Architecture Pattern — Example: Order read model

Pattern: Architecture
Component: examples/example.md
Version: 1.1 | Updated: 2026-07-16
Decision: How should the Storefront read order status without coupling to Orders DB?

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ADR-0142: Storefront order status via projected read model (artifact)

Status: Accepted
Risk: Medium

Context

Storefront needs near-real-time order status. Orders service owns writes. Direct DB reads would couple deploy cycles and break tenancy isolation goals.

Decision drivers

• p99 status read < 150ms
• Orders deploy independent of Storefront
• Tenant isolation enforced in read path
• Eventual consistency ≤ 5s acceptable for status UI

Alternatives

Option	Summary	Fit to QA	Reject reason
A. Sync REST to Orders	Storefront calls Orders API	Simple; adds Orders load	Couples availability; spikes on flash sales
B. Shared Orders DB read	Storefront SQL views	Fast	Rejected — breaks service boundary + tenancy ownership
C. Event-projected read model	Orders emits events; projector fills order_status_rm	Meets latency + decoupling	Chosen

Decision

Use option C: Orders publishes OrderStatusChanged; projector updates order_status_rm; Storefront reads only the read model with org_id predicate.

Architecture diagrams

Trust & failure analysis

Scenario	Impact	Mitigation
Event bus delay > 5s	Stale status	Show “updating…”; alert on lag p99
Projector down	Reads stale	Page on consumer lag; scale projector
Poison event	Projector crash loop	DLQ + skip with metric
Missing org_id filter	Cross-tenant read	DB policy + integration test

Consequences

• Positive: Orders deploy decoupled; Storefront read p99 ~40ms in soak
• Negative: Temporary inconsistency; ops owns lag alerts
• Follow-ups: ADR for archive/TTL of read model rows

Re-review triggers

• Consistency SLO tightens below 1s
• Multi-region active-active for Orders

Approval

Reviewer: K. Ndlovu (independent)  Date: 2026-07-11
Decision: APPROVE WITH CONDITIONS
Conditions: Lag alert + tenant isolation test must land before Storefront cutover.
Author: A. Chen — not approver (SoD OK)

This example is the artifact — not a pointer elsewhere.