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E.3 Consistency of a Distributed System
1
[This clause defines attributes and rules associated
with verifying the consistency of a distributed program. ]
Language Design Principles
1.a
The rules guarantee that remote
call interface and shared passive packages 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
2/1
For a prefixprefix
P that statically denotes a program unit, the following attributes are
defined:
3
- 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.
4
- 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.
5/1
{
8652/0084}
{version (of a compilation unit)} The
version of a compilation unit changes whenever the
version changes
for any compilation unit
changes in a semantically significant way. This
International Standard does not define the exact meaning of "semantically
significant"on which it depends semantically. The version also changes
whenever the compilation unit itself changes in a semantically significant way.
It is
unspecifiedimplementation defined whether there are other
events (such as recompilation) that result in the version of a compilation unit
changing.
{unspecified [partial]}
5.a/1
This paragraph
was deleted.Implementation defined: Events that
cause the version of a compilation unit to change.
5.1/1
{
8652/0084}
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
6
{bounded error (cause) [partial]}
{unit consistency} 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.
{Program_Error
(raised by failure of run-time check)} 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.
6.a
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.
6.b
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.
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