5. Pyro Name Server

• Pyro's hierarchical object naming scheme
• The different Name Server types available in Pyro
• Security plugin features
• The commands for starting a Name Server
• The Name Server Locator
• The special PYRONAME:// and PYROLOC:// URIs
• The methods on the Name Server Pyro object

Pyro's hierarchical object naming scheme

Object names can be absolute or relative. Relative object names are searched in the default group (which has a special name, set by PYRO_NS_DEFAULTGROUP). Absolute names are always searched from the root. Absolute object names start with a special character that signifies the root; the colon (':'). Relative object names don't have this character at the start.

Object names can be simple or compound. Compound object names consist of multiple parts separated by a special group separator character: the dot ('.'). Compound names are used to search within the namespace hierarchy. Each component of the name is the name of a group. The last name component can be the name of a group or an object (usually the latter).

Let's finish with a few examples to clarify all this. Note that by name alone you cannot distinguish a group name or an object name.

:	The namespace root group
:TestObj	The TestObj name in the root (most likely an object name)
:Test.simple.object1	The object1 name in the simple group in the Test group in the root.
Test.simple.object1	The object1 name in the simple group in the Test group in the default group (which is ":Default" if not configured otherwise. This is the Default group in the root).
object1	The object1 name in the default group.

The different Name Server types available

• Regular, non-persistent (Pyro.naming.NameServer)
• Persistent (Pyro.naming.PersistentNameServer)

The persistent NS stores its naming database on disk. Currently this is implemented the easy way; there is a direct mapping between the group names and directories on disk, and between object names + URI's and files in these directories on disk. The database by default is stored in a "Pyro_NS_database" directory that is created in the directory configured by PYRO_STORAGE. There is a special option for the script that specifies another location, if needed. See below.

Usually you don't access the NameServer or PersistentNameServer classes directly: there are scripts to start the right name server.

Connecting objects to the Pyro Daemon with persistent naming

But, when using the persistent name server, there is a complication here: if you didn't explicitly remove your object from the NS, the entry will still be there the next time. Your connect attempt will then fail because your object cannot be registered again in the NS.

The solution is to use the connectPersistent method of the Pyro daemon. Except for the method name, you call it exactly like the regular connect method. It tries to find your object in the NS. If it's there already, the previous URI is used for your object (that also means that the object's GUID is replaced by the previous GUID that was found in the NS). If it isn't there, the regular connect call takes over.

Of course you could always play safe and explicitly unregister any possible previous occurrences from the NS before you connect new instances. This is what all examples do by the way, so you can safely run an example again and again.

For your information, the code that starts the Persistent Name Server uses connectPersistent to connect the name server object to the daemon. Why? because the name server itself is also registered in the NS database, and it is necessary that when the NS restarts, it uses the URI of the previous instance if found in the persistent database.

Security plugin features

The commands for starting a Name Server

ns   (Name Server)

- Arguments: [-h] [-k] [-m] [-n hostname] [-p port] [-b port] [-i identification] [-d [databasefile]] [-s securitymodule]
- Starts the Pyro Name Server. The '-h' argument prints some help, '-k' makes it immune to shutdown requests (a crude form of security against malicious shutdown requests), '-m' allows multiple Name Servers to be started in the same network segment (default=not allowed), '-n hostname' selects the hostname the server should bind on (useful on systems with multiple network adapters/hostnames), '-p port' specifies the Name Server port number, '-b port' specifies the broadcast port number. With '-i identification' you can supply the authentication passphrase that will be required to connect to this server. When it contains spaces, use quotes around it. '-d' starts the persistent server. You can provide an optional filename to the last option to specify the database file. The '-s' option specifies the Python module for security plugins (make sure it is in your PYTHONPATH).

rns   (Restarting Name Server)

This is very much the same as the ns command but the script will restart the name server if it quits or crashes. Do not confuse this with the -d argument, which will start a Name Server that has its database persistent on disk. Of course it is very useful to combine the rns with the -d switch!!!

ns.bat, rns.bat

The Windows scripts for the above.

Consider setting PYRO_CHECKSUM to 1 before starting the NS. It will communicate more reliably and the overhead is very small.

If you want to start the NS from within your own program, you can ofcourse start it by executing the start script mentioned above. You could also use the NameServerStarter class from the Pyro.naming module to start it directly (this is what the script eventually does). Be sure to start it in a separate process or thread because it will run in its own endless loop.

The Name Server Locator

There are essentially three ways to get a reference to the Name Server:

• Use the NameServerLocator's broadcast mechanism. This only works if your network supports broadcasting and the NS is reachable by a broadcast request.

     locator = Pyro.naming.NameServerLocator()
     ns = locator.getNS()

  If your network doesn't allow broadcasts, or the broadcast can't reach the NS, this mechanism doesn't work. There is a simple workaround: just set the PYRO_NS_HOSTNAME config option to the hostname on which your NS can be found. This disables the broadcast lookup and uses the one below instead.
• Use the NameServerLocator's direct host mechanism. This only works if you know the host on which the NS is located. The port argument is optional. If the NS has been started using a non-default port number you can use it to specify the port number.

     locator = Pyro.naming.NameServerLocator()
     ns = locator.getNS(host='hostname', port=7777)

  If you specify the hostname yourself, the locator doesn't attempt to find the NS with the broadcast mechanism, and therefore there is no lookup delay. Also you can specify a port number different from the default port. If you set the PYRO_NS_HOSTNAME config option, the locator automatically uses the specified host for a direct lookup. You don't have to pass Pyro.config.PYRO_NS_HOSTNAME to the getNS method.
• Use the URI string the NS writes to a special file. This always works but is a bit more cumbersome, and your file should be copied or accessible across a network file system. Anyway, assume the NS writes to 'Pyro_NS_URI':

     uri = open('Pyro_NS_URI','r').read()
     uri = Pyro.core.PyroURI(uri)   # convert string to real URI object
     ns = Pyro.naming.NameServerProxy(uri)    # create a static proxy for the NS
     ... you can now invoke methods, such as ns.ping() ...
  

  Note: The URI changes every time a Pyro server or object is created. You cannot use a previously written URI when you have restarted the server.
  Note: pre 1.1 versions allowed you to use a dynamic proxy to access the NS. No longer! You must always use the static proxy! Using a dynamic proxy will screw up the name scheme and default name groups, unless your own code explicitly takes care of this (which is unlikely). This is cumbersome and the static proxy (Pyro.naming.NameServerProxy) contains all necessary logic already. So just use that one. If you use the locator (see above) you will get a correct proxy automatically.
  Note: the constructor of the NameServerProxy has an optional second argument: the connection authentication information. See the Security chapter for more information.

The special PYRONAME:// and PYROLOC:// URIs

    PYRONAME://nshostname:port/objectname

    Pyro.core.getProxyForURI('PYRONAME://:Test.MyObject').getQuote()

There is another special URI, that bypasses the Name Server completely:

    PYROLOC://hostname:port/objectname

    Pyro.core.getProxyForURI('PYROLOC://localhost:MyObject').getQuote()

The methods on the Name Server Pyro object