I'm looking into syncronizing some devices, and I am having a local solution requiring to work without internet.
I have used the System.currentTimeMillis(); but it's good only for one device as system clocks arent totally sync even with automatic time update and same timezones. So I was wondering, what can I use as I cannot use NTP server/internet?
Is the fastest way to get it by setting up a server on a pc, connect via cable to the router and query it for it's timestamp?
Or use a android device as a server and query that?
Is there any other solution? open to everything within java/c++ and wlan/wifi (no internet)
Update,
Have been searching for some hours.. And found that many(almost everyone) have the same issue without NTP. Giving it more thought, I will just create that server solution on a lan connection and fetch a timestamp.. Still open for ideas though :)
you can implement all sort of solutions with distributed memory, but it's exhausting, and you'll be introduced with many different issues...
synchronizing all servers with NTP servers is the way to go.
Related
So I've been tracking a bug for a day or two now which happens out on a remote server that I have little control over. The ins and outs of my code are, I provide a jar file to our UI team, which wraps postgres and provides storage for data that users import. The import process is very slow due to multiple reasons, one of which is that the users are importing unpredictable, large amounts of data (which we can't really cut down on). This has lead to a whole plethora of time out issues.
After some preliminary investigation, I've narrowed it down to the jdbc to the postgres database is timing out. I had a lot of trouble replicating this on my local test setup, but have finally managed to by reducing the 'socketTimeout' of the connection properties to 10s (there's more than 10s between each call made on the connection).
My question now is, what is the best way to keep this alive? I've set the 'tcpKeepAlive' to true, but this doesn't seem to have an effect, do I need to poll the connection manually or something? From what I've read, I'm assuming that polling is automatic, and is controlled by the OS. If this is true, I don't really have control of the OS settings in the run environment, what would be the best way to handle this?
I was considering testing the connection each time it is used, and if it has timed out, I will just create a new one. Would this be the correct course of action or is there a better way to keep the connection alive? I've just taken a look at this post where people are suggesting that you should open and close a connection per query:
When my app loses connection, how should I recover it?
In my situation, I have a series of sequential inserts which take place on a single thread, if a single one fails, they all fail. To achieve this I've used transactions:
m_Connection.setAutoCommit(false);
m_TransactionSave = m_Connection.setSavepoint();
// Do something
m_Connection.commit();
m_TransactionSave = null;
m_Connection.setAutoCommit(true);
If I do keep reconnecting, or use a connection pool like PGBouncer (like someone suggested in comments), how do I persist this transaction across them?
JDBC connections to PostGres can be configured with a keep-alive setting. An issue was raised against this functionality here: JDBC keep alive issue. Additionally, there's the parameter help page.
From the notes on that, you can add the following to your connection parameters for the JDBC connection:
tcpKeepAlive=true;
Reducing the socketTimeout should make things worse, not better. The socketTimeout is a measure of how long a connection should wait when it expects data to arrive, but it has not. Making that longer, not shorter would be my instinct.
Is it possible that you are using PGBouncer? That process will actively kill connections from the server side if there is no activity.
Finally, if you are running on Linux, you can change the TCP keep alive settings with: keep alive settings. I am sure something similar exists for Windows.
I have a Java application, and I need it to be high available.
I was thinking of FastMPJ, like running multiple instances on different PCs. Every minute the app will check if master instance is running, and if not, the other will run instead of it.
I'd like to ask if it is a good solution, or if there is any better.
A more general solution is to use a load-balancing system, that is: you have N instances of the application running with the same privileges (if possible on different hardware), then a redundant load-balancer in front selects one of those based on the actual load for each request/task.
The benefit of this solution is obviously, that hardware is actually used and doesn't sit somewhere idle, waiting on the 0.01% case to jump in. Then the instance is actually tested all the time, and errors will be reported when they happen (like faulty hardware), and you prevent a: "Oh... the backup isn't even working". And on top of that you balance the load between machines adaptively.
In one of my project while implementing a exchange we used Apache Qpid for high availability and my experiense was quite satisfaotory. It scales very well too. I have been running application up to 32 node clusters. Please find further details here and let me know in case u need any further infromation:
http://qpid.apache.org/releases/qpid-0.18/books/AMQP-Messaging-Broker-Java-Book/html/High-Availability.html
Hope it helps:)
One often forgets that there must also be high availability from the application to database as well. It is my experience that the data access layer is where most of the application bottlenecks occur. So make sure you have a good application aware DB load balancer. Oracle has a solid solution but is for Oracle databases only. PostGres has an open source version. Heimdall Data is a commercial solution.
I have seen applications that can detect adjacent networks and desktops and devices attached to them. They can also know the computer/device name that is attached within 30 seconds.
Shall I try runtime.execute ping and net view command to do it, for I find them fast.
How can I capture the output as a result from these commands?
I tried sockets but they are time consuming.. only advantage, that I can also know that they have application installed (in which I created socket, enabling this communication).
Regards
Time-Outs in the initialization of Socket are useful, but you cannot have each connection connected within less than 300 Milli-seconds. On the server side also there is a timeout implementation. There is one sided communication in both. Multi-threading will help.
I'm developing a test app with GWT+Java-AppEngine and the deploys are so heavy and slow.
I read about minimize permutations or parallel compilation of GWT, but my internet connection is not so good and I think that I'm uploading heavy files to App Engine Server.
How can I optimize this? Can I check where is the bottleneck?
The reason that I need several deployments is because I'm using Google API's through OAuth and I can't set localhost as a callback (I do?).
I am not entirely sure about your scenario, so I will try to guess your intentions.
For development purposes, you really should be working on the local server, it comes with all the API's and stubs for things like user login and what not. That is instantaneous. Once you are happy with your local app and it is time to upload, then if AppEngine overlord decides to take time due to AppSize/Slow Connection/Service outage/random act of diety, there is little one can do.
Considering that one doesn't deploy every hour, I think your time would be better spent on the app, instead of tuning the upload time.
I am assuming you are already following http://code.google.com/appengine/docs/java/gettingstarted/uploading.html
I personally have dabbled with appengine but the python version, and it may take a few mins but once upload is complete, you are good to go.
maybe you could get your local machine a dyndns hostname and make it accessible from the internet ? –
I think what Bastian meant was as follows (assuming Dev server can actually serve domains - i am not sure about that)
Have your domain host (example.com) maintain an 'A' record pointing to your development machine IP address [hence when you do example.com, your dev machine responds as the server]
This means if you setup DNS records to point to ghs.google.com or whatever, you will have to change them (DNS records take a while to propagate depending on host)
Once you are happy, and you want to test on google, you still have to 'upload' before you can try it on appspot.com and ofcourse change DNS entries again so example.com works off google servers.
Too much work in my opinion. Better to use dev server in local machine.
Have a break while you are uploading. Have a KitKat to kill time :)
I'm thinking about writing a game which is based around a server, and several client programs connect to it. The game (very) basically consists of a list of items which a user can 'accept', which would remove it from the list on all connected computers (this needs to update very quickly).
I'm thinking about using a Java applet for the client since I would like this to be portable and run from a browser (mostly in Windows), as well as updating fast, and either a C++ or Java server running on Linux (currently just a home server, but possibly to go on a VPS).
A previous 'incarnation' of this game ran in a browser, and used PHP+mySQL for the backend, but this swamped the server quite a bit when several people connected (that was with about 8 people, this would eventually need to handle a lot more).
The users would probably all be in the same physical location (with the same public IP address), and the system would get several requests per second, all of which would require sending the list back to the clients.
Some computers may have firewall restrictions on them, so would you recommend using HTTP traffic, a custom port, or perhaps through SSH or some existing protocol?
Could anyone suggest some tips (threading, multiple requests of one item?), tools, databases (mySQL?), or APIs which would help me get started on this project? I would prefer C++ for the backend as it would be faster, but using Java would allow me to reuse code.
Thanks!
I wouldn't use C++ because of speed alone. It is highly unlikely that the difference in performance will make a real difference to your game. (Your network is likely to cloud any performance difference, unless you have 10 GigE between the client and server) I would use C++ or Java because you will get it working first using that language.
For anyone looking for a good networking API for c++ I always suggest Boost.Asio. It has the advantage of being platform independent, so you can compile a server for linux, windows etc. However, if you are not too familiar with c++ templates/boost the code can be a little overwhelming. Have a look, give it a try.
In terms of general advice. Given the description above, you seem to need a relatively simple server. I would suggest keeping it very basic, single threaded polling loop. Read a message from your connected clients (wait on multiple sockets), and respond appropriately. This eliminates any issue around multiple accesses to your list and other synchronization problems.
I might also suggest, before you re-write your initial incarnation. Try improving it, as you have stated:
and the system would get several requests per second, all of which would require sending the list back to the clients.
Given that each request removes an item from this list, why not just inform your uses which item is removed, rather than sending the entire list over the network time and time again? If this list is of any significant size, this minor change will result in a large improvement.