Coyote Point Systems E350GX, Installation And Administration Manual

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Configuring a Cluster’s Load-Balancing Options
Equalizer Installation and Administration Guide 131
For example, before HTTP 1.1, if a browser wished to retrieve the file index.html from the server
www.coyotepoint.com
, the browser would take the following actions:
1. Browser opens TCP connection to
www.coyotepoint.com
.
2. Browser sends request to server “ GET /index.html ”.
3. Server responds with the content of the page (a bunch of HTML).
4. Server closes connection.
5. Browser determines that there are objects (images) in the HTML document that need to be retrieved, so the
browser repeats Steps 1 to 4 for each of the objects.
There is a lot of overhead associated with opening and closing the TCP connections for each image. The way HTTP
1.1 optimizes this is to allow multiple objects (pages, images, etc) to be fetched and returned across one TCP socket
connection. The client requests that the server keep the connection open by adding the request header
Connection:
keep-alive to the request.
If the server agrees, the server will also include Connection: keep-alive in its response headers, and the client is
able to send the next request over the persistent HTTP connection without the bother of opening additional
connections. This is how Equalizer behaves.
For a Layer 7 cluster, Equalizer evaluates (and possibly changes) both the request and response headers that flow
between the client and server (the request and response bodies are not examined). Match rules are applied to each
client header, cookies may be inserted, and headers may be rewritten. When a client includes
keep-alive in its
headers, there is a fair amount of work required by the Equalizer to determine when the next set of request headers is
ready to be parsed (evaluated), since there may be quite a lot of data going across the connection between sets of
headers.
To reduce this workload, the
once only
flag instructs the Equalizer to evaluate (and potentially modify) only the
first set of headers in a connection. So, in our example above, only the headers in the request for the index.html file
are evaluated; the subsequent requests to obtain the images are not load balanced, but sent to the same server as the
first request.
Enabling
once only
is basically not compatible with persistence and Layer 7 HTTPS cluster (SSL offloading), since
we generally want to examine every request in a connection when persistence or SSL offloading are enabled.
Whether
once only
is enabled or not has a significant effect on how Equalizer routes requests, as summarized in the
following table: