This paper presents a precise executable specification---as a faithful mapping from the pseudocode---of Paxos for System Builders, a practical protocol for replication and consensus in distributed systems. Paxos for System Builders has both a robust implementation in C and a clean pseudocode for critical protocol details. This paper shows how the protocol pseudocode can be expressed easily, essentially line-by-line, in a precise high-level language, DistAlgo, for direct execution in distributed systems. Precise specification and direct execution help significantly in understanding the protocol logic and in automatically checking, tracing, and visualizing protocol runs. They also led to discoveries and fixes of small, difficult-to-catch omissions and liveness bugs in the pseudocode though not the C code. The resulting program also has acceptable performance while having similar size as the pseudocode.

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