MXL - Meddle Expression Language

Overview

MXL (Meddle Expression Language) is a simple expression language used in Meddle connectors for defining conditions and logic.

Used in:

• Trigger Connector
• Alert Connector

Syntax

MXL expressions evaluate to boolean values (true/false) based on payload data.

Basic Structure

field_name operator value

Examples

temperature > 80
pressure < 10
status == "active"
count != 0

Operators

Comparison Operators

Operator	Description	Example
>	Greater than	temperature > 80
<	Less than	pressure < 10
>=	Greater than or equal	humidity >= 60
<=	Less than or equal	speed <= 100
==	Equal to	status == "active"
!=	Not equal to	count != 0

Logical Operators

Operator	Description	Example
&&	AND (both conditions must be true)	temp > 80 && press < 10
`		`

Data Types

Numbers

Compare numeric values:

temperature > 25.5
pressure >= 101.3
count == 100
speed != 0

Strings

Compare string values (use double quotes):

status == "active"
mode != "manual"
error_code == "E001"

Boolean

Compare boolean values:

enabled == true
fault != false

Complex Expressions

Multiple Conditions with AND

All conditions must be true:

temperature > 20 && temperature < 30
pressure > 100 && humidity < 80 && status == "running"

Multiple Conditions with OR

At least one condition must be true:

temperature > 80 || pressure > 150
status == "error" || status == "fault" || status == "stopped"

Combining AND and OR

Use parentheses for grouping (if supported):

(temperature > 80 || pressure > 150) && status == "active"

Common Patterns

Range Check

Check if value is within range:

temperature >= 20 && temperature <= 30

Threshold Alert

Alert when value exceeds threshold:

temperature > 80
pressure > 150
vibration > 5.0

Status Monitoring

Check for specific status values:

status == "error"
status != "running"
mode == "manual"

Multi-Parameter Alert

Alert on multiple conditions:

temperature > 100 || pressure > 150 || vibration > 5

Safety Interlock

Ensure multiple conditions are met:

temperature > 50 && pressure > 100 && flow > 10

Examples by Use Case

Temperature Monitoring

// High temperature warning
temperature > 80

// Temperature out of range
temperature < 15 || temperature > 35

// Critical temperature with pressure
temperature > 100 && pressure > 150

Equipment Status

// Machine stopped unexpectedly
status == "stopped" && runtime > 0

// Error condition
status == "error" || status == "fault"

// Not in automatic mode
mode != "auto"

Quality Control

// Defect detected
defect_count > 0

// Out of specification
dimension < 9.9 || dimension > 10.1

// Multiple quality parameters
hardness < 50 || surface_finish > 2.0 || weight < 100

Process Control

// Process out of control
temperature > 90 || pressure < 95 || flow < 50

// Stable operation
temperature >= 80 && temperature <= 85 && pressure > 100

// Emergency shutdown condition
temperature > 120 || pressure > 200 || vibration > 10

Predictive Maintenance

// Anomaly detected
is_anomaly == true

// High anomaly score
anomaly_score > 0.8

// Multiple indicators
vibration > 5 || temperature > 90 || anomaly_score > 0.7

Best Practices

1. Use Clear Field Names

// Good
temperature > 80
machine_status == "running"

// Avoid
t > 80
s == "r"

2. Use Appropriate Operators

// For equality, use ==
status == "active"

// For inequality, use !=
count != 0

3. Group Related Conditions

// Good - related conditions together
temperature > 80 && pressure > 150

// Good - alternative conditions
status == "error" || status == "fault"

4. Consider Operator Precedence

AND (&&) has higher precedence than OR (||):

// This checks: (A && B) || C
temperature > 80 && pressure > 150 || humidity > 90

// Use parentheses for clarity
(temperature > 80 && pressure > 150) || humidity > 90

5. Test Your Expressions

Test expressions with various input values to ensure they work as expected.

Limitations

• No arithmetic operations: Cannot do temperature + 10 > 90
• No function calls: Cannot do abs(temperature) > 80
• No nested field access: Cannot do sensor.temperature > 80
• String comparison only: No regex or pattern matching

Troubleshooting

Expression Not Triggering

Check:

1. Field names match payload exactly (case-sensitive)
2. Data types match (number vs string)
3. Operator is correct (== not =)
4. String values use double quotes

Always Triggering

Check:

1. Logic is correct (AND vs OR)
2. Threshold values are appropriate
3. Field exists in payload

Syntax Errors

Common mistakes:

• Using = instead of ==
• Missing quotes around strings
• Typos in field names
• Wrong operator precedence

Examples in Context

Trigger Connector

{
  "type": "MeddleTrigger",
  "config": {
    "mode": "DirectDispatch",
    "condition": "temperature > 80 && status == \"active\"",
    "payload": {
      "alert": "high_temperature"
    }
  }
}

Alert Connector

{
  "type": "MeddleAlert",
  "config": {
    "name": "Critical Alert",
    "condition": "temperature > 100 || pressure > 200",
    "minDuration": 10,
    "cooldown": 300,
    "notification": {
      "type": "Email",
      "toEmails": ["alerts@example.com"]
    }
  }
}

Related Documentation

• Trigger Connector - Use MXL for conditional logic
• Alert Connector - Use MXL for alert conditions
• Anomaly Detection - Combine with MXL for smart alerts