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Date: Thu, 04 Jul 2013 16:12:05 +0100
From: Mark Thomas <markt@apache.org>
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To: Tomcat Users List <users@tomcat.apache.org>
Subject: Re: websockets questions
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On 04/07/2013 15:52, André Warnier wrote:
> Mark Thomas wrote:
>> On 04/07/2013 10:17, André Warnier wrote:
>>> To me that means that at some point, there must be on the server side a
>>> process or thread which is dedicated to this websocket client for as
>>> long as it takes, and this process/thread in the meantime is no longer
>>> available to process other HTTP requests.
>>> (That's because the basic idea is that the "websocket application" on
>>> the server side can keep sending messages asynchronously to the client -
>>> and vice-versa - so I don't see how this can work with different
>>> threads/processes on the server; but I'm not that smart, so it may be
>>> that the implementation is smarter).
>>> For that same reason, it would seem that the very concept of
>>> "load-balancing" must be suspended once the websocket connection is
>>> established.
>>
>> The connection has to be kept open but you can use non-blocking IO to
>> only allocate a thread to process data when there is data to process.
>>
>> The exact behaviour varies between Tomcat 7 and Tomcat 8.
>>
>> BIO 7 & 8 - 1 thread per connections, blocking IO, doesn't scale
>>
>> NIO / APR 7 - 1 thread per currently processed frame, non-blocking
>> between frames, scales better
>>
>> NIO / APR 8 - threads only allocated where there is data to process,
>> scales best
> 
> "data to process" meaning an entire websocket "message" I suppose ?

No. Data means bytes on the wire. It doesn't matter if those bytes are
an entire message (multiple frames), a single frame, part of a frame of
even the last few bytes of one frame and the first few bytes of the
next. Tomcat will read what is available, process it as far as possible
given that data it has to hand (for example if the application indicates
it will except it, partial data can be passed to the application if it
is available) and then release the thread and wait for the next set of
bytes to turn up.

> Another question while we're at it :
> As I understand from the specs/docs, there are 2 kinds of messages :
> text or "blob".
> And I found that there are 2 ways of reading that data, corresponding to
> these 2 types of messages.
> However, I do not find anywhere a function or method or call which would
> allow for example the server-side application to find out in advance if
> the data currently available for reading is one or the other type.  Did
> I miss that somewhere, or do I misunderstand the specs/docs ?

Actually, there are three types exposed to applications: Text, Binary, Pong.
The application only finds out what type the message is when the
container calls the appropriate message handler. Note if an application
doesn't have a handler for a particular message type then the message is
ignored.

Mark


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