Ada Reference ManualLegal Information
Contents   Index   References   Search   Previous   Next 

E.1 Partitions

The partitions of a distributed program are classified as either active or passive. 

Post-Compilation Rules

An active partition is a partition as defined in 10.2. A passive partition is a partition that has no thread of control of its own, whose library units are all preelaborated, and whose data and subprograms are accessible to one or more active partitions. 
A passive partition shall include only library_items that either are declared pure or are shared passive (see 10.2.1 and E.2.1).
An active partition shall be configured on a processing node. A passive partition shall be configured either on a storage node or on a processing node.
The configuration of the partitions of a program onto a distributed system shall be consistent with the possibility for data references or calls between the partitions implied by their semantic dependences. Any reference to data or call of a subprogram across partitions is called a remote access

Dynamic Semantics

A library_item is elaborated as part of the elaboration of each partition that includes it. If a normal library unit (see E.2) has state, then a separate copy of the state exists in each active partition that elaborates it. The state evolves independently in each such partition.
An active partition terminates when its environment task terminates. A partition becomes inaccessible if it terminates or if it is aborted. An active partition is aborted when its environment task is aborted. In addition, if a partition fails during its elaboration, it becomes inaccessible to other partitions. Other implementation-defined events can also result in a partition becoming inaccessible. 
For a prefix D that denotes a library-level declaration, excepting a declaration of or within a declared-pure library unit, the following attribute is defined:

Denotes a value of the type universal_integer that identifies the partition in which D was elaborated. If D denotes the declaration of a remote call interface library unit (see E.2.3) the given partition is the one where the body of D was elaborated. 

Bounded (Run-Time) Errors

It is a bounded error for there to be cyclic elaboration dependences between the active partitions of a single distributed program. The possible effects, in each of the partitions involved, are deadlock during elaboration, or the raising of Communication_Error or Program_Error. 

Implementation Permissions

An implementation may allow multiple active or passive partitions to be configured on a single processing node, and multiple passive partitions to be configured on a single storage node. In these cases, the scheduling policies, treatment of priorities, and management of shared resources between these partitions are implementation defined.
An implementation may allow separate copies of an active partition to be configured on different processing nodes, and to provide appropriate interactions between the copies to present a consistent state of the partition to other active partitions. 
In an implementation, the partitions of a distributed program need not be loaded and elaborated all at the same time; they may be loaded and elaborated one at a time over an extended period of time. An implementation may provide facilities to abort and reload a partition during the execution of a distributed program.
An implementation may allow the state of some of the partitions of a distributed program to persist while other partitions of the program terminate and are later reinvoked. 
3  Library units are grouped into partitions after compile time, but before run time. At compile time, only the relevant library unit properties are identified using categorization pragmas.
4  The value returned by the Partition_Id attribute can be used as a parameter to implementation-provided subprograms in order to query information about the partition.

Contents   Index   References   Search   Previous   Next 
Ada-Europe Sponsored by Ada-Europe