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Variables in Expression

You can define variables within your expression to reuse values multiple times. This improves readability and maintainability, especially for complex calculations.

Variables must be assigned before any case statements. This ensures they are properly defined and available for use in your expression.

// Define "var.ratio"
var.ratio = item.usage_memory?.div(item.allocatable_memory)

case var.ratio? > 0.9:
    return CRITICAL, "Memory usage way too high!"
case var.ratio? > 0.8:
    return WARNING, "Hign memory usage"

Handling Optional Metrics (Null Values)

When a variable's value is derived from an optional metric (which might be null), the variable itself can also be null. To correctly handle this in your expressions, you should mark the variable as optional with a ?.

For example, item.usage_memory might be null. In such cases, var.ratio would also be null, requiring var.ratio? in the case statements.

Preventing Errors and Null values

Expressions can encounter issues like division by zero or other errors if source metrics are missing or invalid (e.g., item.allocatable_memory is 0). To gracefully handle such scenarios, you can use the nullish coalescing operator ??.

The ?? operator assigns the value on its right if the expression on its left evaluates to null or an error.

// Define "var.ratio"
var.ratio = item.usage_memory?.div(item.allocatable_memory) ?? 0

case var.ratio > 0.9:
    return CRITICAL, "Memory usage way too high!"
case var.ratio > 0.8:
    return WARNING, "Hign memory usage"

In this example, if item.usage_memory?.div(item.allocatable_memory) results in null or an error (like zero division), var.ratio will be assigned 0 instead. This provides robust error handling.

Chaining Optional Metrics with ??

If multiple metrics involved in a calculation are optional, you can strategically use ?? to ensure valid types for operations.

Suppose item.allocatable_memory is also optional: 

var.ratio = item.usage_memory?.div(item.allocatable_memory?)
// This would cause a type error:
//   function div(..) with argument type NullableInt

Directly passing item.allocatable_memory? (which can be NullableInt) to div() results in an error as div() expects a non-null number.

To solve this, use ?? to provide a default non-null value for optional metrics before using them in calculations:

var.allocatable_memory = item.allocatable_memory? ?? 0
var.ratio = item.usage_memory?.div(var.allocatable_memory) ?? 0

By assigning item.allocatable_memory? ?? 0 to var.allocatable_memory, we ensure that var.allocatable_memory is always a non-null integer (either the actual value or 0). This allows InfraSonar to perform type validation successfully.