I'm new to Oracle Coherence. I read the documentation and done the hands-on using the command prompt. I've no issues in understanding. Then I downloaded the eclipse with oracle coherence tools. I created the application client for the oracle coherence as given below
http://docs.oracle.com/cd/E18686_01/coh.37/e18692/installjdev.htm
I ran the same. It was working fine as I did in my console application. Then I created a new project in the same workspace, created a main class accessed the named cache, put and retrieved some values using the below code,
package coherenceClient;
import com.tangosol.net.CacheFactory;
import com.tangosol.net.NamedCache;
public class Main {
public static void main(String[] args) {
NamedCache cache = CacheFactory.getCache("myCache");
cache.put("MyFirstCacheObject", "This is my first Cache Object");
System.out.println(cache.get("MyFirstCacheObject"));
}
}
I retieved the same value. Then I created another class tried retrieved the same value but it was returning null. Is there are any mistakes in the code?
package coherenceClient;
import com.tangosol.net.CacheFactory;
import com.tangosol.net.NamedCache;
public class Recevier {
public static void main(String[] args) {
NamedCache cache = CacheFactory.getCache("myCache");
System.out.println(cache.get("MyFirstCacheObject"));
}
}
If the coherence cache resides in the JVM (it is not ran as standalone server), then all the data get discarded after your program finishes (you use in-memory storage). Try to put Thread.sleep(200000); to the end of the first program and then run the second instance within the timeout.
In the command prompt you have started the server(as stand-alone) and the clients have joined the server. So all the data in the cache will be available until the server stops, even if the client which inserted the data into the cache leaves the server session.
But in the above case, the coherence cache resides in the JVM(Eclipse) itself and not as stand alone server. So you are getting null value when the program exists.
When you run the second JVM check the original coherence cache server node stdout to see if you actually see the new member joining in the cluster (check the MemberSet). You might just be running two separate JVMs which are completely unaware of each other; hence CacheFactory.getCache("myCache") is creating the cache in each JVM.
The way to go around this is to use cache-server.cmd to start a coherence cache server and then run your eclipse program with a distributed/partitioned or replicated scheme. That way, even when your program exits, the actual data would be live in the the coherence cache server for the second JVM to retrieve when it joins the "same cluster".
Related
I am experiencing database connection errors with an ASP.NET application written in VB, running on three IIS servers. The underlying database is MS Access, which is on a shared network device. It uses Entity Framework, code first implementation and JetEntityFrameworkProvider.
The application is running stable. But, approximately 1 out of 1000 attempts to open the database connection fails with either one of the following two errors:
06:33:50 DbContext "Failed to open connection at 2/12/2020 6:33:50 AM +00:00 with error:
Cannot open database ''. It may not be a database that your application recognizes, or the file may be corrupt.
Or
14:04:39 DbContext "Failed to open connection at 2/13/2020 2:04:39 PM +00:00 with error:
Could not use ''; file already in use.
One second later, with refreshing (F5), the error is gone and it works again.
Details about the environment and used code.
Connection String
<add name="DbContext" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=x:\thedatabase.mdb;Jet OLEDB:Database Password=xx;OLE DB Services=-4;" providerName="JetEntityFrameworkProvider" />
DbContext management
The application uses public property to access DbContext. DbContext is kept in the HttpContext.Current.Items collection for the lifetime of the request, and is disposed at it’s end.
Public Shared ReadOnly Property Instance() As DbContext
Get
SyncLock obj
If Not HttpContext.Current.Items.Contains("DbContext") Then
HttpContext.Current.Items.Item("DbContext") = New DbContext()
End If
Return HttpContext.Current.Items.Item("DbContext")
End SyncLock
End Get
End Property
BasePage inits and disposes the DbContext.
Protected Overrides Sub OnInit(e As EventArgs)
MyBase.OnInit(e)
DbContext = Data.DbContext.Instance
...
End Sub
Protected Overrides Sub OnUnload(e As EventArgs)
MyBase.OnUnload(e)
If DbContext IsNot Nothing Then DbContext.Dispose()
End Sub
What I have tried
Many of the questions on SO which address above error messages, deal with generally not being able to establish a connection to the database – they can’t connect at all. That’s different with this case. Connection works 99,99% of the time.
Besides that, I have checked:
Permissions: Full access is granted for share where .mdb (database) and .ldb (locking file) resides.
Network connection: there are no connection issues to the shared device; it’s a Gigabit LAN connection
Maximum number of 255 concurrent connections is not reached
Maximum size of database not exceeded (db has only 5 MB)
Changed the compile option from “Any CPU” to “x86” as suggested in this MS Dev-Net post
Quote: I was getting the same "Cannot open database ''" error, but completely randomly (it seemed). The MDB file was less than 1Mb, so no issue with a 2Gb limit as mentioned a lot with this error.
It worked 100% on 32 bit versions of windows, but I discovered that the issues were on 64 bit installations.
The app was being compiled as "Any CPU".
I changed the compile option from "Any CPU" to "x86" and the problem has disappeared.
Nothing helped so far.
To gather more information, I attached an Nlog logger to the DbContext which writes all database actions and queries to a log file.
Shared Log As Logger = LogManager.GetLogger("DbContext")
Me.Database.Log = Sub(s) Log.Debug(s)
Investigating the logs I figured out that when one of the above errors occured on one server, another one of the servers (3 in total) has closed the db connection at exactly the same time.
Here two examples which correspond to the above errors:
06:33:50 DbContext "Closed connection at 2/12/2020 6:33:50 AM +00:00
14:04:39 DbContext "Closed connection at 2/13/2020 2:04:39 PM +00:00
Assumption
When all connections of a DbContext have been closed, the according record is removed from the .ldb lock file. When a connection to the db is being opened, a record will be added to the lock file. When these two events occur at the exact same time, from two different servers, there is a write conflict to the .ldb lock file, which results in on of the errors from above.
Question
Can anyone confirm or prove this wrong? Has anyone experienced this behaviour? Maybe I am missing something else. I’d appreciate your input and experience on this.
If my assumption is true, a solution could be to use a helper class for accessing db, which catches and handles this error, waiting for a minimal time period and trying again.
But this feels kind of wrong. So I am also open to suggestions for a “proper” solution.
EDIT: The "proper" solution would be using a DBMS Server (as stated in the comments below). I'm aware of this. For now, I have to deal with this design mistake without being responsible for it. Also, I can't change it in the short run.
I write this as an aswer because of space but this is not really an answer.
It's for sure an OleDb provider issue.
I think that is a sharing issue.
You could do some tries:
use a newer OleDb provider instead of Microsoft.Jet.OLEDB.4.0. (if you have try 64 bits you could already have try another provider because Jet.OLEDB.4.0 is 32 bits only)
Implement a retry mechanism on the new DbContext()
Reading your tests this is probaly not your case. I THINK that Dispose does not always work properly on Jet.OLEDB.4.0 connections. I noted it on tests and I solved it using a different testing engine. Before giving up I used this piece of code
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced, true);
GC.WaitForPendingFinalizers();
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced, true);
As you can understand reading this code, they are tries and the latest solution was changing the testing engine.
If your app is not too busy you could try to lock the db using a different mechanism (for example using a lock file). This is not really different from new DbContext() retries.
In late '90s I remember I had an issue related to disk sharing OS (I were using Novel Netware). Actually I have not experience in using mdb files on a network share. You could try to move the mdb on a folder shared with Windows
Actually I use Access databases only for tests. If you really need to use a single file database you could try other solutions: SQL Lite (you need a library, also this written by me, to apply code first https://www.nuget.org/packages/System.Data.SQLite.EF6.Migrations/ ) or SQL Server CE
Use a DBMS Server. This is for sure the best solution. As the writer of JetEntityFrameworkProvider I think that single file databases are great for single user apps (for this apps I suggest SQL Lite), for tests (I think that for tests JetEntityFrameworkProvider is great), for transfering data or, also, for readonly applications. In other cases use a DBMS Server. As you know, with EF, you can change from JetEntityFrameworkProvider to SQL Server or to MySql without effort.
You went wrong at the design stage: The MS Access database engine is unfit for ASP.Net sites, and this is explicitly stated on multiple places, e.g. the official download page under details.
The Access Database Engine 2016 Redistributable is not intended .... To be used by ... a program called from server-side web application such as ASP.NET
If you really have to work with an Access database, you can run a helper class that retries in case of common errors. But I don't recommend it.
The proper solution here is using a different RDBMS which exhibits stateless behavior. I recommend SQL Server Express, which has limitations, but if you exceed those you will be far beyond what Access supports, and wont cause errors like this.
From the documentation of Symfony lock component
here
I Found That
Unlike other implementations, the Lock Component distinguishes locks instances even when they are created for the same resource. If a lock has to be used by several services, they should share the same Lock instance returned by the Factory::createLock method.
For code example
public function tryLock( $job):
{
$lock = $this->factory->createLock($job->getUniqueId());
$lock->acquire();
return $lock->isAcquired() ;
}
If I run this function 2 times from other class with the same job, it will lock each other
How I can prevent this?! while I’m using this on the same server
And the other problem is How I can use this if I use multiple server instances
We are using spring poller in our application to poll the data from mySQL DB and send to a third party.
The functionality is as such working but when we moved to PRODUCTION since we have mutiple appservers , the job is running from all servers and we need this to be run in all servers to address the request coming in.
Poller time is configured to run #every 5 secs.
But even if we added the update statement, a particular record is being picked up in multiple servers since both are running at the same time.
We have the below configurations
<int-jdbc:inbound-channel-adapter id="datachannel"
query="${sql}"
data-source="dbDataSource" max-rows-per-poll="1" row-mapper="pollerdatamapper"
update="update <table> set flag=1 where id =:Id">
<int:poller fixed-rate="${pollerinterval}">
<int:transactional/>
</int:poller>
</int-jdbc:inbound-channel-adapter>
PollerService class will be invoked as below for each record from the above poller
<int:service-activator input-channel="datachannel"
output-channel="executerchannel" ref="pollerservice" method="getRecordFromPoller">
</int:service-activator>
public class PollerService
{
private static final Logger LOGGER = Logger.getLogger(PollerService.class);
public PollerDataBO getRecordFromPoller(PollerDataBO pollerDataBO)
{
LOGGER.info("call for the Id " + Id);
}
Could you please confirm if there are any transactional setting that we can do restrict the pick of same record in other servers.
Right, SELECT ... FOR UPDATE should be great solution for you:
https://docs.oracle.com/cd/E17952_01/mysql-5.1-en/innodb-locking-reads.html
You also ca try to play with the isolation="SERIALIZABLE" for the <int:transactional/>, but I don't have too much confidence in it.
Also I think we could improve the JdbcPollingChannelAdapter in the code like:
if (this.updatePerRow) {
for (Object row : payload) {
executeUpdateQuery(row);
}
}
And skip those rows which weren't updated.
Feel free to raise a JIRA on the matter.
I am creating dynamic connection strings in my project. They're created on the fly with the information provided specifically for every user. When the application first fires off, if a database doesn't exist (first time user logs on), a new database is created without problems with this initializer:
public DataContext() : base()
{
// ProxyCreation and LazyLoading doesn't affect the situation so
// comments may be removed
//this.Configuration.LazyLoadingEnabled = false;
//this.Configuration.ProxyCreationEnabled = false;
string conStr = GetDb();
this.Database.Connection.ConnectionString = conStr;
}
The problem is, with this method, I have to restart the application pool on the server and the new user should be the first accessor to the application.
I need the same thing without a requirement of restarting the app. Is that possible?
(This is a SPA using AngularJS on MVC views and WebApi as data provider - May be relevant somehow, so thought I should mention)
I already tried this, but this creates an error for EF and the application doesn't start at all...
You could try a little bit different approach to connect directly (and create) the right database.
public class DataContext : DbContext
{
public DataContext(DbConnection connection) : base(connection, true) { }
}
Here you create the DbContext already with the right connection.
Take also care because you need to specify to migrations that the right connection should be used (not the Web.Config connection but the connection that raised the database creation).
See the second overload here https://msdn.microsoft.com/en-US/library/hh829099(v=vs.113).aspx#M:System.Data.Entity.MigrateDatabaseToLatestVersion.
I'm trying to create a bunch of MS visual studio unit tests for my n-tiered web app but for some reason I can't run those tests and I get the following error -
"Object reference not set to an
instance of an object"
What I'm trying to do is testing of my data access layer where I use LINQ data context class to execute a certain function and return a result,however during the debugging process I found out that all the tests fail as soon as they get to the LINQ data context class and it has something to do with the connection string but I cant figure out what is the problem.
The debugging of tests fails here(the second line):
public EICDataClassesDataContext() :
base(global::System.Configuration.ConfigurationManager.ConnectionStrings["EICDatabaseConnectionString"].ConnectionString, mappingSource)
{
OnCreated();
}
And my test is as follows:
TestMethod()]
public void OnGetCustomerIDTest()
{
FrontLineStaffDataAccess target = new FrontLineStaffDataAccess(); // TODO: Initialize to an appropriate value
string regNo = "jonh"; // TODO: Initialize to an appropriate value
int expected = 10; // TODO: Initialize to an appropriate value
int actual;
actual = target.OnGetCustomerID(regNo);
Assert.AreEqual(expected, actual);
}
The method which I call from DAL is:
public int OnGetCustomerID(string regNo)
{
using (LINQDataAccess.EICDataClassesDataContext dataContext = new LINQDataAccess.EICDataClassesDataContext())
{
IEnumerable<LINQDataAccess.GetCustomerIDResult> sProcCustomerIDResult = dataContext.GetCustomerID(regNo);
int customerID = sProcCustomerIDResult.First().CustomerID;
return customerID;
}
}
So basically everything fails after it reaches the 1st line of DA layer method and when it tries to instantiate the LINQ data access class...
I've spent around 10 hours trying to troubleshoot the problem but no result...I would really appreciate any help!
UPDATE:
Finally I've fixed this!!!!:) I dont know why but for some reasons in the app.config file the connection to my database was as follows:
AttachDbFilename=|DataDirectory|\EICDatabase.MDF
So what I did is I just changed the path and instead of |DataDirectory| I put the actual path where my MDF file sits,i.e
C:\Users\1\Documents\Visual Studio 2008\Projects\EICWebSystem\EICWebSystem\App_Data\EICDatabase.mdf
After I had done that it worked out!But still it's a bit not clear what was the problem...probably incorrect path to the database?My web.config of ASP.NET project contains the |DataDirectory|\EICDatabase.MDF path though..
Is LINQDataAccess.EICDataClassesDataContext looking to the web.config or some other outside source of data for its setup?
I can tell you for a fact that you must jump thru hoops to get web.config accessible to your test code.
Update
Ah, yes. I see that you're using ConfigurationManager on the line where your test fails... ConfigurationManager looks to web.config for configuration. This has been a sticking point for me when I write my tests.
You need to either change the code so that the class can be instantiated without web.config, or you need to make it so that your tests can access web.config.
Does your test project have it's own configuration file? This type of behavior usually means the app can't find the connection string. Test projects require their own file since they are not running in the context of the client app.
UPDATE The error you describe after adding an app.config is common when testing web applications built on SQLExpress and attaching an .mdf. SQLExpress cannot be run in more than one process at a time. So if you have previously run your web application it may still be active and will conflict with the attempt to attach the database.
You can use SQL Management Studio to attach the database permanently and then use a more traditional connection string like:
Server=myServer;Database=EICDatabase;Trusted_Connection=True;
For me it seems like your problem is the connection string, which is not set.
I assume your unit test is in a different project than the DAL.
You call the 'new' command on the datacontext constructor without a connection string. So it should usually use its default, when set. But since this setting normally is stored in the web.config of the other project there is no connection string set and you get the error.
If its right, what i assume, you have to get the settings from the DAL project into the unit-testing project. Simplest solution should be to copy web.config connection string to app.config of unit test project.
There are other possibilities for sure, but i think its not easy to get the web.config configuration into your unit-testing project.