There's another way to drive consistency asynchronously across a decentralized network without consensus: Conflict-free Replicated Data Types, or CRDTs.
With CRDTs consistency is inherent in the data structure itself. If different versions of the same data exist across a set of connected nodes (or replicas as they are known in CRDT-speak), they will all eventually converge on the same state. If some data is changed on an offline replica, as soon as connection is re-established this change will propagate to all other replicas, even if the same data was changed on another replica in its absence. At the end of the process, everyone will have the exact same version.
In asynchronous decentralised networks, each node can perceive a different order of events. With CRDTs it is not necessary to try to enforce an order or agree on it before changes can be applied.
CRDTs can range from simple counters to more complex structures, such as documents on which many people collaborate. Because CRDTs follow a strict set of rules based on metadata contained within their structure, conflicts (i.e. the persistence of multiple states of the same data) are automatically avoided without any special code or user intervention required.
Not every data structure can be a CRDT, but where it's possible it makes the job of ensuring consistency significantly easier. Changes can be made locally to CRDTs in complete confidence that they will be valid whatever may be happening elsewhere.
On Autonomi all mutable data types are CRDTs
If a node is seen to be acting unreliably, it is simply ignored by other nodes by being removed from their routing tables and it will not earn any more tokens.
The closest parallels for this come from nature. Stigmergy is a scientific term to describe the mechanisms that ants, termites and bees use to create complex, resilient societies, without requiring central planning, control, or direct communication between the elements. These insects follow simple chemical signals to validate that individuals are who they purport to be, rather than invaders from another colony, and that they are behaving as they should. In the Autonomi Network, deviance is punished by being ignored and unable to earn.
Autonomi is a probabilistic, self-organizing system that reinforces good behaviours by providing a logical pathway for them, self-correcting for unwanted behaviours over time.
With this approach, so long as the rules are carefully written and properly implemented, it is possible to exceed the limit of one-third of nodes being Byzantine inherent in the "Proof of" approaches and still be protected against things like double spends.
Truly decentralized networks can have no master node or oracle to judge what is true or valid and what is not. Instead, the nodes that make up the Network must come to an agreement among themselves about which of many possible versions of “the truth” they will accept and the order in which events occur. This is challenging because each node has a different view of events. They must find a way to converge.
The bitcoin blockchain uses Proof of Work (PoW) to solve these problems of achieving decentralized consensus over the ordering of events to prevent the problem of double-spend – being able to spend the same coin twice – and Byzantine behaviour. Miners battle to verify a block of transactions. If there are two simultaneous winners the chain will split and for a time there are two versions of the truth, i.e. two chains emanating from the disputed block. Ultimately, though, after a few blocks have been added one chain will be longer than the other, and it is this chain that will be accepted as the true state.
PoW has proved largely effective against the Sybil attack, also known as the 51-percent attack, where one entity owns more than half of the mining power, but it is slow, massively energy-intensive and it doesn’t scale. Also, the consensus is probabilistic rather than deterministic: after some time, you might be 99.9 percent sure that all nodes agree on network state, but never 100 percent.
Other blockchains such as ethereum use Proof of Stake (PoS). While much more environmentally friendly, there is a tradeoff in terms of equability in that larger holders of the currency have more power.
Autonomi is naturally secure, meaning that it doesn't need any artificial slowing down or behavioural assumptions to keep data safe. It does not use PoW or PoS for consensus.