Full Stack Software Engineer

Ryan Mahoney

Ryan Mahoney

Director of Product, FirstWho

“The hardest thing about hiring full stack engineers at this level is finding someone who can genuinely own a complete user journey without becoming a liability when complexity spikes. You need someone who writes solid code across the stack, sure, but more importantly, someone who can explain why they picked Postgres over DynamoDB for this specific workload, and walk a junior through that same thinking. The communication bar is higher than most teams want to admit. A candidate who can ship a feature end-to-end but cannot articulate trade-offs to product or push back on unrealistic deadlines will eventually become a bottleneck. You are hiring for judgment spread across layers, not just technical breadth.”

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Competency Questions

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Application Engineering & Architecture

Design and implementation of full-stack application systems, emphasizing end-to-end feature ownership spanning database schemas, RESTful/GraphQL APIs, and modern frontend interfaces with performance optimization.

Backend API & Service Development

Designs RESTful/GraphQL APIs with proper authentication and authorization; implements middleware for logging, security, and request validation; manages database transactions and service integrations.

Interview round: Hiring Manager System Design

Share an experience where you had to debug a production issue that originated in your backend service but manifested elsewhere.

Positive indicators

  • Describes distributed tracing or correlation ID usage
  • Mentions log aggregation and query strategies
  • Shows structured hypothesis formation and testing
  • Discussed post-incident improvements to observability or testing

Negative indicators

  • Blamed other services without investigating their own
  • No systematic approach, just tried things randomly
  • Fixed symptom without understanding root cause

Attitude Questions

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Active Listening

The disciplined cognitive practice of fully concentrating on, comprehending, and retaining communicated information while suspending premature judgment, formulation of response, or solution generation. In technical contexts, this involves parsing both explicit content and implicit signals—detecting gaps between stated and intended meaning, recognizing emotional or political subtext, and verifying understanding through calibrated inquiry—particularly when bridging semantic divides between technical and non-technical stakeholders, or when extracting tacit knowledge that resists formal documentation.

Interview round: Hiring Manager System Design

You're in a squad planning session where a designer is presenting a complex interaction flow. Halfway through, you realize you're not tracking with their reasoning. What do you do?

Positive indicators

  • Proposes brief pause to align
  • Uses paraphrasing technique
  • Offers to continue async if time-constrained
  • Checks understanding with specific questions

Negative indicators

  • Nods along without understanding
  • Waits until implementation to discover gaps
  • Interrupts dismissively or defensively
  • Expects designer to notice their confusion

Progression Framework

This table shows how competencies evolve across experience levels. Each cell shows competency at that level.

Application Engineering & Architecture

5 competencies

CompetencyJuniorMidSeniorPrincipal
Backend API & Service Development

Implements simple REST endpoints following existing schemas; handles basic request validation and error responses.

Designs RESTful/GraphQL APIs with proper authentication and authorization; implements middleware for logging, security, and request validation; manages database transactions and service integrations.

Architects microservices boundaries and API gateways; designs rate limiting and throttling strategies; leads API versioning strategies.

Defines service-oriented and event-driven architecture standards; establishes enterprise API governance and developer portal strategies.

Core Programming & Logic Implementation

Writes functional code following established style guides and patterns; debugs syntax and logical errors with mentorship; executes unit tests for assigned features.

Independently develops complex business logic and features; performs code refactoring for performance and clarity; conducts peer code reviews to ensure maintainability and adherence to standards.

Designs reusable libraries and internal frameworks; establishes coding standards and best practices; architects complex algorithmic solutions.

Defines organization-wide language strategies and paradigms; leads compiler optimization and runtime efficiency initiatives; influences industry standards.

Data Persistence & Schema Design

Writes basic CRUD queries; follows existing schema conventions; uses ORM for simple data retrieval.

Designs normalized relational schemas that balance integrity and performance; implements backward-compatible database migrations; optimizes query performance through strategic indexing and query refactoring.

Architects data sharding and replication strategies; implements event sourcing and CQRS patterns; designs multi-model data solutions.

Sets organization-wide data storage strategy; evaluates distributed databases (CockroachDB, Spanner); drives data consistency models.

Distributed Systems & Communication Patterns

Consumes existing message queue endpoints; handles basic asynchronous operations; understands idempotency concepts.

Implements pub/sub patterns and background job processors for asynchronous workflows; manages circuit breakers and retry logic for resilience; handles distributed caching strategies.

Designs event-driven architectures and saga patterns; implements distributed tracing; manages consistency in distributed transactions.

Establishes distributed systems standards and patterns; researches consensus algorithms; designs multi-region active-active systems.

Frontend Engineering & UI Development

Implements UI components from design specifications; fixes styling and layout issues; learns framework-specific state management basics.

Builds complex interactive features with state management; optimizes bundle size and rendering performance to meet performance budgets; implements responsive and accessible designs adhering to WCAG standards.

Architects frontend state management and component libraries; establishes accessibility (WCAG) and security (CSP) standards; leads frontend migrations.

Sets frontend technology strategy across web and mobile platforms; drives adoption of micro-frontends and edge computing; evaluates emerging frameworks.

Platform Operations & Specialized Systems

6 competencies

CompetencyJuniorMidSeniorPrincipal
AI/ML System Integration

Integrates pre-trained model APIs into applications; handles basic input preprocessing and output formatting.

Deploys custom models to production environments; implements A/B testing frameworks for model variants; manages feature engineering pipelines and prompt engineering workflows.

Architects scalable model serving infrastructure; optimizes inference latency and cost; implements ML monitoring for drift and bias.

Establishes enterprise AI platforms and governance; drives MLOps maturity; ensures ethical AI practices and model version control.

Cloud Infrastructure & Automation

Deploys to pre-configured cloud environments; modifies existing Terraform/CloudFormation templates; monitors basic resource metrics.

Builds CI/CD pipelines with automated testing and deployment gates; manages container orchestration (Kubernetes) and auto-scaling policies; implements infrastructure monitoring and environment parity.

Designs multi-region, highly available infrastructure; implements disaster recovery and backup strategies; optimizes cloud costs and security posture.

Establishes cloud governance frameworks and FinOps practices; drives platform engineering initiatives; evaluates serverless and edge computing strategies.

Data Pipeline Engineering

Executes existing batch jobs; monitors pipeline failures; performs basic data validation checks.

Develops new data transformations and schema evolution handling for analytics; implements incremental loading patterns; optimizes pipeline performance for cost and latency.

Architects streaming data platforms and real-time analytics; ensures data quality, lineage, and GDPR compliance; designs lakehouse architectures.

Establishes data platform strategy and governance; drives data mesh implementations; evaluates novel storage formats (Iceberg/Delta Lake).

Financial Systems Engineering

Implements basic transaction recording and retrieval; ensures idempotency in payment operations; handles currency formatting.

Designs double-entry ledger systems for money movements; implements reconciliation and audit trail mechanisms; manages PCI-DSS compliance requirements for payment processing.

Architects high-volume payment processing systems; designs eventual consistency models for financial data; ensures SOX compliance.

Defines financial architecture standards and ledger strategies; drives real-time payment innovations; establishes fintech partnership integrations.

Observability & System Reliability

Monitors dashboards and escalates alerts; follows runbooks for common incidents; documents system status.

Configures monitoring agents, metrics dashboards, and log aggregation systems; participates actively in on-call rotations and incident response; performs root cause analysis to prevent recurrence.

Designs SLO/SLI frameworks and error budgets; leads blameless post-mortems; implements chaos engineering and automated remediation.

Establishes organizational reliability standards and SRE culture; architects global observability platforms; drives zero-downtime migration strategies.

Technical Leadership & Communication

Documents code and APIs clearly; participates in agile ceremonies; seeks feedback on implementation approaches.

Mentors junior developers through pair programming and constructive code reviews; leads small technical initiatives and spikes; writes Request for Comments (RFCs) and architecture design docs.

Leads cross-functional projects and architecture reviews; drives ADRs and technical roadmaps; manages stakeholder expectations.

Influences executive technical strategy and budget allocation; builds external engineering brand through speaking/writing; establishes mentorship programs.