Online Service with Per-Batch Maximum Delay

Abstract

We study online service with one maximum-waiting-time charge per service batch. The persistent server endpoint prevents a phase-by-phase comparison with the offline optimum: an offline schedule may merge many online phases, share movement globally, and finish at unrelated endpoints. Our main contribution is a metric-independent \emph{group--trajectory certificate framework} that restores such a comparison. For ordered request groups in disjoint time windows, a certificate value is bounded both by the window length and by the metric Steiner cost of the group. After normalizing the offline schedule into consecutive arrival blocks, strictly interior groups are charged to offline delay, while boundary groups induce connectors of congestion at most two along the offline trajectory. One color class therefore has certificate sum at most $2\OPT$; a parity decomposition yields $\sum_h C_h\le4\OPT$. Consequently, any phase rule whose cost is at most $\alpha C_h$ is $4\alpha$-competitive. For visible service, this theorem yields deterministic ratios $10$ on a line, $12$ on a weighted tree, and $20$ on an arbitrary finite metric; the last algorithm is polynomial and uses a phase-local terminal-MST envelope, while an exact metric-Steiner oracle gives ratio $12$. Structurally, elective and automatic schedules can have different event structures but equal offline optimal values. The common value is computable exactly in polynomial time on lines and explicitly represented weighted trees, whereas exact optimization on arbitrary finite metrics is NP-hard. Finally, we use spatial blindness---announced requests whose locations are revealed only when visited---as a stress test: dyadic exploration preserves a constant ratio on a known finite line, while a single hidden request on a star forces a loss linear in its degree.