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#### 8.1.1 Architecture of a Distributed Ada Application

A distributed system is an interconnection of one or more processing nodes and zero or more storage nodes. A distributed program comprises one or more partitions. A partition is an aggregate of library units. Partitions communicate through shared data or RPCs. A passive partition has no thread of control. Only a passive partition can be configured on a storage node. An active partition has zero or more threads of control and has to be configured on a processing node.

The library unit is the core component of a distributed Ada application. The user can explicitly assign library units to a partition. Partitioning is a post-compilation process. The user identifies interface packages at compile-time. These packages are categorized using pragmas. Each of these pragmas supports the use of one of the following classical paradigms:

- Remote subprograms: For the programmer, a remote subprogram call is similar to a regular subprogram call. Run-time binding using access-to-subprogram types can also be used with remote subprograms. These remote subprograms are declared in library units categorized as remote call interface (RCI).
- Distributed objects: Special-purpose access types can designate remote objects. When a primitive dispatching operation is invoked on an object designated by such a remote access, a remote call is performed transparently on the partition on which the object resides. The types of these distributed objects are declared in library units categorized as remote types (RT).
- Shared objects: Global data can be shared among active partitions, providing a repository similar to shared memory, a shared file system or a database. Entryless protected objects allow safe concurrent access and update of shared objects. This feature is orthogonal to the notion of distributed objects, which are only accessed through exported services. These shared objects are declared in library units categorized as shared passive (SP).

The remotely-called subprograms declared in a library unit categorized as remote call interface (RCI) or remote types (RT) may be either statically or dynamically bound. The partition on which a statically bound remote subprogram is executed can be determined before the call. This is a static remote subprogram call. In contrast, a remote method or a dereference of an access to remote subprogram are dynamically bound remote calls, because the partition on which the remote subprogram is executed is determined at runtime, by the actuals of the call.

In the following example, Data_1 and Data_2 are shared passive (SP) library units. Data_1 is configured on a passive partition mapped on a storage node. Partition_1 and Partition_2 are active partitions. Note that under some circumstances, a partition, for instance Partition_2, can be duplicated. To be duplicated, Unit_2 and Unit_3 which are configured on Partition_2 have to provide only dynamically bound remote subprograms. Otherwise, a partition calling a remote subprogram on Unit_2 would not be able to statically determine where to perform the remote call between the two instances of Unit_2.