writes-follow-reads · shipping · also Writes follow reads, WFR session guarantee

Writes-follow-reads

Writes-follow-reads requires every replica incorporating a session write to incorporate and order before it the updates relevant to that session's earlier reads.

Related Observatory article: Writes-follow-reads on hesela.com

The dependency may originate in another session. For MongoDB, cross-session causal transfer uses operation time and cluster time; simply starting another client session does not carry that context. A causally consistent session does not isolate its operations from concurrent clients. Terry et al. section 3.3 separates ordering-only and propagation-only variants; neither alone is the full guarantee. The companion article provides an executable two-write example, not an implementation conformance test.

Relations

related

session-guarantees, monotonic-writes

Citations

1. Douglas B. Terry et al. Session Guarantees for Weakly Consistent Replicated Data. PDIS 1994, sections 3.1-3.4, pp. 142-144. (accessed 2026-10-13)
2. MongoDB Manual: Read Isolation, Consistency, and Recency; client sessions, causal guarantees and isolation. (accessed 2026-10-13)
3. Storage consistency: ordering is not a transaction or a durability promise (accessed 2026-10-13)

JSON: /data/terms/writes-follow-reads.json