Savings Plans have largely replaced Reserved Instances for new AWS EC2 commitments. What can a Reserved Instance still do that a Savings Plan cannot, and when would you buy a standard rather than a convertible RI?

## Why Savings Plans took over Reserved Instances came first and carried a lot of ceremony: you reserved a configuration, and matching it against real usage involved normalisation factors, regional versus zonal scope, and a good deal of spreadsheet work. Savings Plans replaced that with a single number — dollars per hour — and let AWS do the matching. For most EC2 commitments today, that is the right default, and interviewers expect you to say so. But "default" is not "only", and the residual capabilities of RIs are exactly what a well-prepared candidate can name. ## Capability one: capacity reservation The distinction people miss is *scope*. A Reserved Instance is bought as either: - **Regional** — applies anywhere in the region, gives instance size flexibility within the family, and reserves **no** capacity; or - **Zonal** — pinned to a specific Availability Zone and instance type, no size flexibility, and it **does** reserve capacity in that AZ. A Savings Plan is always the first kind of thing: a billing benefit with no capacity guarantee. So if the requirement is "this workload must be able to launch in `eu-west-1a` even during a capacity crunch", a Savings Plan cannot express it. The modern way to express it is an On-Demand Capacity Reservation, which reserves capacity and is billed like an On-Demand instance — and that billed capacity can itself be discounted by a matching Savings Plan. So the good answer is usually "Capacity Reservation for the guarantee, Savings Plan for the discount", not "buy zonal RIs". ## Capability two: exchange (convertible RIs) A **convertible** RI can be exchanged during its term for one or more other convertible RIs, provided the new reservations are of equal or greater total value. That lets you follow a fleet across families, operating systems or tenancies without eating the whole commitment. The price is a smaller discount than a standard RI of the same term. A **standard** RI cannot be exchanged. You can modify limited attributes — Availability Zone, scope between regional and zonal, network platform, or split and merge sizes within a family — but you cannot move it to a different instance family. Savings Plans sit outside this entirely: there is no exchange mechanism. Their flexibility is built into which usage they match, not into changing the contract afterwards. ## Capability three: resale (standard RIs) Standard RIs can be listed on the **Reserved Instance Marketplace** and sold to another AWS customer, subject to conditions such as seller eligibility and a minimum remaining term. Convertible RIs cannot be sold, and neither can Savings Plans. This makes the standard RI the only EC2 commitment with a genuine secondary market — a real, if narrow, escape hatch if your plans change. ## Beyond EC2 Savings Plans cover EC2, Fargate and Lambda. A number of other services still sell commitments only as reserved instances or reserved nodes — Amazon RDS reserved DB instances, ElastiCache reserved nodes, OpenSearch reserved instances, Redshift reserved nodes, DynamoDB reserved capacity. If a large slice of your bill is databases, "we're on Savings Plans" leaves that whole slice at On-Demand rates. Checking which services in your bill are actually eligible is a step candidates routinely skip. ## Standard versus convertible: the decision Think of it as buying an option. The convertible RI's exchange right is an option on your own uncertainty, and you pay for it with a lower discount. - **Standard** when the configuration is genuinely fixed for the term — a licensed platform pinned to a family, a legacy system nobody will fund a rewrite for, a steady control plane. You get the deeper discount and the Marketplace as a fallback. - **Convertible** when you can foresee the family or platform changing but still want a multi-year discount. - **Neither, buy a Compute Savings Plan** when what you actually want is flexibility across families and compute models — the Savings Plan gives you that up front instead of making you exercise an exchange later. ## The framing that scores An interviewer is checking whether you can distinguish *billing* from *capacity*, and whether you understand that every extra freedom (exchange, resale, capacity guarantee) is paid for in discount. A candidate who says "RIs are the old way, always use Savings Plans" is 80% right and will be caught by the follow-up about guaranteeing capacity in an AZ, or about the RDS half of the bill.