The main differences between On-Demand, Spot, and Reserved instances in Amazon EC2 relate to pricing model, flexibility, and cost savings:
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On-Demand Instances:
- Pricing Model: On-Demand instances allow you to pay for compute capacity by the hour or by the second without any long-term commitments or upfront payments.
- Flexibility: On-Demand instances offer maximum flexibility, allowing you to launch instances whenever you need them and terminate them whenever you no longer need them.
- Cost: On-Demand instances typically have the highest hourly rates compared to Spot and Reserved instances, but they provide the greatest flexibility and do not require any upfront payment.
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Spot Instances:
- Pricing Model: Spot instances allow you to bid on unused EC2 capacity, offering significant cost savings compared to On-Demand instances.
- Flexibility: Spot instances are well-suited for applications with flexible start and end times or those that can tolerate interruptions. AWS can reclaim Spot instances with short notice if the current spot price exceeds your bid price or if the capacity is needed by On-Demand or Reserved instances.
- Cost: Spot instances offer the lowest hourly rates among the three pricing models, making them ideal for workloads that can handle interruptions or have variable compute demands.
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Reserved Instances:
- Pricing Model: Reserved instances require a one-time upfront payment and offer a significant discount compared to On-Demand instances, typically up to 75%.
- Flexibility: Reserved instances require a commitment to a specific instance type in a particular region for a one- or three-year term. While they offer lower hourly rates compared to On-Demand instances, they require upfront commitment and are best suited for applications with steady-state usage or predictable workloads.
- Cost: Reserved instances provide the lowest hourly rates among the three pricing models, but they require upfront payment and a commitment to a specific instance type and term.
In summary, On-Demand instances offer maximum flexibility but may have higher costs compared to Spot and Reserved instances. Spot instances provide significant cost savings but come with the risk of interruptions. Reserved instances offer the lowest cost but require upfront commitment and are best suited for predictable workloads. Many users combine different instance types to optimize cost and performance for their workloads.
