Is there a way to inject a variable into a running process without a process listening for RPC requests?
For example if a process was running and using an environment variable, could I change that environment variable at runtime and make the process use the new value?
Are there alternative solutions for dynamically changing variables in a running process? Assume that this process is like a PHP process or a Javascript (node.js) process so I can change the source code... etc.
I think this is similar to passing state or communicating to another process, but I need a really lightweight way of doing so, without going over the network or using libraries or preferably not setting up an RPC server.
Solution does not have to be cross-platform. Prefer Linux.
You can do it it java. Imagine this is your thread class:
public void ThreadClass extends Thread {
Boolean state;
ThreadClass(Boolean b) {
state = b;
}
public void StopThread() {
state = false;
}
public void run() {
while(state) { //Do whatever you want here}
}
}
Now all you have to do is start this thread from your main class:
ThreadClass thread = new ThreadClass(true);
thread.start();
And if you want to change the value of state, call the StopThread method in the thread like so:
try {
thread.StopThread();
} catch (InterruptedException ex) {
Logger.getLogger(NewClass.class.getName()).log(Level.SEVERE, null, ex);
}
This will change the state of the Boolean while the thread is running.
It appears that local IPC implementations like shared memory is the way to go: Fastest technique to pass messages between processes on Linux?
Related
In the past I have done something like this
private static bool AlreadyRunning()
{
var processes = Process.GetProcesses();
var currentProc = Process.GetCurrentProcess();
logger.Info($"Current proccess: {currentProc.ProcessName}");
foreach (var process in processes)
{
if (currentProc.ProcessName == process.ProcessName && currentProc.Id != process.Id)
{
logger.Info($"Another instance of this process is already running: {process.Id}");
return true;
}
}
return false;
}
Which has worked well. In the new dotnet core world everything has a process name of dotnet so I can only run one dotnet app at a time! Not quite what I want :D
Is there an ideal way of doing this in dotnet? I see mutex suggested but I am not sure I understand the possible downsides or error states running on other systems than a windows machine.
.NET Core now supports global named mutex. From PR description, that added that functionality:
On systems that support thread process-shared robust recursive mutexes, they will be used
On other systems, file locks are used. File locks, unfortunately, don't have a timeout in the blocking wait call, and I didn't find any other sync object with a timed wait with the necessary properties, so polling is done for timed waits.
Also, there is a useful note in Named mutex not supported on Unix issue about mutex name, that should be used:
By default, names have session scope and sessions are more granular on Unix (each terminal gets its own session). Try adding a "Global" prefix to the name minus the quotes.
In the end I used a mutex and it seeeeeems okay.
I grabbed the code from here
What is a good pattern for using a Global Mutex in C#?
The version of core I am using does not seem to have some of the security settings stuff so I just deleted it. I am sure it will be fine. (new Mutex only takes 3 parameters)
private static void Main(string[] args)
{
LogManager.Configuration = new XmlLoggingConfiguration("nlog.config");
logger = LogManager.GetLogger("console");
logger.Info("Trying to start");
const string mutexId = #"Global\{{guid-guid-guid-guid-guid}}";
bool createdNew;
using (var mutex = new Mutex(false, mutexId, out createdNew))
{
var hasHandle = false;
try
{
try
{
hasHandle = mutex.WaitOne(5000, false);
if (!hasHandle)
{
logger.Error("Timeout waiting for exclusive access");
throw new TimeoutException("Timeout waiting for exclusive access");
}
}
catch (AbandonedMutexException)
{
hasHandle = true;
}
// Perform your work here.
PerformWorkHere();
}
finally
{
if (hasHandle)
{
mutex.ReleaseMutex();
}
}
}
}
In Apache you can configure that operation will be load using the load_on_startup tag.
I want also when stopping Apache to do several operation using the stop tomcat command.
Is there something like that ?
10x
The closest I can think of is "Finalising a servlet"
http://docs.oracle.com/javaee/6/tutorial/doc/bnags.html
Notifying Methods to Shut Down
To ensure a clean shutdown, your destroy method should not release any shared resources until all the service requests have completed. One part of doing this is to check the service counter. Another part is to notify the long-running methods that it is time to shut down. For this notification, another field is required. The field should have the usual access methods:
public class ShutdownExample extends HttpServlet {
private boolean shuttingDown;
...
//Access methods for shuttingDown
protected synchronized void setShuttingDown(boolean flag) {
shuttingDown = flag;
}
protected synchronized boolean isShuttingDown() {
return shuttingDown;
}
}
If you want to run code to handle Tomcat specific things, use a LifecycleListener, i.e. create a class that implements LifecycleListener.
If you want to to things when your application starts, use a ServletListener.
I'm using nServiceBus 5 and have created a number of host endpoints, two of which listen for database changes. (The specifics of how to do this can be found here). The intention is to have a service running in the background which publishes an event message using the Bus when notified to do so by the database listener.
The code which creates the database listener object and handles events is in the Start method, implemented as part of IWantToRunWhenBusStartsAndStops.
So - Is putting the code here likely to cause problems later on, for example if an exception is thrown (yes, I do have try/catch blocks, but I removed them from the sample code for clarity)? What happens when the Start method finishes executing?
Would I be better off with a constructor on my RequestNewQuoteSender class to instantiate the database listener as a class property and not use the Start method at all?
namespace MySample.QuoteRequest
{
public partial class RequestNewQuoteSender : IWantToRunWhenBusStartsAndStops
{
public void Start()
{
var changeListener = new DatabaseChangeListener(_ConnectionString);
// Assign the code within the braces to the DBListener's onChange event
changeListener.OnChange += () =>
{
// code to handle database change event
changeListener.Start(_SQLStatement);
};
// Now everything has been set up.... start it running.
changeListener.Start(_SQLStatement);
}
public void Stop() { LogInfo("Service Bus has stopped"); }
}
}
Your code seems fine to me.
Just a few small things:
Make changeListener a class field, so that it won't be GC (not 100% sure if it would be but just to make sure);
Unsubscribe from OnChange on the Stop() method;
You may also want to have a "lock" around changeListener.Start(_SQLStatement); and the Stop so that there are no racing conditions (I leave that one up to you to figure out if you need it or not);
Does this make sense ?
In a multi appdomain setup, is there a way to make SetEnvironementVariables and Get.... work within the appdomain only, so each appdomain can have different values for the same variable?
No. :(
This example:
namespace ConsoleApplication
{
class Program
{
static void Main(string[] args)
{
var newDomain = AppDomain.CreateDomain("Alternative");
Proxy proxyObj = (Proxy)newDomain.CreateInstanceAndUnwrap(typeof(Proxy).Assembly.GetName().FullName,
typeof(Proxy).FullName);
Environment.SetEnvironmentVariable("HELLO_MSG", "Hello World", EnvironmentVariableTarget.Process);
proxyObj.ShowEnvironmentVariable();
Console.ReadKey();
}
}
class Proxy : MarshalByRefObject
{
public void ShowEnvironmentVariable()
{
var msg = Environment.GetEnvironmentVariable("HELLO_MSG");
Console.WriteLine(String.Format("{0} (from '{1}' AppDomain)", msg, AppDomain.CurrentDomain.FriendlyName));
}
}
}
Will output:
Hello World (from 'Alternative' AppDomain)
The process is the most specific level of encapsulation for environment variables, and AppDomains will still "live inside" the same process.
Note that this will happen for all other process-level information (such as Directory.GetCurrentDirectory(), command-line args, etc.
One possible solution would create worker processes (".exe" applications spawned from the main process), but that will certainly add some complexity to your application.
In classic ASP.NET I’d persist data extracted from a web service in base class property as follows:
private string m_stringData;
public string _stringData
{ get {
if (m_stringData==null)
{
//fetch data from my web service
m_stringData = ws.FetchData()
}
return m_stringData;
}
}
This way I could simply make reference to _stringData and know that I’d always get the data I was after (maybe sometimes I’d use Session state as a store instead of a private member variable).
In Silverlight with a WCF I might choose to use Isolated Storage as my persistance mechanism, but the service call can't be done like this, because a WCF service has to be called asynchronously.
How can I both invoke the service call and retrieve the response in one method?
Thanks,
Mark
In your method, invoke the service call asynchronously and register a callback that sets a flag. After you have invoked the method, enter a busy/wait loop checking the flag periodically until the flag is set indicating that the data has been returned. The callback should set the backing field for your method and you should be able to return it as soon as you detect the flag has been set indicating success. You'll also need to be concerned about failure. If it's possible to get multiple calls to your method from different threads, you'll also need to use some locking to make your code thread-safe.
EDIT
Actually, the busy/wait loop is probably not the way to go if the web service supports BeginGetData/EndGetData semantics. I had a look at some of my code where I do something similar and I use WaitOne to simply wait on the async result and then retrieve it. If your web service doesn't support this then throw a Thread.Sleep -- say for 50-100ms -- in your wait loop to give time for other processes to execute.
Example from my code:
IAsyncResult asyncResult = null;
try
{
asyncResult = _webService.BeginGetData( searchCriteria, null, null );
if (asyncResult.AsyncWaitHandle.WaitOne( _timeOut, false ))
{
result = _webService.EndGetData( asyncResult );
}
}
catch (WebException e)
{
...log the error, clean up...
}
Thanks for your help tvanfosson. I followed your code and have also found a pseudo similar solution that meets my needs exactly using a lambda expression:
private string m_stringData;
public string _stringData{
get
{
//if we don't have a list of departments, fetch from WCF
if (m_stringData == null)
{
StringServiceClient client = new StringServiceClient();
client.GetStringCompleted +=
(sender, e) =>
{
m_stringData = e.Result;
};
client.GetStringAsync();
}
return m_stringData;
}
}
EDIT
Oops... actually this doesn't work either :-(
I ended up making the calls Asynchronously and altering my programming logic to use MVVM pattern and more binding.