GLOBE DISTRIBUTED SHARED OBJECT

INTRODUCTION  Globe stands for GLobal Object Based Environment.  Globe is different from CORBA and DCOM that it supports a huge number of users across the Internet and still provides distribution transparency.  The most important design goal of Globe is scalability.  Each object is in full control of its distribution strategies across its own replicas.  Hence, such a middleware platform of distributed shared objects is best suited for large scale applications of the future.

ARCHITECTURE

A local object consists of at least four subobjects: Semantics subobject.  This is a local subobject that implements the actual semantics of the distributed object.  For example: GlobeDoc. Communication subobject.  It is responsible for handling communication between parts of the distributed object that reside in different address spaces.  It may offer messaging passing primitives for connection oriented and connectionless communication, multicast facilities.

Replication subobject.  It implements the actual distribution strategy for an object.  The replication subobject has a standard interface. Control subobject.  It bridges the gap between the user defined interfaces of the semantics subobject, and the standard interfaces of the replication subobject.  It takes care of method invocations from client processes.

PROCESS TO OBJECT BINDING The procedure takes place as follows: 1. To bind to an object, a process must pass the name of that object to a naming service. 2. The naming service returns an object handle which is globally unique and location independent. It can be used as a systemwide object reference. 3. The object handle is given to location service which returns a set of contact addresses for a single object. 4. The protocol information is used to load and instantiate classes from a (trusted) class repository. 5. The local object is initialized and through this local object, other local objects are contacted that form a part of the DSO.

COMMUNICATION  To communicate, both parties first bind to some common DSO, and then perform operations on it.  The user just invokes a method on a local object and the object itself manages the replication transparently.  When an invocation completes, no more activity will take place in the DSO, except the activity caused by other invocations.  Hence, Globe objects are said to be passive.

NAMING  The naming service in Globe is based on DNS.  To make use of DNS, a Globe name is first transformed into a DNS name.  For example: we transform a name such as globe://nl/vu/cs/object/foo into foo.object.cs.vu.nl. which is then passed to the local DNS name resolver. The resolution eventually reaches a Globe name server that can handle that part of a name that has not yet been resolved.

LOCATION  The location service stores every DSO’s contact addresses and maintains a mapping of every object handle to a set of contact addresses.  To ensure scalability, the location service should exploit locality.

REPLICATION  Globe incorporates a replication subobject in each local object.  Interfaces of a replication subobject are standardized which results in high degree of flexibility. FAULT TOLERANCE  Fault tolerance in Globe is achieved mainly through replication.  To recover after a failure, globe object server ensures that all local objects are made persistent.

SECURITY  It concentrates on protecting a single DSO against security attacks.  Security is partly handled by a separate security subobject. It communicates with local security services like Kerberos, SESAME and considers various security issues with respect to communication, invocation of semantics subobject and replication.