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title 2020-05-08
description This overview highlights how SCP works end-to-end—quorum sets, blocking sets, federated voting, and how nomination + ballot narrow the network to one ledger value.
authors john
tags
tutorial

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Marta Lokhova walks through the Stellar Consensus Protocol (SCP) in plain language, aiming to give newcomers a mental model before they dive into formal specs. She starts by defining quorum sets (who you trust), thresholds (how many of them must agree), quorum slices (subsets that meet the threshold), and blocking sets (the smallest sets that can prevent you from moving forward).

With those primitives, she introduces statements as SCP’s basic unit of agreement (e.g., “I propose transaction set X for ledger N” or “I’m ready to apply transaction set X”). She then explains federated voting—how nodes progress from vote → accept → confirm on a statement—and uses a step-by-step demo to show how blocking sets can force nodes that initially voted differently to converge on the same outcome.

Finally, she frames SCP as a funnel: the nomination protocol gathers and confirms a candidate set of values (transaction sets), while the ballot protocol prepares and then commits a single value. Nomination continues in the background while ballot runs, letting the system start optimistically but still converge safely if the network needs to adjust.

Key Topics

  • Clear definitions of quorum sets, thresholds, quorum slices, and blocking sets—and how each node’s configuration shapes safety and liveness.
  • Federated voting mechanics (vote/accept/confirm) and why “accepting what your blocking set accepted” prevents deadlock.
  • SCP’s two-phase structure: nomination to build a candidate set, then ballot to prepare/commit one value.
  • Why ballot can start as soon as a node confirms its first nomination, even while nomination continues (optimistic progress with safe fallback).
  • Practical guidance on thresholds: aiming for ~67% across organizations, while internal org nodes may use lower thresholds; recommendation to use Stellar Core’s automatic quorum generation.

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