App Engine app.yaml reference

Region ID

The 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.

Syntax

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.

Example

The following is an example of an app.yaml file:

runtime: go123

instance_class: F2

env_variables:
  BUCKET_NAME: "example-gcs-bucket"

handlers:
- url: /stylesheets
  static_dir: stylesheets

- url: /(.*\.(gif|png|jpg))$
  static_files: static/\1
  upload: static/.*\.(gif|png|jpg)$

- url: /.*
  script: auto

Each service's folder must contain an app.yaml file and one or more Go source files that include the package main statement at the beginning.

Runtime and app elements

Element Description
app_engine_apis

Optional. If you want to use the App Engine legacy bundled services for second-generation runtimes, set this field to true.
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.

Go example: 
runtime: go123

default_expiration: "4d 5h"

handlers:
# ...

For more information, see Cache expiration.

entrypoint

Optional. Overrides the default startup behavior by executing the entrypoint command when your app starts. For your app to receive HTTP requests, the entrypoint element should contain a command which starts a web server that listens on port 8080.

For more information, see Application startup.

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.

Example:

env_variables:
  MY_VAR: "my value"
 where MY_VAR and my value are the name and value of the environment variable that you want to define and each environment variable entry is indented two spaces under the env_variables element. Environment variables not assigned a value default to "None".

For Go, you can then get these values using os.Getenv:

import "os"
//...
if v := os.Getenv("MY_VAR"); v != "" {
  //...
}

Also see the list of runtime environment variables that cannot be overwritten.
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
Optional. The error_code can be one of the following:
over_quota
Indicates the app has exceeded a resource quota
timeout
Served if a deadline is reached before there is a response from your app.

The error_code is optional; if it's not specified, the given file is the default error response for your app.

file
Each file entry indicates a static file that should be served in place of the generic error response. If you specify a file element without a corresponding error_code element, the static file will be the default error page for your app. The custom error data must be less than 10 kilobytes.
Example 
error_handlers:
  - file: default_error.html

  - error_code: over_quota
    file: over_quota.html
handlers

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
Enables warmup requests. See Configuring Warmup Requests.
Example: 
inbound_services:
- warmup
instance_class

Optional. The instance class for this service.

The following values are available depending on your service's scaling:

Automatic scaling
F1, F2, F4, F4_1G
Default: 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.

Basic and manual scaling
B1, B2, B4, B4_1G, B8
Default: B2

Basic and manual instance classes require you to specify either the basic_scaling element or the manual_scaling element.

main

Optional. For Go, the path or fully qualified package name of the main package. This setting only applies if your app uses Go module mode.

You must declare the path to the main package if your package main is not in the same directory as your app.yaml. The main element supports file paths relative to app.yaml or full package names. For example, if your app has this directory structure: 

myapp/
├── app.yaml
├── go.mod
├── cmd
│   └── web
│       └── main.go
└── pkg
    └── users
        └── users.go

Then you should use either: 

main: ./cmd/web

or

main: example.com/myapp/cmd/web
runtime

Required. The name of the runtime environment that is used by your app. For example, to specify the runtime environment, use: 

runtime: go123
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.

Example: 
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.

Example: 
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
String literal. Specify the fully-qualified name of your Serverless VPC Access connector in quotes: 
"projects/PROJECT_ID/locations/REGION/connectors/CONNECTOR_NAME"
egress_setting
Optional. Default is private-ranges-only. The egress_setting can be one of the following:
private-ranges-only
Default. Requests to internal IP addresses are sent through the Serverless VPC Access connector into the connected VPC network. Requests to external IP addresses are sent to the public internet.
all-traffic
All requests are sent through the Serverless VPC Access connector into the connected VPC network.
Example 
vpc_access_connector:
  name: "projects/PROJECT_ID/locations/REGION/connectors/CONNECTOR_NAME"
  egress_setting: all-traffic

Handlers 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.

Element Description
auth_fail_action

To use this element, app_engine_apis must be set to true.

Optional. Describes the action taken when the login element is specified for a handler and the user is not logged in. Has two possible values:

redirect
Default. The user is redirected to the Google sign-in page, or /_ah/login_required if OpenID authentication is used. The user is redirected back to the application URL after signing in or creating an account.
unauthorized
The request is rejected with an HTTP 401 status code and an error message.

If an application needs different behavior, the application itself can implement the handling of user logins. For example, requiring a login for the /profile/ directory or an administrator login for the /admin/ directory. See the Users API for more information.

You can configure a handler to refuse access to protected URLs when the user is not signed in, instead of redirecting the user to the sign-in page, by adding auth_fail_action: unauthorized to the handler's configuration.

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.

Example 
handlers:
- 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.

login

To use this element, app_engine_apis must be set to true.

Optional. Determines whether the URL handler requires that the user is signed in.

This element has three possible values:

optional
Default. Does not require that the user is signed in.
required
If the user has signed in, the handler proceeds normally. Otherwise, the action given in auth_fail_action is taken.
admin
As with required, performs auth_fail_action if the user is not signed in. In addition, if the user is not an administrator for the application, they are given an error message regardless of the auth_fail_action setting. If the user is an administrator, the handler proceeds.

When a URL handler with a login setting other than optional matches a URL, the handler first checks whether the user has signed in to the application using its authentication option. If not, by default, the user is redirected to the sign-in page. You can also use auth_fail_action to configure the app to simply reject requests for a handler from users who are not properly authenticated, instead of redirecting the user to the sign-in page.

Note: the admin login restriction is also satisfied for internal requests for which App Engine sets appropriate X-Appengine special headers. For example, cron scheduled tasks satisfy the admin restriction, because App Engine sets an HTTP header X-Appengine-Cron: true on the respective requests. However, the requests would not satisfy the required login restriction, because cron scheduled tasks are not run as any user.

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
Moved Permanently response code.
302
Found response code.
303
See Other response code.
307
Temporary Redirect response code.
Go example: 
handlers:
- url: /youraccount/.*
  script: auto
  secure: always
  redirect_http_response_code: 301

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.

handlers:
- url: /images
  static_dir: static/images

- url: /.*
  secure: always
  redirect_http_response_code: 301
  script: auto
secure Optional. Any URL handler can use the secure setting, including static file handlers. The secure element has the following possible values:
optional
Both HTTP and HTTPS requests with URLs that match the handler succeed without redirects. The application can examine the request to determine which protocol was used, and respond accordingly. This is the default when secure is not provided for a handler.
never
Requests for a URL that match this handler that use HTTPS are automatically redirected to the HTTP equivalent URL. When a user's HTTPS request is redirected to be an HTTP request, the query parameters are removed from the request. This prevents a user from accidentally submitting query data over a non-secure connection that was intended for a secure connection.
always
Requests for a URL that match this handler that do not use HTTPS are automatically redirected to the HTTPS URL with the same path. Query parameters are preserved for the redirect.
Go example 
handlers:
- url: /youraccount/.*
  script: auto
  secure: 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.

Example: 
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.

Example: 
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
Uses a URL prefix. The regular express pattern should not contain groupings when used with the 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
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.

Scaling elements

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 Description
automatic_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
Optional. Specify a value between 0 and 2147483647, where zero disables the setting.

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
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:

  • A high maximum reduces the number of idle instances more gradually when load levels return to normal after a spike. This helps your application maintain steady performance through fluctuations in request load, but also raises the number of idle instances (and consequent running costs) during such periods of heavy load.
  • A low maximum keeps running costs lower, but can degrade performance in the face of volatile load levels.

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

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 low minimum helps keep your running costs down during idle periods, but means that fewer instances might be immediately available to respond to a sudden load spike.

  • 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
Optional. Specify a value between 0.5 and 0.95. The default is 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
Optional. Specify a value from 0.5 to 0.95. The default is 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:

  • Less than the min_pending_latency you specify, App Engine will not create new instances.
  • More than max_pending_latency, App Engine will try to create a new instance.
  • Between the time specified by 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.
Example 
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
Required. The maximum number of instances for App Engine to create for this service version. This is useful to limit the costs of a service.
idle_timeout
Optional. The instance will be shut down this amount of time after receiving its last request. The default is 5 minutes (5m).
Example 
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
The number of instances to assign to the service at the start.
Example 
manual_scaling:
  instances: 5