# Overview Quorum consensus is... #flashcard A consensus approach that considers values across all nodes part of the decision-making group and determines if they reach a configurable 'quorum'. - If there are *n* replicas (i.e., nodes), every write must be confirmed by *w* nodes and every read by *r* nodes. - As long as *w* + *r* > *n*, then we expect to get up-to-date reads. It guarantees that across all the *n* nodes, there will be at least 1 node where the write and read overlap. - A common choice is n = 3 or 5 and w = r = (n + 1) / 2 (rounded up) - With n = 3, w = 2, r = 2 we can tolerate one unavailable node. - With n = 5, w = 3, r = 3 we can tolerate two unavailable nodes. - If w + r <= n, then you may read stale data, but you have lower latency and higher availability <!--ID: 1751507776940--> # Key Considerations # Pros # Cons # Use Cases # Related Topics