Pool member failback

Hi Kram,

you may use the iRule below as a starting point. The iRule uses the table command to switch between a primary/backup mode to be able to apply different rule sets...

when RULE_INIT {
    set static::primary_pool "pool_a"
    set static::backup_pool "pool_b"
}
when CLIENT_ACCEPTED {
     Check session table if backup mode is currently active
    if { [table lookup "status_$static::primary_pool"] eq "backup" } then {
         Backup mode is currently active. Applying Backup mode availability requirements. Check if primary pool has equal or more than 4 members.
        if { [active_members "$static::primary_pool"] >= 4] } then {
              Primary pool has equal or more than 4 members. Switching back to primary pool and disable Backup mode.
            table delete "status_$static::primary_pool"
            pool "$static::primary_pool"
        } else {
              Primary pool has less than 4 members. Using backup pool and keeping Backup mode active...
            pool "$static::backup_pool"
        }
         Exit the script...
        return
    }
     Backup mode is currently not active (default). Applying normal availability requirements. Check if primary pool has more than 1 members.
    if { [active_members "$static::primary_pool"] > 1] } then {
          Primary pool more than 1 member. Using primary pool and keeping Backup mode disabled...
        pool "$static::primary_pool"
    } else {
          Primary pool less than 2 available member. Using the backup pool and enable Backup mode in session table...
        table add "status_$static::primary_pool" "backup" indefinite indefinite 
        pool "$static::backup_pool"
    }   
}

Update: Changed the order of the script to make it easier to understand. Added additional comments, to explain what the purpose of each line is...

Cheers, Kai