memory-consistency-model-selector

Memory Consistency Model Selector

Overview

When Agent A writes a fact to shared memory, when does Agent B see it? Ch4 names this the memory consistency problem, and getting it wrong produces agents that contradict each other, overwrite each other's conclusions, or act on stale information. This is CAP — the classic consistency/availability trade-off — applied per coordination operation to shared agent memory, not decided once globally. Consistency is chosen per operation because most workflows mix safety-critical decision points with tolerant background work.

Four consistency models, each with a characteristic profile:

• strong (linearizable): every agent sees the latest write before any proceeds. A synchronization barrier after every write — expensive, but required when agents act on shared authoritative state (locks, budgets, inventory) or make safety-critical irreversible decisions. The chapter's example: a drug-interaction finding every agent must see before recommending treatment.
• causal: causally-related writes are ordered; unrelated updates may lag. If A's conclusion depends on B's finding, any reader of A also sees B. The practical default for collaborating agents.
• read_your_writes: an agent always sees its own writes; other agents' writes may arrive later. Session memory for a single agent's continuity.
• eventual: cheapest; all agents converge eventually, but not when. Fine when no single fact is safety-critical and a final synthesis reconciles disagreement (background enrichment, literature accumulation).

The selector scores each model across the operation's requirement axes and recommends one, surfacing escalate_to_strong when the default is not strong but the operation touches authoritative state — the chapter's rule: default to causal, escalate the irreversible decision points to strong.

The cache-divergence helper makes the failure concrete. Ch4's cache-sharing section warns that without a protocol, Agent B acts on a cached read that a newer committed write already superseded. detect_cache_divergence flags exactly that: any cached snapshot older than the latest committed write on the same key.