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<td><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Transactional Data Store Applications</dl></td>
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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 <a href="recovery.html">Recovery procedures</a>.
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. It may simplify matters that Berkeley DB serializes
recovery and creation of a new database 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).
<ol>
<li>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 of control as it chooses. Those threads of control can
either share already open Berkeley DB <a href="../../api_c/env_class.html">DB_ENV</a> and <a href="../../api_c/db_class.html">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.
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 <a href="../../api_c/env_failchk.html">DB_ENV-&gt;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_c/env_failchk.html">DB_ENV-&gt;failchk</a>, or
<a href="../../api_c/env_failchk.html">DB_ENV-&gt;failchk</a> returns <a href="../../ref/program/errorret.html#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), or restarted.
<li>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 <a href="../../api_c/env_failchk.html">DB_ENV-&gt;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_c/env_failchk.html">DB_ENV-&gt;failchk</a>, or
<a href="../../api_c/env_failchk.html">DB_ENV-&gt;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 restarted.
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_c/env_failchk.html">DB_ENV-&gt;failchk</a> method. If the application does not call
<a href="../../api_c/env_failchk.html">DB_ENV-&gt;failchk</a> or if <a href="../../api_c/env_failchk.html">DB_ENV-&gt;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.
<li>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.
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_c/env_class.html">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.
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 <a href="../../api_c/env_open.html">DB_ENV-&gt;open</a> method's <a href="../../api_c/env_open.html#DB_REGISTER">DB_REGISTER</a> flag:
If the <a href="../../api_c/env_open.html#DB_REGISTER">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_c/env_open.html#DB_RECOVER">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_c/env_open.html#DB_RECOVER">DB_RECOVER</a> is not specified, <a href="../../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a> will be
returned. If recovery does not need to be performed, <a href="../../api_c/env_open.html#DB_RECOVER">DB_RECOVER</a>
will be ignored.
There are three additional requirements for the <a href="../../api_c/env_open.html#DB_REGISTER">DB_REGISTER</a>
architecture to work:
<ul type=disc>
<li>First, all applications using the database environment must specify the
<a href="../../api_c/env_open.html#DB_REGISTER">DB_REGISTER</a> flag when opening the environment. However, there
is no additional requirement the application choose a single process to
recover the environment, as the first process to open the database
environment will know to perform recovery.
<li>Second, there can only be a single <a href="../../api_c/env_class.html">DB_ENV</a> handle per database
environment in each process. As the <a href="../../api_c/env_open.html#DB_REGISTER">DB_REGISTER</a> locking is
per-process, not per-thread, multiple <a href="../../api_c/env_class.html">DB_ENV</a> handles in a single
environment could race with each other, potentially causing data
corruption.
<li>Third, the <a href="../../api_c/env_open.html#DB_REGISTER">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_c/env_open.html#DB_REGISTER">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_c/env_open.html#DB_REGISTER">DB_REGISTER</a> flag is
generally used with a test-and-set mutex implementation.
</ul>
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_c/env_failchk.html">DB_ENV-&gt;failchk</a> method.
If <a href="../../api_c/env_failchk.html">DB_ENV-&gt;failchk</a> 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 <a href="../../api_c/env_set_thread_id.html">DB_ENV-&gt;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.)
The two described approaches are different, and should not be combined.
Applications might use either the <a href="../../api_c/env_open.html#DB_REGISTER">DB_REGISTER</a> approach or the
<a href="../../api_c/env_failchk.html">DB_ENV-&gt;failchk</a> approach, but not both 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_c/env_open.html#DB_REGISTER">DB_REGISTER</a> approach for a few reasons: first, it does not
require making periodic calls to the <a href="../../api_c/env_failchk.html">DB_ENV-&gt;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_c/env_failchk.html">DB_ENV-&gt;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.
</ol>
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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