Are DbProviderFactory, DbConnection, DbCommand, and DbDataAdapter Thread-Safe? - c#

Are the .net classes relating to DbProviderFactory thread safe?

from msdn:
Thread Safety
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

When you say "These instances are generated once at run time, and used for the rest of the service's life", do you mean the connection object? Also, do you mean you're keeping the connection object open through out the life of your service? If your service is multi-threaded and you only have one instance of the connection (say for example a singleton or static class), you have to make sure the connection is only used by one thread at a time.
Without seeing much code, it sounds like a problem with how you treat the IDbConnection you get from the factory, instead of the factory itself.
We use the DbProviderFactory very heavily for our multithreaded applications, which connect to Oracle, FoxPro and SqlServer and I haven't seen this issue.
Good luck!
Ricardo.

Related

How to dispose static connections in another AppDomain

I got an windows service application which dynamically loads some other modules in other new appdomain. The problem is they are all using the same static database connections. I can dispose the static connections in the main AppDomain when I shutdown the service. But how can I immediately dispose the other static connections in other AppDomain. The problem is since the other connections still exist, the service application still runs in the Task Manager even I fully stop it.
Thanks
The problem is they are all using the same static database connections.
Yes, that is definitely a problem. Don't do that. Connections are pooled by .NET and are not expensive to create, so the proper pattern is to create them when you need them, use them, and dispose of them when you're done. An effective way of doing that is with using statements.
in general, whatever creates a disposable object is responsible for disposing of it. since your disposable objects are static, there is no way to know what is responsible for disposing of it. So you need to have logic to see if it's already been disposed, if it's open, if it's null, etc. It's much cleaner to just keep all of the creation and disposal logic in one place.

One SQLiteConnection per thread?

I am using SQLite from system.data.sqlite.org
We need to access the database from many threads (for various reasons). I've read a lot about sqlite thread safe capabilities (the default synchronized access mode is fine for me).
I wonder if it is possible to simply open a connection per thread. Is something like this possible? I really don't care about race conditions (request something that hasn't been inserted yet). I am only interested in the fact that it is possible to access the data using one SQLiteConnection object per thread.
Yes. In fact, it's the proper way, as SQLite is not thread safe (by default. You can make it threadsafe compiling with some option). And just to ensure it works: SQLite is being used in some small websites, so multithreading is there :)
Here more information: http://www.sqlite.org/faq.html#q6
Given you use a separate connection per thread you should be fine.
From docs
Note that SQLiteConnection instance is not guaranteed to be thread
safe. You should avoid using the same SQLiteConnection in several
threads at the same time. It is recommended to open a new connection
per thread and to close it when the work is done.

MSpec: How to make static variables thread-safe?

I'm using MSpec for my latest project, and overall I'm really happy with it. However, I do have an issue with concurrency when my tests run in paralel and I'm wondering if anybody has run into this issue or, even better, has a solution?
MSpec heavily relies on static methods and variables to work.
Now it appears when I define static variables in my base classes, that are used by multiple test classes, and I run my tests in paralel, that they share the same static variables and thus interfere with eachother.
I'm using both NCrunch and Resharper as my testrunners and I'm experiencing the problem in both.
Anybody familiar with this problem?
Firstly, I would recommend reading the Thead Safety Guidelines on MSDN. This will give you a good overview of how and why to make methods thread safe in C#.
The following rules outline the design guidelines for implementing threading:
Avoid providing static methods that alter static state. In common server scenarios, static state is shared across requests, which means multiple threads can execute that code at the same time. This opens up the possibility for threading bugs. Consider using a design pattern that encapsulates data into instances that are not shared across requests.
... Adding locks to create thread-safe code decreases performance, increases lock contention, and creates the possibility for deadlock bugs to occur
Be aware of method calls in locked sections. Deadlocks can result when a static method in class A calls static methods in class B and vice versa. If A and B both synchronize their static methods, this will cause a deadlock. You might discover this deadlock only under heavy threading stress.
Be aware of issues with the lock statement (SyncLock in Visual Basic). It is tempting to use the lock statement to solve all threading problems. However, the System.Threading.Interlocked Class is superior for updates that must be atomic ...
As a general note a methodology which I prefer to use (where possible) is to make a method (static or otherwise) immutable. To do this, all variables should be local (created locally on the stack, or passed in as parameters to a method). By ensuring only local variables are used, or member variables are immutable each thread will operate in its own compartment and changes to variables will not affect another thread. This is a methodology I have used extensively in .NET simulation software to allow lock-less and therefore high performance multithreading in C#.
Alternatively, if variables must be member variables and mutable access to them may be protected by lock keywords. Be careful with the use of lock will cause context switching (slow down) and introduces the possibility of a deadlock situation. It also doesn't gaurantee thread safety as the use of lock must protect against the specific scenario you are trying to prevent.
For further reading I would suggest looking these related questions which describe thread safety and immutability in C#:
Designing a Thread Safe class
Achieving Thread Safety
Why are immutable objects thread safe
Best regards,
Static fields are not thread-safe by default. To make them thread-safe you can decorate them with the [ThreadStatic] attribute.
Have a look at ThreadStaticAttribute Class at MSDN for more info.

Which features make a class to be thread-safe?

In MSDN some .NET classes described like this:
"This type is thread safe."
or
"Public static (Shared in Visual Basic) members of this type are thread safe. Instance members are not guaranteed to be thread-safe.".
My question is which features make a class to be thread-safe?
Is there any standard, recommendation or guidelines for thread-safety programming?
When I use lock(C#) keyword, it means my class is thread-safe or not?
How to I evaluate thread-safety of a class? Is there any TESTS to be sure that a class is 100% thread safe?
Example:
public class MyClass
{
public void Method()
{
lock (this)
{
// Now, is my class 100% thread-safe like Microsoft classes?
}
}
type m_member1;
type m_member2;
}
thanks
Is there any standard, recommendation or guidelines for thread-safety
programming?
The most important standard is to ensure that all static members are thread-safe. You will see that all well written APIs including the .NET base class library makes this guarantee across the board. There is a really good reason for this. Since static members are shared across an AppDomain they could be used by many different threads without you even realizing it. It would be awkward at best to provide your own synchronization for every single static member access. Imagine what it would be like if Console.WriteLine were not thread-safe.
As far as recommendations and guidelines there are plenty of well established patterns for doing concurrent programming. The patterns that are out there cover a wide variety of programming problems and use many different synchronization mechanisms. The producer-consumer pattern is one of many well known patterns which happens to solve a large percentage of concurrent programming problems.
Read Threading in C# by Joseph Albahari. It is one of the best and most vetted resources available.
When I use lock(C#) keyword, it means my class is thread-safe or not?
Nope! There is no magic bullet that can make a class thread-safe. The lock keyword is but one of many different tools that can be used to make a class safe for simultaneous access by multiple threads. But, just using a lock will not guarantee anything. It is the correct use of synchronization mechanisms that makes code thread-safe. There are plenty ways to use these mechanisms incorrectly.
How to I evaluate thread-safety of a class? Is there any TESTS to be
sure that a class is 100% thread safe?
This is the million dollar question! It is incredibly difficult to test multithreaded code. The CHESS tool provided by Microsoft Research is one attempt at making life easier for concurrent programmers.
A class is generally considered thread-safe if its methods can be invoked by multiple threads concurrently without corrupting the state of the class or causing unexpected side-effects. There are many reasons why a class may not be thread safe, although some common reasons are that it contains some state that would be corrupted on concurrent access.
There are a number of ways to make a class thread-safe:
Make it immutable, if a class contains no state it is safe to use concurrently from multiple threads.
Employ locking to reduce concurrency. However, this is no guarantee of thread safety, it just ensures that a block of code will not be executed concurrently by multiple threads. If state is stored between method invocations this might still become inconsistent.
How you create a thread-safe class really depends on what you want to do with the class in question.
You also need to ask yourself, do I need to make my class threadsafe? a common model of most UI frameworks is that there is a single UI thread. For example in WinForms, WPF and Silverlight the majority of your code will be executed from the UI thread which means you do not have to build thread-safety into your classes.
First of all, don't use lock(this).
This can cause deadlocks. Because other code can lock that same object from outside the class' scope. You should create a local Object and use it as the class' lock.
Second, thread safety is a complicated issue. There's tons of material about this on the web.
As a rule of thumb, all public methods should be locked and thread safe for the class to be thread-safe.
A Class is considered thread safe if only one thread at a time can modify the state of the objects created from the class OR the class provide such functionality that multiple threads can call various methods of the class at same time.
When I use lock(C#) keyword, it means my class is thread-safe or not?
When you use lock it means that the portion of code inside the lock {} is thread safe. It doesn't guarantee that your class is thread safe. And as Yochai Timmer said it is not a good idea to lock(this)
How to I evaluate thread-safety of a class? Is there any TESTS to be sure that a class is 100% thread safe?
I am not sure there are any tests because it is always possible in multi-threading that you are by chance getting correct results. So in order to be sure you can go through the code of class to see how it is making sure it is thread safe
Very simple explanation:
Thread safe type means you don't need any additional synchronization mechanisms when using your type. Say you can create an instance pass a reference to another thread (or multiple threads) and use methods/properties from both threads without any additional overhead for thread safety.

How do I safely use ADO.NET IDbConnection and IDbCommand to execute multiple database commands concurrently?

The Goal
Use an ADO.NET IDbConnection and IDbCommand to execute multiple commands at the same time, against the same database, given that the ADO.NET implementation is specified at runtime.
Investigation
The MSDN Documentation for IDbConnection does not specify any threading limitations. The SqlConnection page has the standard disclaimer saying "Any instance members are not guaranteed to be thread safe." The IDbCommand and SqlCommand documentation is equally un-informative.
Assuming that no individual instance member is thread-safe, I can still create multiple commands from a connection (on the same thread) and then execute them concurrently on different threads.
Presumably this would still not achieve the desired effect, because (I assume) only one command can execute at a time on the single underlying connection to the database. So the concurrent IDbCommand executions would get serialized at the connection.
Conclusion
So this means we have to create a separate IDbConnection, which is ok if you know you're using SqlConnection because that supports pooling. If your ADO.NET implementation is determined at runtime, these assumptions cannot be made.
Does this mean I need to implement my own connection pooling in order to support performant multi-threaded access to the database?
You will need to manage thread access to your instance members, but most ADO implementations manage their own connection pool. They generally expect that multiple queries will be run simultaneously.
I would feel free to open and close as many connections as is necessary, and handle an exceptions that could be thrown if pooling were not available.
Here's an article on ADO connection pooling
If you create a connection on one thread, you shouldn't use it on a different thread. The same goes for commands.
However, you can create a connection on each of your threads and use those objects safely on their own thread.
Pooling is for when you create lots of short-lived connection objects. It means the underlying ( expensive ) database connections are re-used.
Nick

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