E.3 Consistency of a Distributed System
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AI05-0299-1}
[This subclause defines attributes and rules associated with verifying
the consistency of a distributed program.]
Language Design Principles
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AI05-0248-1}
The rules guarantee that remote call interface and shared passive library
units are consistent among all partitions prior to the execution of a
distributed program, so that the semantics of the distributed program
are well defined.
Static Semantics
For a
prefix
P that statically denotes a program unit, the following attributes are
defined:
P'Version
Yields a value of the predefined
type String that identifies the version of the compilation unit that
contains the declaration of the program unit.
P'Body_Version
Yields a value of the predefined
type String that identifies the version of the compilation unit that
contains the body (but not any subunits) of the program unit.
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8652/0084}
{
AI95-00104-01}
The
version of a compilation unit changes
whenever the compilation unit changes in a semantically significant way.
This International Standard does not define the exact meaning of "semantically
significant". It is unspecified whether there are other events (such
as recompilation) that result in the version of a compilation unit changing.
This paragraph
was deleted.
{
8652/0084}
{
AI95-00104-01}
If P is not a library unit, and P has no completion, then P'Body_Version
returns the Body_Version of the innermost program unit enclosing the
declaration of P. If P is a library unit, and P has no completion, then
P'Body_Version returns a value that is different from Body_Version of
any version of P that has a completion.
Bounded (Run-Time) Errors
In a distributed
program, a library unit is
consistent if the same version of its
declaration is used throughout. It is a bounded error to elaborate a
partition of a distributed program that contains a compilation unit that
depends on a different version of the declaration of a shared passive
or RCI library unit than that included in the partition to which the
shared passive or RCI library unit was assigned.
As
a result of this error, Program_Error can be raised in one or both partitions
during elaboration; in any case, the partitions become inaccessible to
one another.
Ramification: Because a version changes
if anything on which it depends undergoes a version change, requiring
consistency for shared passive and remote call interface library units
is sufficient to ensure consistency for the declared pure and remote
types library units that define the types used for the objects and parameters
through which interpartition communication takes place.
Note that we do not require matching Body_Versions;
it is irrelevant for shared passive and remote call interface packages,
since only one copy of their body exists in a distributed program (in
the absence of implicit replication), and we allow the bodies to differ
for declared pure and remote types packages from partition to partition,
presuming that the differences are due to required error corrections
that took place during the execution of a long-running distributed program.
The Body_Version attribute provides a means for performing stricter consistency
checks.
Wording Changes from Ada 95
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
