0-4-6-understand-split-brain

name	0.4.6-understand-split_brain
description	[0.4.6] Scenario where network partition causes multiple nodes to believe they are leader; resolved by quorum.

understand-split_brain

CALL NUMBER: distributed_consensus_and_replication_protocols.split_brain : deep_consensus(7), deep_raft(6) DEFINITION: Scenario where network partition causes multiple nodes to believe they are leader; resolved by quorum.

Invoke this skill to understand split_brain down to its primitives. The RELATIVE ROOT below is the least-fixed-point closure of everything it bundles from — the full import cone, grouped by the lib each prim comes from. Projected from a prover-typed KB (MAP/SWI-Prolog consistency gate): every reference below resolves.

THE RELATIVE ROOT (the import cone, by lib)

from ?

• agreement (d4): The property that a set of distributed processes reach the same decision outcome on a given value. Uniform agreement requires all processes including those that later crash to decide identically; asymmetric agreement permits different decision classes for different process roles such as primary versus backup.

from deep_consensus

• availability (d1): Probability system remains operational and accessible despite failures.
• safety (d1): Property requiring system never reaches inconsistent or invalid state.
• liveness (d2): Property requiring system to eventually respond or make progress under fair conditions.
• quorum_read (d2): Reading from sufficient replicas to guarantee seeing latest write.
• quorum_write (d2): Writing to sufficient replicas to guarantee subsequent readers see the value.
• replication_factor (d2): Count of replica copies maintained for durability and availability.
• strong_quorum (d2): Quorum where read and write quorums overlap (r + w > n).

from deep_raft

• a007 (d4): Quorum in agreement: a subset of processes sufficient to guarantee agreement can be reached, typically intersecting any valid quorum to ensure overlap.
• a008 (d4): Threshold in agreement: the minimum number of correct processes required to guarantee safety and liveness under given failure assumptions.
• a009 (d5): Asymmetric agreement: one class of processes (e.g., primary) must agree while another class (e.g., backups) may reach different decisions, used in primary-backup protocols.
• a002 (d5): Validity in agreement: the decided value must have been proposed by some process in the system, guaranteeing agreements are grounded in actual proposals.
• a005 (d6): Uniform agreement: all processes including those that later fail must decide on the same value, stricter than ordinary agreement which only binds correct processes.
• a004 (d6): Termination in agreement: every correct process eventually decides some value, ensuring the protocol makes progress and does not stall indefinitely.

from distributed_consensus_and_replication_protocols

• quorum (d1): Subset of nodes sufficient for reads or writes; typically majority (n/2+1) for strong consistency.
• linearizability (d2): Strongest consistency model: every operation appears atomic and ordered in real time across all nodes.
• semi_synchronous_replication (d2): Write waits for quorum of replicas to acknowledge; balances latency and durability.
• synchronous_replication (d2): Write waits for all replicas to acknowledge before returning to client.
• consensus (d3): Agreement on a single value among a set of distributed processes; the core problem consensus protocols solve.
• fault_tolerance (d4): System's ability to continue operating correctly despite component failures.
• replicated_log (d5): Ordered log of commands maintained across replicas; foundation of state machine replication.

CONSUMERS (what needs this)

fencing_token, network_partition

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Projected from the distributed consensus and replication protocols KB (196 concepts / 187 relations) — consistency-typed by MAP; the facet list after the colon IS the cross-lib dependency web.

(leaf — this is an actual skill.)