Atomic broadcast is a group communication primitive to order messages across a set of distributed processes. Atomic multicast is its natural generalization where each message m is addressed to dst(m), a subset of the processes called its destination group. A solution to atomic multicast is genuine when a process takes steps only if a message is addressed to it. Genuine solutions are the ones used in practice because they have better performance. Let 𝒢 be all the destination groups and ℱ be the cyclic families in it, that is the subsets of 𝒢 whose intersection graph is hamiltonian. This paper establishes that the weakest failure detector to solve genuine atomic multicast is 𝜇 = (∧_{g,h ∈ 𝒢} Σ_{g ∩ h}) ∧ (∧_{g ∈ 𝒢} Ω_g) ∧ γ, where Σ_P and Ω_P are the quorum and leader failure detectors restricted to the processes in P, and γ is a new failure detector that informs the processes in a cyclic family f ∈ ℱ when f is faulty. We also study two classical variations of atomic multicast. The first variation requires that message delivery follows the real-time order. In this case, 𝜇 must be strengthened with 1^{g ∩ h}, the indicator failure detector that informs each process in g ∪ h when g ∩ h is faulty. The second variation requires a message to be delivered when the destination group runs in isolation. We prove that its weakest failure detector is at least 𝜇 ∧ (∧_{g, h ∈ 𝒢} Ω_{g ∩ h}). This value is attained when ℱ = ∅.
@InProceedings{sutra:LIPIcs.DISC.2022.35, author = {Sutra, Pierre}, title = {{The Weakest Failure Detector for Genuine Atomic Multicast}}, booktitle = {36th International Symposium on Distributed Computing (DISC 2022)}, pages = {35:1--35:19}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-255-6}, ISSN = {1868-8969}, year = {2022}, volume = {246}, editor = {Scheideler, Christian}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.35}, URN = {urn:nbn:de:0030-drops-172267}, doi = {10.4230/LIPIcs.DISC.2022.35}, annote = {Keywords: Failure Detector, State Machine Replication, Consensus} }
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