What is a NetCache accelerator?

Description of an accelerator

A NetCache appliance can be configured as either of the following types of accelerators:

Web accelerator
Streaming accelerator

When NetCache is configured as an accelerator, it caches content from one or more Web server or streaming servers that you identify, and provides that content to clients that request it. The accelerator is, therefore, an extension of the Web server or streaming server (as applicable). To the outside world, the accelerator is the Web server or streaming server. In contrast, when NetCache is configured as a Web cache or streaming media cache, it acts as an agent for the browser. An accelerator is located close to the origin servers that it services.

Note
An accelerator might also be referred to as a reverse proxy.

Request flow with an accelerator

The following illustration shows the flow of a request from the client to the accelerator, and back to the client.

The following table describes the path that a request travels when an accelerator is deployed.

Stage	Description
1	Users send Web or streaming media requests.
2	Because the accelerator has become an extension of the Web server or streaming server, the accelerator receives the requests.
3	If the accelerator has the requested content in its cache, the accelerator returns that content to the client directly.
4	If the accelerator does not have the requested content in its cache, it fetches the content from the Web server or streaming server, and then returns it to the client while caching the data, if it is cacheable.

In the typical deployment of an accelerator, a Web browser does not have direct communication with the Web server and a media player does not have direct communication with the streaming server.

Advantages of using an accelerator

The advantages of using NetCache as an accelerator include the following:

An accelerator off-loads client traffic from your Web or streaming servers.
An accelerator shields Web servers and streaming servers from the outside world.
Web servers and streaming servers are vulnerable to break-ins, whereas a NetCache appliance is more secure from them. The advantage of using an accelerator is that a Web server's or streaming server's content is always secure because an intruder cannot reach the server. If an intruder breaks into the accelerator, the intruder can only disable the accelerator; the intruder cannot modify the data in any way.

Note
Locating a Web server or streaming server inside the corporate firewall is often desirable to protect the site's content from unauthorized changes. However, depending on its complexity, a corporate firewall might introduce substantial delays or it might have limited bandwidth. NetCache enables you to provide Web content and streaming media content more quickly to users and saves bandwidth because duplicate requests to the same accelerator for the same content are satisfied from the cache.
Work is off-loaded from the Web server or streaming server onto the accelerator. Therefore, you might need fewer Web servers or streaming servers.

What an accelerator caches

A Web accelerator caches only cacheable objects. A Web accelerator proxies noncacheable objects (for example, CGI and private pages).

A streaming accelerator caches on-demand streaming media and splits live streaming media.

Can an accelerator be run in more than one mode

You can run a NetCache appliance as both a Web accelerator and a Web cache. Likewise, you can run a NetCache appliance as both a streaming accelerator and a streaming media cache.

Hit rate when NetCache is an accelerator

The hit rate for a NetCache appliance running as an accelerator is significantly greater than the rate for a NetCache appliance running as a Web cache or streaming media cache. The reason is that the Web server or streaming server that NetCache accelerates has a limited amount of data, compared to the World Wide Web, which has nearly an infinite amount of data. With an accelerator, it is likely that many users will send requests for the same data from the Web server or streaming server.

Load balancing over servers that NetCache is accelerating

If NetCache is configured to accelerate a pool of Web or streaming servers with the same content, NetCache uses a weighting technique based on a history of connection round-trip times to balance the load of requests across the servers. Server CPU load and other server resources are not considered. Initially, all servers have the same chance of being selected. If a server is consistently slower in responding to the NetCache appliance, over time, the chance decreases that the server will be selected.

Scenario

A server that the appliance is configured to accelerate went down. Subsequently, NetCache picks that server from the pool of servers to send the request to. However, the connection fails because the server is down. NetCache records a large round-trip time for this server, which reduces its weight and, therefore, the likelihood that NetCache will pick the server the next time. A chance still exists that NetCache will select that server again because NetCache uses a "weighted random" algorithm. However, over time, the probability that NetCache will select the server decreases. The benefit of this algorithm is that the server is not removed from consideration after a single failure; that failure might be temporary.