PS: you should also combine memcache + ds datastore to reduce cpu.
regards
didier
On Jan 30, 1:10 pm, Artur Downar <artur.dow...@googlemail.com> wrote:
> So, you mean polling on the database.
>
> I wrote:
>
> try {
> for (int i=0; i<100; i++) {
> entity = ofy.find(ToModemInfoEntity.class, "whatever");
> if (entity != null) {
> log.info("got data from db, do my stuff here...");
> } else {
> Thread.sleep(100);
> }
> }
> }catch (Throwable th) {
> th.printStackTrace();
> }
>
> It is cpu-time consuming.
> It polls for about 11seconds and it takes about 1500ms of the cpu time.
> It makes more than 2hours of the CPU time per natural day.
>
> I can extend the Time.sleep(period) but it still seems unacceptable due to
> CPU costs.
>
> On Sun, Jan 30, 2011 at 8:21 AM, Didier Durand <durand.did...@gmail.com>wrote:
>
> > Sorry,
>
> > hit "send" too quicly
>
> > b) why don't you do something like this
>
> > Side reading data:
> > ==================
>
> > do {
> > read new data from ds;
> > if (new data) {
> > mark data as read in ds
> > do what you need
> > return;
> > } else {
> > Thread.sleep(100)
> > }
> > } until (running time near 30s)
> > if (no new data) {
> > give error message to waiting user
> > }
>
> > Side writing data
> > ==================
>
> > write new data to datastore (with corresponding status showing that
> > it's new and not read yet)
>
> > N.B: the 30s is a hard limit imposed by gae. You've to check for it
> > and end gracefully else your servlet will be killed by GAE.
>
> > regards
>
> > didier
>
> > On Jan 30, 8:15 am, Didier Durand <durand.did...@gmail.com> wrote:
> > > Hi,
>
> > > To answer your questions:
>
> > > a) yes, you can say how long you waits Thread.sleep(millis) where
> > > millis says how many milliseconds you want to wait
>
> > > b) why don't you do something like this
>
> > > do {
> > > read new data from ds;
>
> > > }
>
> > > On Jan 29, 10:38 pm, Stephen Johnson <onepagewo...@gmail.com> wrote:
>
> > > > At the moment the Channel API is javascript only. There are a lot of
> > people
> > > > that would like it to be open to other languages for things just like
> > you
> > > > want to do. You might be able to use something like Rhino to run the
> > > > javascript in Java but not sure if that will work. You should take a
> > look at
> > > > XMPP, it's very simple to have the server send instant messages. I did
> > a
> > > > quick search for XMPP libraries in Java and here is one I found. I'm
> > sure
> > > > there are many others. Check outhttp://
> > twit88.com/blog/2009/02/17/java-xmpp-client-library/
> > > > On Sat, Jan 29, 2011 at 2:07 PM, Artur Downar
> > > > <artur.dow...@googlemail.com>wrote:
>
> > > > > Thank you.
>
> > > > > I took a brief look on channel API and it seems I it is the thing I
> > need.
>
> > > > > I have a small question.
>
> > > > > The modem device is a java embedded device and it has no public IP.
> > There
> > > > > is no javascript on it. Only java.
>
> > > > > Is it enough for GAE channel API to communicate with it ?
>
> > > > > On Sat, Jan 29, 2011 at 9:49 PM, Stephen Johnson <
> > onepagewo...@gmail.com
> > > > > > wrote:
>
> > > > >> Sounds like you need more of a Push style. Checkout the Channel API
> > or the
> > > > >> XMPP api.
>
> > > > >> On Sat, Jan 29, 2011 at 1:47 PM, Artur Downar <
> > > > >> artur.dow...@googlemail.com> wrote:
>
> > > > >>> The external device polls for data from GAE.
> > > > >>> It is not specified when the data arrives. To describe it more
> > clearly.
> > > > >>> The device is a GSM modem device that sends an SMS on the user
> > request.
>
> > > > >>> The user launches a web browser and writes a mobile phone number.
> > The
> > > > >>> browser sends data to the GAE and the sms should be send
> > immediately
>
> > > > >>> So I cannot specify how long I can wait in Thread.sleep().
>
> > > > >>> You mention about Objectify. I use that library but I'm quite new
> > to it.
>
> > > > >>> Does it have the mechanism that works like:
>
> > > > >>> process A waits for data until data into database arrives or
> > timeout
> > > > >>> expires
> > > > >>> process B puts the data into database
> > > > >>> process a continues processing the data inserted by process B
>
> > > > >>> ?
>
> > > > >>> On Sat, Jan 29, 2011 at 3:59 PM, Didier Durand <
> > durand.did...@gmail.com>wrote:
>
> > > > >>>> Hi,
>
> > > > >>>> Forgot: you can use Thread.sleep() to wait (java.lang.Thread is
> > also
> > > > >>>> part of the JRE)
>
> > > > >>>> regards
>
> > > > >>>> didier
>
> > > > >>>> On Jan 29, 3:35 pm, Didier Durand <durand.did...@gmail.com>
> > wrote:
> > > > >>>> > Hi,
>
> > > > >>>> > LinkedBlokingQueue is part of JRE WhiteList:
> > > > >>>>http://code.google.com/appengine/docs/java/jrewhitelist.html. So,
> > you
> > > > >>>> > can use it.
>
> > > > >>>> > But, I dont see the need. Why don't you just from on the
> > datastore on
> > > > >>>> > 1 side and read from it on the other. It would be very simple
> > and rely
> > > > >>>> > on the most basic (i.e solid) mechanism of gae. (I would
> > recommend
> > > > >>>> > Objectify for ds read/ write)
>
> > > > >>>> > As you have network round-trips, the read / write time will be
> > > > >>>> > negligible even it can seem high compared to an in-memory
> > mechanism
> > > > >>>> > like a Queue.
>
> > > > >>>> > regards
>
> > > > >>>> > didier
>
> > > > >>>> > On Jan 29, 1:51 pm, arturad <artur.dow...@googlemail.com>
> > wrote:
>
> > > > >>>> > > In my application an embedded device (no public IP) should
> > connect
> > > > >>>> to
> > > > >>>> > > the GAE in order to obtain some data from it. The data are
> > provided
> > > > >>>> by
> > > > >>>> > > the web browser.
>
> > > > >>>> > > The whole data-passing process looks like:
>
> > > > >>>> > > web browser ->
> > GAE ->
> > > > >>>> > > embedded standalone device
>
> > > > >>>> > > I developed a servlet the device connects to. It issues the
> > HTTP
> > > > >>>> GET.
> > > > >>>> > > On the other side the web browser sends data using standard
> > GWT
> > > > >>>> > > RemoteServiceServlet.
>
> > > > >>>> > > In case there is no data for the device the doGet method in
> > the
> > > > >>>> > > servlet should stop for some seconds until user enters data or
> > time
> > > > >>>> > > out expires.
>
> > > > >>>> > > I'm trying to use LinkedBlockingQueue to pass data between
> > two
> > > > >>>> > > servlets. It does not work unfortunately. Seems like the GAE
> > > > >>>> launches
> > > > >>>> > > new JVM for concurrent requests... I pushed the
> > LinkedBlockingQueue
> > > > >>>> > > into memcache and get it by name from concurrent requests.
> > Still
> > > > >>>> does
> > > > >>>> > > not work. Memcache returns NOTnull. But there are no data in
> > the
> > > > >>>> > > queue.
>
> > > > >>>> > > In order to investigate my issue I've made some tests with the
> > > > >>>> > > semaphore. Pushed the semaphore to the memcache and made some
> > > > >>>> > > concurrent operations ... -> does not work -> one process does
> > not
> > > > >>>> > > release the other...
>
> > > > >>>> > > The problem occurs on GAE only. The whole mechanism works on
> > the
> > > > >>>> > > development server.
>
> > > > >>>> > > Is there any way to stop/freeze one process (in the doGet
> > method in
> > > > >>>> > > servlet) and unblock it by another one ?
>
> > > > >>>> > > Or... Is there any other way to solve my problem ?
> > > > >>>> > > Thank you for any suggestions.
> > > > >>>> > > Artur
>
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