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+++ b/README.md
@@ -44,5 +44,10 @@ Bug and feature enhancements for the webseer plugin are handled in GitHub. If
 you find a first search the Cacti forums for a solution before creating an issue
 in GitHub.
 
+## Architecture
+
+The proposed architectural direction for Monitor is documented in the
+[Architecture Decision Record](docs/architecture/0001-hexagonal-monitor.md).
+
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 Copyright (c) 2004-2026 - The Cacti Group, Inc.
diff --git a/docs/architecture/0001-hexagonal-monitor.md b/docs/architecture/0001-hexagonal-monitor.md
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+# ADR-0001: Migrate Monitor toward a hexagonal architecture
+
+**Status:** Proposed
+
+## Context
+
+Monitor currently combines Cacti hook registration, HTTP request handling,
+session state, dashboard rendering, raw SQL construction, poller execution,
+notification policy, email delivery, and persistence in a small number of
+procedural files. It also stores monitoring configuration in Cacti's shared
+`host` table and directly reads optional plugin tables such as Threshold and
+Syslog.
+
+This makes it difficult to test monitoring behavior independently, evolve
+state transitions safely, prevent duplicate background work, or maintain a
+stable extension boundary while keeping compatibility with Cacti 1.2.
+
+## Decision
+
+Monitor will be migrated incrementally to a hexagonal architecture while
+preserving the current Cacti 1.2 hooks, UI routes, device fields, and
+notification behavior during the compatibility period.
+
+The target contains the following responsibilities:
+
+- **Domain:** monitoring profiles, observations, incidents, acknowledgements,
+  notification attempts, and saved dashboard definitions.
+- **Application services:** evaluate a monitoring cycle, reconcile uptime and
+  reboots, dispatch notifications, query a dashboard, and acknowledge or mute
+  an incident.
+- **Ports:** device-status source, threshold-status source, monitor
+  repository, dashboard repository, notification gateway, authorization
+  policy, clock, and worker lease.
+- **Adapters:** Cacti host/Threshold/Syslog APIs, MySQL, Cacti mailer, Cacti
+  poller hook, and the existing Monitor web UI.
+
+Domain code must not access request variables, sessions, globals, SQL, the
+filesystem, or Cacti helper functions. New code must remain PHP 7.4-compatible
+until Cacti changes its supported baseline.
+
+## State and delivery rules
+
+Availability, latency breach, Threshold trigger, acknowledgement, and user UI
+mute state are separate concepts. A monitoring incident has an explicit
+lifecycle; a notification attempt and confirmed delivery are distinct records.
+Only a lease-owning worker may evaluate a cycle or dispatch its notifications.
+
+## Migration plan
+
+1. Add pure value objects and policy functions with characterization tests.
+2. Put Cacti host, Threshold, notification, and authorization access behind
+   adapters used by new application services.
+3. Introduce plugin-owned profile, incident, and notification-attempt storage;
+   mirror existing `host.monitor_*` fields through a compatibility adapter.
+4. Move the poller script to a lease-aware evaluation and delivery workflow.
+5. Move saved dashboard URL state to a versioned dashboard definition, then
+   deprecate legacy direct SQL and request/session coupling.
+
+## Consequences
+
+- Existing hooks and database fields remain supported during migration rather
+  than requiring a flag-day rewrite.
+- New behavior gains unit, integration, and end-to-end test seams.
+- Cacti and optional plugin dependencies become replaceable adapters rather
+  than hidden global dependencies.
+- The migration requires explicit schema/version compatibility and a published
+  deprecation timeline before legacy paths are removed.