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Architecting Transactional Data Store applications
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## Architecting Transactional Data Store applications

When building Transactional Data Store applications, the architecture decisions involve application startup (running recovery) and handling system or application failure. For details on performing recovery, see the Recovery procedures.

Recovery in a database environment is a single-threaded procedure, that is, one thread of control or process must complete database environment recovery before any other thread of control or process operates in the Berkeley DB environment.

Performing recovery first marks any existing database environment as "failed" and then removes it, causing threads of control running in the database environment to fail and return to the application. This feature allows applications to recover environments without concern for threads of control that might still be running in the removed environment. The subsequent re-creation of the database environment is serialized, so multiple threads of control attempting to create a database environment will serialize behind a single creating thread.

One consideration in removing (as part of recovering) a database environment which may be in use by another thread, is the type of mutex being used by the Berkeley DB library. In the case of database environment failure when using test-and-set mutexes, threads of control waiting on a mutex when the environment is marked "failed" will quickly notice the failure and will return an error from the Berkeley DB API. In the case of environment failure when using blocking mutexes, where the underlying system mutex implementation does not unblock mutex waiters after the thread of control holding the mutex dies, threads waiting on a mutex when an environment is recovered might hang forever. Applications blocked on events (for example, an application blocked on a network socket, or a GUI event) may also fail to notice environment recovery within a reasonable amount of time. Systems with such mutex implementations are rare, but do exist; applications on such systems should use an application architecture where the thread recovering the database environment can explicitly terminate any process using the failed environment, or configure Berkeley DB for test-and-set mutexes, or incorporate some form of long-running timer or watchdog process to wake or kill blocked processes should they block for too long.

Regardless, it makes little sense for multiple threads of control to simultaneously attempt recovery of a database environment, since the last one to run will remove all database environments created by the threads of control that ran before it. However, for some applications, it may make sense for applications to have a single thread of control that performs recovery and then removes the database environment, after which the application launches a number of processes, any of which will create the database environment and continue forward.

There are three common ways to architect Berkeley DB Transactional Data Store applications. The one chosen is usually based on whether or not the application is comprised of a single process or group of processes descended from a single process (for example, a server started when the system first boots), or if the application is comprised of unrelated processes (for example, processes started by web connections or users logged into the system).

1. The first way to architect Transactional Data Store applications is as a single process (the process may or may not be multithreaded.)

   When this process starts, it runs recovery on the database environment and then opens its databases. The application can subsequently create new threads as it chooses. Those threads can either share already open Berkeley DB DB_ENV and DB handles, or create their own. In this architecture, databases are rarely opened or closed when more than a single thread of control is running; that is, they are opened when only a single thread is running, and closed after all threads but one have exited. The last thread of control to exit closes the databases and the database environment.

   This architecture is simplest to implement because thread serialization is easy and failure detection does not require monitoring multiple processes.

   If the application's thread model allows processes to continue after thread failure, the DB_ENV->failchk() method can be used to determine if the database environment is usable after thread failure. If the application does not call DB_ENV->failchk(), or DB_ENV->failchk() returns DB_RUNRECOVERY, the application must behave as if there has been a system failure, performing recovery and re-creating the database environment. Once these actions have been taken, other threads of control can continue (as long as all existing Berkeley DB handles are first discarded).

2. The second way to architect Transactional Data Store applications is as a group of related processes (the processes may or may not be multithreaded).

   This architecture requires the order in which threads of control are created be controlled to serialize database environment recovery.

   In addition, this architecture requires that threads of control be monitored. If any thread of control exits with open Berkeley DB handles, the application may call the DB_ENV->failchk() method to detect lost mutexes and locks and determine if the application can continue. If the application does not call DB_ENV->failchk(), or DB_ENV->failchk() returns that the database environment can no longer be used, the application must behave as if there has been a system failure, performing recovery and creating a new database environment. Once these actions have been taken, other threads of control can be continued (as long as all existing Berkeley DB handles are first discarded), or

   The easiest way to structure groups of related processes is to first create a single "watcher" process (often a script) that starts when the system first boots, runs recovery on the database environment and then creates the processes or threads that will actually perform work. The initial thread has no further responsibilities other than to wait on the threads of control it has started, to ensure none of them unexpectedly exit. If a thread of control exits, the watcher process optionally calls the DB_ENV->failchk() method. If the application does not call DB_ENV->failchk() or if DB_ENV->failchk() returns that the environment can no longer be used, the watcher kills all of the threads of control using the failed environment, runs recovery, and starts new threads of control to perform work.

3. The third way to architect Transactional Data Store applications is as a group of unrelated processes (the processes may or may not be multithreaded). This is the most difficult architecture to implement because of the level of difficulty in some systems of finding and monitoring unrelated processes. There are several possible techniques to implement this architecture.

   One solution is to log a thread of control ID when a new Berkeley DB handle is opened. For example, an initial "watcher" process could run recovery on the database environment and then create a sentinel file. Any "worker" process wanting to use the environment would check for the sentinel file. If the sentinel file does not exist, the worker would fail or wait for the sentinel file to be created. Once the sentinel file exists, the worker would register its process ID with the watcher (via shared memory, IPC or some other registry mechanism), and then the worker would open its DB_ENV handles and proceed. When the worker finishes using the environment, it would unregister its process ID with the watcher. The watcher periodically checks to ensure that no worker has failed while using the environment. If a worker fails while using the environment, the watcher removes the sentinel file, kills all of the workers currently using the environment, runs recovery on the environment, and finally creates a new sentinel file.

   The weakness of this approach is that, on some systems, it is difficult to determine if an unrelated process is still running. For example, POSIX systems generally disallow sending signals to unrelated processes. The trick to monitoring unrelated processes is to find a system resource held by the process that will be modified if the process dies. On POSIX systems, flock- or fcntl-style locking will work, as will LockFile on Windows systems. Other systems may have to use other process-related information such as file reference counts or modification times. In the worst case, threads of control can be required to periodically re-register with the watcher process: if the watcher has not heard from a thread of control in a specified period of time, the watcher will take action, recovering the environment.

   The Berkeley DB library includes one built-in implementation of this approach, the DB_ENV->open() method's DB_REGISTER flag:

   If the DB_REGISTER flag is set, each process opening the database environment first checks to see if recovery needs to be performed. If recovery needs to be performed for any reason (including the initial creation of the database environment), and DB_RECOVER is also specified, recovery will be performed and then the open will proceed normally. If recovery needs to be performed and DB_RECOVER is not specified, DB_RUNRECOVERY will be returned. If recovery does not need to be performed, DB_RECOVER will be ignored.

   Prior to the actual recovery beginning, the DB_EVENT_REG_PANIC event is set for the environment. Processes in the application using the DB_ENV->set_event_notify() method will be notified when they do their next operations in the environment. Processes receiving this event should exit the environment. Also, the DB_EVENT_REG_ALIVE event will be triggered if there are other processes currently attached to the environment. Only the process doing the recovery will receive this event notification. It will receive this notification once for each process still attached to the environment. The parameter of the DB_ENV->set_event_notify() callback will contain the process identifier of the process still attached. The process doing the recovery can then signal the attached process or perform some other operation prior to recovery (i.e. kill the attached process).

   The DB_ENV->set_timeout() method's DB_SET_REG_TIMEOUT flag can be set to establish a wait period before starting recovery. This creates a window of time for other processes to receive the DB_EVENT_REG_PANIC event and exit the environment.

   There are three additional requirements for the DB_REGISTER architecture to work:

   - First, all applications using the database environment must specify the DB_REGISTER flag when opening the environment. However, there is no additional requirement if the application chooses a single process to recover the environment, as the first process to open the database environment will know to perform recovery.

   - Second, there can only be a single DB_ENV handle per database environment in each process. As the DB_REGISTER locking is per-process, not per-thread, multiple DB_ENV handles in a single environment could race with each other, potentially causing data corruption.

   - Third, the DB_REGISTER implementation does not explicitly terminate processes using the database environment which is being recovered. Instead, it relies on the processes themselves noticing the database environment has been discarded from underneath them. For this reason, the DB_REGISTER flag should be used with a mutex implementation that does not block in the operating system, as that risks a thread of control blocking forever on a mutex which will never be granted. Using any test-and-set mutex implementation ensures this cannot happen, and for that reason the DB_REGISTER flag is generally used with a test-and-set mutex implementation.

   A second solution for groups of unrelated processes is also based on a "watcher process". This solution is intended for systems where it is not practical to monitor the processes sharing a database environment, but it is possible to monitor the environment to detect if a thread of control has failed holding open Berkeley DB handles. This would be done by having a "watcher" process periodically call the DB_ENV->failchk() method. If DB_ENV->failchk() returns that the environment can no longer be used, the watcher would then take action, recovering the environment.

   The weakness of this approach is that all threads of control using the environment must specify an "ID" function and an "is-alive" function using the DB_ENV->set_thread_id() method. (In other words, the Berkeley DB library must be able to assign a unique ID to each thread of control, and additionally determine if the thread of control is still running. It can be difficult to portably provide that information in applications using a variety of different programming languages and running on a variety of different platforms.)

   A third solution for groups of unrelated processes is a hybrid of the two above. Along with implementing the built-in sentinel approach with the the DB_ENV->open() methods DB_REGISTER flag, the DB_FAILCHK flag can be specified. When using both flags, each process opening the database environment first checks to see if recocvery needs to be performed. If recovery needs to be performed for any reason, it will first determine if a thread of control exited while holding database read locks, and release those. Then it will abort any unresolved transactions. If these steps are successful, the process opening the environment will continue without the need for any additional recocvery. If these steps are unsuccessful, then additional recovery will be performed if DB_RECOVER is specified and if DB_RECOVER is not specified, DB_RUNRECOVERYwill be returned.

   Since this solution is hybrid of the first two, all of the requirements of both of them must be implemented (will need "ID" function, "is-alive" function, single DB_ENV handle per database, etc.)

   The described approaches are different, and should not be combined. Applications might use either the DB_REGISTER approach, the DB_ENV->failchk() or the hybrid approach, but not together in the same application. For example, a POSIX application written as a library underneath a wide variety of interfaces and differing APIs might choose the DB_REGISTER approach for a few reasons: first, it does not require making periodic calls to the DB_ENV->failchk() method; second, when implementing in a variety of languages, is may be more difficult to specify unique IDs for each thread of control; third, it may be more difficult determine if a thread of control is still running, as any particular thread of control is likely to lack sufficient permissions to signal other processes. Alternatively, an application with a dedicated watcher process, running with appropriate permissions, might choose the DB_ENV->failchk() approach as supporting higher overall throughput and reliability, as that approach allows the application to abort unresolved transactions and continue forward without having to recover the database environment. The hybrid approach is useful in situations where running a dedicated watcher process is not practical but getting the equivalent of DB_ENV->failchk() on the DB_ENV->open() is important.

Obviously, when implementing a process to monitor other threads of control, it is important the watcher process' code be as simple and well-tested as possible, because the application may hang if it fails.

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