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Difference between revisions of "Myths"
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→Myth Four: XMPP is unreliable without a bunch of extensions.
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But if you don't get one back, there's no way to tell if it was the message that was lost or the ack, and there's no very obvious way to handle the situation anyway. Do you send another? Or will the client get the message from offline storage? Do you care? | But if you don't get one back, there's no way to tell if it was the message that was lost or the ack, and there's no very obvious way to handle the situation anyway. Do you send another? Or will the client get the message from offline storage? Do you care? | ||
This is where XEP-0198, Stream Management, comes in. The community developed this extension because TCP sessions are a lot less reliable over mobile — now a number of servers support it, and many mobile clients (particularly the Android ones) will use it if available. | |||
XEP-0198 extends TCP's reliability across multiple sessions, and also extends it through to the application. It, too, sends acks on request, but only across a single link. So a client and server can maintain a buffer of sent stanzas, and if they're not acked (and they must be acked in sequence, just like TCP) then when the client reconnects, it can resume the XMPP session (and both ends can resend any missed stanzas). | XEP-0198 extends TCP's reliability across multiple sessions, and also extends it through to the application. It, too, sends acks on request, but only across a single link. So a client and server can maintain a buffer of sent stanzas, and if they're not acked (and they must be acked in sequence, just like TCP) then when the client reconnects, it can resume the XMPP session (and both ends can resend any missed stanzas). | ||
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This mitigates the Two Generals to the point that only the last few messages sent within a session are subject to it, and only then if resuming the session was somehow impossible. | This mitigates the Two Generals to the point that only the last few messages sent within a session are subject to it, and only then if resuming the session was somehow impossible. | ||
The | So wait, doesn't this mean that XMPP, even with all these extensions, is still unreliable in some cases? Isn't the hypothesis - that XMPP stanzas can be lost sometimes - still true? Sure it is. The Two Generals problem is provably insoluble - anyone claiming that their communications system is 100% reliable is making the network communications equivalent of saying they have a perpetual motion machine, and I'm not *that* much of an idiot. | ||
XMPP is, however, as reliable as a protocol can practically be. | |||
== Myth Five : XMPP is unsuited to the web. == | == Myth Five : XMPP is unsuited to the web. == | ||