Connecting ports of components distributed with CORBA

Purpose

Connecting an output port of one component with an input port of another component, where both components are distributed using the CORBA deployer application, deployer-corba.

How it works

Connecting data flow ports of components is done by defining connections (see Naming connections ). When components are distributed using the CORBA deployment component, you need to declare a proxy component in one of the deployers and connect to a port of that proxy. You only need to setup a specific connection in one XML file, the other XML files don't need to repeat the same information.

How it's done

This is your first XML file for component A. We tell that it runs as a Server and that it registers its name in the Naming Service. (See also Using CORBA and the CORBA transport reference manual for setting up naming services)

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE properties SYSTEM "cpf.dtd">
<properties>
  <struct name="ComponentA" type="HMI">
    <simple name="Server" type="boolean"><value>1</value></simple>
    <simple name="UseNamingService" type="boolean"><value>1</value></simple>
  </struct>
</properties>

Save this in component-a.xml and start it with: deployer-corba -s component-a.xml

This is your second XML file for component B. It has one port, cartesianPosition_desi. We add it to a connection, named cartesianPosition_desi_conn. Next, we declare a 'proxy' to Component A we created above, and we do the same for it's port, add it to the connection named cartesianPosition_desi_conn.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE properties SYSTEM "cpf.dtd">
<properties>
  <struct name="ComponentB" type="Controller">
    <struct name="Ports" type="PropertyBag">
      <simple name="cartesianPosition_desi" type="string">
        <value>cartesianPosition_desi_conn</value></simple>
    </struct> 
  </struct>
 
  <!-- ComponentA is looked up using the 'CORBA' naming service -->
  <struct name="ComponentA" type="CORBA">
      <!-- We add ports of A to the connection -->
    <struct name="Ports" type="PropertyBag">
      <simple name="cartesianPosition" type="string">
        <value>cartesianPosition_desi_conn</value></simple>
    </struct> 
  </struct>
</properties>

Save this file as component-b.xml and start it with deployer-corba -s component-b.xml

When component-b.xml is started, the port connections will be created. When ComponentA exits and re-starts, ComponentB will not notice this, and you'll need to restart the component-b xml file as well. Use a streaming based protocol (ROS, POSIX MQueue) in case you want to be more robust against such situations.

Alternative way to do the same

You can also form the connections in a third xml file, and make both components servers like this:

Starting ComponentA: 

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE properties SYSTEM "cpf.dtd">
<properties>
  <struct name="ComponentA" type="HMI">
    <simple name="Server" type="boolean"><value>1</value></simple>
    <simple name="UseNamingService" type="boolean"><value>1</value></simple>
  </struct>
</properties>

Save this in component-a.xml and start it with: cdeployer -s component-a.xml

Starting ComponentB: 

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE properties SYSTEM "cpf.dtd">
<properties>
  <struct name="ComponentB" type="Controller">
    <simple name="Server" type="boolean"><value>1</value></simple>
    <simple name="UseNamingService" type="boolean"><value>1</value></simple>
  </struct>
</properties>

Save this in component-b.xml and start it with: cdeployer -s component-b.xml

Creating two proxies, and the connection: 

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE properties SYSTEM "cpf.dtd">
<properties>
  <!-- ComponentA is looked up using the 'CORBA' naming service -->
  <struct name="ComponentA" type="CORBA">
      <!-- We add ports of A to the connection -->
    <struct name="Ports" type="PropertyBag">
      <simple name="cartesianPosition" type="string">
        <value>cartesianPosition_desi_conn</value></simple>
    </struct> 
  </struct>
 
  <!-- ComponentB is looked up using the 'CORBA' naming service -->
  <struct name="ComponentB" type="CORBA">
      <!-- We add ports of B to the connection -->
    <struct name="Ports" type="PropertyBag">
      <simple name="cartesianPosition_desi" type="string">
        <value>cartesianPosition_desi_conn</value></simple>
    </struct> 
  </struct>
</properties>

Save this in connect-components.xml and start it with: deployer-corba -s connect-components.xml

Further Reading

See deployer and CORBA related Toolchain Reference Manuals.

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