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Region IDThe REGION_ID is an abbreviated code that Google assigns based on the region you select when you create your app. The code does not correspond to a country or province, even though some region IDs may appear similar to commonly used country and province codes. For apps created after February 2020, REGION_ID.r is included in App Engine URLs. For existing apps created before this date, the region ID is optional in the URL.
Learn more about region IDs.
You configure your App Engine app's settings in the app.yaml configuration file. Your config file must specify at least a runtime entry.
Each service in your app has its own app.yaml file, which acts as a descriptor for its deployment. You must first create the app.yaml file for the default service before you can create and deploy app.yaml files for additional services within your app. To learn more about structuring multiple services in your app, see Structuring Web Services in App Engine.
The syntax of app.yaml is the YAML format.
The YAML format supports comments. A line that begins with a pound (#) character is ignored:
# This is a comment.
URL and file path patterns use POSIX extended regular expression syntax, excluding collating elements and collation classes. Back-references to grouped matches (e.g. \1) are supported, as are these Perl extensions: \w \W \s \S \d \D.
The following is an example of an app.yaml file:
build_env_variables
Optional. If you are using a runtime that supports buildpacks, you can define build environment variables in your app.yaml file.
To learn more, see Using build environment variables.
default_expiration
Optional. Sets a global default cache period for all static file handlers for an application. You can also configure a cache duration for specific static file handlers. The value is a string of numbers and units, separated by spaces, where units can be d for days, h for hours, m for minutes, and s for seconds. For example, "4d 5h" sets cache expiration to 4 days and 5 hours after the file is first requested. If omitted, the production server sets the expiration to 10 minutes.
For more information, see Cache expiration.
entrypoint env_variables
Optional. You can define environment variables in your app.yaml file to make them available to your app. Ensure that the key in Environment variable(s) matches the expression '[a-zA-Z_][a-zA-Z0-9_]*' (start with alphabet or "_" followed by any alphanumeric or "_").
Environment variables that are prefixed with GAE are reserved for system use and not allowed in the app.yaml file.
error_handlers
Optional. Used to configure custom error pages that are returned for different error types.
This element can contain the following elements:
error_code
error_code can be one of the following:
over_quota
timeout
The error_code is optional; if it's not specified, the given file is the default error response for your app.
file
file element without a corresponding error_code element, the static file will be the default error page for your app. Warning: Make sure that the path to the error response file does not overlap with static file handler paths. The custom error data must be less than 10 kilobytes.
error_handlers:
- file: default_error.html
- error_code: over_quota
file: over_quota.htmlhandlers
Optional. A list of URL patterns and descriptions of how they should be handled. App Engine can handle URLs by executing application code, or by serving static files uploaded with the code, such as images, CSS, or JavaScript.
See the Handlers and sub-elements syntax.
inbound_services
Optional. Applications must enable those services before it can receive inbound requests. You can enable the service for an app by including an inbound_services section in the app.yaml file.
warmup
inbound_services: - warmup
instance_class
Optional. The instance class for this service.
The following values are available depending on your service's scaling:
F1, F2, F4, F4_1G
F1
Optionally use the automatic_scaling element to change default settings for automatic scaling, such as minimum and maximum number of instances, latency, and concurrent connections.
Note: If instance_class is set to F2 or higher, you can optimize your instances by setting max_concurrent_requests to a value higher than the default value of 10. To determine the optimal value, gradually increase it and monitor the performance of your application.
B1, B2, B4, B4_1G, B8
B2
Basic and manual instance classes require you to specify either the basic_scaling element or the manual_scaling element.
runtime
Required. The name of the runtime environment that is used by your app. For example, to specify the runtime environment, use:
service
Required if creating a service. Optional for the default service. Each service and each version must have a name. A name can contain numbers, letters, and hyphens. The combined length of VERSION-dot-SERVICE-dot-PROJECT_ID, where VERSION is the name of your version, SERVICE is the name of your service, and PROJECT_ID is your project ID, cannot be longer than 63 characters and cannot start or end with a hyphen. Choose a unique name for each service and each version. Don't reuse names between services and versions.
service: service-name
service_account
Optional. The service_account element lets you specify a version-specific service account as the identity for the version. The specified service account is used when accessing other Google Cloud services and executing tasks.
service_account: [SERVICE_ACCOUNT_NAME]@[PROJECT_ID].iam.gserviceaccount.com
vpc_access_connector
Optional. Configures your application to use a Serverless VPC Access connector, enabling the application to send requests to internal resources in your VPC network. For more information, see Connecting to a VPC network.
name
"projects/PROJECT_ID/locations/REGION/connectors/CONNECTOR_NAME"
egress_setting
private-ranges-only. The egress_setting can be one of the following:
private-ranges-only
all-traffic
vpc_access_connector: name: "projects/PROJECT_ID/locations/REGION/connectors/CONNECTOR_NAME" egress_setting: all-trafficHandlers element
The handlers element provides a list of URL patterns and descriptions of how they should be handled. App Engine can handle URLs by executing application code, or by serving static files uploaded with the code, such as images, CSS, or JavaScript.
Patterns are evaluated in the order they appear in the app.yaml file, from top to bottom. The first mapping whose pattern matches the URL is the one used to handle the request.
The following table lists the subelements of the handlers element that control the behavior for static files, static directories, scripts in runtimes other than Node.js, and other settings.
expiration Optional. The length of time a static file served by this handler should be cached by web proxies and browsers. The value is a string of numbers and units, separated by spaces, where units can be d for days, h for hours, m for minutes, and s for seconds. For example, "4d 5h" sets cache expiration to 4 days and 5 hours after the file is first requested. If omitted, the application's default_expiration is used. See Cache expiration for more details. http_headers
Optional. You can set HTTP headers for responses of your static file or directory handlers.
If you need to set HTTP headers in your script handlers, you should instead do that in your app's code. For information about which response headers influence caching, see Caching static content.
For more information about App Engine specific headers, see Request headers and responses.
Examplehandlers:
- url: /images
static_dir: static/images
http_headers:
X-Foo-Header: foo
X-Bar-Header: bar value
vary: Accept-Encoding
# ... CORS Support
One important use of this feature is to support cross-origin resource sharing (CORS), such as accessing files hosted by another App Engine app.
For example, you could have a game app mygame.uc.r.appspot.com that accesses assets hosted by myassets.uc.r.appspot.com. However, if mygame attempts to make a JavaScript XMLHttpRequest to myassets, it will not succeed unless the handler for myassets returns an Access-Control-Allow-Origin: response header containing the value http://mygame.uc.r.appspot.com.
Here is how you would make your static file handler return that required response header value:
handlers:
- url: /images
static_dir: static/images
http_headers:
Access-Control-Allow-Origin: https://mygame.uc.r.appspot.com
# ...
Tip: To allow everyone access to your assets, you can use the wildcard '*', instead of https://mygame.uc.r.appspot.com.
mime_type
Optional. If specified, all files served by this handler will be served using the specified MIME type. If not specified, the MIME type for a file will be derived from the file's filename extension. If the same file is uploaded with multiple extensions, the resulting extension can depend on the order in which the uploads occurred.
For more information about the possible MIME media types, see the IANA MIME Media Types website.
redirect_http_response_code
Optional. redirect_http_response_code is used with the secure setting to set the HTTP response code returned when performing a redirect required by how the secure setting is configured. redirect_http_response_code element has the following possible values:
301
302
303
307
When a user's request is redirected, the HTTP status code will be set to the value of the redirect_http_response_code parameter. If the parameter is not present, 302 will be returned.
script
Optional. Specifies that requests to the specific handler should target your app. The only accepted value for the script element is auto because all traffic is served using the entrypoint command. In order to use static handlers, at least one of your handlers must contain the line script: auto or define an entrypoint element to deploy successfully.
secure Optional. Any URL handler can use the secure setting, including static file handlers. The secure element has the following possible values:
optional
secure is not provided for a handler.
never
always
To target a specific version of your app using the REGION_ID.r.appspot.com domain, you replace the periods that would usually separate the subdomain components of the URL with the string "-dot-", for example:https://VERSION_ID-dot-default-dot-PROJECT_ID.REGION_ID.r.appspot.com
To use custom domains with HTTPS, you must first activate and configure SSL certificates for that domain.
Google Accounts sign-in and sign-out are always performed using a secure connection, unrelated to how the application's URLs are configured.
static_dir
Optional. The path to the directory containing the static files, from the application root directory. Everything after the end of the matched url pattern is appended to static_dir to form the full path to the requested file.
Each file in the static directory is served using the MIME type that corresponds with its filename extension unless overridden by the directory's mime_type setting. All of the files in the given directory are uploaded as static files, and none of them can be run as scripts.
handlers: # All URLs beginning with /stylesheets are treated as paths to # static files in the stylesheets/ directory. - url: /stylesheets static_dir: stylesheets # ...
static_files
Optional. A static file pattern handler associates a URL pattern with paths to static files uploaded with the application. The URL pattern regular expression can define regular expression groupings to be used in the construction of the file path. You can use this instead of static_dir to map to specific files in a directory structure without mapping the entire directory.
handlers: # All URLs ending in .gif .png or .jpg are treated as paths to # static files in the static/ directory. The URL pattern is a # regular expression, with a grouping that is inserted into the # path to the file. - url: /(.*\.(gif|png|jpg))$ static_files: static/\1 upload: static/.*\.(gif|png|jpg)$ # ...
Static files cannot be the same as application code files.
upload
Optional. A regular expression that matches the file paths for all files that will be referenced by this handler. This is necessary because the handler cannot determine which files in your application directory correspond with the given url and static_files patterns. Static files are uploaded and handled separately from application files. The example above might use the following upload pattern: archives/(.*)/items/(.*)
url
Required element under handlers. The URL pattern, as a regular expression that can contain groupings. For example, /profile/(.*)/(.*) would match the URL /profile/edit/manager and use edit and manager as the first and second groupings.
The URL pattern has some differences in behavior when used with the following elements:
static_dir
static_dir element. All URLs that begin with this prefix are handled by this handler, using the portion of the URL after the prefix as part of the file path.
static_files
static_dir to map to specific files in a directory structure without mapping the entire directory.
The elements in following table configure how your application scales. If no scaling type is specified, then automatic scaling is set by default.
To learn more about how App Engine apps scale, see Scaling types.
Element Descriptionautomatic_scaling
Optional. Applicable only for applications that use an instance class of F1 or higher.
Specify this element to change default settings for automatic scaling, such as setting minimum and maximum levels for number of instances, latency, and concurrent connections for a service.
This element can contain the following elements:
max_instances
0 and 2147483647, and deploy a new version or redeploy over an existing version. To disable the maximum instances default configuration setting, specify the maximum permitted value 2147483647.
This parameter specifies the maximum number of instances for App Engine to create for this module version. This is useful to limit the costs of a module.
min_instances
app.yaml file is configured to receive traffic. To learn more about routing traffic to different versions of an app, see Splitting Traffic. Optional. The minimum number of instances for App Engine to create for this module version. These instances serve traffic when requests arrive, and continue to serve traffic even when additional instances are started up as required to handle traffic.
Specify a value from 0 to 1000. You can set the parameter to the value 0 to allow scaling to 0 instances to lower costs when no requests are being served. Note that you are charged for the number of instances specified whether they are receiving traffic or not.
max_idle_instances
Optional. The maximum number of idle instances that App Engine should maintain for this version. Specify a value from 1 to 1000. If not specified, the default value is automatic, which means App Engine will manage the number of idle instances.
Keep the following in mind:
Note: When settling back to normal levels after a load spike, the number of idle instances can temporarily exceed your specified maximum. However, you will not be charged for more instances than the maximum number you've specified.
min_idle_instances
app.yaml file is configured to receive traffic. To learn more about routing traffic to different versions of an app, see Splitting Traffic.
Optional: The number of additional instances to be kept running and ready to serve traffic for this version.
App Engine calculates the number of instances necessary to serve your current application traffic based on scaling settings such as target_cpu_utilization and target_throughput_utilization. Setting min_idle_instances specifies the number of instances to run in addition to this calculated number. For example, if App Engine calculates that 5 instances are necessary to serve traffic, and min_idle_instances is set to 2, App Engine will run 7 instances (5, calculated based on traffic, plus 2 additional per min_idle_instances).
Note that you are charged for the number of instances specified whether they are receiving traffic or not. Keep the following in mind:
A high minimum allows you to prime the application for rapid spikes in request load. App Engine keeps the minimum number of instances running to serve incoming requests. You are charged for the number of instances specified, whether or not they are handling requests.
If you set a minimum number of idle instances, pending latency will have less effect on your application's performance.
target_cpu_utilization
0.6.
This parameter specifies the CPU usage threshold at which new instances will be started to handle traffic, enabling you to balance between performance and cost, with lower values increasing performance and increasing cost, and higher values decreasing performance but also decreasing cost. For example, a value of 0.7 means that new instances will be started after CPU usage reaches 70 percent.
target_throughput_utilization
0.6.
Used with max_concurrent_requests to specify when a new instance is started due to concurrent requests. When the number of concurrent requests reaches a value equal to max_concurrent_requests times target_throughput_utilization, the scheduler tries to start a new instance.
max_concurrent_requests
Optional. The number of concurrent requests an automatic scaling instance can accept before the scheduler spawns a new instance (Default: 10, Maximum: 1000).
Used with target_throughput_utilization to specify when a new instance is started due to concurrent requests. When the number of concurrent requests reaches a value equal to max_concurrent_requests times target_throughput_utilization, the scheduler tries to start a new instance.
We recommend you do not set max_concurrent_requests to less than 10 unless you need single threading. A value of less than 10 is likely to result in more instances being created than you need for a threadsafe app, and that may lead to unnecessary cost.
If this setting is too high, you might experience increased API latency. Note that the scheduler might spawn a new instance before the actual maximum number of requests is reached.
max_pending_latency
The maximum amount of time that App Engine should allow a request to wait in the pending queue before starting additional instances to handle requests so that pending latency is reduced. When this threshold is reached, it is a signal to scale up, and results in an increase in the number of instances.
If not specified, the default value is automatic. This means requests can remain in the pending queue for up to 10s, the maximum pending request time limit, before new instance starts are triggered.
A low maximum means App Engine will start new instances sooner for pending requests, improving performance but raising running costs.
A high maximum means users might wait longer for their requests to be served (if there are pending requests and no idle instances to serve them), but your application will cost less to run.
min_pending_latency
An optional element you can set to specify the minimum amount of time that App Engine should allow a request to wait in the pending queue before starting a new instance to handle it. Specifying a value can lower running costs but increase the time users must wait for their requests to be served.
For free apps, the default value is 500ms. For paid apps, the default value is 0.
This element works together with the max_pending_latency element to determine when App Engine creates new instances. If pending requests are in the queue:
min_pending_latency you specify, App Engine will not create new instances.max_pending_latency, App Engine will try to create a new instance.min_pending_latency and max_pending_latency, App Engine will try to reuse an existing instance. If no instances are able to process the request before max_pending_latency, App Engine will create a new instance.automatic_scaling: target_cpu_utilization: 0.65 min_instances: 5 max_instances: 100 min_pending_latency: 30ms max_pending_latency: automatic max_concurrent_requests: 50
basic_scaling
Applications that use an instance class of B1 or higher must specify either this element or manual_scaling.
This element enables basic scaling of instance classes B1 and higher, can contain the following elements:
max_instances
idle_timeout
5m).
basic_scaling: max_instances: 11 idle_timeout: 10m
manual_scaling
Applications that use an instance class of B1 or higher must specify either this element or basic_scaling.
This element enables manual scaling of instance classes B1 and higher, and can contain the following element:
instances
manual_scaling: instances: 5
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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."],[[["The `app.yaml` file is essential for configuring App Engine app settings, with at least a `runtime` entry required, and each service within the app has its own `app.yaml` file."],["`app.yaml` uses YAML format, supports comments with `#`, and employs POSIX extended regular expression syntax for URL and file path patterns."],["The `handlers` element in `app.yaml` defines how App Engine handles URL patterns, whether by executing application code or serving static files, and patterns are evaluated in the order they appear."],["Scaling within App Engine can be configured using `automatic_scaling`, `basic_scaling`, or `manual_scaling` elements in the `app.yaml`, which dictate how instances are managed based on traffic and performance needs."],["The `REGION_ID` is a code assigned by Google based on the selected region when creating an app and it is included in App Engine URLs for apps created after February 2020."]]],[]]
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