Blockchain & Cryptocurrency Glossary

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Active Set

1 min read
Pronunciation
[ak-tiv set]
Analogy
The Active Set is like the current jury selected from the full jury pool to deliberate on a specific trial.
Definition
The group of validators currently participating in consensus for a given epoch or round, determined by stake weight or election mechanism.
Key Points Intro
Active Set membership influences consensus via:
Key Points

Stake-weighted selection: Validators with more stake more likely included.

Fixed size or dynamic: Protocol defines set size or stake threshold.

Rotation: Membership can change each epoch based on elections.

Consensus participation: Only active validators propose and vote.

Example
In Polkadot, the Active Set consists of the top 300 validators by stake each era, who then produce and validate blocks.
Technical Deep Dive
At the start of each epoch, the staking module ranks validators by total backing (self-stake plus nominations). It selects the top N validators as the Active Set. The consensus engine then uses this list for block production and finality. Changes require updating the validator registry and notifying the consensus layer.
Security Warning
Sudden dropout of many active validators can threaten liveness; ensure sufficient standby validators.
Caveat
Limited Active Set size can centralize power if stake distribution is skewed.
Related Terms
Validator
Nominator
Epoch
Consensus

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