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Preprocess(or)

Wikipedia says:

In computer science, a preprocessor is a program that processes its input data to produce output that is used as input to another program. The output is said to be a preprocessed form of the input data, which is often used by some subsequent programs like compilers. https://en.wikipedia.org/wiki/Preprocessor

Preprocessing for iRules is helpful to eliminate unnecessary code paths or enable specific features.

Example: Debug logging

One of the most common use-cases in iRules for constants is to enable conditional debug logging.

Example: 

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when RULE_INIT {
    set static::debug 1
}
when HTTP_REQUEST {
    # ...
    if {$static::debug} {
        log local0.debug "[HTTP::method] [HTTP::uri] HTTP/[HTTP::version]"
    }
    # ...
}

This is not only bad for the reasons outlined in constants but also from an iRule processing (and performance) standpoint. the if {$static::debug} expression is evaluated whenever the HTTP_REQUEST event is fired.

Using jinja2 we can introduce a condition and preprocess the iRule, so that the log statement is only included when debugging is actually enabled.

Create a configuration.yaml: 

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debug: 1

Place the iRule in example.irule.j2:

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when HTTP_REQUEST {
    # ...
    {% if debug %}
    log local0.debug "[HTTP::method] [HTTP::uri] HTTP/[HTTP::version]"
    {% endif %}
    # ...
}

If debug is indeed 1, it would produce the following iRule: 

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when HTTP_REQUEST {
    # ...
    log local0.debug "[HTTP::method] [HTTP::uri] HTTP/[HTTP::version]"
    # ...
}

If debug is 0, the iRule would not contain the log statement at all.