A Django site (hosted on Webfaction) that serves around 950k pageviews a month is experiencing crashes that I haven't been able to figure out how to debug. At unpredictable intervals (averaging about once per day, but not at the same time each day), all requests to the site start to hang/timeout, making the site totally inaccessible until we restart Apache. These requests appear in the frontend access logs as 499s, but do not appear in our application's logs at all.
In poring over the server logs (including those generated by django-timelog) I can't seem to find any pattern in which pages are hit right before the site goes down. For the most recent crash, all the pages that are hit right before the site went down seem to be standard render-to-response operations using templates that seem pretty straightforward and work well the rest of the time. The requests right before the crash do not seem to take longer according to timelog, and I haven't been able to replicate the crashes intentionally via load testing.
Webfaction says that isn't a case of overrunning our allowed memory usage or else they would notify us. One thing to note is that the database is not being restarted (just the app/Apache) when we bring the site back up.
How would you go about investigating this type of recurring issue? It seems like there must be a line of code somewhere that's hanging - do you have any suggestions about a process for finding it?
I once had some issues like this, and it basically boiled down to my misunderstanding of thread-safety within django middleware. Basically the django middleware is I believe a singleton that is shared among all threads, and these threads were thrashing with the values set on a custom middleware class I had. My solution was to rewrite my middleware to not use instance or class attributes that changed, and to switch the critical parts of my application to not use threads at all with my uwsgi server as these seemed to be an overall performance downside for my app. Threaded uwsgi setups seem to work best when you have views that may complete at different intervals (some long running views and some fast ones).
Since you can't really describe what the failure conditions are until you can replicate the crash, you may need to force the situation with ab (apache benchmark). If you don't want to do this with your production site you might replicate the site in a subdomain. Warning: ab can beat the ever loving crap out of a server, so RTM. You might also want to give the WF admins a heads up about what you are going to do.
Update for comment:
I was suggesting using the exact same machine so that the subdomain name was the only difference. Given that you used a different machine there are a large number of subtle (and not so subtle) environmental things that could tweak you away from getting the error to manifest. If the new machine is OK, and if you are willing to walk away from the problem without actually solving it, you might simply make it your production machine and be happy. Personally I tend to obsess about stuff like this, but then again I'm also retired and have plenty of time to play with my toes. :-)
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I am new to programming and am working on pushing real time data from a PLC to a web page either by deploying HTML 5 on the WAGO or a Modbus driver wrapper. I honestly have tried to research but don't know where to start. it will be a closed private network with little to no influence from the outside web. I am simply looking to display a single piece of live information for proof of concept. basically I'm trying to custom design a Groov program.
You might want to look into using OPC. Kepware & SoftwareToolbox are just 2 of many vendors that offer tools to help you get your data the way you want it.
There is an existing tool to do what you want, but I am under the impression you have to write one from scratch. The existing tool is http://www.softwaretoolbox.com/cogentdatahub/ if you are interested in looking at it for ideas.
I've been able to interface with PLC using python and modbusTCP and an Raspberry pi as the webserver. Python is a quick and easy to learn language. Websockets are the HTML5 component best used for realtime data.
simple connect code (after you install everything):
from pymodbus.client.sync import ModbusTcpClient as ModbusClient
from time import sleep
client = ModbusClient('ip_address_of_modbus_IO')
if(client.connect()):
print(client.read_discrete_inputs(200,1).bits[0])
client.write_coil(0,True)
sleep(100)
client.write_coil(2,True)
found here:
http://simplyautomationized.blogspot.com/2013/09/home-automation-project-2-rpi-light.html
Can create a websocket broadcast server using an example here:
http://simplyautomationized.blogspot.com/2015/09/raspberry-pi-create-websocket-api.html
Fortunately you can not push data to a browser.
The Internet would become an even greater mess if you could.
To solve this, have your webpage contain a timer, written in JavaScript.
Every say 1 sec. it does an AJAX request (e.g. use jQuery implementation) to the server, which then delivers (almost) realtime data.
The webpage then displays that in some DOM element, e.g. an empty DIV.
So it's the browser polling your server.
#BlueDog
The data is "almost" realtime because sampling once a second gives a delay of at least one second. In the ideal case, as soon as data changes, it would be pushed to the browser. Unfortunately the browser has no way of knowing that anything changed, so the best it can do is frequently "ask" for updates (polling).
How much the delay is depends on your poll frequency. If it's once per second one has to add the delays for transmission of the page request and the reply of the server. The transmission time depends on your network (which may be the Internet with all uncertainty involved). If the backbones involved have enough capacity I expect overall delay to be between 1 and 1.5 seconds. With a dedicated network and even more frequent polling, I expect that 0.5 seconds should be possible. These are however estimated averages. If I request a page over the Internet and my provider (again) has a problem, it may be hours before I receive what I want. Also things like virus scanners and OS updates may spoil your game.
So, practically: with a good broadband connection, a stable browser and the right process priorities it should be possible to get below 1 second overall delay (incl. poll time interval) for 95% of the time. Be prepared to reboot the client every few days. Most browsers leak memory and most OS'es do so too.
I'm not after any code in particular but I want to know what is the most efficient way to build a function that will constantly check for updates for things such as messages e.g. Have a chat conversation window and I want live updates such as Facebook.
Currently I have implemented it by putting a while loop in my core code that checks if the view is currently visible run a Task every 5 seconds to get new messages. This works but I don't believe its the most efficient way to do it and I need to consider battery life. *Note I do change visibility when the view goes away e.g. on iOS i do
public override ViewDidDissapper {
Model.SetVisible(false)
}
Has anyone implemented some sort of polling on a cross platform app?
There are many different possible solutions here - which one you prefer depends a lot on your requirements in terms of latency, reliability, efficiency, etc - and it depends on how much you can change server side.
If your server is fixed as a normal http server, then frequent polling may be your best route forwards, although you could choose to modify the 5 seconds occasionally when you think updates aren't likely.
One step up from this is that you could try long polling http requests within your server.
Another step beyond that are using Socket (TCP, UDP or websocket) communications to provide "real time" messaging.
And in parallel to these things, you could also consider using PUSH notifications both within your app and in the background.
Overall, this is a big topic - I'd recommend reading up about PushSharp from #Redth and about SignalR from Microsoft - #gshackles has some blog posts about using this in Xamarin. Also, services like AzureMobileServices, UrbanAirship, Buddy, Parse, etc may help
In my Magento store there is a piece of code that is not closing the DB connection (it seems). Normally I don't see any problems. However the problem arises when I enable caching. After a few hours, the site grounds to a halt and eventually it dies with the MySQL error "Too many connections".
It seems that the "bad" piece of code is cached and reused and therefore gets worse and worse until...death.
I'm scratching my head to find out where this rogue piece of code is being called from(of course there could be more than 1 problem).
I doubt it's a core problem otherwise I probably would have heard of it whilst Googling the issue. So that just leaves 3rd part modules and code I have written.
One option would be to disable all modules and re-enable each one until the problem occurs. But of course it might not happen for several hours (and when it did you just know it would be in the middle of the night ;)). Then of course it might not a 3rd party issue but something I have done. And also I need certain modules in my store for it to function properly (payment gateways etc).
So I'm after suggestions on how to track this down...
I've enabled MySQL logging but it doesn't really tell me all that much.
Any ideas?
Magento 1.7.0.2 and APC with Apache 2
i have always wanted to make a private server but i don't know how i would do this.
i know how a private server works, the game sends data packets to the server. the server will take the data and process it and send data to the other games connected.
my questions are,
how do you edit the game so it will go to your server/change game data.
how do you find what packets do what.
the game will be something like WOW, i have not desided yet.
If you are hoping to embark on creating your own MMORPG then you have a huge task ahead of you, and unfortunately to put it nicely you are probably being too ambitious especially if you are asking these sorts of questions.
You should probably read up on client server architecture.
Also, in answer to your questions about the structure of the data being sent and how it is interpreted, well, that's 100% up to the people that design the system. You will want to simulate the entire game on the server(s) and don't trust the clients at all.
For something as complex as a MMORPG it is really important to create a solid design for the system before anything else, this is very important.
Just to be clear your intent is to create an emulated MMO server to the effect of WOW?
That's not really a trivial task and carries with it its own ethical implications.
Just to get started will require a ton of research, inspection, decoding, an extreme attention to detail.
If you are serious about it, then I would suggest looking up networking tools that can help you inspect traffic across the network and creating a scientific process for operation inspection.
Again, it should be noted this is by no means a trivial task.
This will be fairly difficult as you do not have the communicaton protocol specification for the game's client/server communication.
If you want to start this, then create a server that is simply a pass through. That is, all client requests are forwarded to the particular server. Once you have generated a large enough sample size of packets to study, then you can begin to dissect the meaning of each byte (possibly). Of course, if the packets are encrypted in any way (even a simple XOR encryption) then you will have an even harder time trying to figure out what each byte means. You should capture a sample set using two clients running sniffers so you can see what happens when one client does something and it needs to be sent to all clients.
But if I were you, I would just abandon the idea and work on something else. My two cents..
If you'd like an inside look at how games do networking, there's always Ryzom, which went open-source earlier this year. If you're creating your own MMO you can begin right there, and if you're looking to reverse-engineer one you can practice with your own client and server.
I have web pages that take 10 - 20 database queries in order to get all the required data.
Normally after a query is sent out, the Django thread/process is blocked waiting for the results to come back, then it'd resume execution until it reaches the next query.
Is there's any way to issue all queries asynchronously so that they can be processed by the database server(s) in parallel?
I'm using MySQL but would like to hear about solutions for other databases too. For example I heard that Postgresql has an async client library - how would I use that in this case?
This very recent blog entry seems to imply that it's not built in to either the django or rails frameworks. I think it covers the issue well and is quite worth a read along with the comments.
http://www.eflorenzano.com/blog/post/how-do-we-kick-our-synchronous-addiction/ (broken link)
I think I remember Cal Henderson mentioning this deficiency somewhere in his excellent speech http://www.youtube.com/watch?v=i6Fr65PFqfk
My naive guess is you might be able to hack something with separate python libraries but you would lose a lot of the ORM/template lazy evaluation stuff django gives to the point you might as well be using another stack. Then again if you are only optimizing a few views in a large django project it might be fine.
I had a similar problem and I solved it with javascript/ajax
Just load the template with basic markup and then do severl ajax requsts to execute the queries and load the data. You can even show loading animation. User will have a web 2.0 feel instead of just gloomy page loading. Ofcourse, this means several more HTTP requests per page, but it's up to you to decide.
Here is how my example looks: http://artiox.lv/en/search?query=test&where_to_search=all (broken link)
Try Celery, there's a bit of a overhead of having to run a ampq server, but it might do what you want. Not sure about concurrency of the DB tho. Also, if you want speed for your DB, I'd recommend MongoDB (but you'll need django-nonrel for that).