What happened

On Wednesday, 2022-11-16, at approximately 22:06 UTC, Validators stopped producing blocks at block 1,619,546, and a chain halt occurred. Normal chain operations resumed 18 hours on 2022-11-17 at approximately 15:57 UTC.

Throughout the disruption, Hotspots were not able to fully participate in Proof-of-Coverage. All funds were safe, and data transfer for sensors and devices on the network continued uninterrupted.

However, pending transactions were delayed until the halt was resolved. Validator Operators we’re asked to restart their nodes to kickstart consensus and produce blocks again.

At the same time, the API was down during maintenance, and a database became corrupted. This led to a backup of 170TB of data before the main database could be restored.

Cause

The chain halt was triggered as a result of a corrupted region cache issue for some validators with the release of the latest chain variable.

Because the region was marked as “unknown” to some of the members an election block was produced that could not be absorbed by many nodes, including most of the new consensus group.

This led to a halt until enough nodes had been restarted to clear the cache and absorb the block. After enough of the group had restarted the cache issue was fixed and progress was possible.

Several hours later a second manifestation of the bug occurred. The cache was corrupted again but on fewer nodes, so the chain was able to make partial progress via “skipping” the block proposals where there was disagreement. These skip blocks were the blocks that would show up every half hour to an hour.

New consensus group elections and reward transactions happen together, so the chain was unable to make progress electing a new group. Therefore another restart was needed to repair the issue again. This restart was more difficult because it was harder to reach some operators at that point.

Recovery

Validators were identified and notified to run commands in an effort to resolve the cache issue. Because of the decentralized nature of Validator operations, this message took some time to distribute and for the call to be acted upon.

Initially, to hasten recovery, the team created a rescue block at 5:10 AM UTC.

Resolution

Validator Operators were instructed to perform several actions during the chain halt:

Restart their node, regardless of if they are in consensus or not. After restarting, run the skip command: miner hbbft skip.

Test if your Validator was one with a corrupted region cache by running:

miner eval 'blockchain_region_v1:h3_to_region(611338192892723199,blockchain:ledger()).'

If corrupted, Validators we’re instructed to clear the cache and run two commands: miner ledger cache clear followed by miner ledger cache prewarm

All other Validators were asked to refresh the cache, regardless of whether they were in the Consensus Group or not. Some Validators remained behind on block production, and they were again asked to restart.

18 hours on 2022-11-17 at approximately 15:57 UTC, all Validators were functioning as normal and block production resumed. Validators were notified to take action through Discord and validators-status.helium.com.

Power of Community

Big thanks to the Validator community and those members who acted swiftly to help resolve this issue and provide feedback during the halt.

During the chain halt and resolution, Validators we’re asked to restart, and run commands during late hours.

The entire Helium Community deeply appreciates your continued dedication and support of the Helium Network.

Timeline of Key Announcements

Throughout the disruption, the team worked to keep users up to date on the status and required actions. All below times in UTC.

2022-11-16

00:32 Validator Operators were asked to restart their nodes to kickstart consensus and start producing blocks again.

2:17 The API remained down after scheduled maintenance. The chain remained halted at this time. Both were being fixed in parallel.

4:16 API and chain remained halted.

6:24 The Core Developers issued a rescue block, and it was accepted by the chain. Validator Operators were instructed to monitor block production. The chain was not fully cleared.

12:04 The Core Developers identified the root cause of the chain halt. It was identified that some Validators had a corrupt region cache and were preventing block production. A fix had been issued to them. No updates were required for the entire Validator group, but operators needed to run commands on their affected Validators. The chain remained halted.

14:54 The chain remained halted. The Core Developers were still working with validators to resolve the corrupted region cache issue.

15:23 The API remained down. The Core Developers identified the root cause of this. The API had a database that became corrupted during maintenance. This affected how all services could be brought back up. As a result, over 170TB of data had to be rebuilt from scratch. At this time, the main database was up and running, but in order to service all users and services, The Core Developers needed to create several more instances.

15:51 The chain remained halted. The Core Developers were still working with validators to resolve the corrupted region cache issue.

15:57 Block production resumed, and the chain halt was resolved. The Validator Operator community responded to the call to run commands. The Core Developers continued to monitor the status of block production.

16:49 At this time, the chain was producing blocks, but the API remained down. This meant that there was no way for the App and Explorer UI to display block information, including rewards. Links to community data sources were provided.

19:10 The API was back up and running. At this time, the Explorer and App were about 200 blocks behind and were catching back up to real-time data streams.