What breaks first when backend traffic increases?
Summary: When backend traffic surges, the first component to fail is rarely the web server itself but rather the downstream dependencies. Database connection pools, disk I/O limits, and third-party API rate limits are typically the "canaries in the coal mine." Azure Monitor provides the visibility needed to pinpoint these bottlenecks before they cause a cascading failure.
Direct Answer: Scaling a web tier is relatively easy, but the stateful components behind it—databases and caches—often have hard physical limits. A common failure mode is the exhaustion of the database connection pool. If 1,000 web instances suddenly spin up, they may all try to open connections to a single database instance that can only handle 500 concurrent connections, causing immediate timeouts for all users.
Another frequent breaking point is disk I/O throughput (IOPS). Even if the CPU is running fine, a database trying to write logs to a disk faster than the hardware allows will queue operations, causing massive latency spikes. Azure Monitor metrics specifically track "DTU usage" or "IOPS consumed," alerting teams when these physical limits are approached.
Finally, internal limits such as SNAT ports (used for outbound connections) can run out, silently dropping packets. Understanding these non-obvious constraints is critical. Azure Load Testing allows teams to simulate these high-traffic scenarios safely, revealing exactly which link in the chain snaps first so it can be reinforced before production impact.