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.
Discussion: In most situations, a passive
partition does not execute, and does not have a “real” environment
task. Any execution involved in its elaboration and initialization occurs
before it comes into existence in a distributed program (like most preelaborated
entities). Likewise, there is no concrete meaning to passive partition
termination.
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.
Discussion: For example, an active partition
that includes a unit with a semantic dependence on the declaration of
another RCI package of some other active partition has to be connected
to that other partition by some sort of a message passing mechanism.
A passive partition that is accessible to an
active partition should have its storage addressable to the processor(s)
of the active partition. The processor(s) should be able to read and
write from/to that storage, as well as to perform “read-modify-write”
operations (in order to support entry-less protected objects).
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.]
Ramification: Normal library units cannot
be included in passive partitions.
[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.
Implementation defined: Any events that
can 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:
D'Partition_Id
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
{
AI95-00226-01}
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.
Implementation defined: The scheduling
policies, treatment of priorities, and management of shared resources
between partitions in certain cases.
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.
Ramification: The language does not specify
the nature of these interactions, nor the actual level of consistency
preserved.
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.
Wording Changes from Ada 95
{
AI95-00226-01}
Corrected wording so that a partition that has an elaboration problem
will either deadlock or raise an exception. While an Ada 95 implementation
could allow some partitions to continue to execute, they could be accessing
unelaborated data, which is very bad (and erroneous in a practical sense).
Therefore, this isn't listed as an inconsistency.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
