Sharding 103 - A Question of Control

Community-based, committee-led Proof-of-Stake blockchain is one of the purest forms of democracy. Leaders are elected by the community with checks and balances to ensure they toe the path of the people, by the people, and for the people. Sharding makes it all the better by partitioning and giving each partition autonomy, but who is watching the watcher?

The Art of Knowing

When dealing with events that cannot be undone after committing them, knowing everything and anything at any and all time is important to their successful execution.

Blockchain deals with events that are impossible to Ctrl+Z out of due to its distributed and decentralized nature.

Consensus has to be reached amongst 51% or ⅔ of the network participants, invariably leading to a fork. Sharding compounds this problem by a factor of ten.

With every shard boasting of its own sovereign ecosystem, a fork — accidental or malicious — can have disastrous and unintended consequences for the entire network.

Suddenly, there are two King Louis XIV in the ballroom laying claims to the throne and vying for your sworn allegiance. Meanwhile, emissaries from other shards keep arriving to give and receive tributes — but to which king?

A mediator, or more aptly, someone who does not only possess the genealogy of the kings but also the chronology of the emissaries is the only entity capable of resolving the crisis with minimal friction. However, for such an entity to exist at that moment, they either would have known the exact moment the crisis would erupt or is omniscient.

Therein lies the beauty of a Control Chain.

Watching the Watcher

A Control Chain, also known as Beacon Chain for the Ethereum Community, is a shard that manages the network. It manages consensus and voting rules, the formation of committees, validators and the rewards and penalties applicable to them, assets staked on the network, and registering the states of all shards.

Contrary to the stereotypes the word Beacon or Control Chain may evoke, this shard is the most decentralized shard on the network, consisting of many nodes for the broadest decentralization.

A control shard may have two folds or more the number of nodes available on other shards

Safety and liveness are the two most important parameters for a Control Chain. Safety means the system does not make mistakes.

Liveness means the system will not grind to a halt. With the Control Chain monitoring all shards, safety, and liveness must be balanced in such a way that the network can tolerate a certain level of corruption without jeopardizing integrity or security.

However, the level of tolerance is not the same for both parameters. Safety is focused on malicious attacks, while liveliness deals with network performance and availability.

To ensure safety without any disruption, these two parameters must be balanced in such a way that the level of corruption one can tolerate is different from the other. This means one can set the parameters of the protocols such that safety holds even with high corruption levels.

Concordium’s approach

Concordium is leveraging this insight to develop a blockchain with fewer nodes per shard except for the Control Chain.

This pragmatic and conservative approach allows for a relatively high corruption tolerance due to the fact that the Control Chain is not only responsible for spawning shards, but all shards also use the finality-as-a-service (FaaS) component of the control chain to finalize.

This means finalization on shards is performed by the nodes in the shards together with the finality committee of the control chain, and the control chain keeps track of finalized blocks on all shards.

This greatly improves the scalability of the network while retaining safety amongst shards. The control chain monitors the shards and if any comes to a halt due to corrupted nodes, it resamples the set of nodes running that shard.

This re-establishes liveness.

With safety and liveness in place, we have security and the real King Louis XIV anytime anywhere.

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