Persistence and Communication State Transfer in an Asynchronous Pipe Mechanism - Cloud, Grid and High Performance Computing: Emerging Applications - page 244

Figure 2. Dual π-channel behaviours: (a) when reader-to-writer link is available; and (b) during a
link failure, writer continues transmitting data to the space, delivery to the reader resumes upon link
restoration.

THE Π-CHANNEL API

3. int pi_read(int d, ptr b, int len, dtype_t 1 t):
reads a segment of len elements of type t
into buffer b from the descriptor d, blocks
if no segments are available. It returns the
number of elements successfully read or -1
if end of the pipe is reached.
4. int pi_write(int d, ptr b, int len, dtype_t t):
writes a segment of len elements of type t
from buffer b into the descriptor d.
5. int pi_close(int d), int pi_detach(int d): closes
the “write” and “read” end of the π-channel
d, respectively.
6. int pi_seek(int d, int seg_id): moves the logi-
cal pipe pointer for d to start reading from
segment seg_id, with 0 as the first segment.
This only works at the read end. On success,
it returns 0.
7. pi_tell(int d): returns the segment ID of the
logical pipe pointer of the π-channel.

The π-channels programming interface consists
of the following:

1. pi_attach(str n, int s): attaches to a π-channel
with name n on space s, does not block even if
π-channel is absent. It initiates asynchronous
receive, allowing segments to be buffered
locally even before the pi_read() is issued.
This primitive returns a descriptor represent-
ing the “read” end.
2. int pi_create(str n, int s, int mode): creates
a π-channel with name n on space s. The
mode argument is used to specify if this is a
CREATE - for new π-channels; or APPEND
- to resume writing. On success, it returns
a descriptor representing the “write” end of
the created π-channel.

Table 1. A brief overview of the key π-channels primitives

π-channel Primitive Brief Description

pi_create() Creates a new π-channel.

pi_attach() Attach and retrieve a given π-channel.

pi_write() Writes a pipe segment.

pi_read() Reads a pipe segment.

pi_seek() Seek to a new read segment position.

pi_tell() Return the segment ID of upcoming segment.

pi_close() Closes a π-channel that is opened for writing.

pi_detach() Detach from reading a π-channel.

pi_unlink() Marks a closed π-channel for deletion.