An App Engine app is made up of a single application resourcethat consists of one or moreservices. Each service can be configured to usedifferent runtimes and to operate with different performance settings. Withineach service, you deployversions of that service. Each version then runswithin one or moreinstances, depending on how much traffic you configured itto handle.

Components of an application

Your App Engine app is created under your Google Cloud project when youcreate anapplication resource. The App Engineapplication is a top-level container that includes the service, version, andinstance resources that make up your app. When you create yourApp Engine app, all your resources are created in theregion that you choose, including your app codealong with a collection of settings, credentials, and your app's metadata.

Each App Engine application includes at least one service, thedefaultservice, which can hold many versions, depending on your app's billing status.For more information, seeLimits below.

The following diagram illustrates the hierarchy of an App Engineapp running with multiple services. In this diagram, the app has two servicesthat contain multiple versions, and two of those versions are actively runningon multiple instances:

Other Google Cloud services, for example Datastore, areshared across your App Engine app. For more information, seeStructuring webservices.

Services

Useservices in App Engine to factor your large apps into logicalcomponents that can securely share App Engine features and communicatewith one another. Generally, your App Engine services behave likemicroservices.Therefore, you can run your whole app in a single service or you can design anddeploy multiple services to run asa set of microservices.

For example, an app that handles your customer requests might include separateservices that each handle different tasks, such as:

• API requests from mobile devices
• Internal, administration-type requests
• Backend processing such as billing pipelines and data analysis

Each service in App Engine consists of the source code from your app andthe corresponding App Engine configuration files. The set of files thatyou deploy to a service represent a singleversion of that service and eachtime that you deploy to that service, you are creating additional versionswithin that same service.

Versions

Having multiple versions of your app within each service allows you to quicklyswitch between different versions of that app for rollbacks, testing, or othertemporary events. You can route traffic to one or more specific versions of yourapp bymigrating orsplittingtraffic.

Instances

The versions within your services run on one or moreinstances.By default, App Engine scales your app to match the load. Your apps willscale up the number of instances that are running to provide consistentperformance, or scale down to minimize idle instances and reduces costs.For more information about instances, seeHow Instances areManaged.

Application requests

Each of your app's services and each of the versions within those services musthave a unique name. You can then use those unique names to target and routetraffic to specific resources using URLs, for example:

https://VERSION-dot-SERVICE-dot-PROJECT_ID.REGION_ID.r.appspot.com

Note that the combined length ofVERSION-dot-SERVICE-dot-PROJECT_ID, whereVERSION is the name of your version,SERVICE is the name of your service, andPROJECT_ID is your project ID, cannot be longer than 63 characters and cannot start or end with a hyphen. If the combined length islonger than 63 characters, you might see ErrorDNS address could not be found.

Incoming user requests are routed to the services or versions that areconfigured to handle traffic. You can also target and route requests to specificservices and versions. For more information, seeHandling Requests.

Logging application requests

When your application handles a request, it can also write its own loggingmessages tostdout andstderr. For details about your app's logs, seeWriting ApplicationLogs.

Limits

There is also a limit to the number of instances for each service with basic ormanual scaling:

There is also a limit to the number of characters in the URL of yourapplication.