The Ethereum beacon chain has a concept of committee index, and I do not understand it.

What exactly is the committee index? Why was it needed? How many committee index are there per slot? One or many? And what tasks are assigned to committee index?

1 Answer 1


From Ben Edgington on his Ethereum 2.0 Annotated Specification:

Validators are organised into committees that collectively vote (make attestations) on blocks. Each committee is active at exactly one slot per epoch, but several committees are active at each slot. This type indexes into the list of committees active at a slot.

In Phase 0, validators are members of only one type of committee, and they are shuffled between committees every epoch. The role of the committee is to attest to the beacon block proposed by the selected member of the committee.

The committee index is simply a number assigned to each committee during an epoch. There are 64 active committees per slot. Again from the same reference:

In the beacon chain, the 64 committees active in a slot effectively act as a single committee as far as the fork-choice rule is concerned. They all vote on the proposed block for the slot, and their votes/attestations are pooled. (In a similar way, all committees active during an epoch act effectively as a single committee as far as justification and finalisation are concerned.)

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