Open DragonDev1906 opened 11 months ago
Here is some more information: It looks like the FinalityUpdate changes whenever the attested header changes, even if the sync_committee_size is 1. Though I'm still not sure when this would be useful when requesting a LightClientFinalityUpdate.
/eth/v1/beacon/light_client/finality_update
endpoint, part of this is due to some blocks simply not including a good sync aggregate, in this case you have to wait for the next block to see whether it contains better data. /eth/v1/events
push based mechanism, it would indeed be better if a second LightClientFinalityUpdate
would be pushed in such cases, once an update with supermajority sync committee participation becomes available (in case that the initial one has low participation):
It could of course also be that my script is wrong and doesn't decode the sync committee correctly, hence the question of whether I understood something wrong when reading the consensus-specs.
You can check the next block (attested_header.beacon.slot + 1
) on a block explorer.
For example, https://prater.beaconcha.in/slot/6709376 contains the sync_aggregate
for attested_header.beacon.slot = 6709376
, confirming that there was indeed only a single signature included in that block (out of 512).
Keep in mind that the Goerli network is soon deprecated, Sepolia / Holesky should have better performance in that regard.
For the /eth/v1/beacon/light_client/finality_update endpoint, part of this is due to some blocks simply not including a good sync aggregate, in this case you have to wait for the next block to see whether it contains better data.
You're right. I wasn't expecting the specs to say that create_light_client_update
should completely ignore the sync committee size when creating an update, but it does make sense if the light client is interested in the optimistic header, as the optimistic header is updated with sync_committee_size>=1
. ^1 Also makes sense since the same code is used for create_light_client_finality_update
and create_light_client_optimistic_update
.
For some reason I expected create_light_client_finality_update
to not bother about updates that did not forward the finalized_header, but they can of course still be used to get optimistic header updates. Though unfortunately this means that it can sometimes be impossible to get a finalized_header through the beacon API until the next block is mined.
The spec even says this (apparently I can't read): Full nodes SHOULD provide the LightClientFinalityUpdate with the highest attested_header.beacon.slot
Keep in mind that the Goerli network is soon deprecated, Sepolia / Holesky should have better performance in that regard.
Yeah, that's why I'm currently testing on both Goerli and Sepolia. Higher chance to get unexpected or not quite normal situations that still can happen.
With #5602 (unstable
branch) it should now be possible for you to simply track the eventstream. When monitoring topics=light_client_finality_update
, you will now get an update both when new finality is achieved, and additionally when supermajority is available (if the initial update did not have supermajority participation).
Can also try in browser / curl (but have to wait ~5-10 mins for finality to advance to see something)
Describe the bug When requesting a
LightClientFinalityUpdate
via the beacon_chain RPC the endpoints sometimes returns an update with only a few entries in the sync committee (as shown in the screenshots). I'm not sure if this is a bug in Nimbus, but I can't think of anything where such a FinalityUpdate would be useful.According to the consensus specs light clients should:
process_light_client_update
),store.best_valid_update
store.current_max_active_participants
if the update has more participants set (which should never get this low),LightClientOptimisticUpdate
andPlease correct me if I understood something wrong in the consensus specs or if there is another reason why
LightClientFinalityUpdate
with less than 2/3 of the sync committee would be useful. My current best guess is that it isn't useful at all and probably shouldn't be returned by the/eth/v1/beacon/light_client/finality_update
rpc endpoint. I can easily check for the sync committee size to avoid this issue, instead of just checking the finalized blocknumber, which I mainly do to avoid unnecessary communication internally (I don't care about the optimistic updates/blocks/data).I don't know if this also affects the subscription (I'll likely change it to use the subscription at some point, but for now I'm polling the rpc endpoint).
It could of course also be that my script is wrong and doesn't decode the sync committee correctly, hence the question of whether I understood something wrong when reading the consensus-specs.
To Reproduce Steps to reproduce the behavior:
I additionally gives these updates to a light client if the finalized block number changed. Sometimes this then didn't apply the update, which is how I found this.
Screenshots (logs from my application) Log output of an application polling the RPC endpoint every 12 seconds and outputting the sync committee size:
Log output showing the situation I've found: A finality update with <2/3 sync_committee (the first one I received with a new
finalized_header..execution.block_number
), together with debug logs of the light client showing that the sync committee was the reason that the update was not applied.Additional Info This seems to happen quite regularly, most Epochs give me a FinalityUpdate with less than 2/3 sync committees.