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<title>5.Writing an Event Service Client</title>
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<div><div><h2 class="title" style="clear: both">
<a name="id2458280"></a>5.Writing an Event Service Client</h2></div></div>
<div></div>
</div>
<div class="toc"><dl>
<dt><span class="section"><a href="ar01s05.html#id2458294">5.1. Connecting</a></span></dt>
<dt><span class="section"><a href="ar01s05.html#id2458483">5.2. Using any type</a></span></dt>
<dt><span class="section"><a href="ar01s05.html#id2458621">5.3. Disconnecting</a></span></dt>
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<p>The examples are a great place to start learning about the event
service. Feel free to use them as a starting point for your own clients.
The same examples are available as C++, Python and Java. However, this
section provides a few more general instructions.</p>
<div class="section" lang="en">
<div class="titlepage">
<div><div><h3 class="title">
<a name="id2458294"></a>5.1.Connecting</h3></div></div>
<div></div>
</div>
<p>Here's a list of all the ways clients can locate the omniEvents
server's EventChannelFactory object:</p>
<div class="variablelist"><dl>
<dt><span class="term">by <span class="acronym">IOR</span>. (<span><b class="command">omniEvents
-v</b></span>)</span></dt>
<dd><p>The <tt class="option">-v</tt> option prints the
EventChannelFactory's <span class="acronym">IOR</span>. You can then use a
straightforward <tt class="literal">string_to_object()</tt> method call
to turn this into an object reference.</p></dd>
</dl></div>
<div class="variablelist"><dl>
<dt><span class="term">by <tt class="literal">corbaloc::host:port/omniEvents</tt></span></dt>
<dd><p>The EventChannelFactory is registered in the omniORB INSPOA
as <tt class="literal">omniEvents</tt> which means that you can use the
human readable “<span class="quote">corbaloc</span>” string above instead of the
<span class="acronym">IOR</span>. Replace <tt class="literal">host</tt> with
omniEvents' hostname and <tt class="literal">port</tt> with the
<span class="acronym">TCP</span> port: 11169 (or whatever you chose with the
<tt class="option">-p</tt> option).</p></dd>
</dl></div>
<div class="variablelist"><dl>
<dt><span class="term"><tt class="literal">resolve_initial_references("EventService")</tt></span></dt>
<dd>
<p>If <tt class="filename">omniORB.cfg</tt> is properly configured,
you can use <tt class="literal">resolve_initial_references()</tt> to
find the event service, just as is usually done for the Naming
Service. Just add a line like this:</p>
<pre class="programlisting">InitRef = EventService=corbaloc::host:port/omniEvents</pre>
</dd>
</dl></div>
<div class="variablelist"><dl>
<dt><span class="term">The naming service (<span><b class="command">omniEvents -N
NAME</b></span>)</span></dt>
<dd><p>omniEvents always registers the EventChannelFactory as a
top-level entry in the naming service. Use the <tt class="option">-N</tt>
option to control the context, id & kind.
(<tt class="literal">EventChannelFactory</tt> by default).</p></dd>
</dl></div>
</div>
<div class="section" lang="en">
<div class="titlepage">
<div><div><h3 class="title">
<a name="id2458483"></a>5.2.Using <tt class="literal">any</tt> type</h3></div></div>
<div></div>
</div>
<p>The “<span class="quote">events</span>” pushed and pulled around by the Event
Service are simply CORBA <tt class="literal">any</tt> values. The
<tt class="literal">any</tt> type can hold any <span class="acronym">CORBA</span> type.
The examples simply send a <tt class="literal">long</tt> value, but a more
realistic problem would employ a user-defined struct as the
event.</p>
<p>For user-defined types you first need to define the type in
<span class="acronym">IDL</span>, then compile the <span class="acronym">IDL</span>. For
omniORB with C++ you would use <span><b class="command">omniidl -bcxx -Wba</b></span>.
The <tt class="option">-Wba</tt> generates the operators you will need to use
your type with <tt class="literal">CORBA::Any</tt>.</p>
<p>Here's a small example:</p>
<pre class="programlisting"> module MyMessages {
struct NVPair {
NameType name;
ValueType value;
};
};</pre>
<p>Assuming you compile this <span class="acronym">IDL</span> correctly, you
will have insertion & extraction methods:
<tt class="literal">operator>>=()</tt> and
<tt class="literal">operator<<=()</tt>. Here are examples of how to use
them:</p>
<pre class="programlisting"> CORBA::Any data;
// Insert by value
MyMessages::NVPair inputNvPair = ...
data <<= inputNvPair; // takes a deep copy.
// Insert by pointer
MyMessages::NVPair* inputNvPairPtr = ...
data <<= inputNvPairPtr; // does NOT copy - assumes ownership
//XX delete inputNvPairPtr; <== NO!!
.
.
.
// Extract
const MyMessages::NVPair* outputNvPairPtr = NULL;
if(data >>= outputNvPairPtr)
{
// Use outputNvPairPtr BUT DON'T DELETE IT!!
}
else
{
cerr<<"Wrong type!!"<<endl;
}</pre>
<p>Notice the memory ownership issues. It's best to double check each
time you use <tt class="literal"><<=</tt> or
<tt class="literal">>>=</tt> until you're confident you've got it
right.</p>
</div>
<div class="section" lang="en">
<div class="titlepage">
<div><div><h3 class="title">
<a name="id2458621"></a>5.3.Disconnecting</h3></div></div>
<div></div>
</div>
<p>All Supplier and Consumer objects have a
<tt class="literal">disconnect_*()</tt> method. This means that each
connection has two disconnect methods, one at each end. Which method
should you call to terminate the connection?</p>
<p>The best approach is to call the Proxy's
<tt class="literal">disconnect_*()</tt>, rather than your own. (Either will
work, but instructing the Proxy to disconnect is more efficient.)</p>
<p>The rule for implementing your own servant's
<tt class="literal">disconnect_*()</tt> method is quite simple. Each
<tt class="literal">disconnect_*()</tt> method should call the other
<tt class="literal">disconnect_*()</tt> method. It is the responsibility of
the Event Service end (the Proxy) to ensure that an infinite loop
doesn't occur. So clients don't have to worry - they should always just
call the Proxy's <tt class="literal">disconnect_*()</tt> when their own
<tt class="literal">disconnect_*()</tt> is called.</p>
<p>There is of course NO GUARANTEE that a
<tt class="literal">disconnect_*()</tt> method will only be called once. You
should never assume that your servants' methods will not be called until
the object has actually been deactivated in the POA.</p>
<p>It is possible to connect to ProxyPushConsumer &
ProxyPullSupplier objects without providing an address for callbacks.
When you do that, the proxies cannot call your disconnect method. If you
choose to connect to these proxies without providing an address for
callbacks, then you must clean up your own objects without help from the
Event Service.</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>These semantics only apply to Event Service v1.1 implementations
such as omniEvents v2.6. Earlier specifications were vague about
disconnection semantics, so you must be VERY careful if you want to
interoperate with an older Event Service implementation (such as
omniEvents v2.4 and earlier).</p>
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