhost:5052
2. lighthouse vc --beacon-nodes http://localhost:5052,http://192.168.1.1:5052

In the first example, the validator client will attempt to contact http://localhost:5052 to perform duties. If that node is not contactable, not synced or unable to serve the request then the validator client may fail to perform some duty (e.g. produce a block or attest).

However, in the second example, any failure on http://localhost:5052 will be followed by a second attempt using http://192.168.1.1:5052. This achieves redundancy, allowing the validator client to continue to perform its duties as long as at least one of the beacon nodes is available.

There are a few interesting properties about the list of --beacon-nodes:

• Ordering matters: the validator client prefers a beacon node that is earlier in the list.
• Synced is preferred: the validator client prefers a synced beacon node over one that is still syncing.
• Failure is sticky: if a beacon node fails, it will be flagged as offline and won't be retried again for the rest of the slot (12 seconds). This helps prevent the impact of time-outs and other lengthy errors.

Note: When supplying multiple beacon nodes the http://localhost:5052 address must be explicitly provided (if it is desired). It will only be used as default if no --beacon-nodes flag is provided at all.

Configuring a redundant Beacon Node

In our previous example, we listed http://192.168.1.1:5052 as a redundant node. Apart from having sufficient resources, the backup node should have the following flags:

• --staking: starts the HTTP API server and ensures the execution chain is synced.
• --http-address 0.0.0.0: this allows any external IP address to access the HTTP server (a firewall should be configured to deny unauthorized access to port 5052). This is only required if your backup node is on a different host.
• --subscribe-all-subnets: ensures that the beacon node subscribes to all subnets, not just on-demand requests from validators.
• --import-all-attestations: ensures that the beacon node performs aggregation on all seen attestations.

Subsequently, one could use the following command to provide a backup beacon node:

lighthouse bn \
  --staking \
  --http-address 0.0.0.0 \
  --subscribe-all-subnets \
  --import-all-attestations

Resource usage of redundant Beacon Nodes

The --subscribe-all-subnets and --import-all-attestations flags typically cause a significant increase in resource consumption. A doubling in CPU utilization and RAM consumption is expected.

The increase in resource consumption is due to the fact that the beacon node is now processing, validating, aggregating and forwarding all attestations, whereas previously it was likely only doing a fraction of this work. Without these flags, subscription to attestation subnets and aggregation of attestations is only performed for validators which explicitly request subscriptions.

There are 64 subnets and each validator will result in a subscription to at least one subnet. So, using the two aforementioned flags will result in resource consumption akin to running 64+ validators.

Redundant execution nodes

Lighthouse previously supported redundant execution nodes for fetching data from the deposit contract. On merged networks this is no longer supported. Each Lighthouse beacon node must be configured in a 1:1 relationship with an execution node. For more information on the rationale behind this decision please see the Merge Migration documentation.

To achieve redundancy we recommend configuring Redundant beacon nodes where each has its own execution engine.