Overview

The Helium blockchain team has been working for several weeks on some incremental upgrades to our Proof of Coverage (PoC) and election systems, to make them more scalable and fair. Changes here include:

  • adjustments to make consensus groups more geographically diverse.
  • changes to limit the length of PoC paths, which grow exponentially more expensive to validate (O(n²) for the computer scientists in the audience) as they grow longer.
  • changes to PoC challenge generation to make it impossible for an unsynced hotspot to participate in a PoC challenge.

What Happened

PoCv2 Issues

On October 10th around noon PDT, we submitted the chain variable transaction to activate these changes, which had gone, disabled, into production over the preceeding few weeks. Shortly thereafter, things began to go badly wrong. We were immediately aware of the impact and began analysis of the situation. While we were able to determine quickly that block gossip has stopped functioning effectively, it was unclear why. Our public-facing API server, which allows our mobile apps to communicate with the hotspot blockchain network, was also not functioning correctly.

First, we discovered that the code version on our cloud based full nodes was too old, and they had stopped syncing the chain and participating in block gossip, slowing things down quite a bit. This was remedied by 3pm, but did not help as much as expected, so we continued to look at hotspots to figure out what the true root of the issue was. It was clear that validating PoC receipt transactions was a major part of the problem, but it was unclear how they had become problematic.

By 8PM PDT the ledger on the API had been reset and it resumed normal service, and the team was deep into discussions of how to resolve the issue. At 11PM PDT a transaction was prepared that rolled back part of the configuration change, and this seemed, initially, to be effective, so the team went to sleep. Overnight, at approximately 1:30AM, the chain halted again, for similar reasons.

The next morning was also dedicated to analysis of the problem. We prepared and partially rolled out a patch by 3pm that set a deadline on how long a transaction would have to have to validate, preventing extremely long block times. It was declared GA at 4:20PM PDT and seemed to help somewhat, but not enough. By 6PM PDT, it was decided that this was not helping enough and the full suite of PoC changes should be removed via another chain variable change transaction. This had the desired effect of restabilizing the chain.

Ledger Fork

However, just after 8PM PDT, the chain experienced another halt. Investigation revealed that a large number of hotspots considered the block invalid and could not continue syncing the chain. While we initially feared that this was related to the earlier problems, it turned out to be unrelated. We have been dealing with long-term determinism issues surrounding retained floating point values and list ordering diverging over time for a while now, and this was another in the same category.

Our initial response was to skip past the invalid block that had been generated, but the block that the consensus group subsequently generated proved impossible (for the aforementioned determinism issues) for a large percentage of the hotspot fleet to progress past. However, the rest of the fleet managed to continue making blocks and holding elections. Over the weekend, we spent time refining the process for safely bringing hotspots back into line with the group. This involved reconstructing the ledger data from blocks on disk, which was moderately time-consuming and not particularly safe. By Monday, we had improved the existing code around this enough to safely fix the remaining hotspots.

API Recovery

The API server essentially is a blockchain consumer and was affected by the ledger fork as well. A conflicting view of the ledger wouldn’t allow the API server to ingest blocks and update the necessary data in the back-end database required by the app to render different screens.

Before we realized that the root issue was the ledger fork, we tried a few cleanup methods on the API server:

  • Rebooted the API server on all the AWS instances, between 1–2 PM PDT Thursday, Oct 10
  • Rebuilt the API server from scratch with updated blockchain dependencies, between 2–5 PM PDT Thursday, Oct 10

None of which actually resolved the issue. However, once we knew that we had a conflicting ledger, it was a simple matter of:

  • Pausing the API server’s sync process
  • Resetting the API server’s copy of the ledger
  • Resuming the blockchain sync process

These steps worked as intended and the API server was back up and running around 8PM PDT, Thursday, Oct 10.

Continued Struggles with Pathing

Through the weekend and for most of the following week we struggled with block production and a phenomenon where PoC rate would fall terribly low, causing a large number of issues as many active hotspots were assumed down due to how long it had been since they’d issued a PoC challenge.

We eventually realized that the issue had nothing, strictly, to do with the PoC code, but that in our more crowded cities, changes to how long a hotspot would be judged a poor network participant for failing PoC challenges was lessened, leading to many new hotspots being in the neighborhood all at once. This lead to many new and exceptionally long paths.

Since paths grow exponentially more expensive with their length, this would lead to situations where our PoC challenge generation state machine would be stuck so long generating challenges that no challenge that it generated could ever succeed, as their validity is height-limited.

Since challenge generation took longer than the rate at which new challenges would be issues, this state machine entered a spiral from which it would never recover, until either restarted or fixed by a code change, which was made on the 17th.

Resolution

There were a number of operational and code fixes undertaken over the course of the week. We added time budgets to all of the places where a PoC challenge could be generated or validated, and added new path limitation code to trim paths as soon as they grow too long, rather than computing them in full before truncating them. The new PoC code was re-enabled on Thursday, October 17th, along with these changes.

Operationally, our team worked to reset all of the nodes that had been effected by the ledger fork and return them to proper working order.

Impact

Over the course of the incident, block production was massively slowed down, and with them, elections, so token production was slowed down. We were regularly seeing block times of well over half an hour, many of which took manual intervention to resolve.

A number of hotspots had their PoC challenge code fall behind such that they could no longer issue challenges, impacting their ability to make tokens.

Due to the ledger fork, 65 hotspots ceased to stay synchronized with the chain, making it so that they couldn’t, temporarily, participate in PoC processes or elections, making them effectively unable to earn tokens.

These changes also altered the topology of the network, leading to unstable block and election times.

Next steps

We’ve taken action to continually measure determinism drift on the ledger, and when we detect any, we’ll work to reduce it until there is none.

Work on restablizing block and election times is ongoing. Since our consensus groups run on appliances using consumer internet, this is will be an ongoing challenge.

We have put in place procedures for slower and more careful roll out of new chain variables, and re-prioritized the reconstruction of our separated test cluster.

This blog is a first step in clearly documenting both incidents and our work to remediate them, as well as the place to announce planned changes, deployments of those changes, and any maintenance of the app and associated services.

We apologize for the instability as we work to improve everyone’s ability to participate in the growth of the network.