Design Pattern: Local Multicast

Problem Statement

Provide efficient intraprocess multicasting of data without resorting to more expensive interprocess communication methods such as UDP multicast.

Framework

In contrast to the PUBLISH and SUBSCRIBE types described in Annex F.2.1 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 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, say n_P, transmitted by a PUBL_n instance on a given channel need not match the number of output elements, say n_S, of a SUBL_n instance on the same channel. If n_P>n_S, the SUBL_n instance will ignore the extra data elements; if n_P<n_S 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.

Methodology

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:

• The ID input must be an appropriate identifier for a distributed multicast group, such as a UDP multicast group address.
• 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.

For examples of the application of this methodology see the documentation of the layered MVC design pattern.