Pass variable between HTTP_REQUEST and HTTP_REQUEST_DATA

that means variable set in one iRules event (e.g. HTTP_REQUEST) will not be available in other iRules event (e.g. HTTP_REQUEST_DATA) ?

no. it is available as long as it is on the same connection.

 config

root@(ve11a)(cfg-sync In Sync)(Active)(/Common)(tmos) list ltm rule qux
ltm rule qux {
    when HTTP_REQUEST {
  log local0. "client=[IP::client_addr]:[TCP::client_port] request=[HTTP::request]"
  if { [HTTP::uri] starts_with "/test" } {
    set variable 1234
  }
  if { [HTTP::method] equals "POST" } {
    HTTP::collect [HTTP::header content-length]
  }
}
when HTTP_REQUEST_DATA {
  log local0. "client=[IP::client_addr]:[TCP::client_port]"
  log local0. "variable=$variable"
}
}

 test1 (curl -d "key1=value1" http://172.28.24.10/test/something)

Oct  7 02:39:55 ve11a info tmm[15094]: Rule /Common/qux HTTP_REQUEST: client=172.28.24.1:43070 request=POST /test/something HTTP/1.1  User-Agent: curl/7.15.5 (i686-redhat-linux-gnu) libcurl/7.15.5 OpenSSL/0.9.8b zlib/1.2.3 libidn/0.6.5  Host: 172.28.24.10  Accept: */*  Content-Length: 11  Content-Type: application/x-www-form-urlencoded
Oct  7 02:39:55 ve11a info tmm[15094]: Rule /Common/qux HTTP_REQUEST_DATA: client=172.28.24.1:43070
Oct  7 02:39:55 ve11a info tmm[15094]: Rule /Common/qux HTTP_REQUEST_DATA: variable=1234

 test2 (curl -d "key1=value1" http://172.28.24.10/something)

Oct  7 02:40:04 ve11a info tmm1[15094]: Rule /Common/qux HTTP_REQUEST: client=172.28.24.1:43071 request=POST /something HTTP/1.1  User-Agent: curl/7.15.5 (i686-redhat-linux-gnu) libcurl/7.15.5 OpenSSL/0.9.8b zlib/1.2.3 libidn/0.6.5  Host: 172.28.24.10  Accept: */*  Content-Length: 11  Content-Type: application/x-www-form-urlencoded
Oct  7 02:40:04 ve11a info tmm1[15094]: Rule /Common/qux HTTP_REQUEST_DATA: client=172.28.24.1:43071
Oct  7 02:40:04 ve11a err tmm1[15094]: 01220001:3: TCL error: /Common/qux HTTP_REQUEST_DATA - can't read "variable": no such variable     while executing "log local0. "variable=$variable""