Abstracting Interactions Based on Message Sets

ing Interactions Based on Message Sets Svend Frr 1 and Gul Agha 2. 1 Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, CA 94303 2 University of Illinois, 1304 W. Springfield Avenue, Urbana, IL 61801 Abs t rac t . An important requirement of programming languages for distributed systems is to provide abstractions for coordination. A common type of coordination requires reactivity in response to arbitrary communication patterns. We have developed a communication model in which concurrent objects can be activated by sets of messages. Specifically, our model allows direct and abstract expression of common interaction patterns found in concurrent systems. For example, the model captures multiple clients that collectively invoke shared servers as a single activation. Furthermore, it supports definition of individual clients that concurrently invoke multiple servers and wait for subsets of the returned reply messages. Message sets are dynamically defined using conjunctive and disjunctive combinators that may depend o n the patterns of messages. The model subsumes existing models for multiRPC and multi-party synchronization within a single, uniform activation framework. 1 I n t r o d u c t i o n Distributed objects are often reactive, i.e. they carry out their actions in response to received response. Tradit ional object-oriented languages require one to one correspondence between response and a receive message: i.e. each response is caused by exactly one message. However, many coordination schemes involve object behaviors whose logical cause is a set of messages rather than a single message. For example, consider a transaction manager in a distributed database system. In order to commit a distributed transaction, the manager must coordinate the action taken at each site involved in the transaction. A two-phase commit protocol is a possible implementation of this coordination pattern. In carrying out a two-phase commit protocol, the manager first sends out a status inquiry to all the sites involved. In response to a status inquiry, each site sends a positive reply if it can commit the transaction; a site sends back a negative reply if it cannot commit the transaction. After sending out inquiries, the manager becomes a reactive object waiting for sites to reply. The logical structure of the manager is to react to a set of replies rather than a single reply: if a positive reply is received from all sites, the manager decides to commit the transaction; if a negative reply is received from any site, the manager must abort the transaction. In tradit ional object-oriented languages, the programmer must implement a response to a set of messages in terms of sequences of responses to single messages. * The reported work was carried out while the first author was affiliated with the University of Illinois. The current emaJl addresses are f rolund@hpl .hp. corn and [email protected], edu