URL based load balancing. How?

If you just want to do simple pool forwarding based on URI, then you don't need to use iRules anymore. You can use LTM Traffic Profiles, ideal for simple logic especially if you don't know iRules. Can all be configured in the GUI

https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-profiles-reference-12-0-0/2.html

If you're not good at writing iRules, use LTM policy with 2 rules

• HTTP Host equals example.com --> forward pool primary_pool
• HTTP Host equals subdomain.example.com --> forward pool subdomain_pool

Here's an iRule I use for this type of thing. It uses a "Datagroup" that you can update the http host header to pool mapping to do what you want.

1. Create a Datagroup "HOST_2_POOL_DataGroup" with entries like:

   "expample.com" := "APP_Pool01",
   "subdomain.example.com" := "APP_Pool02",
   

   `

2. Here is the iRule "HTTP-Host-2-Pool-MAP"

   `when HTTP_REQUEST {
   set pool_select_value [class lookup [string tolower [HTTP::host]] HOST_2_POOL_DataGroup]
   if {[active_members $pool_select_value] > 0 } {
       log local0.notice "available pool members"
       pool $pool_select_value
       } else {
           HTTP::respond 503 content "No active pool members in pool"
       }
   }