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<html>
<head>
<meta http-equiv="Content-Type"
content="text/html; charset=iso-8859-1">
<meta name="GENERATOR" content="Microsoft FrontPage Express 2.0">
<title>TAO Implementation Repository</title>
</head>
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
vlink="#CC0000">
<hr>
<h1>TAO Implementation Repository </h1>
<p>This document describes the proposed design of the TAO
Implementation Repository (also known as the reactivator or
activation service). </p>
<p>This is the second major revision of our plans for
implementation of this service for TAO.</p>
<hr>
<h2>CORBA Spec</h2>
<p>What does the CORBA spec say about the Implementation
Repository? In the CORBA Specification Revision 2.2,
Section 2.1.14 states:</p>
<table border="0" width="100%">
<tr>
<td width="10%" height="17"> </td>
<td width="80%" height="17">The Implementation Repository
contains information that allows the ORB to locate and
activate implementations of objects. Although most of the
information in the Implementation Repository is specific
to an ORB or operating environment, the Implementation
Repository is the conventional place for recording such
information. Ordinarily, installation of
implementations and control of policies related to the
activation and execution of object implementations is
done through operations on the Implementation
Repository. In addition to its role in the
functioning of the ORB, the Implementation Repository is
a common place to store additional information associated
with implementations of ORB objects. For example,
debugging information, administrative control, resource
allocation, security, etc., might be associated with the
Implementation Repository.</td>
<td width="10%" height="17"> </td>
</tr>
</table>
<p>And also in section 15.2.1:</p>
<table border="0" width="100%">
<tr>
<td width="10%"> </td>
<td width="80%">. . .CORBA does not specify the interface
or characteristics of the Implementation Repository or
the ImplementationDef. As such, implementation typing and
descriptions vary from ORB to ORB and are not part of
this specification.</td>
<td width="10%"> </td>
</tr>
</table>
<hr>
<h2>Goals of the Implementation Repository</h2>
<p>The Implementation Repository will add support to TAO's
architecture so that servers using Persistent Object References,
as defined in the Portable Object Adapter (POA) specification,
can be restarted. This service will allow servers to come and go
without invalidating the Persistent Object References that they
use. </p>
<h3>Constraints when using the Implmentation Repository</h3>
<ul>
<li>When a server is restarted by the Implementation
Repository, it must be able to recreate enough state to
deal correctly with the request issued by a client on the
Persistent Object Reference. Unless dynamic servant and
POA activation is used, the restarted server must also
recreate the POA in which the Persistent object was
registered and register the persistent object with that
POA. </li>
<li>The Implementation Repository will track all processes it
has restarted. It will ensure that multiple instances of
same server processes are not started simultaneously.
This will be implemented by a "ping" call made
by the server at a certain interval.</li>
<li>The use of the Implementation Repository in TAO will be
optional. Real-time applications can choose not to use
the Implementation Repository. IIOP 1.1 helps in this
respect by grouping all the extra information in
components.</li>
</ul>
<h3>Alternate Implementations </h3>
<p>Other ORB vendors use alternative techniques for
Implementation Repository. These techniques usually require new
naming techniques for persistent objects and new client-side APIs
to bind to persistent objects. TAO's Implementation Repository
will not require such extensions. </p>
<p>One implementation of an Implementation Repository could use
an Object Reference that points to the Implementation Repository
instead of pointing directly to the persistent object. This extra
level of indirection is used by the Implementation Repository to
start the server, and then use the Location Forwarding mechanism
to forward the client request to the server. This technique
forces clients to use the Implementation Repository (at least
once) even when the server is already running. </p>
<hr>
<h2>Implementation </h2>
<h3>IOR Changes</h3>
<p>Currently, TAO IORs are set up as the following:</p>
<table border="0">
<tr>
<td><table border="1">
<tr>
<td>Type ID</td>
<td>Host/Port</td>
<td>T/P Flag</td>
<td>TimeStamp</td>
<td>POA ID</td>
<td>OBJ ID</td>
</tr>
</table>
</td>
</tr>
</table>
<p>For Persistent IORs, the T/P Flag is set for perisistant and
the TimeStamp is ignored. </p>
<p>We plan on implementing IIOP 1.1 to make the implementation
and integration of the Implementation Repository easier.
Persistant IORs will have extra profiles pointing to
Implementation Repository. If the connection attempt to the
first profile fails, then the next profile (the Implementation
Repository) will be contacted it it will return a Location
Forward to the server. The new IORs will contain multiple
profiles and tags for such things as the ORB type and server
name.</p>
<table border="1" width="100%">
<tr>
<td rowspan="3">Type ID</td>
<td><table border="0" width="100%">
<tr>
<td width="25%"><em>Profile 1</em></td>
<td width="75%"><table border="1" width="100%">
<tr>
<td>Host</td>
<td>Port</td>
<td>Object Key</td>
<td><em>Components</em> <table border="1"
width="100%">
<tr>
<td>TAG_ORB_TYPE</td>
</tr>
<tr>
<td>tag_orb_version</td>
</tr>
<tr>
<td>tag_server_name</td>
</tr>
<tr>
<td>tag_timestamp</td>
</tr>
<tr>
<td>tag_priority</td>
</tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td><table border="0" width="100%">
<tr>
<td width="25%"><em>Profile 2(Impl Repo)</em></td>
<td width="75%"><table border="1" width="100%">
<tr>
<td>Host</td>
<td>Port</td>
<td>Object Key</td>
<td><em>Components</em> <table border="1"
width="100%">
<tr>
<td>TAG_ORB_TYPE</td>
</tr>
<tr>
<td>tag_orb_version</td>
</tr>
<tr>
<td>tag_server_name</td>
</tr>
<tr>
<td>tag_timestamp</td>
</tr>
<tr>
<td>tag_priority</td>
</tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td><table border="0" width="100%">
<tr>
<td width="25%"><em>Profile 3 (Impl Repo)</em></td>
<td width="75%"><table border="1" width="100%">
<tr>
<td>Host</td>
<td>Port</td>
<td>Object Key</td>
<td><em>Components</em> <table border="1"
width="100%">
<tr>
<td>TAG_ORB_TYPE</td>
</tr>
<tr>
<td>tag_orb_version</td>
</tr>
<tr>
<td>tag_server_name</td>
</tr>
<tr>
<td>tag_timestamp</td>
</tr>
<tr>
<td>tag_priority</td>
</tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
</table>
<p> </p>
<h3>Preliminary Interface</h3>
<p>Here is a preliminary interface of the Implementation
Repository in IDL: </p>
<pre>module TAO
{
// ....
exception Already_Registered {};
// Object already bound in the Implementation Repository
exception Cannot_Activate
{
string reason_;
};
exception Not_Found {};
// Object not found in the Implementation Repository
struct Environment_Variable
{
string name_;
string value_;
};
// One environment variable
struct INET_Addr
{
unsigned short port_;
unsigned long host_;
};
// The location of a server
typedef sequence<Environment_Variable> Environment;
// Complete environment
typedef sequence<string> Command_Line_Options;
// Command line options
struct Process_Options
{
string executable_name_;
// Executable name
Command_Line_Options command_line_options_;
// Command line options
Environment environment_;
// Environment
string working_directory_;
// Working directory
unsigned long creation_flags_;
// Creation flags
};
interface Ping_Object
{
void ping ();
// Used for checking for liveness of a server.
};
interface Implementation_Repository
{
Object activate_object (in Object obj)
raises (Not_Found,
Cannot_Activate);
// Restart server that will contain this persistent object and return the
// new Object reference.
//
// The <Not_Found> exception is raised when <obj> is not found
// in the Implementation Repository. The <Cannot_Activate> exception
// is raised when <obj> is found in the Repository but could not be
// activated.
INET_Addr activate_server (in string server)
raises (Not_Found,
Cannot_Activate);
// Restart server that is named <server> and return the host/port
//
//
// The <Not_Found> exception is raised when <server> is not found
// in the Implementation Repository. The <Cannot_Activate> exception
// is raised when <server> is found in the Repository but could not be
// activated.
void register_server (in string server,
in Process_Options options)
raises (Already_Registered);
// Restart server process when client is looking for <server>.
//
// The <Already_Registered> exception is raised when <server> has
// already been registered with the Implementation Repository.
//
// The <Object_Not_Persistent> exception is raised when <server> is
// not a Persistent Object Reference.
void reregister_server (in string server,
in Process_Options options)
raises (Already_Registered);
// Restart server process when client is looking for <server>.
//
// The <Already_Registered> exception is raised when <server> has
// already been registered with the Implementation Repository.
//
// The <Object_Not_Persistent> exception is raised when <server> is
// not a Persistent Object Reference.
void remove_server (in string server)
raises (Not_Found);
// Remove <server> from the Implementation Repository.
//
// The <Not_Found> exception is raised when <server> is not found
// in the Implementation Repository.
void server_is_running (in string server,
in INET_Addr addr,
in Ping_Object ping);
// Used to notify the Implementation Repository that <server> is alive and
// well at <addr>.
void server_is_shutting_down (in string server);
// Used to tell the Implementation Repository that <server> is shutting
// down.
};
};</pre>
<hr>
<h2>Accessing the Implementation Repository </h2>
<p>The Implementation Repository will be transparent to the
clients and the servers.</p>
<ul>
<li><strong>Use a Helper Application</strong><br>
A helper application will be part of the Implementation
Repository. It will be a command-line utility that will
assist users with adding and removing Persistent Object
References from the Implementation Repository. </li>
</ul>
<h3>Locating an instance of Implementation Repository </h3>
<h4>Server side</h4>
<p>The Implementation Repository will be located on the same host
as the server. If not, host information can be specified through
command line options or environment variables. The default port
of the Implementation Repository can be overriden through command
line options or environment variables. </p>
<h4>Client side</h4>
<p>The correct Implementation Repository will be stored in
another profile in the IOR. Other Implementation
Repsoitories can also be located by multicasting (on a default
multicast group) the server name of the Persistent Object the
client is interested in. The default multicast group can be
override through command line options or environment variables.
The default port of the Implementation Repository can be
overriden through command line options or environment variables. </p>
<hr>
<h2>How the Client Side works</h2>
<p>The first part should work on all clients that support IIOP
1.1: </p>
<ul>
<li>Client obtains a Persistent Object Reference, which
contains multiple profiles to both regular objects and
Implementation Repositories.</li>
<li>It will now make a requestion on the first profile, which
is the last known profile for the object.</li>
<li>If this fails, then the next one will be tried. And
if that fails, the next. One of the other profiles
will be a reference to the Implementation Repository, in
which it will either return NOT_FOUND or will start up
the server and return a Location Forward message.</li>
</ul>
<p>TAO clients will also have added functionality since other
Implementation Repositories can be specified on the command line,
in environment variables, or found through multicast. </p>
<ul>
<li>If all of the profiles fail, then contact the other
Implementation Repositories. First get those
specified on the command line or in environment
variables.</li>
<li>Then, if multicast is available: <ul>
<li>Multicast the Object Reference to a group of
Implementation Repositories </li>
<li>Wait until response or a timeout. The response
will contain the Object Reference of a
Implementation Repository that knows about the
Object Reference </li>
</ul>
</li>
<li>Now connect to any Implementation Repositories that have
been found.</li>
<li>Call <i>activate ()</i> passing the Persistent Object
Reference.</li>
<li>If a new Object Reference was sent back then retry the
request using the it. If this request fails, then fail
(no more retries).</li>
<li>If a null reference was sent back, then fail.</li>
</ul>
<p>TAO clients will (eventually) have an optimization where if
there are several IORs that have the same server name, and one of
them gets forwarded, then the client will be able to change its
other IORs without going through the overhead of contacting
Implementation Repository.</p>
<hr>
<p>Back to the <a
href="http://www.cs.wustl.edu/~schmidt/ACE_wrappers/TAO/docs/index.html">TAO
documentation</a> page. <!--#include virtual="/~schmidt/cgi-sig.html" --> </p>
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