Elastic Container Instance supports preemptible instances. You can run stateless applications that feature high scalability and fault tolerance and short-lived jobs on preemptible instances to reduce usage costs of instances. This topic describes how to create a preemptible Elastic Container Instance-based pod in a Kubernetes cluster.
Background information
Preemptible elastic container instances are cost-effective. You can bid for unused Alibaba Cloud resources to create preemptible instances. When the market price exceeds your bid price for a preemptible instance or when the inventory resources are insufficient, the resources consumed by the instance are reclaimed and the instance is released.
Preemptible instances can be used to run short-lived jobs and stateless applications that feature high scalability and fault tolerance, such as scalable website services, image rendering, big data analysis, and large-scale parallel computing. Preemptible instances are suitable for applications that require wide distribution, high scalability, and high fault tolerance. Preemptible instances help you reduce costs and increase the throughput for these applications. For more information, see Preemptible instances.
Basic concepts
Before you create a preemptible instance, take note of the following items:
Billing
The market price of a preemptible instance fluctuates based on the supply and demand for its instance specification. When you create a preemptible instance, you must specify a bid policy for the instance. If your bid price is higher than the real-time market price and the resource inventory of the instance specification is sufficient, the preemptible instance is created. After a preemptible instance is created, it enters a protection period during which the instance is billed based on the market price at the time of purchase. By default, the protection period is 1 hour. After the protection period expires, the instance is billed based on the real-time market price.
NotePreemptible instances are billed at lower prices compared with pay-as-you-go instances. The price of a preemptible instance fluctuates based on the supply and demand for its instance specification. A preemptible instance is billed based on its actual duration of use. For more information, see the "Billing methods" section of the Preemptible instances topic.
Reclaim mechanism
After the protection period of a preemptible instance expires, the system checks the market price and resource inventory of the instance specification every 5 minutes. If the real-time market price exceeds your bid price or the resource inventory is insufficient, the preemptible instance is released.
NoteA ready-for-release (SpotToBeReleased) event is generated about 3 minutes before the resources of a preemptible instance are reclaimed.
After the resources of a preemptible instance are reclaimed, the information about the instance is retained, the billing on the instance stops, and the status of the instance changes to Expired.
Precautions
Before you use preemptible instances, take note of the following items:
Select a suitable instance specification and place a suitable bid.
You can call the following API operations of Elastic Compute Service (ECS) to query information about preemptible instances over the last 30 days. Then, you can select a suitable instance specification and place a bid.
DescribeSpotPriceHistory: queries the historical prices of preemptible instances.
DescribeSpotAdvice: queries information such as the average release rate and average discount rate of preemptible instances.
ImportantWhen you place a bid, take into consideration the fluctuations in market prices and the price range that you are comfortable with. A good bid is usually higher than the average market price, but still within the range of your business requirements. In this way, requests for creating preemptible instances can be accepted and processed, and the created instances will not be released due to changes in market prices.
We recommend that you store important data in a storage medium that will not be affected if preemptible instances are released. For example, you can use independent disks that are not to be released along with their associated instances, or use external storage resources such as File Storage NAS file systems.
Creation methods
You can create a preemptible instance by specifying ECS instance types or the number of vCPUs and memory size.
Specify ECS instance types
The preemptible instance is billed at the real-time discounted pay-as-you-go price of the specified ECS instance type.
Specify the number of vCPUs and memory size of the instance
This method has the same effect as the method of specifying ECS instance types. The system automatically selects an ECS instance type that meets your requirements for the number of vCPUs, memory size, and prices. The created preemptible instance is billed at the real-time discounted market price of the used ECS instance type, instead of the pay-as-you-go prices of the vCPU and memory resources that are used by the elastic container instance.
Only the instances that provide two or more vCPUs can be created by using this method. The following table lists the supported vCPU and memory specifications. If the specifications that you specify are not supported by Elastic Container Instance, the system automatically uses supported higher specifications.
Number of vCPUs
Memory (GiB)
2
2, 4, 8, and 16
4
4, 8, 16, and 32
8
8, 16, 32, and 64
12
12, 24, 48, and 96
16
16, 32, 64, and 128
24
24, 48, 96, and 192
32
32, 64, 128, and 256
52
96, 192, and 384
64
128, 256, and 512
Configuration description
You can add annotations to the metadata section in the configuration file of the pod to create a preemptible elastic container instance. The following table describes the annotations:
Annotation | Example | Required | Description |
k8s.aliyun.com/eci-spot-strategy | SpotAsPriceGo | Yes | Specifies the bidding policy for the preemptible instance. Valid values:
|
k8s.aliyun.com/eci-spot-price-limit | "0.5" | No | Specifies the maximum hourly price of the preemptible instance. This value can be accurate to up to three decimal places. This annotation is valid only when k8s.aliyun.com/eci-spot-strategy is set to SpotWithPriceLimit. |
k8s.aliyun.com/eci-spot-duration | "0" | No | Specifies the protection period of the preemptible instance. Unit: hour. Default value: 1. A value of 0 indicates no protection period. |
k8s.aliyun.com/eci-spot-fallback | "true" | No | Specifies whether to automatically create a pay-as-you-go instance to ensure that an instance is created if inventory resources that meet the specification requirements for the preemptible instance are insufficient. Default value: false. |
Annotations must be added to the metadata of pods. For example, when you create a job, you must add annotations in the spec.template.metadata section.
To use features of Elastic Container Instance, you can add annotations only when you create Elastic Container Instance-based pods. If you add or modify annotations when you update pods, these annotations do not take effect.
Example 1: Specify an ECS instance type and use the SpotWithPriceLimit bid policy
Example 2: Specify the number of vCPUs and memory size and use the SpotAsPriceGo bid policy
Example 3: Set no protection period
Example 4: Automatically create a pay-as-you-go instance if inventory resources that meet the requirements for the instance specification are insufficient
apiVersion: batch/v1
kind: Job
metadata:
name: test
spec:
template:
metadata:
labels:
app: perl
alibabacloud.com/eci: "true"
annotations:
k8s.aliyun.com/eci-use-specs : "ecs.c6.large" # Specify an ECS instance type.
k8s.aliyun.com/eci-spot-strategy: "SpotWithPriceLimit" # Specify SpotWithPriceLimit as the bid policy.
k8s.aliyun.com/eci-spot-price-limit: "0.05" # Specify a maximum hourly price for the preemptible instance.
k8s.aliyun.com/eci-spot-fallback: "true" # Automatically create a pay-as-you-go instance if inventory resources that meet the requirements for the preemptible instance specification are insufficient.
spec:
containers:
- name: pi
image: registry.cn-shanghai.aliyuncs.com/eci_open/perl:5
command: ["perl", "-Mbignum=bpi", "-wle", "print bpi(2000)"]
restartPolicy: NeverThe preceding YAML code can create a preemptible instance based on the ecs.c6.large instance type.
If inventory resources that meet the requirements for the instance type and maximum hourly price are sufficient, a preemptible instance is created. After the instance is created, you are guaranteed to use the instance for one hour. After the one-hour protection period elapses, if the market price is higher than your bid price or the inventory resources that meet the instance specification are insufficient, the preemptible instance is released.
If inventory resources that meet the requirements for the instance type and maximum hourly price are insufficient, a pay-as-you-go instance is created. After the instance is created, the system does not automatically release the instance.
After the instance is created, you can run the
kubectl describe podcommand to view the events of the corresponding pod to check whether the instance is a pay-as-you-go instance. If a SpotDegraded event is displayed, the instance is a pay-as-you-go instance.
Release of preemptible instances
After a preemptible instance is created, it can run normally during the protection period. After the protection period expires, the preemptible instance is released if the market price is higher than your bid price or the inventory resources are insufficient. You can understand the release status of preemptible instances by performing the following operations:
SpotToBeReleased event
A SpotToBeReleased event is generated approximately 3 minutes before a preemptible instance is released.
ImportantElastic Container Instance sends the event to the list of Kubernetes events. During the three-minute period, you can take measures to ensure that your business is not affected by the release of the preemptible instance. For more information, see Graceful termination.
Run the
kubectl describecommand to view the details of the preemptible instance. The SpotToBeReleased event is displayed in the Events section of the command output. Example:Events: Type Reason Age From Message ---- ------ ---- ---- ------- Warning SpotToBeReleased 3m32s kubelet, eci Spot ECI will be released in 3 minutesRun the
kukubectl get eventscommand to view the event information. The SpotToBeReleased event is displayed in the command output. Example:LAST SEEN TYPE REASON OBJECT MESSAGE 3m39s Warning SpotToBeReleased pod/pi-frmr8 Spot ECI will be released in 3 minutes
Status of the preemptible instance after it is released
After a preemptible instance is released, the instance information is still retained. The status of the instance is changed to Failed and the cause of the failure is BidFailed.
Run the
kubectl get podcommand to view the instance information. The instance status that is displayed in the command output changes to BidFailed. Example:NAME READY STATUS RESTARTS AGE pi-frmr8 1/1 BidFailed 0 3h5mRun the
kubectl describecommand to view the details of the preemptible instance. Then, view the details about the instance status in the command output. Example:Status: Failed Reason: BidFailed Message: The pod is spot instance, and have been released at 2020-04-08T12:36Z
Graceful termination
A SpotToBeReleased event is generated approximately 3 minutes before a preemptible instance is released. Meanwhile, the ContainerInstanceExpired parameter in the pod conditions is set to true. To avoid service interruptions caused by reclamation of preemptible instances, you can use notification mechanisms to gracefully terminate preemptible instances and rotate pods.
Virtual nodes allow you to configure graceful termination for preemptible instances. You can add the k8s.aliyun.com/eci-spot-release-strategy: api-evict annotation to the metadata of a preemptible Elastic Container Instance-based pod. This way, when the virtual node receives a notification about a SpotToBeReleased event, the virtual node calls the Eviction API to evict the preemptible instance.
To enable notifications on preemptible instance interruptions by using pod conditions and evict preemptible instances by using the Eviction API, you must update ack-virtual-node to 2.11.0 or later. For more information, see ACK Virtual Node.
The Eviction API is called in compliance with the configurations specified by the PodDisruptionBudget (PDB) and the terminationGracePeriodSeconds parameter configured for pods. Creating an Eviction object for a pod by calling the API is similar to performing the DELETE operation defined by the control policy of the pod. Procedure of graceful termination:
Make an API call
When a virtual node receives a notification about a SpotToBeReleased event, the node calls the Eviction API.
Check the PDB
The API server checks the PDB configured for the pod to be evicted.
Evict the preemptible instance
After the API server validates the eviction request, the pod is evicted through the following steps:
The pod updates its deletion timestamp so that the API server can recognize the pod to be terminated. The pod is also stamped with the grace period that you configured.
The kubelet of the virtual node on which the pod runs recognizes the pod and initiates graceful termination for the pod.
When the kubelet terminates the pod, the control plane of the cluster disassociates the pod from relevant Endpoints and Endpointslices. In this case, the pod controller no longer recognizes the pod as valid.
When the grace period ends, the kubelet forcefully terminates the pod.
The kubelet notifies the API server to delete the pod.
The API server deletes the pod.
Reconcile workloads
If a pod is managed by a controller, such as a ReplicaSet or StatefulSet, or a Job, SparkApplication, or workflow that is configured with fault tolerance settings, the controller automatically creates a new pod after the original pod is evicted.
If the PDB used by the pod is incorrectly configured or a large number of pods are not in the Ready state when the node calls the Eviction API, the eviction process becomes stuck. If the eviction process is not completed when the preemptible instance expires, the instance is directly released.