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The View Discovery protocol addresses two main difficulties: (1) it enables processes joining and leaving the system to learn about the current membership of the system, (2) it is crucial to ensure the consistency, validity and totality properties of DBRB since it supplies information about views “instantiated” by the protocol and associated quorum systems. We formally discuss the View Discovery protocol in the full version of the paper [14]. 4.2 Broadcast In order to broadcast some message m, processes in DBRB use the following types of messages: PREPARE: When a correct process s invokes a DBRB-BROADCAST(m) operation, the algorithm creates & Mprepare = (PREPARE, ™, CUs) message, where cv, is the current view of the system of process s. Message Mprepare is sent to every process that is a member of cvs (Algorithm 4). Process s disseminates the PREPARE message if cu, is installed by s; otherwise, s does not disseminate the message to members of cv, (Algorithm 4), but rather waits to install some view and then disseminates the PREPARE message (Algorithm 4). ACK: When a correct process q receives Mprepare Message, q firstly checks whether view specified in Mprepare is equal to the current view of g (Algorithm 4). If that is the case, q checks whether it is allowed to send an ACK message for m (see Consistency paragraph in §5; Algorithm 4) and if it is, g sends Mack = (ACK, m, 0, cvg) message to process s (i.e., the Pe: eee igre S wake 2. wees EO De pl pel ed eed ee ee eee i ce A RR a oN,

Table 3 The View Discovery protocol addresses two main difficulties: (1) it enables processes joining and leaving the system to learn about the current membership of the system, (2) it is crucial to ensure the consistency, validity and totality properties of DBRB since it supplies information about views “instantiated” by the protocol and associated quorum systems. We formally discuss the View Discovery protocol in the full version of the paper [14]. 4.2 Broadcast In order to broadcast some message m, processes in DBRB use the following types of messages: PREPARE: When a correct process s invokes a DBRB-BROADCAST(m) operation, the algorithm creates & Mprepare = (PREPARE, ™, CUs) message, where cv, is the current view of the system of process s. Message Mprepare is sent to every process that is a member of cvs (Algorithm 4). Process s disseminates the PREPARE message if cu, is installed by s; otherwise, s does not disseminate the message to members of cv, (Algorithm 4), but rather waits to install some view and then disseminates the PREPARE message (Algorithm 4). ACK: When a correct process q receives Mprepare Message, q firstly checks whether view specified in Mprepare is equal to the current view of g (Algorithm 4). If that is the case, q checks whether it is allowed to send an ACK message for m (see Consistency paragraph in §5; Algorithm 4) and if it is, g sends Mack = (ACK, m, 0, cvg) message to process s (i.e., the Pe: eee igre S wake 2. wees EO De pl pel ed eed ee ee eee i ce A RR a oN,

Related Figures (6)

) Figure 1 State transition diagram for correct processes.
) Figure 1 State transition diagram for correct processes.
) Figure 2 Protocol overview for DBRB-JOIN or DBRB-LEAVE (left), and DBRB-BROADCAST (right).
) Figure 2 Protocol overview for DBRB-JOIN or DBRB-LEAVE (left), and DBRB-BROADCAST (right).
© Algorithm 2 DBRB-JOIN and DBRB-LEAVE implementations at process p.
© Algorithm 2 DBRB-JOIN and DBRB-LEAVE implementations at process p.
all deliver message m once they collect a quorum (with respect to v1) of matching DELIVER messages. In this execution p; has successfully executed a certificate collection phase in vp and then reused the message certificate to relay an appropriate COMMIT message to processes that are members of v1 (since the system has reconfigured to v,). Note that Figure 3 depicts the described execution. For presentation simplicity, Figure 3 just shows the COMMIT and DELIVER messages that allow process p; to deliver m. ) Figure 3 Example of a broadcast operation in DBRB algorithm, considering a dynamic membership. DBRB Algorithm Correctness
all deliver message m once they collect a quorum (with respect to v1) of matching DELIVER messages. In this execution p; has successfully executed a certificate collection phase in vp and then reused the message certificate to relay an appropriate COMMIT message to processes that are members of v1 (since the system has reconfigured to v,). Note that Figure 3 depicts the described execution. For presentation simplicity, Figure 3 just shows the COMMIT and DELIVER messages that allow process p; to deliver m. ) Figure 3 Example of a broadcast operation in DBRB algorithm, considering a dynamic membership. DBRB Algorithm Correctness
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