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:
- 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
to greater than
0, you might encounter warmup requests when those instances
first start, but they will remain available after that time.
Enabling warmup requests
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
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
directive in your
app.yaml file, for example:
inbound_services: - warmup
Creating your handler
Create a handler that will process the requests that are sent to
Your handler should perform any warmup logic that is needed by your app.