I am looking for a sample java code which reads time from a particular server (could be a webservice or could be some bytes using TCP) and sets its on client machine. This could be achieved easily if you dont consider latency and compromise on those millisecond/nanosecond differences. I would like to have a pseudo-code for server and client, or an algorithm to achieve this.
Also I would like to know how NTP servers set time on client servers? Is there any white paper that explains this?
Answers are here. For more results you may google with keywords like "NTP architecture" or similar
https://en.wikipedia.org/wiki/Network_Time_Protocol
http://www.tldp.org/HOWTO/TimePrecision-HOWTO/ntp.html
Related
sorry if my english isn't perfect.
I'm trying to make an app and I need to exchange information between more devices.
I thought that could be a solution connect the devices on a server but I really don't have the idea where start.
What language I need to study to make this? There is a better solution?
This highly depends on what you are trying to achieve in the first place. It would be helpful if you could tell what you are trying to do, but I will still outline some general aspects:
You need to decide, what information is going to be exchanged and how this should happen
What information: Figure out, what exactly needs to be sent and received. Generic text messages? Images? Byte Streams?
How should this be done: Generally spoken, there are two approaches of getting information as a client: Polling and subscribing.
Polling: This approach means to periodically check an endpoint for new data. For example, HTTP uses this way: A web browser or any other client (REST-Client for example) periodically requests information from a HTTP-Server, using a connection just for this single request.
Subscribing / Sync / Notification: In some way or another, the client tells the server that it is interested in the information and wants to get notified when there is something new. The connection is initiated at the beginning and held open for further usage. The benefit of this approach is that changes are received immediately, but on the other hand a permanent connection needs to be maintained.
Things to study
At the beginning, get a good understanding of the TCP/IP Protocol, how Sockets work, how common Protocols do their job (e.g. HTTP, WebSockets)
Take a look at specific Protocols working on top of the basic ones
Tip: REST: Most common WebServices Protocol, providing a common way to exchange stateless data. Uses Polling.
WebSockets: Socket connection using Web Browsers. Commonly used to update information without needing to poll.
There is no specific language to learn for connections. It's more about understanding what the difficulties are and what ways have been invented to address this. Once you get to this point and know what you want to do, it's possible in every language.
Recommendation: As you seem to use Java/Android, I would try to use REST. A really great client-side library for REST on Android is Retrofit. For the server side use what fits for you .. common Java way would be to use Jersey, but you are free to choose from a lot of choices. If using Jersey is too hard for the beginning, maybe take a look at the JS/NodeJS world, those guys invented Express, which allows you to create a REST service out of just a database, wihtout having to code a lot.
First you need to decide if you want to go for an Android or an iOS application. There are other various mobile operating systems as well, but these are widely used . If you want to go for android which is most widely used in my opinion, then you need to learn Java. If you want to go for iOS application, then you need to learn swift or objectiveC. These languages provide the API to connect with various types of services such as Facebook, Firebase and Amazon etc. If you want to connect to some other local server who’s IP is known to you, then you can use socket programming to send messages.
There could be many ways you can implement this. One way will be using Web services. Of course REST might be a better option, if you follow this approach. You can implement Your service(server side code) with any language. I will recommend you use java since you are already using android.
Aside from this You might need to go through the basics of REST, its specifications and
some reference implementations for language of your preference.
I'm developing a distributed application, and I need to connect a client Java based to a server C++ based. Both of them will need to send information to each other, but I need them to be able to do things while waiting for the information, and they don't know when they are gonna get new information, or send information.
How can I achieve this? Now I'm trying to implement a basic communication with Sockets, but I don't really get to communicate them. I have read that using sockets + threads is usually a good approach for client-server apps.
Could you please recommend me some web or book to read about this, or send me some example code to learn?
Do you think that i should use other approach, better than sockets? maybe a higher level library (i would need it for c++ and java) or a totally different way?
EDIT:
I will add some extra information.
What I would love to achieve is the following:
My C++ program has a main loop, where I would like to have a call like GetUpdatedDataFromRemoteDevice() where I read the new values of some numerical variables that previously got updated from the net (the socket, for example).
Eventually, the C++ program will need to send a message to the remote device, to tell him to send other kind of data, and after that, keep getting the updated values.
From the Java program (remote device) the application running is an interactive touchable screen, that cant get blocked by the network transmissions, because it must keep working for the user, so all the networking should be done in a separated thread.
That thread, should connect to the server, and when a button is pushed, start to send the data (4 changing numerical values) in a loop until another event happens.
It would be nice also to be easily re-connectable to the server.
ICE is a modern and good library for distributed applications:
many languages as C++ and Java
many platforms
GNU GPL
good performance
easy to use
First, you define the messages you want to exchange between server and client.
Then, you implement the C++ and Java source code to handle these messages.
More info at http://zeroc.com/ice.html
Have fun ;-)
EDIT: I have to use ACE in some projects. I can tell ACE is very old, maybe mature, but uses outdated C++ coding rules :-(
Therefore ACE is not as easy to use as STL or BOOST. Moreover, ACE is not really efficient... I prefer ICE ;-)
I don't know what your application is but robust client server socket programming is pretty hairy task to do properly. Hardware byte order, String encoding, Network errors, retries, duplicate messages, acks etc.. require lots of good design and careful programming. You need to get it work well as single-threaded before even thinking using multiple threads.
Unless you need instant notifications from server to client I suggest that you use HTTP as protocol between client and server. Client can poll server occasionally for new messages.
Anyway the problem has been solved multiple times already.
http://activemq.apache.org/
http://www.rabbitmq.com/devtools.html
http://www.cs.wustl.edu/~schmidt/ACE-overview.html
I did something of this sort once. In my case it was easier to connect my C++ app to a local Java app using JNI and then have the two Java apps talk to each other.
I am trying to set up a webserver on an old machine of mine. I have installed ubuntu server edition and aim to use it for the following:
I want to run a java program on the server. I want to be able to retrieve data from the program from another computer/phone using an internet connection. I also want to be able to give the program data, and get a response saying whether or not the data has been received correctly.
So for example:
A .jar program runs on my server and holds a variable x
I want to be able to query the value of x from another device (over the internet).
I want to be able to set the value of x remotely from another device, and get a response saying it was successful in altering the value.
What are my options here? I would like to try and keep things simple. It is perhaps worth mentioning that I will be the only one using the system. The server will be used exclusively for dealing with the two requests outline above.
Is it simply the case of creating a java program that listens out for incoming requests and running that on the server?
As you mentioned, you can start with custom ServerSocket wrapper which will decode incoming requests and do as it's bid. Currently, whole frameworks are done to encapsulate common code of this task -- see my 3rd point.
Old-school java solution: use RMI. See RMI tutorial.
New-school java solution: devise some simple text-based protocol with 2 commands:
Read()
Set(newVal)
Then implement that protocol over some new trendy Java framework, like Apache MINA, which is created specifically to facilitate quick development of network apps in Java.
I, personally, started with RMI for such kind of tasks. Since RMI is considered Core Java technology, it's wise to learn it.
Is there an already written Java DNS Server that only implements authoritative responses. I would like to take the source code and move it into a DNS server we will be developing that will use custom rule sets to decide what TTL to use and what IP address to publish.
The server will not be a caching server. It will only return authoritative results and only be published on the WHOIS record for the domains. It will never be called directly.
The server will have to publish MX records, A records and SPF/TXT records. The plan is to use DNS to assist in load balancing among gateway-servers on multiple locations (we are aware that DNS has a short reach in this area). Also it will cease to publish IP addesses of gateway-servers when they go down (on purpose or on accident) (granted, DNS will only be able to help during extended outages).
We will write the logic for all this ourselves.. but I would very much like to start with a DNS server that has been through a little testing instead of starting from scratch.
However, that is only feasible if what we copy from is simple enough. Otherwise,, it could turn out to be a waste of time
George,
I guess what you need is a java library which implements DNS protocol.
Take a look at dnsjava
This is very good in terms of complete spec coverage of all types of records and class.
But the issue which you might face with a java based library is performance.
DNS servers would be expected to have a high throughput. But yes, you can solve that by throwing more hardware.
If performance is a concern for you , I would suggest to look into unbound
http://www.xbill.org/dnsjava/
Unfortunately, the documentation states "jnamed should not be used for production, and should probably not be used for testing. If the above documentation is not enough,
please do not ask for more, because it really should not be used."
I'm not aware of any better alternatives, however.
You could take a look at Eagle DNS:
http://www.unlogic.se/projects/eagledns
It's been around for a few years and it's quite well tested by now.
I have Google'd my butt off, and I can't find anything on this topic.
I am trying to create a download client using Java, and I have figured out how to download files with Java, but I want to accelerate the download speed. I know how this works (opening several connections to the download server), but how can I achieve this?
I am looking for either some detailed explanation of such an algorithm or some code examples.
This is only possible if the server side supports range requests. You can determine that by checking using a HEAD request if the HTTP response header contains Accept-Ranges: bytes. If that is the case, then you can just spawn several threads which downloads the file in parts using the Range header. The URLConnection and ExecutorService are helpful in this.
Keep in mind that you also take the limitation in amount of threads and network bandwidth of your own machine into account.
Related questions:
Reading first part of file using HTTP
How to use URLConnection to fire and handle HTTP requests
Make simultaneous web requests in Java
BalusC described the trick and here is a reference to some source-code you can review and start with:
JDownLoader[Java]: http://svn.jdownloader.org/projects/show/jd
Free Download Manager[CPP]: http://freedownload.svn.sourceforge.net/viewvc/freedownload/
#BalusC Nice Work
I'm a bit unclear, are you writing a Java client that will talk to a server (perhaps a Java servlet?), so you control both sides of the data transfer? If so, you can do nearly anything you want. Java has java.util.zip, which has functions to do the compression.
If you want to download four (or N) files at once, just start up N threads and pass the HTTP requests to the server in parallel. This may not actually improve things, depending on link speed, network congestion, etc.
Writing your own client and making it properly multi-thread safe is a whole lot of work, which is why people just use the Apache HTTP client code. Its rock solid.