One feature of some operating systems is the ability to run several threads of execution within a single process. This allows an application to handle asynchronous events, and makes it easier to create event-driven applications without resorting to polling schemes. This section discusses how the database manager works with multiple threads, and lists some design guidelines that you should keep in mind. To determine if your platform supports the multithreading feature, refer to the Application Building Guide.
This section assumes that you are familiar with the terms relating to the development of multithreaded applications (such as critical section and semaphore). If you are not familiar with these terms, consult the programming documentation for your operating system.
A DB2 application can execute SQL statements from multiple threads using contexts. A context is the environment from which an application runs all SQL statements and API calls. All connections, units of work, and other database resources are associated with a specific context. Each context is associated with one or more threads within an application.
For each executable SQL statement in a context, the first run-time services call always tries to obtain a latch. If it is successful, it continues processing. If not (because an SQL statement in another thread of the same context already has the latch), the call is blocked on a signaling semaphore until that semaphore is posted, at which point the call gets the latch and continues processing. The latch is held until the SQL statement has completed processing, at which time it is released by the last run-time services call that was generated for that particular SQL statement.
The net result is that each SQL statement within a context is executed as an atomic unit, even though other threads may also be trying to execute SQL statements at the same time. This action ensures that internal data structures are not altered by different threads at the same time. APIs also use the latch used by run-time services; therefore, APIs have the same restrictions as run-time services routines within each context.
By default, all applications have a single context that is used for all database access. While this is perfect for a single threaded application, the serialization of SQL statements makes a single context inadequate for a multithreaded application. By using the following DB2 APIs, your application can attach a separate context to each thread and allow contexts to be passed between threads:
Contexts may be exchanged between threads in a process, but not exchanged between processes. One use of multiple contexts is to provide support for concurrent transactions. For the details of how to use these context APIs, refer to the Administrative API Reference and Concurrent Transactions.
Follow these guidelines when accessing a database from multiple thread applications:
Applications must ensure that user-defined data structures used by SQL statements and database manager routines are not altered by one thread while an SQL statement or database manager routine is being processed in another thread. For example, do not allow a thread to reallocate an SQLDA while it was being used by an SQL statement in another thread.
It may be easier to give each thread its own user-defined data structures to avoid having to serialize their usage. This is especially true for the SQLCA, which is used not only by every executable SQL statement, but also by all of the database manager routines. There are three alternatives for avoiding this problem with the SQLCA:
On AIX, Solaris, HP-UX, SCO UnixWare 7, and Silicon Graphics IRIX, changes have been made to the functions that are used for run time querying of the code page and country code to be used for a database connection. They are now thread safe but can create some lock contention (and resulting performance degradation) in a multithreaded application which uses a large number of concurrent database connections.
A new environment variable has been created (DB2_FORCE_NLS_CACHE) to eliminate the chance of lock contention in multithreaded applications. When DB2_FORCE_NLS_CACHE is set to TRUE the code page and country code information is saved the first time a thread accesses it. From that point on the cached information will be used for any other thread that requests this information. By saving this information, lock contention is eliminated and in certain situations a performance benefit will be realized.
DB2_FORCE_NLS_CACHE should not be set to true if the application changes locale settings between connections. If this is done then the original locale information will be returned even after the locale settings have been changed. In general, multithreaded applications will not change locale settings. This ensures that the application remains thread safe.
An application that uses multiple threads is, understandably, more complex than a single-threaded application. This extra complexity can potentially lead to some unexpected problems. When writing a multithreaded application, exercise caution with the following:
Each context in an application has its own set of database resources, including locks on database objects. This makes it possible for two contexts, if they are accessing the same database object, to deadlock. The database manager will detect the deadlock and one of the contexts will receive SQLCODE -911 and its unit of work will be rolled back.
Be careful with any programming techniques that establish inter-context dependencies. Latches, semaphores, and critical sections are examples of programming techniques that can establish such dependencies. If an application has two contexts that have both application and database dependencies between the contexts, it is possible for the application to become deadlocked. If some of the dependencies are outside of the database manager, the deadlock is not detected, thus the application gets suspended or hung.
As an example of this sort of problem, consider an application that has two contexts, both of which access a common data structure. To avoid problems where both contexts change the data structure simultaneously, the data structure is protected by a semaphore. The contexts look like this:
context 1
SELECT * FROM TAB1 FOR UPDATE....
UPDATE TAB1 SET....
get semaphore
access data structure
release semaphore
COMMIT
context 2
get semaphore
access data structure
SELECT * FROM TAB1...
release semaphore
COMMIT
Suppose the first context successfully executes the SELECT and the UPDATE statements while the second context gets the semaphore and accesses the data structure. The first context now tries to get the semaphore, but it cannot because the second context is holding the semaphore. The second context now attempts to read a row from table TAB1, but it stops on a database lock held by the first context. The application is now in a state where context 1 cannot finish before context 2 is done and context 2 is waiting for context 1 to finish. The application is deadlocked, but because the database manager does not know about the semaphore dependency neither context will be rolled back. This leaves the application suspended.
Because the database manager cannot detect deadlocks between threads, design and code your application in a way that will prevent deadlocks (or at least allow them to be avoided). In the above example, you can avoid the deadlock in several ways:
Change the code for context 1 to perform a commit before it gets the semaphore.
Change the code for context 2 to release the semaphore before doing the SELECT.
Change the code for context 1 to obtain the semaphore before running the SELECT statement. While this technique will work, it is not highly recommended because the semaphores will serialize access to the database manager, which potentially negates the benefits of using multiple threads.
While this will not prevent the deadlock, it will allow execution to resume. Context 2 is eventually rolled back because it is unable to obtain the requested lock. When handling the roll back error, context 2 should release the semaphore. Once the semaphore has been released, context 1 can continue and context 2 is free to retry its work .
The techniques for avoiding deadlocks are shown in terms of the above example, but you can apply them to all multithreaded applications. In general, treat the database manager as you would treat any protected resource and you should not run into problems with multithreaded applications.