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Gregory Burd eff277ac23 Release 5.2.28 on 6/10/2011
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<h2 class="title" style="clear: both"><a id="cam_app"></a>Architecting Data Store and Concurrent Data Store applications</h2>
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<p>When building Data Store and Concurrent Data Store applications, the
architecture decisions involve application startup (cleaning up any
existing databases, the removal of any existing database environment
and creation of a new environment), and handling system or application
failure. "Cleaning up" databases involves removal and re-creation
of the database, restoration from an archival copy and/or verification
and optional salvage, as described in <a class="xref" href="cam_fail.html" title="Handling failure in Data Store and Concurrent Data Store applications">Handling failure in Data Store and Concurrent Data Store applications</a>.</p>
<p>Data Store or Concurrent Data Store applications without database
environments are single process, by definition. These applications
should start up, re-create, restore, or verify and optionally salvage
their databases and run until eventual exit or application or system
failure. After system or application failure, that process can simply
repeat this procedure. This document will not discuss the case of these
applications further.</p>
<p>Otherwise, the first question of Data Store and Concurrent Data Store
architecture is the cleaning up existing databases and the removal of
existing database environments, and the subsequent creation of a new
environment. For obvious reasons, the application must serialize the
re-creation, restoration, or verification and optional salvage of its
databases. Further, environment removal and creation must be
single-threaded, that is, one thread of control (where a thread of
control is either a true thread or a process) must remove and re-create
the environment before any other thread of control can use the new
environment. It may simplify matters that Berkeley DB serializes creation of
the environment, so multiple threads of control attempting to create a
environment will serialize behind a single creating thread.</p>
<p>Removing a database environment will first mark the environment as
"failed", causing any threads of control still running in the
environment to fail and return to the application. This feature allows
applications to remove environments without concern for threads of
control that might still be running in the removed environment.</p>
<p>One consideration in removing 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 to remove and re-create a environment, since the
last one to run will remove all environments created by the threads of
control that ran before it. However, for some few applications, it may
make sense for applications to have a single thread of control that
checks the existing databases and removes the environment, after which
the application launches a number of processes, any of which are able
to create the environment.</p>
<p>With respect to cleaning up existing databases, the database environment
must be removed before the databases are cleaned up. Removing the
environment causes any Berkeley DB library calls made by threads of control
running in the failed environment to return failure to the application.
Removing the database environment first ensures the threads of control
in the old environment do not race with the threads of control cleaning
up the databases, possibly overwriting them after the cleanup has
finished. Where the application architecture and system permit, many
applications kill all threads of control running in the failed database
environment before removing the failed database environment, on general
principles as well as to minimize overall system resource usage. It
does not matter if the new environment is created before or after the
databases are cleaned up.</p>
<p>After having dealt with database and database environment recovery after
failure, the next issue to manage is application failure. As described
in <a class="xref" href="cam_fail.html" title="Handling failure in Data Store and Concurrent Data Store applications">Handling failure in Data Store and Concurrent Data Store applications</a>, when a thread of control in a Data Store or
Concurrent Data Store application fails, it may exit holding data
structure mutexes or logical database locks. These mutexes and locks
must be released to avoid the remaining threads of control hanging
behind the failed thread of control's mutexes or locks.</p>
<p>There are three common ways to architect Berkeley DB Data Store and Concurrent
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 logging into the system).</p>
<div class="orderedlist">
<ol type="1">
<li>The first way to architect Data Store and Concurrent Data Store
applications is as a single process (the process may or may not be
multithreaded.)
<p>When this process starts, it removes any existing database environment
and creates a new environment. It then cleans up the databases and
opens those databases in the environment. 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_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 the process to continue after
thread failure, the <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV-&gt;failchk()</a> method can be used to determine if
the database environment is usable after the failure. If the
application does not call <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV-&gt;failchk()</a>, or
<a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV-&gt;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, removing the
environment and creating a new environment, and cleaning up any
databases it wants to continue to use. 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.</p></li>
<li>The second way to architect Data Store and Concurrent Data Store
applications is as a group of related processes (the processes may or
may not be multithreaded).
<p>This architecture requires the order in which threads of control are
created be controlled to serialize database environment removal and
creation, and database cleanup.</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-&gt;failchk()</a> method to determine if the
database environment is usable after the exit. If the application does
not call <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV-&gt;failchk()</a>, or <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV-&gt;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, removing the environment and creating a new
environment, and cleaning up any databases it wants to continue to use.
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.</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, removes and creates the database environment, cleans
up the databases 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-&gt;failchk()</a> method. If the application does not call
<a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV-&gt;failchk()</a>, or if <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV-&gt;failchk()</a> returns
<a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">DB_RUNRECOVERY</a>, the environment can no longer be used, the
watcher kills all of the threads of control using the failed
environment, cleans up, and starts new threads of control to perform
work.</p></li>
<li>The third way to architect Data Store and Concurrent 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.
<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 open/create
the database environment, clean up the databases 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, cleans
up the environment and databases, 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, cleaning up the environment.</p><p>If it is not practical to monitor the processes sharing a database
environment, it may be 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-&gt;failchk()</a> method. If <a href="../api_reference/C/envfailchk.html" class="olink">DB_ENV-&gt;failchk()</a> returns
<a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">DB_RUNRECOVERY</a>, the watcher would then take action, cleaning up
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-&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.)</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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