Chapter 21. Handling Overload

Avoiding overload is a goal of load balancing policies. But no matter how efficient your load balancing policy, eventually some part of your system will become overloaded. Gracefully handling overload conditions is fundamental to running a reliable serving system.

One option for handling overload is to serve degraded responses: responses that are not as accurate as or that contain less data than normal responses, but that are easier to compute. For example:

  • Instead of searching an entire corpus to provide the best available results to a search query, search only a small percentage of the candidate set.

  • Rely on a local copy of results that may not be fully up to date but that will be cheaper to use than going against the canonical storage.

However, under extreme overload, the service might not even be able to compute and serve degraded responses. At this point it may have no immediate option but to serve errors. One way to mitigate this scenario is to balance traffic across datacenters such that no datacenter receives more traffic than it has the capacity to process. For example, if a datacenter runs 100 backend tasks and each task can process up to 500 requests per second, the load balancing algorithm will not allow more than 50,000 queries per second to be sent to that datacenter. However, even this constraint can prove insufficient to avoid overload when you’re operating at scale. At ...

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