Serverless platforms handle concurrency by automatically scaling function instances to process multiple requests simultaneously. When a function is triggered—such as by an HTTP request or an event—the platform creates a new instance of that function to handle the request. If additional requests arrive while existing instances are busy, the platform spins up more instances in parallel. Each instance operates independently, ensuring that workloads don’t block each other. For example, AWS Lambda allocates a concurrency limit per function or account, which defines the maximum number of instances that can run at once. If the limit is reached, new requests may be throttled or queued until capacity frees up.
The scaling behavior depends on the platform’s configuration and the type of trigger. For event-driven workloads, like processing messages from a queue, serverless platforms often scale instances proportionally to the number of pending events. If a queue has 100 messages, the platform might create up to 100 instances to process them concurrently. However, platforms also apply safeguards to prevent overloading downstream resources. For instance, Azure Functions lets developers set a maxConcurrentRequests threshold for HTTP triggers to limit simultaneous connections. Similarly, Google Cloud Functions uses adaptive scaling, adjusting instance counts based on traffic patterns while respecting regional compute quotas.
Under the hood, serverless platforms isolate function instances using lightweight containers or virtual machines. This isolation ensures that one function’s performance issues (like a memory leak) don’t affect others. Cold starts—the delay when initializing a new instance—can impact concurrency during sudden traffic spikes, but platforms mitigate this by keeping some instances “warm” for reuse. Developers can further optimize by using provisioned concurrency (e.g., AWS Lambda’s feature to pre-initialize instances) or by designing functions to minimize startup time. Overall, the combination of automatic scaling, resource isolation, and configurable limits allows serverless platforms to balance concurrency efficiently without manual intervention.