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Concurrent Systems/notes/9 - Consensus.md

@@ -21,3 +21,19 @@ This object is a **consensus object**: a *one-shot object* (every process can ac
 - **Integrity:** every process decides at most once
 - **Agreement:** the decided value is the same for all processes
 - **Wait-freedom:** every invocation of propose by a correct process terminates
+
+Conceptually, we can implement a consensus object by a register X, initialized at ⊥, for which the propose operation is atomically defined as:
+```
+propose(v) :=
+	if X = ⊥ then
+		X <- v
+		return X
+```
+
+Universality of consensus holds as folows:
+- given an object O of type Z
+- each participant runs a local copy of O, all initialized at the same value
+- create a total order on the operations O, by using consensus objects
+- force all processes to follow this order to locally simulate O
+	- all local copies are consistent
+