Use warmup requests to avoid request and response latency during the time when your app's code is being loaded to a newly created instance. To learn how to configure warmup requests, see Configuring Warmup Requests to Improve Performance.
App Engine frequently needs to load your app's code into a fresh instance. Loading an instance can happen in the following situations:
- When you redeploy a version of your app.
- When new instances are created due to the load from requests exceeding the capacity of the current set of running instances.
- When maintenance and repairs of the underlying infrastructure or physical hardware occur.
Loading your app's code to a new instance can result in loading requests. Loading requests can result in increased request latency for your users, but you can avoid this latency using warmup requests. Warmup requests load your app's code into a new instance before any live requests reach that instance.
If warmup requests are enabled for your application, App Engine attempts to detect when your application needs a new instance and initiates a warmup request to initialize a new instance. However, these detection attempts do not work in every case. As a result, you might encounter loading requests, even if warmup requests are enabled in your app. For example, if your app is serving no traffic, the first request to the app will always be a loading request, not a warmup request.
Warmup requests use instance hours like any other request to your App Engine application. In most cases where warmup requests are enabled, you won't notice an increase in instance hours because your application is simply initializing in a warmup request instead of a loading request. Your instance hour usage can increase if you decide to do more work, such as pre-caching during a warmup request. If you set a minimum number of idle instances, you might encounter warmup requests when those instances first start, but they will remain available after that time.For more details, see Configuring warmup requests.