On 30 June 2010 22:55, Aaron Fransen <aaron.fran...@gmail.com> wrote: > > > On Wed, Jun 30, 2010 at 6:26 AM, Graham Dumpleton > <graham.dumple...@gmail.com> wrote: >> >> On 30 June 2010 21:35, Aaron Fransen <aaron.fran...@gmail.com> wrote: >> > >> > >> > On Tue, Jun 29, 2010 at 6:17 PM, Graham Dumpleton >> > <graham.dumple...@gmail.com> wrote: >> >> >> >> On 30 June 2010 02:14, Aaron Fransen <aaron.fran...@gmail.com> wrote: >> >> > Couple more things I've been able to discern. >> >> > >> >> > The first happened after I "fixed" the html code. Originally under >> >> > mod_python, I guess I was cheating more than a little bit by sending >> >> > <html></html> code blocks twice, once for the incremental notices, >> >> > once >> >> > for >> >> > the final content. Once I changed the code to send a single properly >> >> > parsed >> >> > block, the entire document showed up as expected, however it still >> >> > did >> >> > not >> >> > send any part of the html incrementally. >> >> > >> >> > Watching the line with Wireshark, all of the data was transmitted at >> >> > the >> >> > same time, so nothing was sent to the browser incrementally. >> >> > >> >> > (This is using the write() functionality, I haven't tried watching >> >> > the >> >> > line >> >> > with yield yet.) >> >> >> >> Use a variation of WSGI middleware wrapper in: >> >> >> >> >> >> >> >> http://code.google.com/p/modwsgi/wiki/DebuggingTechniques#Tracking_Request_and_Response >> >> >> >> using it to 'print' returned data to Apache log and then tail Apache >> >> error log to see when that data is output. Alternatively, change the >> >> code there to output a time stamp against each chunk of data written >> >> to the file recording the response content. >> >> >> >> This will show what data is returned by WSGI application, before >> >> mod_wsgi truncates anything greater than content length specified, >> >> plus also show whether it is your WSGI application which is delaying >> >> output somehow, or whether Apache output filters are doing it. >> >> >> >> Graham >> > >> > I've actually tried a variation on this already using a built-in logging >> > facility in the application that writes date/time values to an external >> > log >> > file with comments, and in the case of testing wsgi I actually included >> > some >> > time.sleep() statements to force a delay in the application. >> > >> > To give you an idea of the flow, here's essentially what's going on: >> > >> > def application(environ,start_response): >> > mydict = {} >> > mydict['environ']=environ >> > mydict['startresponse'] = start_response >> > # run program in another .py file that has been imported >> > RunTest(mydict) >> > >> > Then in the other module you would have something like: >> > >> > def RunTest(mydict): >> > status = '200 OK' >> > response_headers = [('Content-type','text/html')] >> > writeobj = detail['startresponse'](status,response_headers) >> > writeobj('<html><body>Fetching sales for 2009...') >> > time.sleep(2) >> > writeobj('<br>Fetching sales for 2010...') >> > >> > ...then finally... >> > >> > writeobj('5000 results returned.</body></html>') >> > return >> > >> > This is obviously a truncated (and fake) example, but it gives you an >> > idea >> > of the flow. >> >> Now go try the following two examples as illustrated instead. >> >> In both cases, do not use a web browser, instead telnet to the port of >> the web server and enter HTTP GET directly. If you are not using >> VirtualHost, use something like: >> >> telnet localhost 80 >> GET /stream-yield.wsgi HTTP/1.0 >> >> If using a VirtualHost, use something like: >> >> telnet localhost 80 >> GET /stream-yield.wsgi HTTP/1.1 >> Host: tests.example.com >> >> Ensure additional blank line entered to indicate end of headers. >> >> First example uses yield. >> >> # stream-yield.wsgi >> >> import time >> >> def application(environ, start_response): >> status = '200 OK' >> >> response_headers = [('Content-type', 'text/plain')] >> start_response(status, response_headers) >> >> for i in range(10): >> yield '%d\n' % i >> time.sleep(1) >> >> Second example uses write: >> >> # stream-write.wsgi >> >> import time >> >> def application(environ, start_response): >> status = '200 OK' >> >> response_headers = [('Content-type', 'text/plain')] >> write = start_response(status, response_headers) >> >> for i in range(10): >> write('%d\n' % i) >> time.sleep(1) >> >> return [] >> >> For me, using stock standard operating system supplied Apache on Mac >> OS X, I see a line returned every second. >> >> If I use Safari as a web browser, in both cases the browser only shows >> the response after all data has been written and the socket connection >> closed. If I use Firefox however, they display as data comes in. >> >> This delay in display is thus possibly just the behaviour of a >> specific browser delaying the display until the socket is closed. >> >> The example for multipart/x-mixed-replace which others mention is: >> >> import time >> >> def application(environ, start_response): >> status = '200 OK' >> >> response_headers = [('Content-Type', 'multipart/x-mixed-replace; >> boundary=xstringx')] >> start_response(status, response_headers) >> >> yield '--xstrinx\n' >> >> for i in range(10): >> >> yield 'Content-type: text/plain\n' >> yield '\n' >> yield '%d\n' % i >> yield '--xstringx\n' >> >> time.sleep(1) >> >> With telnet you will see the various sections, but with Safari again >> only shows at end, although you will find that it only shows the data >> line, ie., the number and not all the other stuff. So, understands >> multipart format but doesn't support x-mixed-replace. It was always >> the case that only certain browsers supported that mime type. In the >> case of Firefox, it doesn't seem to like it at all and seems to give >> up and not display anything, not even replacing the previously >> displayed page contents. >> >> What this means is that you cant rely on browsers to handle multipart >> mixed replace alone. If you were really going to use that format, you >> really want to use JavaScript and AJAX stuff to process it. The same >> applies for progressive display of plain text content when streamed >> over time. >> >> In summary, you really want to be using some JavaScript/AJAX stuff on >> browser side to get uniform behaviour on all the browsers. >> >> Graham > > I can see that this could potentially get very ugly very quickly. > > Using stock Apache on the current Ubuntu server, using yield produced a > response error
What error? If you aren't going to debug it enough to even work out what the error is in the browser or Apache error logs and post it here for comment so can say what may be wrong on your system, then we cant exactly help you much can we. > and using write() (over the telnet interface) returned the 0 > only and disconnected. Similar behavior in Firefox. All the scripts I provided you are conforming WSGI applications and work on mod_wsgi. If you are having issues, then it is likely going to be the way your Apache/Python is setup or how you configured mod_wsgi to host the scripts. Again, because you are providing no details about how you configured mod_wsgi we cant help you work out what is wrong with your system. > How odd that nobody's come up with a simple streaming/update schema (at > least to my mind). For response content they have and it can be made to work. Just because you cant get it working or don't understand what we are saying about the need to use a JavaScript/AJAX type client (eg. comet style) to make use of it as opposed to trying to rely on browser functionality that doesn't exist doesn't change that. Request content streaming is a different matter as I will explain below but you haven't even mentioned that as yet that I can see. > It would have been nice to be able to provide some kind of in-stream > feedback for long running jobs, but it looks like I'm going to have to > abandon that approach. The only issue with either of the other solutions is > that each subsequent request depends on data provided by the prior, so the > amount of traffic going back & forth could potentially become a problem. > > Alternatively I could simply create a session database that saves the > required objects then each subsequent request simply fetches the required > one from the table and... > > Well, you can see why streaming seemed like such a simple solution! Back to > the drawing board, as it were. I'll try one last time to try and summarise a few issues for you, although based on your attitude so far, I don't think it will change your opinion or help your understanding. 1. Streaming of responses from a WSGI application works fine using either yield or write(). If it doesn't work for a specific WSGI hosting mechanism then that implementation may not be conforming to WSGI requirements. Specifically, between a yield and/or write() it is required that an implicit flush is performed. This should ensure that the data is written to the HTTP client connection and/or ensure that the return of such data to the client occurs in parallel to further actions occurring in that request. 2. A WSGI middleware that caches response data can stuff this up. One cant outright prohibit a WSGI middleware holding on to response data, albeit that for each yield or write() technically it is supposed to still pass on at least an empty string down the chain so as to allow control to get back to the underlying WSGI implementation, which may uses such windows to swap what request context it is operating on so as to allow a measure of concurrency in situation where threads may not be getting used. 3. Where a WSGI adapter on top of an existing web server is used, eg. various options that exist with Apache and nginx, then an output filter configured into the web server may also stuff this up. For example, an output filter that compresses response data may buffer up response data into large blocks before compressing them and returning them. 4. Although response content can be streamed subject to above caveats, streaming of request content is a totally different matter. First off, WSGI requires that the request content have a Content-Length specified. Thus technically a HTTP client cant leave out Content-Length and instead used chunked request content. Further, the way in which many web servers and WSGI servers are implemented would prohibit streaming of request content anyway. This is because many implementations, especially where proxying occurs, eg. cgi, fastcgi, scgi, ajp, uwsgi, mod_proxy (??), and mod_wsgi daemon mode, expect that the whole request content can be read in and written across the proxy connection before any attempt is made to start reading any data returned from the web application. The request content therefore cannot be open ended in length because most implementations will never switch from reading that content to expecting response from the application. Thus it isn't possible to use WSGI as both way streaming mechanism where some request content is written, some response content returned and then the client sends more request content based on that etc etc. So what does this all mean. First up is that response content streaming should be able to be made to work, however since request content streaming isn't technically allowed with WSGI, if you need that you are out of luck if you want to conform to WSGI specification. Second, you can however with mod_wsgi embedded mode slightly step outside of strict WSGI conformance and have request content streaming. You are then bound to Apache/mod_wsgi, but whether you want to do that is debatable for reasons below. The bigger problem with both way streaming or long polling applications which use the same HTTP request is that WSGI servers tend to use processes and threads for concurrency. When you use this mechanisms they will tie up a process or thread for the whole time. Thus if you have lots of concurrent request you need huge numbers of processes and/or threads, which just isn't usually practical because of resource usage such as memory. For that reason, one would instead on the server usually use special purpose web servers for these types of applications and use HTTP directly and avoid WSGI, due to WSGI blocking nature. Instead these servers would use an event driven system model or other system which allows concurrency without requiring a process or thread per application. In short, this is what Comet and dedicated servers for that are about. Allowing large numbers of concurrent long requests with minimal resources. That they are dedicated systems also allows them to avoid limitations in other high level web application interfaces such as CGI, FASTCGI, SCGI, AJP etc which have an expectation that can read whole request content before trying to deal with any response from a web application that is handling the requests. Anyway, hopefully that explains things better. You can do what you want, you just need to select the correct tool for the job. Graham _______________________________________________ Web-SIG mailing list Web-SIG@python.org Web SIG: http://www.python.org/sigs/web-sig Unsubscribe: http://mail.python.org/mailman/options/web-sig/archive%40mail-archive.com