Provide efficient intraprocess multicasting of data without
resorting to more expensive interprocess communication methods such
as UDP multicast.
In contrast to the PUBLISH and SUBSCRIBE types
described in Annex E of IEC
61499-1, which use communication networks for exchange of data,
instances of PUBL_n and SUBL_n service interface
types are used for local (within the same runtime process) multicast
publishing and subscribing of data. The service interface types and
associated service sequences for n=1 are illustrated below.
In addition to increased speed of data transfer vs. PUBLISH/SUBSCRIBE
, these blocks offer the following convenience features:
- The local multicast group is identified by the ID
input. For convenience, the default value of this input is set to
the function block's instance name. This can be overridden by
connection to a non-default variable or configuration parameter.
- The number of data elements is given by the n
suffix of the function block type. The number of data elements,
, transmitted by a PUBL_n instance on a given channel
need not match the number of output elements, say
, of a SUBL_n instance on the same channel. If n
, the SUBL_n instance will ignore the extra data
elements; if n
the extra outputs of the SUBL_n instance will remain at
their initial values.
- If a RD_i output of a SUBL_n block is
not connected or is connected to a variable of a different data
type than that of the corresponding SD_i input of a PUBL_n
block on the same local mulitcast channel, the SD_i value
will not be copied to the RD_i variable and no error
condition will result.
As noted above, the use of the local
multicast function block types PUBL_n/SUBL_n is subject to
substantially fewer restrictions than the use of the corresponding
distributed multicast types PUBLISH_n/SUBSCRIBE_n . As a
result, care must be exercised when converting from a design which
uses local multicast to a design using distributed multicast. In
particular, for distributed multicast:
For other examples of the application of this methodology see the
documentation of the layered MVC design pattern.
- The ID input must be an appropriate identifier
for a distributed multicast group, such as a UDP multicast group
- The number of data elements given by the n suffix
of the function block type must be identical for all publishers
and subscribers in distributed multicast group.
- The SD_i (i=0,1,...,n-1) inputs of PUBLISH_n
instances must be connected to variables of concrete type, i.e.,
not the abstract ANY type, in order to permit their
proper encoding for transmission. Similar restrictions apply to
the corresponding RD_i (i=0,1,...,n-1) outputs of SUBSCRIBE_n
instances, and the types of each SD_i variable and the
corresponding RD_i variable must match at both ends of
the multicast connection for each value of i.
In addition to the sharing of local multicast channels across Resources
in the same Device, the FBRT implementation of this design
pattern permits the sharing of channels among Devices in the
same Java™ runtime process, for example Devices that are
launched from the same SystemManager. This usage can be documented
through the use of a LocalBus segment in the System diagram;
for an example see the ASM_VIEWL System configuration.