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how instances are managed
Instances are the basic building blocks of App Engine, providing all the resources needed to successfully host your application. At any given time, your application can be running on one or many instances with requests being spread across all of them. Each instance includes a security layer to ensure that instances cannot inadvertently affect each other.
App Engine can automatically create and shut down instances as traffic fluctuates, or you can specify a number of instances to run regardless of the amount of traffic. To determine how and when new instances are created, you specify a scaling type for your app. The scaling settings are applied at the App Engine version level as part of the app.yaml file.
App Engine applications that use basic or automatic scaling are powered by any number of dynamic instances at a given time, depending on the volume of incoming requests. As requests for your application increase, the number of dynamic instances may increase as well.
If you use basic scaling, App Engine attempts to keep your cost low, even though that may result in higher latency as the volume of incoming requests increases.
When none of the existing instances are available to serve an incoming request, App Engine starts a new instance. Even after starting a new instance, some requests may need to be queued until the new instance completes its startup process. If you require the lowest latency possible consider using automatic scaling, which creates new instances preemptively to minimize latency.
If you use automatic scaling, each instance in your app has its own queue for incoming requests. Before the queues become long enough to have a noticeable effect on your app's latency, App Engine automatically creates one or more new instances to handle the increasing load.
Note: If your service is infrequently used, standard environment scales to zero instances. To reduce the impact of cold-start latency for infrequently used services, you can change the configuration to allow a minimum of one instance to always be active or enable warmup requests.
You can configure the settings for automatic scaling to achieve a trade-off between the performance you want and the cost you can incur. The following table describes these settings.
Automatic scaling settings | Description --------------------------- | ----------- Target CPU Utilization | Sets the CPU utilization ratio threshold to specify the CPU usage threshold at which more instances will be started to handle traffic. Target Throughput Utilization | Sets the throughput threshold for the number of concurrent requests after which more instances will be started to handle traffic. Max Concurrent Requests | Sets the max concurrent requests an instance can accept before the scheduler spawns a new instance.
Watch the App Engine Scheduler Settings video to see the effects of these settings.
When request volumes decrease, App Engine reduces the number of instances. This downward scaling helps ensure that all of your application's current instances are being used for optimal efficiency and cost effectiveness.
When an application is not being used at all, App Engine turns off its associated dynamic instances, but readily reloads them as soon as they are needed. Reloading instances can result in loading requests and additional latency for users.
You can specify a minimum number of idle instances. Setting an appropriate number of idle instances for your application based on request volume allows your application to serve every request with little latency, unless you are experiencing abnormally high request volume.
If your app uses automatic scaling, it takes approximately 15 minutes of
inactivity for the idle instances to start shutting down. To keep one or more
idle instances running, set the value of
min_idle_instances
to 1 or higher.
If you are sending batches of requests to your services, for example, to a task queue for processing, a large number of instances will be created quickly. We recommend controlling this by rate limiting the number of request sent per second, if possible. For example, if you use {{tasks_name}}, you can control the rate at which tasks are pushed.
An instance of an auto-scaled service is always running. However, an instance of a manual or basic scaled service can be either running or stopped. All instances of the same service and version share the same state. You change the state of your instances by managing your versions. You can:
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Use the Versions page in the Google Cloud console
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Use
[gcloud app versions start](/sdk/gcloud/reference/app/versions/start)and[gcloud app versions stop](/sdk/gcloud/reference/app/versions/stop)commands -
Use the {{modules_api_name}}
Each service instance is created in response to a start request, which is an
empty HTTP GET request to /_ah/start. App Engine sends this request to bring
an instance into existence; users cannot send a request to /_ah/start. Manual
and basic scaling instances must respond to the start request before they can
handle another request. The start request can be used for two purposes:
- To start a program that runs indefinitely, without accepting further requests.
- To initialize an instance before it receives additional traffic.
Manual, basic, and automatically scaling instances startup differently. When you
start a manual scaling instance, App Engine immediately sends a /_ah/start
request to each instance. When you start an instance of a basic scaling service,
App Engine allows it to accept traffic, but the /_ah/start request is not sent
to an instance until it receives its first user request. Multiple basic scaling
instances are only started as necessary, in order to handle increased traffic.
Automatically scaling instances do not receive any /_ah/start request.
When an instance responds to the /_ah/start request with an HTTP status code
of 200–299 or 404, it is considered to have successfully started and can
handle additional requests. Otherwise, App Engine terminates the instance.
Manual scaling instances are restarted immediately, while basic scaling
instances are restarted only when needed for serving traffic.
The shutdown process might be triggered by a variety of planned and unplanned events, such as:
- There are too many instances and not enough app requests (traffic).
- You manually stop an instance.
- You deploy an updated version to the service.
- The instance exceeds the maximum memory for its configured
instance_class. - Your application runs out of Instance Hours quota.
- Your instance is moved to a different machine, either because the current machine that is running the instance is restarted, or App Engine moved your instance to improve load distribution.
One of the benefits standard environment's "pay for only what you need" platform as described earlier in Scaling Down is that the system autoscales the number of instances down to zero when there is no traffic. This helps make App Engine a cost-effective solution for small applications that don't receive continuous requests. When an instance needs to be shut down, new incoming requests are routed to other instances (if any) and requests that are currently being processed are given time to complete.
When an instance needs to be shut down, App Engine sends a KILL (SIGKILL)
signal, terminating the instance.
When App Engine creates a new instance for your application, the instance must first load any libraries and resources required to handle the request. This happens during the first request to the instance, called a Loading Request. During a loading request, your application undergoes initialization which causes the request to take longer.
The following best practices allow you to reduce the duration of loading requests:
- Load only the code needed for startup.
- Access the disk as little as possible.
- In some cases, loading code from a zip or jar file is faster than loading from many separate files.
Warmup requests are a specific type of loading request that load application
code into an instance ahead of time, before any live requests are made. Manual
or basic scaling instances do not receive an /_ah/warmup request.
To learn more about how to use warmup requests, see Configuring warmup requests.
App Engine attempts to keep manual and basic scaling instances running indefinitely. However, at this time there is no guaranteed uptime for manual and basic scaling instances.
Hardware and software failures that cause early termination or frequent restarts can occur without prior warning and can take considerable time to resolve; thus, you should construct your application in a way that tolerates these failures.
Here are some good strategies for avoiding downtime due to instance restarts:
- Reduce the amount of time it takes for your instances restart or for new ones to start.
- For long-running computations, periodically create checkpoints so that you can resume from that state.
- Your app should be "stateless" so that nothing is stored on the instance.
- Use queues for performing asynchronous task execution.
- If you configure your instances to manual scaling:
- Use load balancing across multiple instances.
- Configure more instances than required to handle normal traffic.
- Write fall-back logic that uses cached results when a manual scaling instance is unavailable.
The standard environment has network time protocol (NTP) services which use Google NTP servers. However, the NTP service is not editable.