website: add page on blsync (#30057)

Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>
Co-authored-by: Felix Lange <fjl@twurst.com>

docs/fundamentals/blsync.md

@@ -0,0 +1,70 @@
+---
+title: Beacon light client
+description: Running geth with integrated beacon light client
+---
+
+`blsync` is a beacon chain light client. Integrated within Geth, blsync eliminates the necessity of running a separate [consensus client](/docs/getting-started/consensus-clients), making it ideal for use-cases that do not require full validation capabilities. It comes with very low resource requirements and can sync the beacon chain within seconds. `blsync` can be run in two modes: integrated or standalone. In standalone mode it is possible to use it for driving other execution clients.
+
+<note>Important: blsync is not suitable for running a validator. It is also not recommended for nodes handling any amount of money or used in production settings due to its lower security guarantees compared to running a full consensus client.</note>
+
+## Usage
+
+### Integrated mode
+
+To run blsync as part of Geth, you need to specify a public HTTP endpoint and a checkpoint:
+
+- **Choose an Endpoint**: Select a reliable and available endpoint from the [Light Sync Endpoints](https://s1na.github.io/light-sync-endpoints/) list. These nodes are community-maintained.
+
+- **Specify the Checkpoint**: Obtain a weak subjectivity checkpoint from a trusted node operator. The checkpoint should be less than 2 weeks old.
+
+#### Checkpoint
+
+A checkpoint is the block root of the first proposed slot of a finalized beacon epoch. In this guide we use [beaconcha.in](https://sepolia.beaconcha.in) to find a checkpoint:
+
+- Visit sepolia.beaconcha.in.
+- Navigate to the latest finalized epoch that is ideally 1 hour old.
+![Finding a suitable epoch](/images/docs/blsync1.png)
+- Open the epoch details and find the first proposed slot at the end of the page.
+![Finding the first slot](/images/docs/blsync2.png)
+- Copy the block root field.
+![Copy the block root](/images/docs/blsync3.png)
+
+#### Example
+
+The following command can be used to start Geth with blsync on the Sepolia network. Note that the checkpoint root will be outdated two weeks after the writing of this page and a recent one will have to be picked according to the guide above:
+
+```terminal
+./build/bin/geth --sepolia --beacon.api https://sepolia.lightclient.xyz --beacon.checkpoint 0x0014732c89a02315d2ada0ed2f63b32ecb8d08751c01bea39011b31ad9ecee36
+```
+
+### Running `blsync` as a Standalone Tool
+
+As mentioned before, `blsync` can be run in standalone mode. This will be similar to running a consensus client with low resource requirements and faster sync times. In most cases Geth users can use the integrated mode for convenience. The standalone mode can be used e.g. to drive an execution client other than Geth.
+
+#### Installing
+
+Depending on your [installation method](/docs/getting-started/installing-geth) either you have access to the `blsync` binary or you will have to build it from source by:
+
+```terminal
+go build ./cmd/blsync
+```
+
+#### Running
+
+Blsync takes the same flags as above to configure the HTTP endpoint as well as checkpoint. It additionally needs flags to connect to the execution client. Specifically `--blsync.engine.api` to configure the Engine API url and `--blsync.jwtsecret` for the JWT authentication token.
+
+Again to sync the Sepolia network in this mode, first run Geth:
+
+```terminal
+./build/bin/geth --sepolia --datadir light-sepolia-dir
+```
+
+The logs will indicate the Engine API path which is by default `http://localhost:8551` and the path to the JWT secret created which is in this instance `./light-sepolia-dir/geth/jwtsecret`. Now blsync can be run:
+
+```terminal
+./blsync --sepolia --beacon.api https://sepolia.lightclient.xyz --beacon.checkpoint 0x0014732c89a02315d2ada0ed2f63b32ecb8d08751c01bea39011b31ad9ecee36 --blsync.engine.api http://localhost:8551 --blsync.jwtsecret light-sepolia-dir/geth/jwtsecret
+
+INFO [06-23|15:06:33.388] Loaded JWT secret file                   path=light-sepolia-dir/geth/jwtsecret crc32=0x5a92678
+INFO [06-23|15:06:34.130] Successful NewPayload                    number=6,169,314 hash=d4204e..772e65 status=SYNCING
+INFO [06-23|15:06:34.130] Successful ForkchoiceUpdated             head=d4204e..772e65 status=SYNCING
+```

docs/getting-started/consensus-clients.md

@@ -7,6 +7,8 @@ Geth is an [execution client](https://ethereum.org/en/glossary/#execution-client
 
 There are five consensus clients available, all of which connect to Geth in the same way. This page will outline how Geth can be set up with a consensus client to form a complete Ethereum node.
 
+<note>As an alternative it is possible to run with an integrated beacon light client for non-production settings. Please refer to [this](/docs/fundamentals/blsync) guide.</note>
+
 ## Configuring Geth {#configuring-geth}
 
 Geth can be downloaded and installed according to the instructions on the [Installing Geth](/docs/getting-started/installing-geth) page. In order to connect to a consensus client, Geth must expose a port for the inter-client RPC connection.

src/data/documentation-links.yaml

@@ -20,6 +20,8 @@
       to: /docs/fundamentals/security
     - id: Sync-modes
       to: /docs/fundamentals/sync-modes
+    - id: Beacon light sync
+      to: /docs/fundamentals/blsync
     - id: Account management
       to: /docs/fundamentals/account-management
     - id: Databases