This content explains the critical architectural principle that horizontal scaling (adding more instances) in cloud environments only works if applications are designed without local state dependencies. The core argument is that scaling isn't just about adding hardware - it's about removing assumptions about locality. Two concrete examples illustrate the problem: (1) File uploads - if an avatar is saved to a container's local disk, it's only available to that specific instance, causing failures when load balancers route traffic to other containers; (2) Scheduled jobs - a cron task that sends daily reports will execute multiple times (once per container) when scaled to multiple instances. The content frames this as a coordination problem rather than a scaling problem, emphasizing that senior engineers understand the need to move state and coordination to centralized systems rather than keeping them local to individual instances. The key insight is that true scalability requires architectural changes to eliminate local state, not just infrastructure expansion.
In cloud settings, horizontal scaling only improves performance if your code doesn't maintain local state
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Applications that keep state locally (files, sessions, scheduled jobs) don't gain capacity from adding instances
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Files saved to a container's local disk are only available to that specific instance
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When multiple containers run the same cron job, the job executes once per container
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Scaling problems are actually coordination problems that require moving state to centralized systems
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Senior engineers understand that scaling requires removing assumptions about locality, not just adding hardware
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