Coyote Point Systems Equalizer Spezifikationen PDF herunterladen (Seite 305)

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:

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 (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 1.0 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.

For HTTP/1.1, persistent connections are the default.

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 can be incompatible with persistence and Layer 7 HTTPS clusters (which rewrite HTTP to

HTTPS links in server response headers), since in these cases we generally want to examine every request in a

connection. However, in configurations where examining the headers in every transaction in a connection is not

required, enabling once only can significantly improve performance.

Whether once only is enabled or not has a significant effect on how Equalizer routes requests, as summarized in

the following table:

Requests

in a single

keep-alive

connection

once only enabled once only disabled

First Request

persist

enabled

If request contains a cookie and there is no match

rule hit, send request to the server in the cookie.

If request contains a cookie and there is a match rule

If request contains a cookie and there is no match

rule hit, send request to the server in the cookie.

If request contains a cookie and there is a match rule

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Equalizer Administration Guide

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