Rules engine

The rules engine evaluates a CEL expression against every validated telemetry point and dispatches actions when it matches. It is the lightweight, per-point reactive layer: simple condition-to-action logic that runs in under 5ms. For multi-step graph processing — enrichment, branching, connectors, dead-letter handling — see Rule chains.

A rule is a condition (a CEL expression), a device group it applies to, and one or more actions to fire when the condition is true.

How evaluation works

The engine consumes the same enriched, validated telemetry stream your devices produce when sending telemetry, and evaluates matching rules against each point. Evaluation must complete within 5ms per point.

flowchart TD
  point([Validated point<br/>telemetry.validated.T1]) --> scope{Device in the<br/>rule's group?}
  scope -->|no| skip([Rule not evaluated])
  scope -->|yes| eval{CEL condition true?}
  eval -->|no| none([No action])
  eval -->|yes| act["Dispatch actions"]
  act --> event["Publish rule.triggered.T1.{rule}"]
  act --> metric["Increment rules_triggered_total"]

CEL expression variable scope

Conditions are written in CEL (Common Expression Language). Only the variables below are in scope; referencing anything else is a compile-time error, so a broken rule can never be saved.

Variable	Type	Description
data	map(string, double)	Telemetry numeric values, e.g. data.temperature
data_str	map(string, string)	Telemetry string values, e.g. data_str.firmware_status
device.id	string	Device ID
device.name	string	Display name
device.groups	list(string)	Group membership
device.tags	map(string, string)	Tag key-value pairs
device.firmware	string	Firmware version
device.status	string	online / offline
device.last_seen	int	Unix timestamp (seconds) of last telemetry
timestamp	int	Unix timestamp (seconds) of the current point
tenant_id	string	Tenant ID

Compile-time validation

When a rule is created or updated, the CEL expression is compiled and validated before storage. Two classes of error are caught up front:

• Syntax errors. A malformed expression like data.temperature >> is rejected with INVALID_ARGUMENT and a human-readable parse error.
• Undeclared references. Using a variable that isn’t in scope — e.g. data.temperature > threshold — fails with “undeclared reference to ‘threshold’” and INVALID_ARGUMENT.

This is why rules are fast at runtime: by the time a rule is stored it is already a validated, compiled artifact, not a string to be parsed per point.

Tip

The UI’s “Validate Condition” button runs this same compile step via the API before you save, so you find scope and syntax errors in the editor rather than at deploy.

Group scoping

Every rule targets a device group. A point is only evaluated against rules whose target group contains the originating device. A rule for group furnace is never evaluated for a device in group hvac — the engine skips it entirely rather than evaluating-then-discarding. This keeps per-point work proportional to the rules that actually apply.

Example rules

Temperature threshold

Alert when any furnace device exceeds 80 °C:

data.temperature > 80

Target group: furnace. Action: webhook to your alerting endpoint.

Tag-scoped condition

Only fire for EU-region devices over threshold — combining numeric data with a device tag:

data.temperature > 80 && device.tags["region"] == "eu"

String telemetry

React to a string-valued metric, e.g. firmware reporting it needs an update:

data_str.firmware_status == "update_required"

Composite

Combine multiple metrics and device state:

data.temperature > 75 && data.pressure > 4.0 && device.status == "online"

Action types

A rule fires one or more of these actions when its condition is true:

Action	Behavior
Webhook	HTTP POST carrying the telemetry point plus rule metadata. Retried 3 times with exponential backoff; on final failure it is logged with trace context and rule_action_failures_total{type="webhook"} is incremented.
MQTT command	Publishes a command payload to the EMQX topic devices/{device_id}/commands — e.g. {"set_fan": "high"}.
Email	Sends a notification via SMTP.
Redpanda event	Publishes an event to a Redpanda subject for downstream consumers.

The rule.triggered event and metrics

Every time a rule fires, in addition to its actions, the engine:

• Publishes an event to rule.triggered.{tenant_id}.{rule_id} carrying rule_id, device_id, and the trigger values. This is what powers live rule-trigger feeds and the per-rule trigger history in the UI.
• Increments rules_triggered_total{tenant="T1",rule="R1"}.

If a rule does not match, no action is dispatched and no event is published.

The compiled-rule cache

Compiled rules are cached in Aerospike namespace rules, set compiled, keyed by {tenant_id}:{rule_id}, with a 600-second (10-minute) TTL. Evaluation reads the ready-to-run compiled form from this cache rather than recompiling per point. A practical consequence: after you edit or disable a rule, the change is fully in effect within the cache TTL window. Disabling a rule sets its enabled flag to false so subsequent evaluations skip it.

Conflict detection (advisory)

When you create or update a rule, the engine checks for potentially conflicting rules — for example, two rules on the same device group with overlapping conditions. Conflicts are advisory, not blocking: the rule is saved and the response includes a warnings array noting the overlap.

Caution

There is no automatic conflict resolution in v1 — no priority ordering, no mutual exclusion. If two rules both match a point, both fire. For example, one rule setting the fan to high above 80 °C and another setting it to low above 75 °C will both trigger at 85 °C; a warning is logged and corem_rules_concurrent_triggers_total is incremented, but neither is suppressed. Design overlapping rules with that in mind, or move the logic into a rule chain where branching is explicit.

When to reach for rule chains instead

Use the rules engine for simple, stateless condition → action logic. Reach for rule chains when you need to enrich a message with asset or related-entity data, branch on multiple conditions, gate on blockchain finality, fan out to external connectors with retries and a dead-letter queue, or create and manage alarms as first-class entities.

Where to go next

Rule chains Graph-based processing: nodes, connectors, versioning, DLQ, and debug sessions.

Alarms & notifications Turn rule outcomes into managed alarms and routed notifications.

Sending telemetry The validated stream the rules engine consumes.