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Dynamically reallocating computing resources to handle bursty workloads is a common practice for web applications (e.g., e-commerce) in clouds. However, our empirical analysis on a standard n-tier benchmark application (RUBBoS) shows that simply scaling an n-tier application by reallocating hardware resources without fast adapting soft resources (e.g., server threads, connections) may lead to large response time fluctuations. This is because soft resources control the workload concurrency of component servers in the system: adding or removing hardware resources such as Virtual Machines (VMs) can implicitly change the workload concurrency of dependent servers, causing either under- or over-utilization of the critical hardware resource in the system. To quickly identify the optimal soft resource allocation of each server in the system and stabilize response time fluctuation, we propose a novel Scatter-Concurrency-Throughput (SCT) model based on the monitoring of each server's real-time concurrency and throughput. We then implement a Concurrency-aware system Scaling (ConScale) framework which integrates the SCT model to fast adapt the soft resource allocations of key servers during the system scaling process. Our experiments using six realistic bursty workload traces show that ConScale can effectively mitigate the response time fluctuations of the target web application compared to the state-of-the-art cloud scaling strategies such as EC2-AutoScaling.