app.yaml Reference

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You configure your App Engine app's settings in the app.yaml file. This file specifies how URL paths correspond to request handlers and static files. The app.yaml file also contains information about your app's code, such as the application ID and the latest version identifier.

An individual app.yaml file acts as a descriptor for the deployment of an individual service in your app. 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.

The app.yaml file of a service must be located in the root directory of that service. To learn more about structuring multiple services in your app, see Configuration Files.

Example

The following is an example of an app.yaml file for a Python application:

runtime: python27
api_version: 1
threadsafe: true

handlers:
- url: /
  script: home.app

- url: /index\.html
  script: home.app

- url: /stylesheets
  static_dir: stylesheets

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

- url: /admin/.*
  script: admin.app
  login: admin

- url: /.*
  script: not_found.app

A script: directive can contain either a file path ending in .py, which means the script uses CGI, or a Python module path, with package names separated by dots, which means the script uses WSGI.

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
application	The recommended approach is to remove the application element from your app.yaml file and instead, use a command-line flag to specify your application ID: To use the gcloud app deploy command, you must specify the --project flag: gcloud app deploy --project [YOUR_PROJECT_ID] To use the appcfg.py update command, you specify the -A flag: appcfg.py update -A [YOUR_PROJECT_ID] For more information about using these commands, see Deploying Your App. The application ID is the GCP Console project ID that you specified when you created the application in the Google Cloud Platform Console.
api_version	Required. The version of the API in the given runtime environment that is used by your app. When Google announces support for a new version of a runtime environment's API, your deployed app will continue to use the one for which it was written. To upgrade your app to a new version of the API, you change this value and then redeploy your app to App Engine. When you specify the 1 value, the latest supported runtime environment is used each time you deploy that app (currently ). At this time, App Engine has one version of the python runtime environment: 1
auto_id_policy	Optional. If you are setting entity identifiers automatically, you can change the method employed by setting the auto ID policy. The following are valid options: default Default. Uses scattered auto IDs that are large well-distributed integers that are small enough to be represented by 64-bit floats. legacy The legacy option will be deprecated in a future release and will eventually be removed. For more information, see the blog post announcing this change.
builtins	Optional. The Python SDK includes a number of built-in handlers for common application functions. The builtins directive allows you to include specific handlers in app.yaml. The following built-in handlers are available for your use: appstats Enables Appstats at /_ah/stats/, which you can use to measure your application's performance. In order to use Appstats, you also need to install the event recorder. deferred Enables the deferred handler at /_ah/queue/deferred. This builtin allows developers to use deferred.defer() to simplify the creation of Task Queue tasks. Also see Background work with the deferred library. remote_api Enables the remote_api builtin at /_ah/remote_api/. This builtin allows remote applications with the proper credentials to access the datastore remotely. Example: builtins: - deferred: on - appstats: on The builtins directive is a special instance of the includes directive. Each builtin directive is equivalent, in Python, to an includes directive with an expanded path. For example: builtins: - name: on Is equivalent to: includes: - $PYTHON_LIB/google/appengine/ext/builtins/name/ When you use builtins in your app.yaml file, any handlers that are defined by in the built-in include.yaml file will supersede any handlers that you define in your app.yaml file. However, if you include a file that then uses builtins or includes, the handlers are added by order of the include hierarchy. In other words, the handlers of the "parent" include are added before the builtins of the "child" includes, and so on. For example, consider the following app.yaml, which uses the built-in appstats handlers: handlers: - url: /.* script: main.app builtins: - appstats: on The resulting list of handlers is: [/_ah/stats, /.*] If the app.yaml uses an includes directive: includes: - included.yaml And the included.yaml file uses builtins: handlers: - url: /.* script: main.app builtins: - appstats: on The resultant list of handlers is now: [/.*, /_ah/stats] The order of placement of the builtins clause in a .yaml file doesn't change the behavior.
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 example: runtime: python27 api_version: 1 threadsafe: true default_expiration: "4d 5h" handlers: # ... For more information, see Static cache expiration.
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. These variables will be available in the os.environ dictionary: env_variables: DJANGO_SETTINGS_MODULE: 'myapp.settings'
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 dos_api_denial Indicates that the request was blocked by your app's DoS Protection configuration. 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	Required. 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
includes	Optional. The includes directive allows you to include the configuration file for any library or service throughout your application. For example, you might include a user administration library as follows: includes: - lib/user_admin.yaml App Engine resolves the included path in the following order: Absolute or relative path to the working directory. The specified path resolves to a file. Relative to the application's directory, which is also known as the basepath. The basepath and path resolve to a file. Relative to the file that included the current file. The location of the referring file and the include path resolve to the included file. If the include directive specifies a directory, then App Engine looks in that directory for a file called include.yaml. If the include directive is a file, then that specific file is included. Using includes retrieves only the following types of directives from the destination file (if present): builtins includes handlers skip_files Included skip_files patterns are added to those in the including app.yaml, or to the default list if there is no explicit list in app.yaml. Note that skip_files compares absolute paths.
inbound_services	Optional. Before an application can receive email, the application must be configured to enable the service. You enable the service for a Python app by including an inbound_services section in the app.yaml file. The following inbound services are available: mail Allows your application to receive mail. warmup Enables warmup requests. See Configuring Warmup Requests. Example: inbound_services: - mail - 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 is assigned if you do not specify an instance class along with the automatic_scaling element. Basic and manual scaling B1, B2, B4, B4_1G, B8 Default: B2 is assigned if you do not specify an instance class along with the basic_scaling element or the manual_scaling element. 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 10, which is the default. To find the optimal value, gradually increase it and monitor the performance of your application.
libraries	Optional. The Python 2.7 runtime includes some third-party libraries. Some of these are available by default; others are only available if configured. You can specify which version you want to use by specifying the name and version values. libraries: - name: PIL version: "1.1.7" - name: webob version: "latest" Note than when you specify latest, the SDK determines the latest library version at deployment time. Once deployed, the library version will not change. The only way to get a different version of the library is to deploy again. If you're developing an application that doesn't have users yet: you don't need to track new versions. But if your application is being actively used, beware: you might be surprised that your application starts using a new not-backward-compatible library version. For a list of the included third-party libraries, see Third-party Libraries. You can use additional pure-python third-party libraries by installing them into a local directory. If you are using the flexible environment, see Using Python libraries in the flexible environment.
module	Note: Modules are now named Services. To manage your app with the gcloud tool, use the service element instead. To use the appcfg tool, services must be declared in app.yaml files as modules, for example: module: service-name 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. It 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.
runtime	Required. The name of the runtime environment that is used by your app. To specify Python, use python. runtime: python
service	Services were formerly known as Modules. Supported only by the gcloud tool or Cloud SDK-based plugins, for example: gcloud app deploy . To use the appcfg tool, see module. 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. It 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 Note: The gcloud app deploy command is backwards compatible and supports existing app.yaml files that include services declared as modules, for example: module: service_name
skip_files	Optional. The skip_files element specifies which files in the application directory are not to be uploaded to App Engine. The value is either a regular expression, or a list of regular expressions. Any filename that matches any of the regular expressions is omitted from the list of files to upload when the application is uploaded. Filenames are relative to the project directory. The skip_files has the following default: skip_files: - ^(.*/)?#.*#$ - ^(.*/)?.*~$ - ^(.*/)?.*\.py[co]$ - ^(.*/)?.*/RCS/.*$ - ^(.*/)?\..*$ The default pattern excludes Emacs backup files with names of the form #...# and ...~, .pyc and .pyo files, files in an RCS revision control directory, and Unix hidden files with names beginning with a dot (.). To extend the above regular expression list, copy and paste the above list into your app.yaml and add your own regular expressions. For example, to skip files whose names end in .bak in addition to the default patterns, add an entry like this for skip_files: skip_files: - ^(.*/)?#.*#$ - ^(.*/)?.*~$ - ^(.*/)?.*\.py[co]$ - ^(.*/)?.*/RCS/.*$ - ^(.*/)?\..*$ - ^(.*/)?.*\.bak$ To skip a full directory, add the directory name to the list. For example, to skip a directory named logs, add the following line to the previously described ones: skip_files: - logs/
threadsafe	Required. Configures your application to use concurrent requests. If using Python's threading library, the thread-local data, as returned by threading.local(), is cleared after each request. threadsafe: [true | false] Note: The threadsafe directive is required for Python applications. threadsafe: true requires that all script handlers be WSGI ones. That is, each script must be specified in a script: directive a using Python module path, with package names separated by dots. The last component of a script: directive using a Python module path is the name of a global variable in the service: that variable must be a WSGI app, and is usually called app by convention.
version	The recommended approach is to remove the version element from your app.yaml file and instead, use a command-line flag to specify your version ID: To use the gcloud app deploy command, you specify the -v flag: gcloud app deploy -v [YOUR_VERSION_ID] To use the appcfg.py update command, you specify the -V flag: appcfg.py update -V [YOUR_VERSION_ID] For more information about using these commands, see Deploying Your App. An identifier for the version of your application code that you deploy to App Engine. The version ID can contain lowercase letters, digits, and hyphens. It cannot begin with the prefix ah- and the names default and latest are reserved and cannot be used. Note: Version names should begin with a letter, to distinguish them from numeric instances which are always specified by a number. This avoids the ambiguity with URLs like 123.my-service.appspot.com, which can be interpreted two ways: If version “123” exists, the target will be version “123” of the given service. If that version does not exist, the target will be instance number 123 of the default version of the service. Each version of an application retains its own copy of app.yaml. When an application is uploaded, the version mentioned in the app.yaml file being uploaded is the version that gets created or replaced by the upload. An administrator can change which version of the application is serving traffic by using the Google Cloud Platform Console, and can also test other versions before configuring them to receive traffic.

Handlers element

The handlers element is a required element in the app.yaml. The 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
application_readable	Optional. Boolean. By default, files declared in static file handlers are uploaded as static data and are only served to end users. They cannot be read by an application. If this field is set to true, the files are also uploaded as code data so your application can read them. Both uploads are charged against your code and static data storage resource quotas.
auth_fail_action	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. See the Users API for more information. The following example requires a login for the /profile/ directory and an administrator login for the /admin/ directory: handlers: - url: /profile/.* script: user_profile.app login: required - url: /admin/.* script: admin.app login: admin - url: /.* script: welcome.app 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: handlers: - url: /secure_api/.* script: api_handler.app login: required auth_fail_action: unauthorized
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.
login	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. 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 handlers: - url: /youraccount/.* script: accounts.app login: required 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 the path to the script from the application root directory: handlers: # The root URL (/) is handled by the WSGI application named # "app" in home.py. No other URLs match this pattern. - url: / script: home.app # The URL /index.html is also handled by the home.py script. - url: /index\.html script: home.app # A regular expression can map parts of the URL to the # path of the script. - url: /browse/(books|videos|tools) script: \1.catalog.app # All other URLs use the WSGI application named in "app" # in not_found.py. - url: /.* script: not_found.app A script: directive must be a python import path, for example, package.module.app that points to a WSGI application. The last component of a script: directive using a Python module path is the name of a global variable in the module: that variable must be a WSGI app, and is usually called app by convention. Note: just like for a Python import statement, each subdirectory that is a package must contain a file named __init__.py.
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 handlers: - url: /youraccount/.* script: accounts.app login: required secure: always The development web server does not support HTTPS connections. It ignores the secure parameter, so paths intended for use with HTTPS can be tested using regular HTTP connections to the development web server. 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. All files in this directory are uploaded with your app as static files. App Engine stores and serves static files separately from your app's files. Static files are not available in the app's file system by default. This can be changed by setting the application_readable option to true. 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)$ App Engine stores and serves static files separately from application files. Static files are not available in the application's file system by default. This can be changed by setting the application_readable option to true. Static files cannot be the same as application code files. If a static file path matches a path to a script used in a dynamic handler, the script will not be available to the dynamic handler.
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](#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

To learn more about how App Engine apps scale, see Scaling dynamic instances.

The following table lists the options for defining how you can specify that your application should scale.

Note: For a comparison of the scaling types see Scaling types and instance classes.

Element	Description
automatic_scaling	Optional. Automatic scaling is assumed by default with a default instance class of F1 unless specified otherwise. The automatic_scaling element sets minimum and maximum levels for number of instances, latency, and concurrent connections for a service. This element can contain the following elements: 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. Important: If you use appcfg from the App Engine SDK for Python to deploy, you cannot use this parameter in your app.yaml. Instead, set the parameter as described in Setting Autoscaling Parameters in the API Explorer, or by using the App Engine Admin API. 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 starts a new instance. Important: If you use appcfg from the App Engine SDK for Python to deploy, you cannot use this parameter in your app.yaml. Instead, set the parameter as described in Setting Autoscaling Parameters in the API Explorer, or by using the App Engine Admin API. 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. Important: If you use appcfg from the App Engine SDK for Python to deploy, you cannot use this parameter in your app.yaml. Instead, set the parameter as described in Setting Autoscaling Parameters in the API Explorer, or by using the App Engine Admin API. 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. Important: If you use appcfg from the App Engine SDK for Python to deploy, you cannot use this parameter in your app.yaml. Instead, set the parameter as described in Setting Autoscaling Parameters in the API Explorer, or by using the App Engine Admin API. 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 starts a new instance. You might experience increased API latency if this setting is too high. Note that the scheduler might spawn a new instance before the actual maximum number of requests is reached. 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 10, which is the default. To find the optimal value, gradually increase it and monitor the performance of your application. max_idle_instances The maximum number of idle instances that App Engine should maintain for this version. 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. 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". App Engine can create an instance at any time between the time specified in `min-pending-latency` and `max-pending-latency`. In other words, App Engine will not create an instance to serve a pending request before the time specified in `min-pending-latency`, but App Engine will create an instance after `max-pending-latency` is reached. 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_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. For this feature to function properly, you must make sure that warmup requests are enabled and that your application handles warmup 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. 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 service: my-service runtime: python27 api_version: 1 instance_class: F2 automatic_scaling: target_cpu_utilization: 0.65 min_instances: 5 max_instances: 100 min_pending_latency: 30ms # default value max_pending_latency: automatic max_concurrent_requests: 50
basic_scaling	Optional. The basic_scaling element sets the number of instances for a service. This element can contain the following elements: idle_timeout Optional. The instance will be shut down this amount of time after receiving its last request. The default is 5 minutes. 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. Example service: my-service runtime: python27 api_version: 1 instance_class: B8 basic_scaling: max_instances: 11 idle_timeout: 10m
manual_scaling	Optional. The manual_scaling element enables manual scaling for a service and sets the number of instances for a service. This element can contain the following elements: instances The number of instances to assign to the service at the start. Example service: my-service runtime: python27 api_version: 1 instance_class: B8 manual_scaling: instances: 5

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.

Note: Script handlers can set cache durations by returning the appropriate HTTP headers to the browser.

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.