Expression Debugging

In many cases, expressions are evaluated in an autonomous environment that seriously complicates their testing. For example, when a system administrator creates an expression for an alert's event trigger, he cannot correctly test it from inside the Expression Builder since there are no at-hand instances of triggering events that could provide "real" data. In other words, the default table will not be defined in the Resolution Environment, and the expression will not produce a correct result when calculated inside the visual editor.

Using Evaluation and Error Log

Each expression evaluation is reported as a separate Evaluation event in the Expression Builder event log. Similarly, expression evaluation failures are logged as Evaluation Error events in the Expression Builder event log.

Evaluation of some expressions is not bound to any server context. This debugging method will not work for such expressions.

Using Trace Function

The Iotellect expression language includes a very handy trace() function that was designed for expression debugging. This function takes any value, logs it (to make it viewable in the Event Log), and returns it unchanged, resulting to any other side effects.

Methods of viewing values logged by trace function differ:

  • If an expression is calculated inside an Iotellect Server, values can be viewed by activating the Expression Tracing event filter (which is a part of the Iotellect core installation bundle)

  • If an expression is calculated inside a dashboard, values can be viewed in an event log component.

  • In other cases, values will be logged to the standard logging facility (and can be accessed from the console, file, etc.)

Example: Let's assume that we want to create an alert that will be triggered if pressure is under 20 units and temperature is above 50 units. Our initial variable trigger's expression will look like this:

{pressure} < 20 && {temperature} > 50

However, if the alert does not raise, it's hard to find out whether the "pressure part" of expression is false or its "temperature part" is.

To enable easy debugging, we can modify our expression as follows:

trace({pressure}, "Pressure") < 20 && trace({temperature}, "Temperature") > 50

Once this is done, we'll see the following output in Event Log every time our alert's performs the check:

  • Pressure: 25.3

  • Temperature: 52.7

This helps to quickly find out current values and see why alert setup is incorrect.

Another option is to track the state of individual parts of the expression instead of the "raw" metric values:

trace({pressure} < 20, "Pressure Condition") && trace({temperature} > 50, "Temperature Condition")

The output will be as follows:

  • Pressure Condition: false

  • Temperature Condition: true

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