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You can use warmup requests to reduce request and response latency during the time when your app's code is being loaded to a newly created instance.
App Engine frequently needs to load your app's code into a fresh instance. Loading an instance can happen in the following situations:
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. Warmup requests, like other requests, are subject to the request timeout.
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 min_idle_instances to greater than 0, you might encounter warmup requests when those instances first start, but they will remain available after that time.
Warmup requests are used by the App Engine scheduler, which controls the auto scaling of instances based on user-supplied configuration. With warmup requests enabled, App Engine issues GET requests to /_ah/warmup. You can implement handlers for this request to perform application-specific tasks, such as pre-caching application data.
The scheduler starts up instances when it determines that more instances are needed. Warmup requests may appear in logs even if they are disabled because the scheduler uses them to start instances.
Note that warmup requests are not guaranteed to be called. In some situations loading requests are sent instead: for example, if the instance is the first one being started up, or if there is a steep ramp-up in traffic. However, there will be a "best effort" attempt to send requests to already warmed-up instances if warmup requests are enabled.
To enable warmup requests, add the warmup element under the inbound_services directive in your app.yaml file, for example:
inbound_services:
- warmup
Create a handler that will process the requests that are sent to /_ah/warmup. Your handler should perform any warmup logic that is needed by your app.
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Last updated 2025-08-07 UTC.
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-07 UTC."],[[["Warmup requests help reduce latency by loading your app's code into new instances before live requests arrive, avoiding delays caused by loading requests."],["App Engine uses warmup requests to initialize new instances in situations like deployments, increased traffic, or infrastructure maintenance, but they are not guaranteed in every scenario."],["Enabling warmup requests in your `app.yaml` file with the `inbound_services: - warmup` directive allows App Engine to send `GET` requests to `/_ah/warmup` for instance initialization."],["You can create a handler for the `/_ah/warmup` path to execute application-specific tasks, such as pre-caching data, during the warmup process."],["Warmup requests do consume instance hours, however in most cases they will replace loading requests and will not result in an increase in instance hours, and an increase in instance hour usage may occur if more tasks are done during warmup."]]],[]]
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