app.yaml Configuration File

You configure your App Engine app's settings in the app.yaml file. The app.yaml file also contains information about your app's code, such as the runtime and the latest version identifier.

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.

For Go 1.12+, the app.yaml is required to contain at least a runtime: go entry, for a brief overview, see Defining Runtime Settings.

Directory structure

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. To learn more about structuring multiple services in your app, see Structuring Web Services in App Engine.

Example

The following is an example of an app.yaml file for a Go 1.12+ application:

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.

Runtime and app elements

Element	Description
`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. Example: For more information, see Static 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.
`env_variables`	Optional. You can define environment variables in your `app.yaml` file to make them available to your app. 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. 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. 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. 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 a Go 1.12+ 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 classses require you to specify either the `basic_scaling` element or the `manual_scaling` element.
`main`	Optional. The path or fully qualified package name of the main package. 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. To specify Go 1.12+, use: runtime: go Other available runtime values include:
`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. Combined length of service and version 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

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 scripts, static files, static directories, and other settings.

Element	Description
`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 Static 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. Example handlers: - url: /images static_dir: static/images http_headers: X-Foo-Header: foo X-Bar-Header: bar value # ... 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.appspot.com` that accesses assets hosted by `myassets.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.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: http://mygame.appspot.com # ... Note: if you wanted to allow everyone to access your assets, you could use the wildcard `'*'`, instead of `http://mygame.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` Moved Permanently response code. `302` Found response code. `303` See Other response code. `307` Temporary Redirect response code. Example 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 script handlers and 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. Example To target a specific version of your app using the 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-[YOUR_PROJECT_ID].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. The expression can contain groupings that can be referred to in the file path to the script with regular expression back-references. 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. To learn more about how App Engine apps scale, see Scaling types.

Element Description

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` Warning: For this feature to function properly, you must make sure that warmup requests are enabled and that your application handles warmup requests. 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, or `automatic`. The default value is `automatic`. 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` Warning: For this feature to function properly, you must make sure that warmup requests are enabled and that your application handles warmup requests. The number of instances to be kept running and ready to serve traffic. Note that you are charged for the number of instances specified whether they are receiving traffic or not. This setting only applies to the version that receives most of the traffic. 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. Because App Engine keeps idle instances in reserve, it is unlikely that requests will enter the pending queue except in exceptionally high load spikes. You will need to test your application and expected traffic volume to determine the ideal number of instances to keep in reserve. `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: 80). 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. The default value is `30ms`. 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. This element works together with the `min_pending_latency` element to determine when App Engine creates new instances. If pending requests are in the queue: Less than `min_pending_latency`, App Engine will not create new instances. More than `max_pending_latency`, App Engine will try to create 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. `min_pending_latency` 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. When this threshold is reached, it is a signal to scale down, and results in a decrease in the number of instances. The default value is `30ms`. A low minimum means requests must spend less time in the pending queue when all existing instances are active. This improves performance but increases the cost of running your application. A high minimum means requests will remain pending longer if all existing instances are active. This lowers running costs but increases the time users must wait for their requests to be served. Example
`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
`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

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, or automatic. The default value is automatic. 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

The number of instances to be kept running and ready to serve traffic. Note that you are charged for the number of instances specified whether they are receiving traffic or not. This setting only applies to the version that receives most of the traffic. 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. Because App Engine keeps idle instances in reserve, it is unlikely that requests will enter the pending queue except in exceptionally high load spikes. You will need to test your application and expected traffic volume to determine the ideal number of instances to keep in reserve.

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: 80).

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. The default value is 30ms.

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.

This element works together with the min_pending_latency element to determine when App Engine creates new instances. If pending requests are in the queue:

Less than min_pending_latency, App Engine will not create new instances.
More than max_pending_latency, App Engine will try to create 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.

min_pending_latency

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. When this threshold is reached, it is a signal to scale down, and results in a decrease in the number of instances. The default value is 30ms.

A low minimum means requests must spend less time in the pending queue when all existing instances are active. This improves performance but increases the cost of running your application.
A high minimum means requests will remain pending longer if all existing instances are active. This lowers running costs but increases the time users must wait for their requests to be served.

Example

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

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

Static cache expiration

Unless told otherwise, web proxies and browsers retain files they load from a website for a limited period of time.

You can define a global default cache period for all static file handlers for an application by including the top-level default_expiration element. You can also configure a cache duration for specific static file handlers.

The expiration time will be sent in the Cache-Control and Expires HTTP response headers, and therefore, the files are likely to be cached by the user's browser, as well as by intermediate caching proxy servers such as Internet Service Providers. After a file is transmitted with a given expiration time, there is generally no way to clear it out of intermediate caches, even if the user clears their own browser cache. Re-deploying a new version of the app will not reset any caches. Therefore, if you ever plan to modify a static file, it should have a short (less than one hour) expiration time. In most cases, the default 10-minute expiration time is appropriate.

Directory structure

Example

Syntax

Runtime and app elements

Handlers element

CORS Support

Scaling elements

Static cache expiration

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