7.3System Architecture on Cloud Services

Unlike an in-house data center, IaaS offers servers, storage, and networking as resources provisioned through an API. The HA and scalability ideas from earlier sections still apply, but understanding cloud-specific building blocks—ephemeral storage, tenant isolation, autoscalers—matters both in practice and on the exam.

7.3.1Storage and IP address types

• Ephemeral storage is tied to an instance's lifecycle: terminate the instance and the data disappears. Use it for things you can rebuild (the OS, caches). Persistent storage has a lifecycle independent of any instance (e.g., a block-storage volume) and survives instance termination—the right place for data you cannot afford to lose, like database files.
• Fixed IP is an IP address permanently assigned to an instance. Floating IP is an address reserved independently of any instance and reattached to whichever node needs it—reattaching it to a healthy node on failure achieves failover without changing the IP address clients use.

7.3.2Network isolation and supporting services

• A tenant network gives each user (tenant) a logically isolated network space while sharing the cloud provider's physical infrastructure—traffic from other tenants is invisible and never mixes with yours. On top of that, firewalls and security groups (permit rules applied per instance, typically stateful so return traffic for an allowed connection passes automatically) restrict traffic to only what is needed.
• Object storage handles files as key-value objects accessed over an HTTP API (suited to large volumes of images, backups, logs; unlike block storage, it is not mounted as a filesystem).
• Messaging systems/queues mediate service-to-service communication asynchronously: the sender just places a message on the queue, decoupling it from the receiver's processing speed or temporary outages. An autoscaler automatically adds or removes instances based on metrics like CPU utilization—cloud automation of the scale-out/in idea from the previous section.

The key discipline in IaaS design is always asking "what disappears, and what survives?" An autoscaler adds and removes web-server instances based on load, but if DB data or user-uploaded images are stored directly on those instances' ephemeral storage, the data vanishes the instant a scale-in event terminates one. The standard division of labor is therefore: keep the database on persistent storage (a dedicated instance or managed service), and keep user uploads in object storage. For failover, designing around a floating IP lets you reattach just the address from a failed node to a healthy one—traffic switches instantly with no DNS-propagation wait (with only fixed IPs, swapping out a whole node means reconfiguring the address itself). On the networking side, because a cloud shares physical infrastructure across many customers, tenant network isolation is the baseline, and inside that you layer security groups for least-privilege rules—"web servers: only 80/443," "DB server: only port 3306 from web servers." If services communicate only via synchronous HTTP calls, one service stalling or going down drags the whole chain to a halt; inserting a messaging system/queue to go asynchronous means messages simply pile up in the queue while the receiver is briefly unavailable, and the sender keeps working regardless—a real gain in resiliency.

7.3.3Section summary

• Ephemeral = tied to the instance's fate / persistent = survives independently. Fixed IP = permanent / floating IP = reattachable for IP-stable failover
• Tenant network + security groups give logical isolation and least-privilege traffic. Object storage, queues (async decoupling), and autoscalers are the standard cloud building blocks