RAM caching and queue parallel requests

It's possible to do something like this with an iRule. The following example for 11.x has been tested up to a few hundred requests/sec and seems to operate adequately if not perfectly. A few points to note;

• It uses a subtable per core to track cache entries to queue
• The maximum queue delay is configurable, the example uses 5 sec, adjust this to suit
• If you get cache-busting query-strings, add these to HTTP_REQUEST_SEND

As always, this may or may not work in your environment and there are no guarantees.

...David

 HTTP Parallel Request Queueing for 11.x v1.0 10/7/15 DIH

when HTTP_REQUEST {
  set path [HTTP::path]
  set query [HTTP::query]
 Need to match the Accept-Encoding value if it exists
 The server probably sets the Vary header...
  if {[HTTP::header exists "Accept-Encoding"]} {
    set uri [HTTP::uri]:[HTTP::header "Accept-Encoding"]
  } else { set uri [HTTP::uri] }
 Check the queue flag, if set we need to queue in front of the cache
  if {[table lookup -notouch -subtable cq_[TMM::cmp_unit] $uri] == 1} {
    set i 1
   Bail out of the loop if the cache fills
    while {[table lookup -notouch -subtable cq_[TMM::cmp_unit] $uri] == 1} {
     Only queue for a maximum of 5 seconds
      if { $i > 25 } {
        log local0. "Exceeded max queue time on core [TMM::cmp_unit] for $uri"
        break
      }
      after 200
      incr i
    }
   After dequeueing a connection, need a short wait for the cache to fill
    after 200
  }
}
when CACHE_REQUEST {
  if {[table lookup -notouch -subtable cq_[TMM::cmp_unit] $uri] == 1} {
    if { [CACHE::age] > 0 } {
     Got a cache hit but the queue flag was set - don't panic!
      log local0. "Failsafe: reset table on core [TMM::cmp_unit] for $uri"
     Fix it up and carry on
      table replace -notouch -subtable cq_[TMM::cmp_unit] $uri 0
    }
  }
}
when HTTP_REQUEST_SEND {
 Skip cache-busting URIs here
  if { not ($query contains "ca=n") } {
   Cache miss - set the queue flag
    table replace -notouch -subtable cq_[TMM::cmp_unit] $uri 1
  }
}
when HTTP_RESPONSE {
 Only enable the RAM cache if the header is set
  if { [HTTP::header exists "CacheOn"] } {
    CACHE::enable
   Create one table entry per core, match the cache timeout
    table add -subtable cq_[TMM::cmp_unit] $uri 0 3600
  } else {
    CACHE::disable
  }
  HTTP::header remove "CacheOn"
}
when HTTP_RESPONSE_RELEASE {
 Server reply - clear the queue flag
  table replace -notouch -subtable cq_[TMM::cmp_unit] $uri 0
}