Deployment Diagrams
Where software actually runs — regions, zones, clusters, networks — and therefore what fails together.
Definition
A deployment diagram maps logical components onto physical or virtual infrastructure: which regions, which availability zones, which clusters, which networks, and what runs where.
What it uniquely answers
What fails together.
A logical diagram showing three replicas suggests redundancy. A deployment diagram showing all three in one availability zone shows that the redundancy is against instance failure only. That difference is invisible at every other level and is the most common gap between intended and actual resilience.
It also answers:
- Where does traffic cross a boundary? Cross-zone and cross-region hops carry latency and transfer charges, and they are frequently unintentional.
- What is the blast radius of losing a zone, a region, an account or a cluster?
- Where are the trust boundaries in the network?
- Which components share fate through a common dependency — a NAT gateway, a load balancer, a shared cluster.
What to mark
- Failure domain boundaries clearly — zone, region, account, cluster.
- Every network hop that crosses one, with its direction.
- Shared components in the path: gateways, load balancers, meshes, proxies.
- What is single-instance, which is the fastest way to find unintended single points of failure.
- The control plane — identity, secrets, pipelines, observability — and where each lives. This is the omission that makes disaster recovery plans inoperable.
The review it enables
Walk each failure domain and ask what survives its loss. That exercise, done on a deployment diagram before an incident, finds most resilience gaps — and it consistently finds that the control plane is single-region while the workload is not.
Failure scenarios
- Logical redundancy drawn as physical, hiding co-location.
- The control plane omitted, so a regional failover plan cannot be executed.
- Generated from configuration and never read, which is accurate and useless.
- Drawn for the target state while the current state differs, with no indication which is which.
Interview question
"What does a deployment diagram show that a container diagram cannot?"