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Skill 24 · Architect For Startups
Subchapter 24.3
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| Stage | Default Compute | Why |
|---|---|---|
| Pre-seed / MVP | Lambda + API Gateway | $0 at zero traffic. Ship in hours, not days. |
| Seed / Early traction | Lambda OR ECS Fargate | Fargate if you need WebSockets or >15min processing. ALB setup cost is worth avoiding a forced re-platform later. Lambda function URLs + response streaming cover some gaps. |
| Series A / Steady traffic | ECS Fargate | Predictable costs at steady-state; Savings Plans eligible |
| Series B+ / Team has K8s | EKS only if team already knows it | Never adopt K8s as a startup unless you’re hiring K8s engineers |
Counterintuitive: ECS Fargate at seed stage feels heavy, but the ALB + target group + task definition setup is a one-time cost (~1 day). You avoid a forced re-platform when you outgrow Lambda’s 15-minute timeout or need persistent connections. For simpler cases, Lambda function URLs with response streaming can bridge the gap without a full container deploy.
| Stage | Default Database | Why NOT the “proper” choice |
|---|---|---|
| MVP validation | RDS PostgreSQL t4g.micro | $12/mo. SQL gives you joins, ad-hoc queries, and flexibility to iterate on your data model without re-engineering |
| MVP with auto-scaling needs | Aurora Serverless v2 | Scales to zero ACU… but minimum is 0.5 ACU ($43/mo). Worth it only if traffic is spiky and unpredictable |
| Confirmed key-value access patterns | DynamoDB on-demand | $0 at zero traffic. Only choose this if you’ve confirmed you don’t need relational queries |
| Need PostgreSQL but cost-sensitive | RDS PostgreSQL t4g.micro/small | $12-25/mo. Single-AZ is FINE until you have paying customers with uptime commitments |
DynamoDB single-table design: Every blog post says do it. DON’T at a startup. It’s a premature optimization that makes your data model rigid before you know your access patterns. Use multiple simple tables. Refactor to single-table design when you have proven query patterns AND DynamoDB costs justify the optimization.
| Gotcha | Impact | What to Do Instead |
|---|---|---|
| NAT Gateway as default | $32/mo + $0.045/GB — often your #2 line item | VPC endpoints for S3/DynamoDB. Public subnets + SGs for Lambda. Only add NAT if you actually need private subnet egress |
| Aurora Serverless v2 “scales to zero” | Minimum 0.5 ACU = $43/mo even idle | RDS t4g.micro at $12/mo is cheaper until you need auto-scaling |
| Multi-AZ everything | 2x cost on RDS, ElastiCache | Single-AZ until you have paying customers with uptime commitments (SLA or contractual). If credits cover it and you’re past MVP, enabling early is fine as insurance — but don’t let it become a hard dependency before you need it |
| CloudFront for API | $0 minimum but adds debugging complexity | Skip until you need geographic distribution or WAF |
| Secrets Manager per secret | $0.40/secret/mo adds up | SSM Parameter Store SecureString is free. Use Secrets Manager only for rotation |
| Cross-AZ data transfer | $0.01/GB between AZs | Chatty microservices in different AZs = hidden cost. Colocate or go single-AZ |
| Decision | With Credits | Without Credits |
|---|---|---|
| RDS instance size | Right-size to actual need (credits don’t change this) | Same — don’t over-provision just because credits cover it |
| Multi-AZ | Enable if credits cover it AND you’re past MVP — fine as early insurance | Defer until paying customers with uptime commitments |
| Reserved Instances / Savings Plans | Do NOT buy while on credits — wait until credits expire to see real spend patterns | Buy after 3 months of stable, post-credits usage |
| Managed services vs DIY | Always prefer managed services. EKS is the exception — only adopt it if your team already has Kubernetes expertise, and understand you’re taking on significant operational overhead | Same |
| Graviton instances | Prefer Graviton unless you have native x86 dependencies. 20% cheaper AND credits last longer. Test your container on ARM before committing | Same |
Critical: Never commit to Savings Plans or Reserved Instances while on credits. You can’t see your real usage patterns. Wait until 3 months AFTER credits are exhausted.
Before finalizing any architecture, ask: “If traffic 10x’s, what happens to my bill?”
| Service | 10x Behavior | Startup Risk |
|---|---|---|
| Lambda | 10x invocations = ~10x cost | Low risk — linear and predictable |
| DynamoDB on-demand | 10x reads/writes = ~10x cost | Medium risk — costs scale linearly with reads/writes. Design access patterns early to avoid surprise bills at traction |
| RDS | Doesn’t auto-scale (unless Aurora Serverless) | Low cost risk, HIGH availability risk |
| NAT Gateway | 10x data = 10x data processing charges | High risk — this is where surprise bills come from |
| S3 | 10x storage = linear. 10x requests = linear | Low risk |
| CloudFront | 10x requests = ~8x cost (volume discounts) | Low risk |
| ECS Fargate | 10x tasks = 10x cost. No volume discounts | Medium risk — but you control the scaling |
| AWS Best Practice | Startup Reality | Do This Instead |
|---|---|---|
| Multi-AZ for all stateful services | Costs 2x, you have 50 users | Single-AZ until paying customers with uptime commitments |
| Separate microservices from day 1 | You have 2 engineers and 1 service | Monolith or modular monolith. Split at pain points. If on Lambda, a well-organized single-repo with shared layers achieves the same cohesion as a container monolith |
| Use CloudTrail + GuardDuty + Security Hub | $20-50+/mo for security tooling | CloudTrail only until Series A. Add GuardDuty at first enterprise deal |
| VPC with private subnets + NAT | NAT costs $32+/mo minimum | Public subnets + security groups until you have compliance requirements |
| Custom KMS keys for encryption | $1/key/mo + API costs | AWS-managed keys (free) until compliance requires CMK |
| Detailed CloudWatch dashboards | You’re iterating weekly, dashboards are stale | 3 alarms (error rate, p99 latency, cost threshold). Add dashboards when you have an SRE |