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Horizontal versus vertical scaling, load-balancer strategies, and stateless service design.
5 questions in System Design.
Thumbnail generation system design should return one displayable image immediately while every durable variant is produced asynchronously. Persist the original, write metadata, show a client-side preview or first small thumbnail, and expose variant readiness so the UI never asks for an image size that is not ready.
Scale up while one machine can still hold the workload, because it costs no distributed complexity; scale out once you need redundancy or exceed the largest instance. Scaling out only works if the tier is stateless, which means externalising session state rather than pinning users to a node.
Presence at 10 million users is a lease system, not a connection-state table. Clients renew short-lived keys, the dot expires when renewals stop, and the real cost is bounded fan-out to visible watchers rather than every contact.
Reactive scaling has a latency of its own — metric window, evaluation, provisioning, warm-up, health checks — and a surge that arrives faster than that latency is served entirely by the capacity you already had. The fix is a scheduled floor plus a saturation-based signal, not a more aggressive policy.
Multi-tenant placement at scale should use measured resource cost, not tenant count. Keep the long tail in shared pools, move heavy tenants to reserved or dedicated capacity, and design tenant migration as a normal operation because every placement decision ages quickly.