Persistence and Communication State Transfer in an Asynchronous Pipe Mechanism - Cloud, Grid and High Performance Computing: Emerging Applications - page 246

are explicitly posted and retrieved by processes
(Figure 4). A π-Space is a logical name space for
π-channels, providing dynamic binding of chan-
nel endpoints.
Persistence fits elegantly with the model of
shared spaces. When a π-channel is created, a
copy is automatically posted on the π-Space.
During a pi_attach(), the π-Space is accessed to
retrieve the named π-channel. Within a single
π-Space, a name is bound to at most one π-channel.
To reuse an existing name, the π-channel has to
be marked for deletion. The programming model
includes operations to create, access, and close
π-Spaces, enabling the use of multiple spaces
within a single application.

2. The sink application (reader) leaves the
workflow. The writer continues streaming
to the cache (π-Space), ignoring the loss of
the reader. When the reader recovers, it can
resume reading from the π-Space.
3. The link between applications is severed.
Assuming that the π-Space is implemented
as a reliable service; this is considered as
a combination of source failure - from the
perspective of the sink; and sink failure -
from the perspective of the source.

An application may leave a workflow vol-
untarily or involuntarily. A voluntary departure
occurs when it migrates to another resource.
Communication state transfer is employed to re-
establish connections with migrated applications.
An involuntary departure may be due to machine
failure and/or application crash. We assume the
fail-stop failure model, i.e., the process crashes
and performs no further communication. A con-
nection loss is treated as a component failure
and is detected when a communication operation
cannot be completed.

Support for Fault-Tolerance
and Application Migration

During a workflow execution, the following
events may occur:

1. The source application (writer) leaves the
workflow. Unless the downstream compo-
nents do not require any further data from this
writer, the entire workflow may be stalled.
This also occurs when all outstanding data
segments has been consumed.

Figure 4. Shared space communication in π-Space/π-channels