How Spot Instances Work
To use Spot instances, create a Spot instance request or a Spot fleet request. The request includes the maximum price that you are willing to pay per hour per instance (your bid price), and other constraints such as the instance type and Availability Zone. If your bid price is greater than the current Spot price for the specified instance, and the specified instance is available, your request is fulfilled immediately. Otherwise, the request is fulfilled whenever the Spot price falls below your bid price or the specified instance becomes available. Spot instances run until you terminate them or until Amazon EC2 must terminate them (also known as a Spot instance interruption).
When you use Spot instances, you must be prepared for interruptions. Amazon EC2 can interrupt your Spot instance when the Spot price rises above your bid price, when the demand for Spot instances rises, or when the supply of Spot instances decreases. When Amazon EC2 marks a Spot instance for termination, it provides a Spot instance termination notice, which gives the instance a two-minute warning before it terminates. Note that you can't enable termination protection for Spot instances. For more information, see Spot Instance Interruptions.
Note that you can't stop and start an Amazon EBS-backed instance if it is a Spot instance, but you can reboot or terminate it.
Shutting down a Spot instance on OS-level results in the Spot instance being terminated. It is not possible to change this behavior.
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Supply and Demand in the Spot Market
AWS continuously evaluates how many Spot instances are available in each Spot instance pool, monitors the bids that have been made for each pool, and provisions the available Spot instances to the highest bidders. The Spot price for a pool is set to the lowest fulfilled bid for that pool. Therefore, the Spot price is the price above which you must bid to fulfill a Spot request for a single Spot instance immediately.
For example, suppose that you create a Spot instance request, and that the corresponding Spot instance pool has only five Spot instances for sale. Your bid price is $0.10, which is also the current Spot price. The following table shows the current bids, ranked in descending order. Bids 1-5 are fulfilled. Bid 5, being the last fulfilled bid, sets the Spot price at $0.10. Bid 6 is unfulfilled. Bids 3-5, which share the same bid price of $0.10, are ranked in random order.
| Bid | Bid price | Current Spot price | Notes |
|---|---|---|---|
|
1 |
$1.00 | $0.10 | |
|
2 |
$1.00 | $0.10 | |
|
3 |
$0.10 | $0.10 | |
|
4 |
$0.10 | $0.10 |
Your bid |
|
5 |
$0.10 | $0.10 |
Last fulfilled bid, which sets the Spot price. Everyone pays the same Spot price for the period. |
|
— — — |
— — — |
Spot capacity cutoff | |
|
6 |
$0.05 |
Now, let's say that the size of this pool drops to 3. Bids 1-3 are fulfilled. Bid 3, the last fulfilled bid, sets the Spot price at $0.10. Bids 4-5, which also are $0.10, are unfulfilled. As you can see, even though the Spot price didn't change, two of the bids, including your bid, are no longer fulfilled because the Spot supply decreased.
| Bid | Bid price | Current Spot price | Notes |
|---|---|---|---|
|
1 |
$1.00 | $0.10 | |
|
2 |
$1.00 | $0.10 | |
|
3 |
$0.10 | $0.10 |
Last fulfilled bid, which sets the Spot price. Everyone pays the same Spot price for the period. |
|
— — — |
— — — |
Spot capacity cutoff | |
|
4 |
$0.10 |
Your bid | |
|
5 |
$0.10 | ||
|
6 |
$0.05 |
To fulfill a Spot request for a single instance from this pool, you must bid above the current Spot price of $0.10. If you bid $0.101, your request will be fulfilled, the Spot instance for bid 3 would be interrupted, and the Spot price would become $0.101. If you bid $2.00, the Spot instance for bid 3 would be interrupted and the Spot price would become $1.00 (the price for bid 2).
Keep in mind that no matter how high you bid, you can never get more than the available number of Spot instances in a Spot instance pool. If the size of the pool drops to zero, then all the Spot instances from that pool would be interrupted.
Launching Spot Instances in a Launch Group
Specify a launch group in your Spot instance request to tell Amazon EC2 to launch a set of Spot instances only if it can launch them all. In addition, if the Spot service must terminate one of the instances in a launch group (for example, if the Spot price rises above your bid price), it must terminate them all. However, if you terminate one or more of the instances in a launch group, Amazon EC2 does not terminate the remaining instances in the launch group.
Note that although this option can be useful, adding this constraint can lower the chances that your Spot instance request is fulfilled. It can also increase the chance that your Spot instances will be terminated.
If you create another successful Spot instance request that specifies the same (existing) launch group as an earlier successful request, then the new instances are added to the launch group. Subsequently, if an instance in this launch group is terminated, all instances in the launch group are terminated, which includes instances launched by the first and second requests.
Launching Spot Instances in an Availability Zone Group
Specify an Availability Zone group in your Spot instance request to tell the Spot service to launch a set of Spot instances in the same Availability Zone. Note that Amazon EC2 need not terminate all instances in an Availability Zone group at the same time. If Amazon EC2 must terminate one of the instances in an Availability Zone group, the others remain running.
Note that although this option can be useful, adding this constraint can lower the chances that your Spot instance request is fulfilled.
If you specify an Availability Zone group but don't specify an Availability Zone in the Spot instance request, the result depends on whether you specified the EC2-Classic network, a default VPC, or a nondefault VPC. For more information about EC2-Classic and EC2-VPC, see Supported Platforms.
EC2-Classic
Amazon EC2 finds the lowest-priced Availability Zone in the region and launches your Spot instances in that Availability Zone if the lowest bid for the group is higher than the current Spot price in that Availability Zone. Amazon EC2 waits until there is enough capacity to launch your Spot instances together, as long as the Spot price remains lower than the lowest bid for the group.
Default VPC
Amazon EC2 uses the Availability Zone for the specified subnet, or if you don't specify a subnet, it selects an Availability Zone and its default subnet, but it might not be the lowest-priced Availability Zone. If you deleted the default subnet for an Availability Zone, then you must specify a different subnet.
Nondefault VPC
Amazon EC2 uses the Availability Zone for the specified subnet.
Launching Spot Instances in a VPC
To take advantage of the features of EC2-VPC when you use Spot instances, specify in your Spot request that your Spot instances are to be launched in a VPC. You specify a subnet for your Spot instances the same way that you specify a subnet for your On-Demand instances.
The process for making a Spot instance request that launches Spot instances in a VPC is the same as the process for making a Spot instance request that launches Spot instances in EC2-Classic—except for the following differences:
You should base your bid on the Spot price history of Spot instances in a VPC.
[Default VPC] If you want your Spot instance launched in a specific low-priced Availability Zone, you must specify the corresponding subnet in your Spot instance request. If you do not specify a subnet, Amazon EC2 selects one for you, and the Availability Zone for this subnet might not have the lowest Spot price.
[Nondefault VPC] You must specify the subnet for your Spot instance.
