Indirect Binding via an Implementation Repository in Software

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144 Indirect Binding via an Implementation Repository
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Most general-purpose ORBs provide an implementation repository that supports indirect binding for persistent references Indirect binding solves the problems associated with direct binding of persistent references, at the cost of slightly reduced performance for the first request from a client to an object The implementation repository typically also provides automatic server start-up on demand and may provide different activation modes (see Section 146) 1441 Standards Conformance of Implementation Repositories The CORBA specification does not standardize the implementation repository and only suggests some functions that vendors may choose to implement This lack of standardization is deliberate Implementation repositories are intimately related to their underlying platform For example, implementation repositories must deal with details such as process creation and termination, threads, and signal handling These functions vary widely among operating systems, so implementation repositories are inherently not portable The CORBA specification permits ORB implementations for environments ranging from embedded systems to global enterprise systems It is not feasible to provide a specification that covers all possible environments because the exact functionality offered by an implementation repository varies dramatically for different environments Features such as object migration, scalability, performance, and load balancing all depend on the implementation repository It therefore provides a major point at which ORB vendors can provide additional features and tailor repositories to target environments
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Despite the lack of standardization, interoperability among ORBs from different vendors is still guaranteed CORBA strictly specifies how an implementation repository interacts with clients during binding, so a client using vendor A's ORB can interoperate with an implementation repository from vendor B Proprietary mechanisms exist only between servers and their respective implementation repositories This means that a server written for vendor A's ORB requires an implementation repository from the same vendor However, the interactions between servers and their repositories are not visible to clients and other servers and so do not compromise interoperability Proprietary mechanisms between servers and their implementation repositories are confined to the ORB configuration, and the POA mapping ensures that server source code portability is preserved across ORBs from different vendors Because implementation repository features are vendor-dependent, the explanations that follow may not apply to all ORBs, and you will probably find that your particular ORB's repository differs somewhat from what we describe here However, most general-purpose ORBs have implementation repositories that provide features along the lines we describe, so the explanations that follow should still be useful 1442 Implementation Repository Structure An implementation repository has the following responsibilities It maintains a registry of known servers It records which server is currently running on which host and at which port number It starts servers on demand if they are registered for automatic start-up Each implementation repository must run as a process that listens for requests on a fixed host and at a fixed port number ORB vendors can reserve port numbers for their exclusive use through the Internet Assigned Numbers Authority (IANA) In addition, the implementation repository must run permanently This means that implementation repositories are daemon processes that are usually started by a start-up script at boot time
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Table 141 Example population of an implementation repository's server table Logical Server Name POA Name Start-Up Command Host and Port CCS CCS CCS NameService Payroll Stock Stock thermometer thermostat controller ns_poa PR_V1 dept_1 dept_2 rsh bobo /opt/CCS/CCS_svr /opt/myorb/bin/name_svr -v fifiacmecom:1253 boboacmecom:1780 boboacmecom:1780 boboacmecom:1799
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An implementation repository maintains a data structure known as a server table to keep track of servers Table 141 shows an example For each server, the implementation repository records the following
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Logical server name The logical server name identifies what we think of as "the server" In other words, it identifies a process that implements one or more POAs when it is instantiated as a running process POA name The POA name serves as a primary key into the table during binding Whereas the logical server name serves mainly as an administrative handle to all the information about a server, the POA name occurs in object references and identifies at what address its server can be found The start-up command records how a server can be started on demand if it is not running at the time a client invokes a request Note that a single logical server can use several POAs If it does, there need not be a start-up command registered for every POA For example, in Table 141, the CCS server registers a start-up command only for the controller POA but not for the thermometer and thermostat POAs In that case, only requests to the controller, but not thermometers and thermostats, will result in automatic activation of the server Registration of a start-up command is optional For example, the Stock and Payroll servers in Table 141 do not have a start-up command Absence of a start-up command means that these servers will not be started by the implementation repository on demand Instead, they must be started by hand[2]
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[2] Earlier versions of the CORBA specification used to call such servers persistent servers Unfortunately, the term persistent as applied to servers had nothing to do with persistent IORs Instead, the term denoted a server that must be started manually Because of the potential confusion with persistent references, the term persistent server no longer exists in the specification (but you may come across it in older literature on CORBA)
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Also note that the server that is started by the implementation repository need not run on the same machine as the repository itself For example, the CCS server is started on a different machine via the remote shell Using rsh to start a server remotely is only one possible option Some ORBs also allow you to directly nominate a host for a server, and the ORB takes care of starting the server on that host for you In addition, some ORBs also allow you to specify a specific port number for the server to use Host and port This column records the address at which a server is currently running No entry in this column indicates that the server is currently down Note that if a server uses multiple POAs, different POAs may be listening for requests on the same port or may use different ports The choice depends on your ORB vendor Some ORBs map all POAs in a server to the same port number, whereas others assign a different port to each POA or POA manager The choice does not affect how you write your server code The main point of interest is that for each instantiated POA, the
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