iRule to Change Default Pool (not redirect to another pool)?

Hi there! Transitioning from NetScaler to F5's Big-IP LTM can indeed feel like drinking from a firehose, but I will try to help! Your scenario is quite common in environments with blue/green deployments, and you've outlined a clear strategy for handling dynamic traffic routing using iRules.

For your specific challenge, you want to dynamically change the default pool based on a STATIC variable while maintaining the existing LTM monitoring. Unfortunately, directly changing the default pool of a Virtual Server via an iRule is not supported because iRules are designed to handle traffic flow rather than configuration changes.

However, you can achieve your goal by employing a combination of iRules and LTM features. Below are a couple of approaches to consider:

Approach 1: Using iRule with Pool Selection Logic

Instead of changing the default pool, you can use an iRule to dynamically select the pool based on the value of your STATIC variable. Here's an example of how you might write such an iRule:

when RULE_INIT {
    set static::deployment "blue"  ;# Initialize with default value
}

when HTTP_REQUEST {
    switch -exact [static::deployment] {
        "blue" {
            pool pool_blue
        }
        "green" {
            pool pool_green
        }
        default {
            reject
        }
    }
}

In this example, the iRule directs traffic to pool_blue or pool_green based on the value of static::deployment. You can update the STATIC variable using an API call to Big-IP, which should update the variable without requiring developers to change their pipelines.

Approach 2: Using a Data Group and iRule

Another method is to use a Data Group to store the current deployment status and reference it within your iRule. Here’s how you can do it:

1. Create a Data Group (e.g., deployment_status) with an entry like current_deployment := blue.
2. Update the Data Group using an API call when you switch deployments.
3. Reference the Data Group in your iRule to direct traffic accordingly.

when HTTP_REQUEST {
    set deployment_status [class lookup current_deployment deployment_status]
    switch -exact $deployment_status {
        "blue" {
            pool pool_blue
        }
        "green" {
            pool pool_green
        }
        default {
            reject
        }
    }
}

Monitoring and Failover

To handle the monitoring aspect, ensure that each pool has appropriate health monitors configured. This way, if all nodes in the active pool go down, the LTM can automatically fail over to the backup pool based on the health monitor status.

In summary, while you can't change the default pool directly via an iRule, you can use iRules to dynamically select the appropriate pool based on a variable. Additionally, using Data Groups can provide a more structured and easily manageable approach to store and update deployment status.

I hope this helps! If you have any further questions or need additional clarification, feel free to ask.