(DoD) under DARPA/NASA subcontract NAG2-593 administered by the NASA

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The ISIS toolkit is a distributed programming environment based on virtually synchronous process groups and group communication We present a new family of protocols in support of this model Our approach revolves around a multicast primitive, called CBCAST, which implements fault-tolerant, causally ordered message delivery. CBCAST can be used directly, or extended into a totally ordered multicast primitive, called ABCAST. It normally delivers messages immedi-ately upon reception, and imposes a space overhead proportional to the size of the groups to which the sender belongs, usually a small number. Both protocols have been implemented as part of a recent version of ISIS and we discuss some of the pragmatic issues that arose and the performance achieved. Our work leads us to conclude that process groups and group communica-tion can achieve performance and scaling comparable to that of a raw message transport layer–a finding contradicting the widespread concern that this style of distributed computing may be unacceptably costly,