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Common Object Request Broker Architecture

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The Common Object Request Broker Architecture (CORBA) is a standard defined by the Object Management Group (OMG) that enables software components written in multiple computer languages and running on multiple computers to work together.

Contents

General overview

CORBA is a mechanism in software for normalizing the method-call semantics between application objects that reside either in the same address space (application) or remote address space (same host, or remote host on a network). CORBA uses an interface definition language (IDL) to specify the interfaces that objects will present to the outside world. CORBA then specifies a “mapping” from IDL to a specific implementation language like C++ or Java. Standard mappings exist for Ada, C, C++, Lisp, Smalltalk, Java, COBOL, PL/I and Python. There are also non-standard mappings for Perl, Visual Basic, Ruby, Erlang, and Tcl implemented by object request brokers (ORBs) written for those languages. The CORBA specification dictates that there shall be an ORB through which the application interacts with other objects. In practice, the application simply initializes the ORB, and accesses an internal Object Adapter which maintains such issues as reference counting, object (& reference) instantiation policies, object lifetime policies, etc. The Object Adapter is used to register instances of the generated code classes. Generated Code Classes are the result of compiling the user IDL code which translates the high-level interface definition into an OS- and language-specific class base for use by the user application. This step is necessary in order to enforce the CORBA semantics and provide a clean user processes for interfacing with the CORBA infrastructure. Some IDL language mappings are more hostile than others. For example, due to the very nature of Java, the IDL-Java Mapping is rather trivial and makes usage of CORBA very simple in a Java application. The C++ mapping is not trivial but accounts for all the features of CORBA, i.e. exception handling. The C-mapping is even more strange (since it's not an OO language) but it does make sense and handles the RPC semantics just fine. (Red Hat Linux delivers with the GNOME UI system, which has its IPC built on CORBA) A "language mapping" requires that the developer ("user" in this case) create some IDL code representing the interfaces to his objects. Typically a CORBA implementation (either an Open Source or commercial product) comes with a tool called an IDL compiler. This compiler will convert the user's IDL code into some language-specific generated code. The generated code is then compiled using a traditional compiler to create the linkable-object files required by the application. This diagram illustrates how the generated code is used within the CORBA infrastructure:

This figure illustrates the high-level paradigm for remote interprocess communications using CORBA. Issues not addressed here, but that are accounted-for in the CORBA specification include: data typing, exceptions, network protocol, communication timeouts, etc. For example: Normally the server side has the Portable Object Adapter (POA) that redirects calls either to the local servants or (to balance the load) to the other servers. Also, both server and client parts often have interceptors that are described below. Issues CORBA (and thus this figure) does not address, but that all distributed systems must address: object lifetimes, redundancy/fail-over, naming semantics (beyond a simple name), memory management, dynamic load balancing, separation of model between display/data/control semantics, etc. In addition to providing users with a language and a platform-neutral remote procedure call specification, CORBA defines commonly needed services such as transactions and security, events, time, and other domain-specific interface models.

Key features

Objects By Reference

Objects are used in an application "by reference". This reference is either acquired though a "stringified" URI string, NameService lookup (similar to DNS), or passed-in as a method parameter during a call. Object references are "lightweight" objects matching the interface of the "real object" (remote or local). Method calls on the reference result in subsequent calls to the ORB and blocking on the thread while waiting for a reply, success or failure. The parameters, return data (if any) , and exception data are marshaled internally by the ORB according the local language/OS mapping.

Data By Value

The CORBA Interface Definition Language provides the language/OS-neutral inter-object communication definition. CORBA Objects are passed by reference, while data (integers, doubles, structs, enums, etc) are passed by value. The combination of Objects by reference and data-by-value provides the means to enforce strong data typing while compiling clients and servers, yet preserve the flexibility inherent in the CORBA problem-space.

Objects by Value (OBV)

Apart from remote objects, the CORBA and RMI-IIOP define the concept of the OBV. The code inside the methods of these objects is executed locally by default. If the OBV has been received from the remote side, the needed code must be either a priori known for both sides or dynamically downloaded from the sender. To make this possible, the record, defining OBV, contains the Code Base that is a space separated list of URLs from where this code should be downloaded. The OBV can also have the remote methods. The OBV's may have fields that are transferred when the OBV is transferred. These fields can be OBV's themselves, forming lists, trees or arbitrary graphs. The OBV's have a class hierarchy, including multiple inheritance and abstract classes.

CORBA Component Model (CCM)

CORBA Component Model (CCM) is an addition to the family of CORBA definitions. It was introduced with CORBA 3 and it describes a standard application framework for CORBA components. Though not dependent on "language independent Enterprise Java Beans (EJB)", it is a more general form of EJB, providing 4 component types instead of the 2 that EJB defines. It provides an abstraction of entities that can provide and accept services through well-defined named interfaces called ports. The CCM has a component container, where software components can be deployed. The container offers a set of services that the components can use. These services include (but are not limited to) notification, authentication, persistence and transaction management. These are the most-used services any distributed system requires, and, by moving the implementation of these services from the software components to the component container, the complexity of the components is dramatically reduced.

External links

Portable interceptors

Portable interceptors are the "hooks", used by CORBA and RMI-IIOP to mediate the most important functions of the CORBA system. The CORBA standard defines the following types of interceptors:

  1. IOR interceptors mediate the creation of the new references to the remote objects, presented by the current server.
  2. Client interceptors usually mediate the remote method calls on the client (caller) side. If the object Servant (CORBA) exists on the same server where the method is invoked, they also mediate the local calls.
  3. Server interceptors mediate the handling of the remote method calls on the server (handler) side.

The interceptors can attach the specific information to the messages being sent and IORs being created. This information can be later read by the corresponding interceptor on the remote side. Interceptors can also throw forwarding exceptions, redirecting request to another target.

General InterORB Protocol (GIOP)

Main article: GIOP

The GIOP is an abstract protocol by which Object request brokers (ORBs) communicate. Standards associated with the protocol are maintained by the Object Management Group (OMG.). The GIOP architecture provides several concrete protocols:

  1. Internet InterORB Protocol (IIOP) - The Internet Inter-Orb Protocol, is a protocol for communication between CORBA ORBs that has been published by the Object Management Group. IIOP is an implementation of the GIOP for use over an internet, and provides a mapping between GIOP messages and the TCP/IP layer.
  2. SSL InterORB Protocol (SSLIOP) - SSLIOP is IIOP over SSL, providing encryption and authentication.
  3. HyperText InterORB Protocol (HTIOP) - HTIOP is IIOP over HTTP, providing transparent proxy bypassing.
  4. and many more...

Data Distribution Service (DDS)

The Object Management Group (OMG) has a related standard known as the Data Distribution Service (DDS) standard. DDS is a publish-subscribe data distribution model, in contrast to the CORBA remotely-invoked object model.

VMCID (Vendor Minor Codeset ID)

Each standard CORBA exception includes a minor code to designate the subcategory of the exception. Minor exception codes are of type unsigned long and consist of a 20-bit “Vendor Minor Codeset ID” (VMCID), which occupies the high order 20 bits, and the minor code which occupies the low order 12 bits. Minor codes for the standard exceptions are prefaced by the VMCID assigned to OMG, defined as the unsigned long constant CORBA::OMGVMCID, which has the VMCID allocated to OMG occupying the high order 20 bits. The minor exception codes associated with the standard exceptions that are found in Table 3-13 on page 3-58 are or-ed with OMGVMCID to get the minor code value that is returned in the ex_body structure (see Section 3.17.1, “Standard Exception Definitions,” on page 3-52 and Section 3.17.2, “Standard Minor Exception Codes,” on page 3-58). Within a vendor assigned space, the assignment of values to minor codes is left to the vendor. Vendors may request allocation of VMCIDs by sending email to tagrequest@omg.org. The VMCID 0 and 0xfffff are reserved for experimental use. The VMCID OMGVMCID (Section 3.17.1, “Standard Exception Definitions,” on page 3-52) and 1 through 0xf are reserved for OMG use. The Common Object Request Broker: Architecture and Specification (CORBA 2.3)

Corba Location

CorbaLoc means Corba Location and refers to a stringified object reference for a CORBA object that looks similar to a URL. All CORBA products must support two OMG-defined URLs: "corbaloc:" and "corbaname:". The purpose of these is to provide a human readable/editable way to specify a location where an IOR can be obtained. An example of corbaloc is shown below:

corbaloc::160.45.110.41:38693/StandardNS/NameServer-POA/_root

A CORBA product may optionally support the "http:", "ftp:" and "file:" formats. The semantics of these is that they provide details of how to download a stringified IOR (or, recursively, download another URL that will eventually provide a stringified IOR).

External links

Benefits

CORBA brings to the table many benefits that no other single technology brings in one package. These benefits include language- and OS-independence, freedom from technology-linked implementations, strong data-typing, high level of tunability, and freedom from the details of distributed data transfers.

Language Independence

CORBA at the outset was designed to free engineers from the hang-ups and limitations of considering their designs based on a particular software language. Currently there are many languages supported by various CORBA providers, the most popular are Java and C++. There are also C-only, SmallTalk, Perl, Ada, and Python implementations, just to mention a few.

OS Independence

CORBA's design is also meant to be OS-independent. CORBA is available in Java (of course also OS-independent), Linux/Unix, Windows, Sun, Mac, etc.

Freedom from Technologies

One of the main implicit benefits is that CORBA provides a neutral playing field for engineers to be able to normalize the interfaces between various new and legacy systems. When integrating C/C++, Java, Fortran, Python, and any other language/OS in to a single cohesive system design model, CORBA provides the means to level the field and allow disparate teams to develop systems and unit tests that can later be joined together into a whole system. Of course this does not rule out the need for basic system engineering decisions, such as threading, timing, object lifetime, etc. (These issues are part of any system regardless of technology) But the main issue is to allow system elements to be normalized into a single cohesive system model. For example, the design of a Multitier architecture is made simple using Java Servlets in the web server and various CORBA servers containing the business logic and wrapping the database accesses. This allows the implementations of the business logic to change (obviously the interface changes would need to be handled as in any other technology). For example, a database wrapped by a server can have its database schema change for the sake of improved disk usage or performance (or even whole-scale database vendor change) without affecting the external interfaces. At the same time, C++ legacy codes can talk to C/Fortran legacy codes and Java database codes, and can provide data to the web interface.

Strong Data Typing

CORBA provides flexible data typing (for example an "ANY" type) but also it enforces tightly coupled data-typing, reducing human errors. In a situation where Name-Value pairs are passed around, it's conceivable that a server provides a number where a string was expected. CORBA Interface Definition Language provides the mechanism to ensure that user-code "conforms" not only to the method-names, but also the return-, parameter-types, and exceptions.

High Tune-ability

There are many implementations available (for example: OmniORB, Open source C++ and Python implementation) that have many options for tuning the threading and connection management features. Of course not all implementations have the same features. (That's not CORBA's fault, it's the implementor's responsibility)

Freedom From Data Transfer Details

While handling the low-level connection and threading semantics, CORBA also provides for a high-level of detail about "when things go wrong" with a method-call. This is defined in the CORBA-defined standard exception set, and the implementation-specific extended exceptions. Through the exceptions, the application can determine whether a call failed for a "small problem, so try again" or "the server is dead", or "the reference doesn't make sense." The general rule is: "no exception means method call is guaranteed." This is a very powerful design feature.

Problems and criticism

While CORBA promises to deliver much in the way code is written and software constructed, it was much criticized during its history, and some implementations have failed to deliver some of its features.[1] Some of its failures are due to the implementations and the process by which CORBA was created as a standard, others reflect problems in the politics and business of implementing a software standard. These problems, unfortunately, have led to a significant decline in CORBA use and adoption in new projects and areas, and have caused the technology to be "replaced" by Java-centric technologies. [2]

Fundamental flaws

CORBA's notion of location transparency has been criticized; that is, that objects residing in the same address space and accessible with a simple function call are treated the same as objects residing elsewhere (different processes on the same machine, or different machines). This notion is flawed if one requires that all local accesses be as complex as the most complex remote scenario. However CORBA does not place a restriction on the complexity of the calls. Many implementations provided for recursive thread/connection semantics (i.e. Obj A calls B, which then calls A back before returning to A)

Design and process deficiencies

The creation of the CORBA standard is also often cited for its process of design by committee. There is no process to arbitrate between conflicting proposals, or to decide on the hierarchy of problems to tackle. Thus the standard was often created by taking a union of features in all proposals, with no regard to their coherence.[3] This made the specification very complex, prohibitively expensive (if at all possible) to implement entirely, and often ambiguous. Also a design committee composed largely of vendors of the standard implementation creates a disincentive to a comprehensive standard, because standards and interoperability increase competition and ease customers' movement between alternative implementations. This leads to much political fighting within the committee, and frequently release of revisions of the CORBA standard that are impossible to use without proprietary extensions.[1]

Problems with implementations

Through its history (like any other standards-process), CORBA was plagued by shortcomings of its implementations. Often, there were few implementations matching all of the critical elements of the specification,[3] and existing implementations were incomplete or inadequate. As there is no requirement to provide a reference implementation, members are free to propose features which were never tested for usefulness or implementability. Implementations are further hindered by the general tendency of the standard to be verbose, and the common practice of setting compromise as a sum of all submitted proposals, which often creates APIs that are incoherent, quirky or even impossible to use, even if the individual proposals were perfectly reasonable. Working implementations of CORBA had been very difficult to acquire in the past, but are now much easier to find. (The Java SDK comes with CORBA already.) Some poorly designed implementations had been found to be complex, slow, incompatible and incomplete. Commercial versions can be very expensive. This changed significantly as commercial-, hobbyist-, and government-funded high quality free implementations became available. Perhaps the main reason "CORBA has fallen out of favor" was the advent of Java soon after CORBA's introduction. Of course CORBA attempted a goal of tall-order. Java was definitely able to cover the issues but only if your entire system is in Java. But this is not the case for integrating legacy systems with new system elements, or developing new high-performance C++/Fortran codes. Since Java has been the "hot item", new systems could be developed in Java alone, and thus RMI/J2EE could be used exclusively.

Firewalls

CORBA (more precisely, GIOP) uses binary formats in order to transmit data. This is more efficient than a textual format (such as XML), since the amount of data to be transmitted is smaller and less processing has to be done to encode and decode data. However, it has been difficult to get such binary messages through firewalls. At one time it was difficult even to force implementations to use a single standard port - they tended to pick multiple random ports instead. Due to such difficulties, some users have made increasing use of web services instead of CORBA. These communicate using XML via port 80, which is normally left open for web browsing via HTTP. Recent CORBA implementations, though, support SSL and can be easily configured to work on a single port. Also, more CORBA-friendly firewalls are now commercially available.

External links

Notes

  1. ^ a b Trouble with CORBA An historical essay, outlining some of the problems that eventually allowed Java's popularity to over-shadow the problem CORBA was trying to solve
  2. ^ Response to the Rise and Fall of CORBA, the other side of the story
  3. ^ a b Rise and Fall of CORBA, a comprehensive analysis of CORBA failures, by one of its former architects, Michi Henning

CORBA implementations

OMG trademarks

CORBA, IIOP and OMG are the registered marks of the Object Management Group and should be used with care. Differently, the GIOP is not a registered OMG trademark. Hence in some cases may be more appropriate just to say that the application uses or implements the GIOP-based architecture.

See also

Wikibooks
Wikibooks' Programming has more about this subject:
CORBA

References

External links

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Common Object Request Broker Architecture from Wíkipedia. ©2006 by Wíkipedia. Licensed under the GNU Free Documentation License. View a list of authors or edit this article.

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