What happened

On Monday 2022-02-07 5:46 AM UTC the team announced a chain halt caused by enlarged block 121528. Normal chain operations were announced at 3:04 PM UTC, less than 10 hours later.

Throughout the disruption, devices continued to have coverage and operate normally as state channels remained open processing packets. There were also no risks to Hotspot owners or HNT holders but they did experience a delay in transaction processing and Proof-of-Coverage activity.

Cause

A roaming server (see Roaming Server background below) crash created a disparity between its state channel record and the record held by state channel participants (Hotspots).

Due to this disparity, the state channel participants submitted a large number of dispute transactions which increased the size of block 121528. Low-performance validators in the Consensus Group were unable to consume the large block, ultimately causing the chain halt.

Recovery

The team worked with the validator community to execute a command for validators in the Consensus Group. This command allowed the validators to clear the pending dispute transactions and replace them with a chain variable that set the state channel grace period to 1 block.

The state channel grace period is the window when valid state channel disputes can be submitted after state channels close. Typically, this grace period is set at a number of blocks which allows state channel participants time for dispute transactions. By limiting it to 1 block, essentially no valid disputes can be submitted.

Resolution

A fix was implemented and deployed with the latest Validator release v1.7.0 which limits valid dispute transactions. This fix provides the foundation for a future chain variable which will effectively eliminate the possibility that a large amount of valid dispute transactions can be submitted. This upgrade is designed to increase network resiliency but also impacts transfer rewards if disputes occur so an accompanying HIP will be proposed soon.

The roaming servers have been upgraded and additional monitoring has been implemented to anticipate and prevent future crashes.

The team is further considering whether to adjust the maximum block size.

In addition, to upgrade communication channels with the Validator community Helium Status includes an additional page specifically for validator operators.

To reduce the time to coordinate responses to situations like this in the future, operators hosting large numbers of validators are highly encouraged to subscribe to the Helium Validator Status page.

This is purely an opt-in channel, but will allow validator operators to subscribe and receive email, sms, or webhook notifications.

Additionally, an opt-in “bat signal”, a way to notify the validator community that help is needed, so core developers can work with validator operators in real-time during future incidents is also getting worked out.

Roaming Server Background

Helium has been partnering with several leading LoRaWAN Network Service Providers to provide roaming services. These service providers value the amount and growth of network coverage the Helium community has provided to augment their own coverage for their customers.

To enable this service, Helium has set up roaming servers that accept and purchase packets on behalf of the interested partners. These roaming servers use a simplified version of Console’s backend (Router), called Packet Purchaser. Because of the explosive growth of the Network, the number of packets “seen” by these roaming servers continues to rapidly increase.

A spike in traffic caused one of the roaming servers to go down, and when it restarted there were differences between the state channel record of the packet purchaser and the records held by a large number of state channel participants (Hotspots). These Hotspots saw the difference in the packet purchaser state channel and submitted dispute transactions. A large number of these dispute transactions were added to the block which became difficult for low-performance validators in the Consensus Group to consume.

Community Powered

Many THANKS to our overall validator community, but specifically to the validator operators who quickly responded to the core team’s requests for aid and helped ensure the chain recovered and returned to normal operations.

Scaling challenges are related to the enormous demand not only to provide coverage (nearing 600K Hotspots), but also the increased usage of the Network.

The Network is still in its early stages and we appreciate your patience and support as we continue to work together to build the world’s largest contiguous wireless network.

Timeline of Announcements

Throughout the disruption, the team worked to provide frequent updates and report on status:

02/07 Monday 5:56 AM (all times UTC)
Helium Validator operators and core developers are investigating a potential chain halt @ block 1215286. The Consensus Group has not produced a block in about 46 minutes.

More information to come as we uncover more details.

Validator community, please come online if you have validators currently in the Consensus Group.

Proof of Coverage may be degraded, and other transactions may be queued until the Consensus Group resumes block production. Data Transfer remains unaffected.

6:32 AM
The Consensus Group is still struggling to produce the next block with transactions and, as a result, the chain remains halted. Data Transfer continues to be unaffected but all other blockchain activity is delayed, including transaction processing.

7:49 AM
We are continuing to monitor the Consensus Group. It is making slow progress but has not produced a block yet.

9:09 AM
The Validator community continues to monitor the Consensus Group as it tries to agree upon a block. Once this is completed, the core developers will prepare a chain variable to address the State Channel issue that may be affecting the blockchain. Although we are prepared to issue a Rescue block, we are hoping to avoid doing this as Data Transfer continues to be unaffected. Other transactions (including Hotspot onboarding, Payments, and Proof of Coverage) remain degraded at this time.

1:36 PM
The Validator community is still trying to get the chain started again. As mentioned before we are hoping to avoid a Rescue Block since Data Transfer is still unaffected. Pending transaction posts should be avoided and will remain disabled, and other transactions (including Hotspot onboarding, Payments, and Proof of Coverage) remain degraded at this time.

3:04 PM
Validators have resumed block production and the chain is back to normal operations. Pending transactions have been re-enabled and already cleared.

A large block was created and the skip of the halting block included wiping the transaction buffer and replacing with a chain var to temporarily reduce the state channel grace period to mitigate the issue.

However, state channel closes will be negatively impacted including rewards for packet transfer and high overages. Another chain var will be issued to return the state channel back to normal operations.

The team is performing further investigation to determine root cause.