The r7g.medium pairs 1 vCPU with 8 GiB of RAM, the 8.00 GiB-per-vCPU ratio that marks a memory-optimized instance. In the name, "r" stands for RAM, "7" is the generation, "g" marks AWS Graviton (ARM) processors and "medium" is the size within the r7g family. It runs on AWS Graviton3 Processor at 2.5 GHz, and AWS introduced it in February 2023. AWS has since shipped r8g and r9g; r8g.medium is the direct replacement at this size, scoring 30,926 CoreMark/s against this instance's 26,471 — roughly 17% more throughput for the same vCPU count. mh.vs2.medium is the closest Spot shape: 2 vCPU and 16 GiB, so 1 more vCPU and 8 GiB more memory, about 79% below the US East (N. Virginia) on-demand rate.
Match qualityx86 substitution
r7g.medium is Graviton (arm64). Rackspace Spot does not have arm64 servers, so mh.vs2.medium is an x86_64 substitute with similar specs.
You will need x86 builds of your container images. Most software rebuilds without changes, but anything with compiled native code needs a real rebuild rather than a retag. Speed per core also differs between the two chips, so it is worth testing before deciding how many nodes to run.