This simulator places two independent server nodes in parallel, each able to carry the full service demand, in front of a single service terminal. You choose the mission duration and failure rate, fail either node by hand, and watch the routing and the survival probability respond.
• A 3D server A rack, server B rack, independent service paths and a service terminal. • Sliders for mission duration (100-3000 h) and component failure rate (0.05-2 per 1000 h), two fault switches (server A, server B) and a motion-marker toggle. • Five live readouts: individual mission survival, at-least-one survival, system failure probability, live healthy branches and whether live service is available. • Two presets: Lose A (B continues to serve the terminal) and Lose both (service stops).
Each branch survives with Ri = exp(-lambda T). Service is lost only when both fail, so R_parallel = 1 - (1 - R1)(1 - R2). If each branch has a 0.9 survival probability, the pair gives 1 - 0.1 x 0.1 = 0.99. Adding a second independent, full-capacity branch therefore cuts the unreliability by multiplying the two unreliabilities together.
Two active, independent, full-capacity branches are assumed with perfect routing. This is mission reliability, not repairable availability, and the manual faults are illustrative states. Shared-cause and routing failures are excluded, which is exactly why the result is optimistic when real branches share power, cooling or software. The Common-Mode Failure lab shows how a shared cause defeats that assumption.
At least one healthy full-capacity branch. Either independent branch is sufficient to keep the terminal served.
R_parallel = 1 - (1 - R1)(1 - R2): multiply the two unreliabilities and subtract from 1. With exponential branches each R is exp(-lambda T).
The formula assumes the branches fail separately. A shared cause, such as common power or routing, can defeat both at once and make the real reliability lower than this calculation.
It excludes repair, routing failures and shared-cause failures, and it assumes each branch can carry the full load alone.