Caching the Server Birth Address in Software

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Caching the Server Birth Address
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Normally, a server simply writes the address of the implementation repository into the profile body of each persistent reference If an ORB uses multicomponent profiles, the server can additionally embed its own host and port number (known as the server birth address) in one of the components of the object key When a client written for the same ORB receives the reference, it can extract the server birth address from the reference and attempt to connect directly to the server The server may no longer be running at the address it used when the reference was created In that case, the attempt to reach the server at its birth address fails and the client then rebinds using the implementation repository The birth address optimization reduces the load on the implementation repository, especially if servers tend to be long-lived The optimization is particularly useful for servers that are always started on the same port number (many implementation repositories offer this as an option) Of course, a server may eventually move to a different host or port number, perhaps months later To avoid futile attempts to reach a server at its birth address on the first call via each reference, the client can maintain a table of birth addresses that are known to be stale With such a table, a non-functional birth address will be tried only once Note that the client cannot update a stale birth address in a reference after it has learned the new location of the server from the repository Doing this would be useful for
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IT-SC book: Advanced CORBA Programming with C++
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example, if the client in turn passes the reference to another process However, the CORBA specification makes it illegal: no component of a CORBA system is allowed to modify any part of an object reference after the reference is created The Object::hash operation guarantees that the hash value for a reference will not change while that reference denotes an existing object If the client were to update the server birth address inside an IOR, the hash value would change
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Proprietary Protocols between Server and Implementation Repository
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Figure 141 illustrates the messages exchanged between client, implementation repository, and server Note that steps 1, 4, 5, and 7 use IIOP In other words, all interactions that involve the client are portable whether client and server use the same vendor's ORB or different ORBs On the other hand, steps 2 and 3 are restricted to the interaction between the server and its implementation repository The server and the implementation repository always use an ORB from the same vendor, and their interactions are invisible to clients Therefore, an ORB is free to use any protocol and communication mechanism it likes for these interactions Frequently, the implementation repository has an IDL interface that the server uses to send its address details In other words, the implementation repository can appear as an ordinary CORBA object to the server, and the server communicates with it using IIOP However, there are other options open to ORB vendors to improve performance Some ORBs use a UDP-based protocol between server and implementation repository When the server starts up, instead of connecting to the repository using an address picked up from the ORB configuration, the server can send a UDP broadcast to dynamically acquire the address of the repository One or more repositories that know the server's POA names and command line respond with their address details to the server, and the server chooses one of the repository addresses it receives to embed in IORs This mechanism not only simplifies configuration but also can provide simple fault tolerance For example, if several repositories reply to the server, the server can create IORs with multiple profiles, one for each repository The assumption is that the repositories are mirrors of each other, and if one repository becomes unreachable or crashes, the client has a second address in the IOR it can use to bind a request (However, no commercial ORBs currently implement this feature) Using UDP to communicate with the repository can also be more efficient because UDP is a lightweight protocol with less overhead than IIOP If the server and the implementation repository reside on the same host, they can also use a completely different transport for communication, such as a UNIX domain socket or shared memory, which can be faster in some environments
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