maintenance page for VIP with different hosts point to different pools if available else redirect to http://$host/maintenance

try "string tolower" instead of "string length"

Expanding on @arpydays' answer, the conditional clause is incorrect in a few ways. I'm reproducing your code here in a marked up block with comments after:

when HTTP_REQUEST {
     sets the timer to return client to host URL set stime 10
     Check if the URI is /maintenance
    switch [HTTP::uri] {
        "/maintenance" {
             Send an HTTP 200 response with a Javascript meta-refresh pointing to the host using a refresh time
            HTTP::respond 200 content \
                " \
                Sorry! This site is down for maintenance." "Content-Type" "text/html" 
            return
        }
    }
    
     If the respective pool is down, redirect to the maintenance page else 
     send the request to the respective pool 
    if { [ string length [HTTP::host] eq test.com ] && [ active_members test_PROD_V ] < 1 } { 
        log local0. $host 
        HTTP::redirect "http://$host/maintenance" 
        return 
    } else {
        [ pool test_PROD_V ] 
    } 
}

You have used the evaluation operator (the square brackets -- [...]) in a number of places where it will have an unintended effect and will produce runtime errors.

To explain, the evaluation operator assumes its contents are a valid Tcl statement and evaluates it, substituting the result of the evaluation in place of the operator. From there, normal Tcl processing continues. So, let's say you do this:

if { [string tolower [HTTP::host]] eq "test.com" } {

the runtime expands inside-out. So, HTTP::host is evaluated first. That command results in a string. Let's say that it yields the value "test.com". If so, after this first expansion, the statement now looks like this:

if { [string tolower test.com] eq "test.com" } {

The remaining evaluation operator is expanded, yielding this:

if { test.com eq "test.com" } {

The runtime then executes the if command. That's what you expect and want. But let's consider your final statement:

[pool test_PROD_V]

This evaluates the pool command, which results in something (I'm not sure what it is, but I'm guessing it's a integer). Let's assume it is an integer. After the expansion, you'll have something like this:

1

Well, the runtime treats every line as a statement. Every statement is a command followed by one or more argument. So, it tries to evaluate the command called "1". There is no such command, so a runtime error is thrown.

It's important to understand that the evaluation operator is not needed to force Tcl to execute a command. Every time Tcl sees a line, it will treat it as a command followed by zero or more arguments. The evaluation operator is only needed when you want to have Tcl evaluate a command that is embedded inside of a statement.

Given all of this, your if clause should look like this:

if { [string tolower [HTTP::host]] eq "test.com" && [active_members test_PROD_V] < 1 } {

and, as I say, your final statement should simply be:

pool test_PROD_V

Another point to simplify the configuration. use relative links in redirect instead of absolute if it is on the same Web service...

Replace :

HTTP::redirect "http://$host/maintenance"

By :

HTTP::redirect "/maintenance"

And with the auto refresh in the javascript, redirect to the previous URL for a better user experience..

when HTTP_REQUEST {
     Check if the URI is /maintenance
    switch [HTTP::uri] {
        "/maintenance" {
             sets the timer to return client to host URL 
            set stime 10
             Send an HTTP 200 response with a Javascript meta-refresh pointing to the host using a refresh time
            HTTP::respond 200 content \
                " \
                Sorry! This site is down for maintenance." "Content-Type" "text/html" 
            return
        }
    }

     If the respective pool is down, redirect to the maintenance page else 
     send the request to the respective pool 
    if { [ string length [HTTP::host] eq test.com ] && [ active_members test_PROD_V ] < 1 } { 
        log local0. $host 
        HTTP::redirect "http://$host/maintenance?uri=[HTTP::uri]" 
        return 
    } else {
        pool test_PROD_V
    } 
}

The following irule is one configured for some customers:

when HTTP_REQUEST {
    switch [HTTP::path] {
        "/maintenance/logo.png" {
            HTTP::respond 200 content [ifile get "logo.png"] "Content-Type" "image/png"
            return
        }
        "/maintenance/maintenance.html" {
            HTTP::respond 200 content [ifile get "maintenance.html"] "Content-Type" "text/html"
            return
        }
        "/maintenance/error.html" {
            HTTP::respond 200 content [ifile get "error.html"] "Content-Type" "text/html"
            return
        }
        default {
            set dpool [LB::server pool]
            if { $dpool equals "" } { HTTP::redirect "/maintenance/error.html" ; unset dpool; return}
            if { [active_members $dpool] == 0 } { HTTP::redirect "/maintenance/maintenance.html"; unset dpool; return}
            unset dpool
        }
    }
}

when HTTP_RESPONSE {
  if { [HTTP::status] eq "404" } { HTTP::redirect "/maintenance/error.html" }
}

Do you mean this irule ?

when HTTP_REQUEST {
     Check if the URI is /maintenance
    switch [HTTP::uri] {
        "/maintenance" {
             sets the timer to return client to host URL 
            set stime 10
             Send an HTTP 200 response with a Javascript meta-refresh pointing to the host using a refresh time
            HTTP::respond 200 content \
                " \
                Sorry! This site is down for maintenance." "Content-Type" "text/html" 
            return
        }
    }

     If the respective pool is down, redirect to the maintenance page else 
     send the request to the respective pool 
    if { [ active_members [LB::server pool] ] < 1 } { 
        log local0. $host 
        HTTP::redirect "/maintenance?uri=[HTTP::uri]" 
        return 
    }
}