On 14/06/2023 15:21, Nikolai Zhubr wrote:
Hi all,
I'm trying to migrate my servlet previously running on Tomcat 7 for
ages, to Tomcat 9.0.36 as per openSuse 15.4, and facing some problem.
The servlet in question is using websocket, basically as a
security-enhanced http-friendly replacement of plain-old TCP socket.
That is, as a bi-directional binary stream with guaranteed data
integrity and ordering.
By default Tomcat usually tries to spread the load over multiple threads
as much as reasonably possible. With normal http(s) requests, this all
works quite well out-of-the-box in most cases with no substantial
drawbacks. Now, websocket is somewhat different. Trying to automatically
split incoming websocket data over multiple threads is both useless and
harmfull, at least in some specific simple use-cases, because (1) the
servlet is not prepared to parallelize incoming commands from the same
client connection (it would make no sense due to application logic) so
it only adds unnecessary locking burden, and (2) more importantly, it
sometimes breaks the ordering of data in incoming websocket stream,
because OS thread scheduling is non-deterministic, so some portions
might arrive to the servlet in wrong order, from time to time.
There are multiple things that do not make sense in the above paragraph.
Servlets play no role in processing WebSocket data.
Incoming WebSocket frames are processed synchronously on a single thread
for as long as there is data to read. Processing may switch to a
different thread between messages but that won't change the order in
which messages are received on onMessage() is called.
The Jakarta WebSocket specification explicitly states that "In all
cases, the implementation must not invoke an endpoint instance with more
than one thread per peer at a time"
This all
was observed ages ago and for Tomcat 7 a simple fix was found:
replace "Protocol=HTTP/1.1" with
"Protocol=org.apache.coyote.http11.http11protocol"
Java class names and XML attribute names are case sensitive. And the
quotes are in the wrong place. The above would never have worked. I
assume you meant:
protocol="org.apache.coyote.http11.Http11Protocol"
which is the deprecated (and removed in 8.5.x) BIO connector.
in respective <Connector> item.
This made Tomcat 7 not spread one connection between multiple threads
and all worked just fine then.
Now in Tomcat 9, the "http11protocol" option no longer exists. Instead,
there is say "http11NioProtocol". But, it does not disable multithreaded
websocket.
Again, case is important. The above values are not correct.
So, my question is, is it possible to somehow prevent multithreading in
handling one connection in Tomcat 9?
There is no multi-threading within a single WebSocket connection. It is
explicitly not allowed by the Jakarta WebSocket specification and Tomcat
follows that rule.
How do I configure that?
N/A.
You need to fix whatever it is in your application that is causing
messages to be processed out of order.
Mark
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