I wrote a Windows Service that polls data on some timeintervalls and writes them in database.
But if there occures an Error writing to Database and the service stops.
I want a robust solution that goes on even if an error occures.
I put try/catch bocks around every action that is done by the service in OnStart, OnStop and Dispose but it sopped anyway.
is try/catch the correct approach?
And where do I have to put it?
Will it help to put try/catch in Main method?
Many thanks.
Try wrapping your call in a TransactionScope, this will roll back any failed changes you are trying to make and should not break on failure.
using (var scope = new TransactionScope())
{
DeleteStuff();
UpdateStuff();
scope.Complete();
}
Related
All,
I am using Change Feed Processor Library.Want to know the best way to handle service failure along with the exceptions/errors scenario's in ProcessChangesAsync method. Below are the events am referring to.
1) Service failure - Service having the processor library crashed in the middle of some operation. How to start the process from the same document(doc on failure instance)? is there any inbuilt mechanism where change feed will start with the last failed documents? E.g. Let assume,in current batch we have 10 docs.5 processed successfully and then service breaks because of network failure or by some other reasons.Will my process starts with 6th document once service is re-started? How to achieve this?
2) Exception and Errors- Any errors in ProcessChangesAsync method can be handle using try catch at the global level but how to persist those failure records and make them available for the next batch? Again,looking for any available inbuilt mechanism in change feed process.
1) The Processor Library, by default, checkpoints after a successful run of ProcessChangesAsync. In the latest library version, you can customize the Checkpointer to do manual checkpoints in case you need it. If for some reason the processor shuts down before checkpointing, then it will start processing next from the the last successful checkpoint stored in the Leases collection. In your case, it will start with the first document again, so you will never lose a change but you could experience double processing (this is an "at least once" model).
2) There is no built-in mechanism that you can leverage, handling exceptions within the ProcessChangesAsync is your responsibility. You could not only add a global try/catch but, in the case you are looping over the documents, add a try/catch inside the loop, to handle a failing document (maybe send it to queue for later analysis/post-process) without losing the batch. If you require logging for those errors (I'm assuming that's what you mean by persisting errors?), then the latest version is compatible with LibLog, so plugging your own custom logging is as simple as:
using Microsoft.Azure.Documents.ChangeFeedProcessor.Logging;
var hostName = "SampleHost";
var tracelogProvider = new TraceLogProvider(); //You can use any provider supported by LibLog
using (tracelogProvider.OpenNestedContext(hostName))
{
LogProvider.SetCurrentLogProvider(tracelogProvider);
// After this, create IChangeFeedProcessor instance and start/stop it.
}
Source
Extra info for the comments
To avoid exceptions halting the batch or causing a batch to be reprocessed, you can have handling like this:
public async Task ProcessChangesAsync(IChangeFeedObserverContext context, IReadOnlyList<Document> documents, CancellationToken cancellationToken)
{
try
{
foreach(var document in documents)
{
try
{
// Do your work for the document
}
catch(Exception ex)
{
// Something happened with the current document, handle it, send it to a queue / another storage to analyze, log it. This catch will make the loop continue with the next.
}
}
}
catch(Exception ex)
{
// Something unhandled happened, log it and avoid throwing it again so the next batch is processed
}
}
We are using the SQLite.NET PCL in a Xamarin application.
When putting the database under pressure by doing inserts into multiple tables we are seeing BUSY exceptions being thrown.
Can anyone explain what the difference is between BUSY and LOCKED? And what causes the database to be BUSY?
Our code uses a single connection to the database created using the following code:
var connectionString = new SQLiteConnectionString(GetDefaultConnectionString(),
_databaseConfiguration.StoreTimeAsTicks);
var connectionWithLock = new SQLiteConnectionWithLock(new SQLitePlatformAndroid(), connectionString);
return new SQLiteAsyncConnection (() => { return connectionWithLock; });
So our problem turned out to be that although we had ensured within the class we'd written that it only created a single connection to the database we hadn't ensured that this class was a singleton, therefore we were still creating multiple connections to the database. Once we ensured it was a singleton then the busy errors stopped
What I've take from this is:
Locked means you have multiple threads trying to access the database, the code is inherently not thread safe.
Busy means you have a thread waiting on another thread to complete, your code is thread safe but you are seeing contention in using the database.
...current operation cannot proceed because the required resources are locked...
I am assuming that you are using async-style inserts and are on different threads and thus an insert is timing out waiting for the lock of a different insert to complete. You can use synchronous inserts to avoid this condition. I personally avoid this, when needed, by creating a FIFO queue and consuming that queue synchronously on a dedicated thread. You could also handle the condition by retrying your transaction X number of times before letting the Exception ripple up.
SQLiteBusyException is a special exception that is thrown whenever SQLite returns SQLITE_BUSY or SQLITE_IOERR_BLOCKED error code. These codes mean that the current operation cannot proceed because the required resources are locked.
When a timeout is set via SQLiteConnection.setBusyTimeout(long), SQLite will attempt to get the lock during the specified timeout before returning this error.
Ref: http://www.sqlite.org/lockingv3.html
Ref: http://sqlite.org/capi3ref.html#sqlite3_busy_timeout
I have applied the following solution which works in my case(mobile app).
Use sqlitepclraw.bundle_green nugget package with SqlitePCL.
Try to use the single connection throughout the app.
After creating the SQLiteConnection.
Apply busytime out using following call.
var connection = new SQLiteConnection(databasePath: path);
SQLite3.BusyTimeout(connection.Handle, 5000); // 5000 millisecond.
I am using java API to interact with MQ.
When I try to get a message from an empty queue, I get exception.
Ok, but when I control it with try catch, I am expecting not prompted any error message in console.
but I get!!
try {
queue.get(getMessage, new MQGetMessageOptions());
return getMessage.readUTF();
} catch (Exception e) {
return "";
}
get in console as "MQJE001: Completion Code '2', Reason '2033'."
How can I disable this information output?
how can I check message availability or current queue size?
Thanks
First question: How can I disable this information output?
This link might help: Hide Java Output
Second question: How can I check message availability or current queue size?
MQQueue.getCurrentDepth() method will get you the current queue size. But you must note that this may not represent the correct queue depth at all times as messages could be consumed by other applications from the same queue. Actually you should not worry about queue depth. It is best practice to keep consuming messages and handle 2033 (MQJE001: MQRC_NO_MSG_AVAILABLE) exception which is thrown when there are no messages in the queue in your application.
Easiest method is using MQException.log=null line in your application.
Details bellow:
https://www.ibm.com/support/knowledgecenter/SSFKSJ_7.5.0/com.ibm.mq.dev.doc/q031000_.htm
I have one piece of code that gets run on Application_Start for seeding demo data into my database, but I'm getting an exception saying:
The ObjectContext instance has been disposed and can no longer be used for operations that require a connection
While trying to enumerate one of my entities DB.ENTITY.SELECT(x => x.Id == value);
I've checked my code and I'm not disposing my context before my operation, Below is an outline of my current implementation:
protected void Application_Start()
{
SeedDemoData();
}
public static void SeedDemoData()
{
using(var context = new DBContext())
{
// my code is run here.
}
}
So I was wondering if Application_Start is timing out and forcing my db context to close its connection before it completes.
Note: I know the code because I'm using it on a different place and it is unit tested and over there it works without any issues.
Any ideas of what could be the issue here? or what I'm missing?
After a few hours investigating the issue I found that it is being caused by the data context having pending changes on a different thread. Our current implementation for database upgrades/migrations runs on a parallel thread to our App_Start method so I noticed that the entity I'm trying enumerate is being altered at the same time, even that they are being run on different data contexts EF is noticing that something is wrong while accessing the entity and returning an incorrect error message saying that the datacontext is disposed while the actual exception is that the entity state is modified but not saved.
The actual solution for my issue was to move all the seed data functions to the database upgrades/migrations scripts so that the entities are only modified on one place at the time.
I have WebService that is hosted by ASP.NET web site. Inside the TransactionScope object is used to handle transactions:
using (TransactionScope scope = new TransactionScope())
{
...
scope.Complete();
}
The problem is that during debugging, when I am going through each line in step-by-step mode,
transaction timeout is occurred and any attempt to access DB crashed with '' error, and as a result: further debugging is prohibited.
How could I handle that without deleting mentioned lines of code?
P.S. I've tried to find, how to increase a time-out of created transaction, but didn't find something helpful.
Any thoughts are welcome.
Thanks.
You can specify an infinite timeout for the Transaction by passing in a zero length TimeSpan as part of the constructor:
using (TransactionScope scope = new TransactionScope(TransactionScopeOption.Required, new TimeSpan(0)))
The TransactionScopeOption of Required is what is used as default with your parameterless constructor.
See http://msdn.microsoft.com/en-us/library/ms172152(VS.90).aspx for more information.