Memory hook: Preserve the right identity, separate the right failure domains, and distinguish resuming a machine from recovering an application.
Must remember
Addresses and interfaces
- Private IPv4 addresses belong to the VPC address space. An instance's primary private IP normally remains with its primary ENI through stop/start; terminating resources is a different lifecycle operation.
- An automatically assigned public IPv4 is not a durable endpoint and can change on stop/start. Reboot and stop/start are not equivalent operations.
- An Elastic IP (EIP) is a static regional IPv4 allocation that can be reassociated with supported resources. A static address simplifies allowlists or appliance failover, but allocation itself supplies no redundant application capacity.
- Public IPv4 addresses incur charges, including allocated Elastic IPs kept idle. Prefer an appropriate load-balancer/DNS endpoint when the real requirement is replaceable application servers rather than a single machine's address.
- An elastic network interface (ENI) contains private IPs, a MAC address and security-group associations. It belongs to a subnet and therefore to one AZ.
- A secondary ENI can move between compatible instances in the same AZ, allowing an appliance to retain a private network identity. The primary ENI cannot simply be detached from a running instance.
- Network-identity failover still needs detection, reassociation permissions, a ready replacement and application recovery. Moving an interface does not copy files, RAM or transactions to another machine.
Placement strategies
- Cluster placement packs supported instances close together in one AZ for tightly coupled, low-latency, high-throughput communication. Think HPC, not geographic disaster tolerance.
- Spread placement separates a small critical set across distinct underlying hardware. It reduces correlated hardware failure; it is not a replication or backup service.
- Partition placement separates groups of instances onto different underlying hardware. Partition-aware distributed applications such as Kafka, Cassandra or Hadoop can distribute replicas across those failure domains.
- Choose based on the application's communication pattern and failure model. A distributed data platform should know where it places replicas; separation is useful only if the application uses it.
- Placement/type support differs. Empty groups supply no running capacity, and this lab's burstable instance cannot demonstrate every strategy.
Hibernation and lifecycle
- Hibernation saves RAM to the encrypted EBS root volume, stops the instance, and restores the memory image on resumption. It suits expensive initialization held in memory.
- It requires supported instance/AMI configurations, prior enablement and adequate root-volume space. It is not interchangeable with an ordinary stop, which does not preserve RAM.
- Retained EBS storage continues to cost money. A stopped/hibernated machine is not an active standby serving users.
- Reboot restarts the OS. Stop ends compute while retaining eligible EBS state. Terminate destroys the instance, subject to volume-retention settings.
- Important instance-store data is not protected by hibernation or a durable-looking IP address. Storage lifecycle and replicas remain separate design decisions; review disks and shared files.
Choose under exam pressure
| Requirement or clue | Decision and reason |
|---|---|
| Low inter-node latency in tightly coupled HPC | Cluster placement on supported instances |
| Separate a small set of critical replicas | Spread placement plus application replication |
| Isolate groups in a distributed data platform | Partition placement and aware replica assignment |
| Retain a private appliance identity within one AZ | Secondary ENI failover |
| Fixed regional public IPv4 required | EIP, with explicit failover and cost consideration |
| Resume a long in-memory initialization | Supported hibernation |
| Continue operating through an AZ loss | Cross-AZ application/data design, not ENI movement |
Traps
- An ENI cannot be moved across AZs. Its subnet association is part of its placement boundary.
- Identity is not state. An IP address, hostname or MAC does not preserve an application's data.
- Suspension is not availability. Hibernation preserves working memory but does not serve requests while stopped or create a second failure domain.
Active recall
1. An appliance fails and its replacement is in another AZ. Can moving the secondary ENI restore the original private identity?
No. An ENI is tied to its subnet/AZ. Cross-AZ recovery needs a different addressing and traffic-steering design plus application recovery. A same-AZ interface reassociation is not a regional or multi-AZ failover strategy.
2. A distributed data system wants to isolate replica groups, not just individual machines. Which placement strategy fits?
Partition placement, with the application assigning data replicas across partitions. Spread separates a smaller set of individual instances; cluster prioritizes close networking. Placement alone does not decide where the application stores each replica.
3. An instance retains its EIP after stop/start but loses application memory. Did the EIP fail?
No. The EIP preserves an address allocation, not RAM. Ordinary stop/start does not restore in-memory application state. Use appropriate persistence or eligible hibernation when resuming memory is the actual requirement.
4. A workload needs minimal latency between tightly coupled workers and also regional disaster recovery. Is cluster placement sufficient?
No. Cluster placement addresses intra-AZ communication performance. Regional recovery requires additional copies, capacity and failover procedures elsewhere. The two requirements operate at different scopes and may require separate architectures.
5. Why can a hibernated fleet still generate charges while being unavailable to clients?
Its saved memory and disks remain on EBS, and other allocated resources can persist. Hibernation pauses compute rather than deleting storage or maintaining active serving replicas. Cost and availability must both be evaluated across the full resource set.
Terraform anchor: Stable for_each keys preserve resource addresses; changing a descriptive map key can propose replacement even when the intended workload is unchanged.
Sources
- EC2 instance addressing — private and public addressing lifecycle.
- Elastic network interfaces — interface identity, attachment and scope.
- EC2 placement groups — placement strategies and compatibility.
- EC2 hibernation — memory preservation and requirements.