I have a server/client application and I need to synchronize the time of the clients with the time of the server. Thereby I need to implement an NTP Server (or similar) on the server application and an NTP client on the client side.
NTP is not a requirement, but it has to be accurate for at least 50ms (then it would be already a bit critical, but still usable, until I figure out a better solution).
Does anyone have any advices, if there is a good implementation that could help me, or if it's doable to implement this by myself (with the desired precision).
Edit: More information about the project
The server application runs on a customers Home PC. The computer of the customer does not have to change the time, only the client (which is running on a dedicated computer, running on Ubuntu).
This is an IoT project, there are many clients that are very small hardwar devices and should have the same date/time as the server. Because the client won't fiddle with the client's configuration, and he does not have access to it (it's a blackbox), I can run my application using admin rights, if needed.
I don't want to install an NTP Server on my client's home PC. This is the reason I want to deliver this functionality inside of my Java Application.
There's a NtpClient in apache commons-net that you can use for this. You can use this to figure out the clock drift at least of your client pc and try to compensate for that.
Related
I have a web application that consists of a java part and a php part. When a user does a request the php process will open a tcp/ip connection to the java process. It will keep this connection open for the duration of the request and this connection will be used to send a lot of information back and forth. This application runs very well as long as its hosted on either a dedicated server or on a VM that uses OpenVZ.
As soon as I try to host it on a KVM VM it becomes extremely slow. The reason for this is that within a single user request the php process can easily do up to 1 or 2 thousand tcp-ip sends to the java process. Now since this is all done over the same connection It really should not be a problem but on KVM VM's it seems each send gets about 20 milliseconds worth of delay so now a request that would normally take 0.1 seconds takes 20 seconds instead.
I'm not 100% sure KVM is to blame, But I have tested this on 3 different hosting provdiders using OpenVZ and another 3 different hosting providers using KVM. It runs perfectly fine on all the OpenVZ hosts and the send delay problem is present on all the KVM hosts.
O and I have tcpnodelay set on both the java and the php side.
Any idea what I could try to make this work on KVM?
So to answer my own question. It seems it seems you wont be able to avoid that send latency since even though its on localhost it still has to go from the virtualization layer down to the network layer and back up.
However, instead of creating TCP sockets on localhost the solution was to use Unix sockets instead. Since Unix sockets do not access the network layer in any way.
And as a bonus Using Unix sockets instead of TCP sockets gave my application a nice across the board performance boost. Including on setups were it worked fine before.
I am incredibly new to the topic of websockets and am trying to figure out the correct way to handle the communication between a device and a server.
Here is my scenario: I have a thermostat (very similar to the Nest) that needs to communicate with a web server. Every time you change the temperature on the thermostat, I need to send data to the web server to update it's "current stats" in the database. Easy, I can do that.
The part that I am confused about, and think websockets might be a use-case is when the user changes the temperature from the web interface. The thermostat has to pull in that information from the server to see "Oh, okay you want it to be 66 degrees."
I've thought of having the thermostat long-polling the server every 2-5 seconds to see what the "current stats" are in the database to change the temperature, but that seems like overkill.
Is there a way to open a connection between the thermostat and the server to listen for messages?
I began down the road of reading about websockets, however, I believe it is unfortunately browser-based only.
As I'm fairly new to the game with regards to these types of connections, if anyone could point me in the right direction regarding protocols, communication, etc. I would greatly appreciate it!
Tech Specs
Server is written in Ruby on Rails
Thermostat is written in Java
Websockets can be used between any two programs which need to communicate, they are certainly not restricted to the browser. That said, should you be using websockets is a different question. one thing to think about is that websockets involves a persistent connection. this may not scale (if you have lots of devices) and it may also be overkill. if you are expecting the temperature to be changed once a day, having a persistent connection for the entire day is an enormous waste of resources. websockets are typically used when communication needs to be "fast" and relatively frequent. unless you really need instantaneous updates in the thermostat, i would just have it ping the server every few minutes for updates.
Side note, websockets is fairly new, so any libraries you end up using may be a bit on the immature side.
We prototyped some java to java websockets a while not too long ago. We used the ning async library on the client side and the atmosphere library (built on netty) on the server side.
WebSockets is just a specification for tunneling something similar to TCP sockets over HTTP; it's not confined to the browser, and client libraries are available for most common languages.
This sounds like a reasonable use case for a long-running connection, but I would generally prefer a raw TCP connection to a WebSockets connection unless you have a specific restriction in mind (e.g., most home Internet connections have no problem with connecting to a server at an arbitrary port).
I just created a free ec2 account. I want to test few things but i want to make sure that i am using ONLY the free version.
I want to run a java application in windows 2008 R2 server with TOMCAT as web server. Ec2 by default doesn't have java SDK and TOMCAT installed.
SO here are the few queries
Will i be charged for installing TOMCAT and JAVA also for deploying my application?
How do i install TOMCAT and JAVA ? can someone provide a link?
I tried doing a remote desktop ut could not connect to the server. Looks like a firewall issue. But then how do i disable ?
Can't answer all your questions, but I'll take a shot.
If you want to watch your ec2 cost, setup a billing alert in AWS that will notify you when your account exceeds a certain threshold, and set your threshold very low if you are very concerend. You can always monitor your outstanding charges on your billing page on a daily basis if you are really concerned.
Pick a small or micro instance if you want to keep your costs low; micro instances I have found are mostly useless for any serious work, but they are very, very cheap and maybe ok for trying things out.
To install any software, you are going to need to get remote desktop working. Once the server is spun up, remote in and you can install anything you want; there is no extra cost to install software (except buying licenses if you need them, but that is not aws's concern, they won't be charging you for installing tomcat.
You need to open up the windows firewall on your 2008 server AND you need to make sure the AWS security group also allows incoming connections on port 3389 - its essentially there firewall that runs in addition to the EC2 instances firewall - BOTH need to be open. You can either modify the security group that was assigned to your instance, or create a new one and associated it with your instance.
I need to test a functionality internal to my company's server whose benefit is evident only when clients run slow (as of latency and packet loss). To that extent, I need to simulate clients on a slow and/or lossy connection (TCP/HTTP). I'm using a Mac, Mountain Lion, and ideally I'd need to run both server and client locally.
One approach I tried to pursue -- unsuccessfully -- was to get hold of some java APIs that allow me to build clients with slow connections. I know JMeter has got something called SlowSockets (or something similar), but I was looking for APIs more focused on slow-performing clients. Any ideas of useful APIs?
Another approach I tried consisted in using a proxy to act as a middleman between client and server. In that case, the proxy should provide functionalities for simulating slow links. I've tried Charles proxy (Mac) and Apache TCPMon, however I seem to miss something when I try to get them at work. With TCPMon, for instance, when I start it in 'Proxy' mode (which is the mode that offers the 'simulate slow link' functionality) I define port for the local proxy, but I can't see how to define the remote host and port. Something similar happens with Charles Proxy; I can set the local port in the Proxy settings, but I can't understand how to define the remote end of the proxy (in fact connections fail saying the remote server is not responding). Anyone having ideas what I'm doing wrong?
One further approach I have tried to pursue is by using lower-level (e.g. OS-based) means; in this case, I tried Apple's Network Link Conditioner. I switched it on and defined my slowness parameters, but when I ping I don't seem to see the expected RTT etc. I've got a feeling NLC has a tight relationship with XCode and iOS testing, anyone capable of putting it at work for testing other (e.g. Java) applications? I've also tried ipfw on Mac, however the manual says ipfw is now deprecated and I don't want to dedicate time to get to know a tool that won't be available soon.
Any idea/help will be highly appreciated.
Thanks in advance.
I would like to have the clients query each other through the server without delay ( = no polling interval ).
Example: Server S, clients A and B
Client A wants to request Client B.
Client A will make a request to the server S, no problem there.
Then Server S needs to be able to request Client B but how to do that without polling?
All the node.js/APE (for PHP) technos are designed for the web, however I don't use a web server for that. Does Java has something close to a push technology/framework that is not web?
I would really prefer a solution that doesn't require each client to use their own reserved port (I don't want to end up with 1 WebService per client for example)
Note: all the clients are on the same machine.
A couple of options...
Plain socket communication. java.net.Socket, java.net.ServerSocket. Maximum flexibility but requires knowledge of low level TCP/IP API/concepts.
The good old RMI. Java based RPC layer on top of TCP/IP. Works good when client and server are both in Java and generally in same subnet. May give problems when client and/or server are natted.
Spring Remoting, it's actually pretty decent.
Bi-Directional Web Services. i.e. clients host their own WSes which the Server calls when it needs to do a callback.
JMS as someone already mentioned.
Distributed Data Structures, Check out http://www.hazelcast.com/
Lots of options to chose from, no need for webserver.
If you really don't want to use a web server then I would check out JMS. That being said, all the cool kids are using web servers these days since the protocols are so ubiquitous.
Your use case requires a messaging protocol. We don't really know the scope of your problem but you already said you want a server to exchange requests between clients, so I would go with an existing solution rather than a roll your own approach.
JMS has been mentioned and is certainly a viable Java based solution, another would be XMPP which is a real time communication protocol commonly used for instant messaging.
It is an open standard that has both server and client support in every major language and platform. This would allow you to have standalone apps, web based ones and apps for mobile devices all being able to communicate with each other. The only potential gotcha for your use case is that it is text based. Since you haven't said what requests you want to pass back and forth, I don't know if this will fit your bill or not.
You can use Smack for client side development in Java and any OS server you want.