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<h2 class="nocount" style="page-break-before: always">Table of Contents</h2>
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<div id="contents" class="contents">
<h2>support for copy and monitor</h2>
<p><b>copy</b> and <b>monitor</b> are not used in this project.
They are intended for use by pvAccess and by pvAccess servers.
They are provided with this project because the code depends only on
pvData itself.
</p>
<p>This document describes C++ specific code.
<a href="http://epics-pvdata.sourceforge.net/informative/pvRequest.html">
pvRequest.html</a>
provides a language independent overview of <b>copy</b> and <b>monitor</b>.
</p>
<p>
<b>NOTE:pvRequest.html</b> must be updated since it is based on an earlier version of pvCopy that
had knowledge of PVRecord. The C++ version was implemented in pvDatabaseCPP
and the Java version on pvIOCJava.
At present only the C++ version of the new API for pvCopy is implemented.
</p>
<p>Copy provides:
<dl>
<dt>createRequest</dt>
<dd>
The Channel create methods in pvAccess all have an argument
<b>PVStructure pvRequest</b>.<br />
Given an ascii string createRequest creates a PVStructure that provides
a pvData representation of the information from the ascii string.
It is this structure that can be passed to the channel create methods.<br />
The information in a pvRequest selects an arbitrary subset of the
fields in a top level structure that resides in the server.
In addition options can be specified. Both global and field specific
options can be specified.
</dd>
<dt>pvCopy</dt>
<dd>This is a facility used by channel providers.
It provides client specific code that manages a copy of an arbitrary
subset of the fields in a top level structure that resides in the
provider. It also allows provider access to options specified
by the client.
</dd>
</dl>
Monitor provides:
<dl>
<dt>monitor</dt>
<dd>This is support code for channel providers that implement channel
monitor. It, together with the queue facility, provides support for
monitor queues.
</dd>
<dt>monitorPlugin</dt>
<dd>This is support for implementing monitor plugins.
A monitor plugin can be developed that has no knowledge
of pvAccess but only pvData.
</dd>
</dl>
</p>
<h2>support for copy</h2>
<p><b>copy</b> provides the ability to create a structure that has
a copy of an arbitrary subset of the fields in an existing top level
structure. In addition it allows global options and field specific options.
It has two main components: <b>createRequest</b> and <b>pvCopy</b>.
Given a string createRequest creates a pvRequest, which is a PVStructure
that has the format expected by <b>pvCopy</b>.
</p>
<h3>createRequest</h3>
<p>This is mainly used by pvAccess clients. Given a request string it creates
a pvRequest structure that can be passed to the pvAccess create methods.
In turn pvAccess passes the pvRequest to a local channel provider which
then passes it to pvCopy.
</p>
<p>The definition of the public members is:</p>
<pre>
class CreateRequest {
...
static CreateRequestPtr create();
virtual PVStructurePtr createRequest(std::string const &amp;request);
std::string getMessage();
};
</pre>
<p>An example of how it is used is:</p>
<pre>
CreateRequestPtr createRequest = CreateRequest::create();
PVStructurePtr pvRequest = createRequest-&gt;createRequest(request);
if(pvRequest==NULL) {
std::string error = createRequest-&gt;getMessage();
// take some action
} else {
//success do something
}
</pre>
<h3>pvCopy</h3>
<p>The definition of the public members is:</p>
<pre>
class epicsShareClass PVCopyTraverseMasterCallback
{
...
virtual void nextMasterPVField(PVFieldPtr const &amp;pvField);
};
class class epicsShareClass PVCopy
{
...
static PVCopyPtr create(
PVStructurePtr const &amp;pvMaster,
PVStructurePtr const &amp;pvRequest,
std::string const &amp; structureName);
PVStructurePtr getPVMaster();
void traverseMaster(PVCopyTraverseMasterCallbackPtr const &amp; callback);
StructureConstPtr getStructure();
PVStructurePtr createPVStructure();
size_t getCopyOffset(PVFieldPtr const &amp;masterPVField);
size_t getCopyOffset(
PVStructurePtr const &amp;masterPVStructure,
PVFieldPtr const &amp;masterPVField);
PVFieldPtr getMasterPVField(std::size_t structureOffset);
void initCopy(
PVStructurePtr const &amp;copyPVStructure,
BitSetPtr const &amp;bitSet);
void updateCopySetBitSet(
PVStructurePtr const &amp;copyPVStructure,
BitSetPtr const &amp;bitSet);
void updateCopyFromBitSet(
PVStructurePtr const &amp;copyPVStructure,
BitSetPtr const &amp;bitSet);
void updateMaster(
PVStructurePtr const &amp;copyPVStructure,
BitSetPtr const &amp;bitSet);
PVStructurePtr getOptions(std::size_t fieldOffset);
...
};
</pre>
where
<dl>
<dt>PVCopyTraverseMasterCallback::nextMasterPVField</dt>
<dd>
<b>PVCopyTraverseMasterCallback</b> is a callback which must
be implemented by the code that uses pvCopy, normally
the channel provider. It has the single method <b>nextMasterPVField</b>
<br />
<b>nextMasterPVField</b> is called for each field in the master
as a result of a call to <b>traverseMaster</b>.
</dd>
<dt>create</dt>
<dd>
This is the method for creating a PVCopy instance.<br/>
<dl>
<dt>pvMaster</dt>
<dd>the top level structure managed by the server.</dd>
<dt>pvRequest</dt>
<dd>selects the set of subfields desired
and options for each field.</dd>
<dt>structureName</dt>
<dd>the name for the top level of any PVStructure created.
</dd>
</dl>
</dd>
<dt>getPVMaster</dt>
<dd>
Gets the top level structure from pvMaster.
</dd>
<dt>traverseMaster</dt>
<dd>
Traverse all fields of the top level structure of pvMaster.
For each field the callback is called.
</dd>
<dt>getStructure</dt>
<dd>
Get the introspection interface for a PVStructure for e copy.
</dd>
<dt>createPVStructure</dt>
<dd>Create a copy instance.
Monitors keep a queue of monitor elements.
Since each element needs a PVStructure, multiple top level structures
will be created.
</dd>
<dt>getCopyOffset</dt>
<dd>Given a field in pvMaster.
return the offset in copy for the same field.
A value of std::string::npos means that the copy does not have this field.
Two overloaded methods are provided. The first is called if
the field of master is not a structure. The second is for
subfields of a structure.
</dd>
<dt>getMasterPVField</dt>
<dd>
Given a offset in the copy get the corresponding field in pvMaster.
</dd>
<dt>initCopy</dt>
<dd>
Initialize the fields in copyPVStructure
by giving each field the value from the corresponding field in pvMaster.
bitSet will be set to show that all fields are changed.
This means that bit set will have the value <b>{0}</b>.
</dd>
<dt>updateCopySetBitSet</dt>
<dd>
Set all fields in copyPVStructure to the value of the corresponding field
in pvMaster. Each field that is changed has it's corresponding
bit set in bitSet.
</dd>
<dt>updateCopyFromBitSet</dt>
<dd>
For each set bit in bitSet set the field in copyPVStructure to the value
of the corresponding field in pvMaster.
</dd>
<dt>updateMaster</dt>
<dd>
For each set bit in bitSet set the field in pvMaster to the value
of the corresponding field in copyPVStructure.
</dd>
<dt>getOptions</dt>
<dd>
Get the options for the field at the specified offset.
A NULL is returned if no options were specified for the field.
If options were specified,PVStructurePtr is
a structure with a set of PVString subfields that specify name,value
pairs. name is the subField name and value is the subField value.
</dd>
</dl>
<h2>support for monitor</h2>
<p>This consists of two components:
<dl>
<dt>monitor</dt>
<dd>Used by code that implements pvAccess monitors.</dd>
<dt>monitorPlugin</dt>
<dd>Code that provides special semantics for monitors.</dd>
</dl>
</p>
<h3>monitor</h3>
<pre>
class MonitorElement {
MonitorElement(PVStructurePtr const &amp; pvStructurePtr);
PVStructurePtr pvStructurePtr;
BitSetPtr changedBitSet;
BitSetPtr overrunBitSet;
};
class Monitor {
virtual Status start() = 0;
virtual Status stop() = 0;
virtual MonitorElementPtr poll() = 0;
virtual void release(MonitorElementPtr const &amp; monitorElement) = 0;
};
class MonitorRequester : public virtual Requester {
virtual void monitorConnect(Status const &amp; status,
MonitorPtr const &amp; monitor, StructureConstPtr const &amp; structure) = 0;
virtual void monitorEvent(MonitorPtr const &amp; monitor) = 0;
virtual void unlisten(MonitorPtr const &amp; monitor) = 0;
};
</pre>
<h4>monitorElement</h4>
<p><b>MonitorElement</b> holds the data for one element of a monitor queue.
It has the fields:
<dl>
<dt>pvStructurePtr</dt>
<dd>A top level structure with data values at the time the monitors occurs.</dd>
<dt>changedBitSet</dt>
<dd>Shows which fields have changed since the previous monitor.</dd>
<dt>overrunBitSet</dt>
<dd>Shows which fields have changed more han once since the previous monitor.</dd>
</dl>
</p>
<h4>monitorElement queue</h4>
<p>
A queue of monitor elements must be implemented by any channel provider that implements
<b>Channel::createMonitor</b>.
For an example implementation look at pvDatabaseCPP.
It has the following:
<pre>
typedef Queue&lt;MonitorElement&gt; MonitorElementQueue;
typedef std::tr1::shared_ptr&lt;MonitorElementQueue&gt; MonitorElementQueuePtr;
class MultipleElementQueue :
public ElementQueue
{
public:
POINTER_DEFINITIONS(MultipleElementQueue);
virtual ~MultipleElementQueue(){}
MultipleElementQueue(
MonitorLocalPtr const &amp;monitorLocal,
MonitorElementQueuePtr const &amp;queue,
size_t nfields);
virtual void destroy(){}
virtual Status start();
virtual Status stop();
virtual bool dataChanged();
virtual MonitorElementPtr poll();
virtual void release(MonitorElementPtr const &amp;monitorElement);
...
};
</pre>
<h4>Monitor</h4>
<p><b>Monitor</b> must be implemented by any channel provider that implements
<b>Channel::createMonitor</b>.
Remote PVAccess also implements Monitor on the client side.
Note that each client has it's own queue that is not shared with other client.
</p>
<p>Monitor has the following methods:</p>
<dl>
<dt>start</dt>
<dd>
Start monitoring.
This will result in a an initial monitor that has the current value
of all fields.
</dd>
<dt>stop</dt>
<dd>
Stop monitoring.
</dd>
<dt>poll</dt>
<dd>
Called to get a monitor element.
If no new elements are available then a null pointer is returned.
</dd>
<dt>release</dt>
<dd>
Release the monitor element.
The caller owns the monitor element between the calls to poll and release.
</dd>
<dl>
</dl>
<h4>MonitorRequester</h4>
<p>This must be implemented by a pvAccess client.
It has the methods:</p>
<dl>
<dt>monitorConnect</dt>
<dd>
A monitor has either connected of disconnected.
</dd>
<dt>monitorEvent</dt>
<dd>
A new monitor element is available.
</dd>
<dt>unlisten</dt>
<dd>
The channel is going away. The client cam no longer access the monitor.
</dd>
</dl>
<h3>monitorPlugin</h3>
<pre>
class MonitorPlugin
{
virtual std::string const &amp; getName() = 0;
virtual bool causeMonitor(
PVFieldPtr const &amp;pvField,
PVStructurePtr const &amp;pvTop,
MonitorElementPtr const &amp;monitorElement) = 0;
virtual void monitorDone(
MonitorElementPtr const &amp;monitorElement);
virtual void startMonitoring();
virtual void stopMonitoring();
virtual void beginGroupPut();
virtual void endGroupPut();
};
class MonitorPluginCreator
{
virtual MonitorPluginPtr create(
FieldConstPtr const &amp;field,
StructureConstPtr const &amp;top,
PVStructurePtr const &amp;pvFieldOptions) = 0;
virtual std::string const &amp; getName() = 0;
}
class MonitorPluginManager
{
static MonitorPluginManagerPtr get();
bool addPlugin(
std::string const &amp;pluginName,
MonitorPluginCreatorPtr const &amp;creator);
MonitorPluginCreatorPtr findPlugin(std::string const &amp;pluginName);
void showNames();
};
</pre>
<h4>MonitorPlugin</h4>
<p><b>MonitorPlugin</b> must be implemented by the plugin implementation.
It has methods:</p>
<dl>
<dt>getName</dt>
<dd>Get the name of the plugin.</dd>
<dt>causeMonitor</dt>
<dd>
Should the value of pvField cause a monitor to be raised.
pvField and pvTop are fields in the top level structure
being monitored. monitorElement has the top level structure
for the copy</b>.
The implementation should <b>not</b> modify the fields in the structure
being monitored.
Called with pvTop locked.
</dd>
<dt>monitorDone</dt>
<dd>
Called just before monitorElement will be given to client.
The plugin can change the data values and bitSets in monitorElement.
Called with pvTop unlocked.
</dd>
<dt>startMonitoring</dt>
<dd>
Monitoring is starting.
</dd>
<dt>stopMonitoring</dt>
<dd>
Monitoring is being stopped.
</dd>
<dt>beginGroupPut</dt>
<dd>
A set of puts is starting.
Called with pvTop locked.
</dd>
<dt>endGroupPut</dt>
<dd>
The set of puts is complete.
Called with pvTop locked.
</dd>
</dl>
<h4>MonitorPluginCreator</h4>
<p><b>MonitorPluginCreator</b> must also be implemented by the plugin implementation.
It is called for each field instance that has options of the from
<b>[plugin=name...]</b> where <b>name</b> is the name of the plugin.
Note that a plugin instance will belong to a single client.
It has methods:</p>
<dl>
<dt>getName</dt>
<dd>Get the name of the plugin.</dd>
<dt>create</dt>
<dd>
Create a new plugin instance.
If the arguments are not compatible with the plugin a NULL shared pointer is
returned.<br/>
pvFieldOptions is
a structure with a set of PVString subfields that specify <b>name,value</b>
pairs. name is the subField name and value is the subField value.<br/>
Note that a plugin will below to a single client.
</dd>
<dl>
<h4>MonitorPluginManager</h4>
<p><b>MonitorPluginManager</b> has the methods:</p>
<dl>
<dt>get</dt>
<dd>
MonitorPluginManager is a singleton.
The first call to get will create the single instance.
Further calls will return the single instance.
</dd>
<dt>addPlugin</dt>
<dd>
Add a new plugin.
</dd>
<dt>findPlugin</dt>
<dd>
Find a plugin. A NULL shared pointer is returned if it has not been added.
</dd>
<dt>showNames</dt>
<dd>
Show the names of all plugins that have been added.
</dd>
</dl>
<p><b>NOTE:</b>
Should the method <b>causeMonitor</b>
have arguments <b>pvField</b> and <b>pvTop</b>
be defined so that they can not be modified.
This would be possible if the following was defined:
<pre>
typedef std::tr1::shared_ptr&lt;const PVField&gt; PVFieldConstPtr;
typedef std::tr1::shared_ptr&lt;const PVStructure&gt; PVStructureConstPtr;
</pre>
then the definition for causeMonitor could be:
<pre>
virtual bool causeMonitor(
PVFieldConstPtr const &amp;pvField,
PVStructureConstPtr const &amp;pvTop,
MonitorElementPtr const &amp;monitorElement) = 0;
</pre>
But just adding these definitions is not sufficient.
In addition all methods defined in pvDataCPP must be checked.
In particular many of the methods in <b>Convert</b> must have
their arguments modified.
Big job.
</p>
<h2>monitorPlugin example</h2>
<h3>Example Plugin Overview</h3>
<p>This section describes an example plugin that:</p>
<ul>
<li>Only raises monitors when a field changes value.<br />
If no plugin is provided
the default is to raise a monitor when a put is issued to a field.</li>
<li>Optionally a change will not raise a monitor.<br />
The change will, however,
appear if a put to another field raise a monitor.</li>
</ul>
<p>As an example assume that a channel provided by pvAccess has a top level structure
that represents a power supply.</p>
<pre>
structure powerSupply
structure alarm
structure timeStamp
structure power
double value
structure alarm
structure display
structure voltage
double value
structure alarm
structure display
structure current
double value
structure alarm
structure display
</pre>
<p>A pvAccess client wants to create a monitor on the powerSupply as follows:
The client wants a top level structure that looks like:
<pre>
structure powerSupply
structure alarm
structure timeStamp
structure power
double value
structure voltage
double value
structure current
double value
</pre>
In addition the client wants monitors to occur only when one of the monitored
fields changes value but not just because a put occured.
Also if only the timeStamp changes value then that should not cause a monitor.
</p>
<p>The example monitor plugin implements the semantics the
client wants. It can be attached to any field via the following options:
<pre>
[plugin=onChange,raiseMonitor=value]
</pre>
This plugin will trigger a monitor for the field only if the field changes
value. In addition <b>value</b> equals <b>false</b> means do not raise a monitor
for changes to this field.
But if a change to another field does cause a monitor the change to this field
will be passed to the client.
</p>
<p>
Assume that the client has already connected to the channel.
The client can then issue the commands:</p>
<pre>
std::string request("field(alarm[plugin=onChange]");
request += ",timeStamp[plugin=onChange,raiseMonitor=false]";
request += ",power.value[plugin=onChange";
request += ",voltage.value[plugin=onChange";
request += ",current.value[plugin=onChange";
PVStructurePtr pvRequest = createRequest-&gt;createRequest(request);
MonitorPtr monitor = channel-&gt;createMonitor(monitorRequester,pvRequest);
</pre>
<h3>Example Plugin Code</h3>
<p>The header file to create the example has the definition:</p>
<pre>
class ExampleMonitorPlugin{
public:
static void create();
};
</pre>
<p>The implementation is:</p>
<pre>
class OnChangePlugin : public MonitorPlugin
{
public:
virtual ~OnChangePlugin(){}
OnChangePlugin() {}
bool init(
FieldConstPtr const &amp;field,
StructureConstPtr const &amp;top,
PVStructurePtr const &amp;pvFieldOptions)
{
pvField = getPVDataCreate()-&gt;createPVField(field);
raiseMonitor = true;
if(pvFieldOptions!=NULL) {
PVStringPtr pvString =
pvFieldOptions-&gt;getSubField&lt;PVString&gt;("raiseMonitor");
if(pvString!=NULL) {
std::string value = pvString-&gt;get();
if(value.compare("false")==0) raiseMonitor = false;
}
}
return true;
}
virtual std::string &amp;getName(){return pluginName;}
virtual bool causeMonitor(
PVFieldPtr const &amp;pvNew,
PVStructurePtr const &amp;pvTop,
MonitorElementPtr const &amp;monitorElement)
{
bool isSame = convert-&gt;equals(pvNew,pvField);
if(isSame) return false;
convert-&gt;copy(pvNew,pvField);
return raiseMonitor;
}
private:
PVFieldPtr pvField;
bool raiseMonitor;
};
class OnChangePluginCreator : public MonitorPluginCreator
{
public:
virtual std::string &amp;getName(){return pluginName;}
virtual MonitorPluginPtr create(
FieldConstPtr const &amp;field,
StructureConstPtr const &amp;top,
PVStructurePtr const &amp;pvFieldOptions)
{
OnChangePluginPtr plugin(new OnChangePlugin());
bool result = plugin-&gt;init(field,top,pvFieldOptions);
if(!result) return MonitorPluginPtr();
return plugin;
}
};
void ExampleMonitorPlugin::create()
{
static OnChangePluginCreatorPtr plugin;
static Mutex mutex;
Lock xx(mutex);
if(plugin==NULL) {
plugin = OnChangePluginCreatorPtr(new OnChangePluginCreator());
MonitorPluginManager::get()-&gt;addPlugin(pluginName,plugin);
}
}
</pre>
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