How Big, How Fast?

Marimba's Castanet Software Distribution System approaches the Web-performance problem from two sides: scalability and browser performance. Because browser performance typically degrades as the number of users increases, these are two sides of the same coin. The Castanet architecture approaches the problem by simplifying distribution through proxies and repeaters, thus splitting up bandwidth issues among a number of distributed systems. Instead of a single transmitter serving all tuners, repeaters are placed where they are needed to keep all channels up-to-date for local users and to improve users' performance by eliminating the need for transoceanic and transcontinental Web links.

Castanet balances loads by mak ing all tuners connect initially to a main transmitter when subscribing to a channel. If the transmitter has a repeater that's located closer to the subscriber, it points the tuner to that repeater. Channel updates are done transparently, between the tuner and the repeater, which is kept up-to-date by the transmitter ( see the figure ). If the repe ater is down, the tuner is smart enough to go back to the original uniform resource locator (URL) and obtain the update directly from the transmitter (or get referred to another functional repeater).

Bandwidth is an issue for subscribers, too, if they happen to be sitting inside a corporate firewall. A Castanet proxy server placed outside the firewall acts as a special kind of repeater, a single point of contact for all corporately subscribed channels. The proxy server can relay updates directly to users, minimizing the number of direct connections to each Castanet transmitter.

Finally, TCP virtual circui ts for every file and graphic sent over the Web really drag down browser performance. Marimba's patent-pending Application Distribution Protocol (ADP) establishes a single TCP circuit between tuner and transmitter and uses it to update all channels at once, eliminating a huge amount of protocol overhead. Incremental updating of code and data also helps to reduce the amount of network traffic. Marimba says that it's patenting ADP to retain control over its early development, much as Sun retained control over Java.

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Castanet's Distribution Structure

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Castanet tries to relieve Web bottlenecks by distributing the load to server-b ased repeaters.