Chapter 4. Concurrency
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## Chapter 4. Concurrency

DB offers a great deal of support for multi-threaded and multi-process applications even when transactions are not in use. Many of DB's handles are thread-safe, or can be made thread-safe by providing the appropriate flag at handle creation time, and DB provides a flexible locking subsystem for managing databases in a concurrent application. Further, DB provides a robust mechanism for detecting and responding to deadlocks. All of these concepts are explored in this chapter.

Before continuing, it is useful to define a few terms that will appear throughout this chapter:

- Thread of control

  Refers to a thread that is performing work in your application. Typically, in this book that thread will be performing DB operations.

  Note that this term can also be taken to mean a separate process that is performing work — DB supports multi-process operations on your databases.

  Also, DB is agnostic with regard to the type or style of threads in use in your application. So if you are using multiple threads (as opposed to multiple processes) to perform concurrent database access, you are free to use whatever thread package is best for your platform and application. That said, this manual will use pthreads for its threading examples because those have the best chance of being supported across a large range of platforms.

- Locking

  When a thread of control obtains access to a shared resource, it is said to be locking that resource. Note that DB supports both exclusive and non-exclusive locks. See Locks for more information.

- Free-threaded

  Data structures and objects are free-threaded if they can be shared across threads of control without any explicit locking on the part of the application. Some books, libraries, and programming languages may use the term thread-safe for data structures or objects that have this characteristic. The two terms mean the same thing.

  For a description of free-threaded DB objects, see Which DB Handles are Free-Threaded.

- Blocked

  When a thread cannot obtain a lock because some other thread already holds a lock on that object, the lock attempt is said to be blocked. See Blocks for more information.

- Deadlock

  Occurs when two or more threads of control attempt to access conflicting resource in such a way as none of the threads can any longer make further progress.

  For example, if Thread A is blocked waiting for a resource held by Thread B, while at the same time Thread B is blocked waiting for a resource held by Thread A, then neither thread can make any forward progress. In this situation, Thread A and Thread B are said to be deadlocked.

  For more information, see Deadlocks.

## Which DB Handles are Free-Threaded

The following describes to what extent and under what conditions individual handles are free-threaded.

- `DbEnv`

  Free-threaded so long as the `DB_THREAD` flag is provided to the environment `open()` method.

- `Db`

  Free-threaded so long as the `DB_THREAD` flag is provided to the database `open()` method, or if the database is opened using a free-threaded environment handle.

- `Dbc`

  Cursors are not free-threaded. However, they can be used by multiple threads of control so long as the application serializes access to the handle.

- `DbTxn`

  Access must be serialized by the application across threads of control.

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