http redirect irule

John, I still can't see when are removing the complete log statement line, the iRule stops to work. But, if you are just trying to run "[substr [HTTP::uri] 7]" on this line, it's really wrong thing. You need to store back on HTTP::uri.

HTTP::uri [substr [HTTP::uri] 7]
pool testlab

This way, on client side the URI should be:

/abcdef/api/

/api/

Anyway, If you are facing issue when you do strip to uri value, and the traffic have an unexpected redirection because of the irule 1 execution, you can try to set priority to this irule. This way, it will execute before the iRule that you are rewriting the uri. e.g.

irule 1
when HTTP_REQUEST priority 300 { 
    if { [HTTP::host] contains "testlab.john.com" and [HTTP::uri] starts_with "/api/" }
    {
        HTTP::redirect "https://f5.com[HTTP::uri]"
    }
}

irule 2
when HTTP_REQUEST {
    if { [HTTP::host] contains "testlab.john.com" and [HTTP::uri] starts_with "/abcdef/api/" } {
        HTTP::uri [substr [HTTP::uri] 7]
        pool testlab
    }
}

I think I have no more ideas to this.