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Berkeley DB Transactional Data Store Applications
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<h2 class="title" style="clear: both"><a id="transapp_app"></a>Architecting Transactional Data Store applications</h2>
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<p>
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 <a class="xref" href="transapp_recovery.html" title="Recovery procedures">Recovery procedures</a>.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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).
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
The first way to architect Transactional Data Store
applications is as a single process (the process may or may not
be multithreaded.)
</p>
<p>
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 <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> and <a href="../api_reference/C/db.html" class="olink">DB</a>
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.
</p>
<p>
This architecture is simplest to implement because thread
serialization is easy and failure detection does not require
monitoring multiple processes.
</p>
<p>
If the application's thread model allows processes to continue
after thread failure, the <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> method can be used to
determine if the database environment is usable after thread
failure. If the application does not call <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a>, or
<a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> returns
<a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">DB_RUNRECOVERY</a>,
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).
</p>
</li>
<li>
<p>
The second way to architect Transactional Data Store
applications is as a group of related processes (the processes
may or may not be multithreaded).
</p>
<p>
This architecture requires the order in which threads of control are
created be controlled to serialize database environment recovery.
</p>
<p>
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 <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a>
method to detect lost mutexes and locks and determine if the
application can continue. If the application does not call
<a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a>, or <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> 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
</p>
<p>
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 <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> method. If the application
does not call <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> or if <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> 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.
</p>
</li>
<li>
<p>
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.
</p>
<p>
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
<a href="../api_reference/C/env.html" class="olink">DB_ENV</a> 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.
</p>
<p>
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.
</p>
<p>
The Berkeley DB library includes one built-in implementation of this approach,
the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method's <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a> flag:
</p>
<p>
If the <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a> 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 <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> is also specified, recovery will be performed
and then the open will proceed normally. If recovery needs to
be performed and <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> is not specified,
<a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">DB_RUNRECOVERY</a>
will be returned. If recovery does not need to be performed,
<a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> will be ignored.
</p>
<p>
Prior to the actual recovery beginning, the <a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_REG_PANIC" class="olink">DB_EVENT_REG_PANIC</a>
event is set for the environment. Processes in the application using
the <a href="../api_reference/C/envevent_notify.html" class="olink">DB_ENV->set_event_notify()</a> method will be notified when they do their next
operations in the environment. Processes receiving this event should
exit the environment. Also, the <a href="../api_reference/C/envevent_notify.html#event_notify_DB_EVENT_REG_ALIVE" class="olink">DB_EVENT_REG_ALIVE</a> 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
<a href="../api_reference/C/envevent_notify.html" class="olink">DB_ENV->set_event_notify()</a> 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).
</p>
<p>
The <a href="../api_reference/C/envset_timeout.html" class="olink">DB_ENV->set_timeout()</a> method's <a href="../api_reference/C/envset_timeout.html#set_timeout_DB_SET_REG_TIMEOUT" class="olink">DB_SET_REG_TIMEOUT</a> 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.
</p>
<p>
There are three additional requirements for the <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a>
architecture to work:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
First, all applications using the database environment must
specify the <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a> 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.
</p>
</li>
<li>
<p>
Second, there can only be a single <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> handle per database
environment in each process. As the <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a> locking is
per-process, not per-thread, multiple <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> handles in a single
environment could race with each other, potentially causing
data corruption.
</p>
</li>
<li>
<p>
Third, the <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a> 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 <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a> 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 <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a> flag is generally
used with a test-and-set mutex implementation.
</p>
</li>
</ul>
</div>
<p>
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 <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> method.
If <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> returns that the environment can no longer be
used, the watcher would then take action, recovering the
environment.
</p>
<p>
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 <a href="../api_reference/C/envset_thread_id.html" class="olink">DB_ENV->set_thread_id()</a> 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.)
</p>
<p>
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 <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> methods <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a> flag, the <a href="../api_reference/C/envopen.html#envopen_DB_FAILCHK" class="olink">DB_FAILCHK</a> 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 <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> is specified and if <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> is not
specified, <a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">DB_RUNRECOVERY</a>will be returned.
</p>
<p>
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 <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> handle per database, etc.)
</p>
<p>
The described approaches are different, and should not be
combined. Applications might use either the <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a>
approach, the <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> 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 <a href="../api_reference/C/envopen.html#envopen_DB_REGISTER" class="olink">DB_REGISTER</a> approach for a
few reasons: first, it does not require making periodic calls
to the <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> 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 <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> 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 <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV->failchk()</a> on the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> is important.
</p>
</li>
</ol>
</div>
<p>
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.
</p>
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