What is PoI (EigenTrust++)?

Proof of Importance with EigenTrust++ (PoI with EigenTrust++) is an enhanced version of the Proof of Importance consensus mechanism used by the NEM blockchain's Symbol platform. It extends the original PoI concept by incorporating EigenTrust++, a reputation algorithm adapted from peer-to-peer network research, to provide a more robust assessment of node behavior and trustworthiness. The importance score combines stake-based factors (token balance and vesting), transaction activity, and node reputation as computed by the EigenTrust++ algorithm. This reputation component evaluates nodes based on their historical behavior — how reliably they have participated in block production, validated transactions, and propagated data through the network.

The addition of EigenTrust++ to the importance calculation provides stronger Sybil resistance and better alignment of consensus participation with genuine network contribution. Nodes that consistently behave well — producing valid blocks when selected, maintaining uptime, and honestly propagating transactions — accumulate higher reputation scores and are more likely to be selected for block harvesting. Nodes that behave maliciously or unreliably see their reputation decrease, reducing their influence on consensus. This creates a dynamic system where consensus power reflects both economic commitment (stake) and demonstrated reliability (reputation). The multi-factor approach makes it more difficult for attackers to gain outsized influence through any single vector.

PoI with EigenTrust++ is used by NEM's Symbol blockchain, which launched in March 2021 as the next-generation NEM platform. Symbol (XYM) was designed as an enterprise-friendly blockchain with features including multi-signature accounts, aggregate transactions, and namespace/mosaic systems for asset management. The enhanced PoI mechanism reflects Symbol's focus on building a network where active and reliable participation is rewarded, rather than simply holding large token balances. The EigenTrust++ reputation system was adapted from academic research on trust in decentralized systems, adding a formal mathematical framework for evaluating node behavior. Both the original NEM NIS1 chain and the newer Symbol chain continue to operate, with the PoI variants representing the evolution of the importance-based consensus concept.