Backend System Design

Act as a staff backend engineer writing the system-design document for a new service, endpoint, or backend feature, so the architecture is decided and reviewable before any slicing or code.

Goal: Produce a 12-section backend system-design RFC for the active task, grounded in the task's decisions and constraints, and persist it as BACKEND_SYSTEM_DESIGN.md. The command is capability-routed: it designs backend systems on any stack (a monolith route, a service, a queue worker, a serverless function) and is not tied to one language or framework. It is the backend sibling of frontend-system-design, and it composes with slo-define, performance-budget, api-contract-review, migration-safety-steward, and release-plan rather than duplicating them.

Mandatory context bootstrap (before any output):

• Read these sections in WORKFLOW_OPERATING_SYSTEM.md first:
• ## LLM execution contract
• ## Editor mode policy (mode definitions only; the tool mapping table is lazy-loaded in wos/editor-mode-mappings.md and needed only for non-Claude-Code tools)
• ## Global output contract (including Adaptive handoff and Mode selection rule)
• ## Cross-cutting workflow guardrails
• Bootstrap tiers (ADR-0025): the light-weight commands (branch-commit, what-next, where-we-at, slice-closure, compact-task-memory) may skip ## Editor mode policy good-fits lists and ## Cross-cutting workflow guardrails sequencing heuristics, reading only the mode definitions and the core guardrail rules (routing memory, command-less input triage, official command names, material change, no-op). This reduces bootstrap from ~6,750 to ~3,500 tokens for these commands.
• Read additional sections only when relevant to this command's role.
• Read the commands/ directory command inventory to ensure command names and availability are current.
• Align all routing recommendations and next-command suggestions with the current command set.
• Official next-command names only: every recommended next command (including the handoff Run now line) MUST be the basename of an existing commands/.md file in this workflow repository. Never invent names.

Required inputs:

• active task folder path
• TASKSTATE.md, SOURCEOF_TRUTH.md, DECISIONS.md (the design must respect locked decisions)
• IMPACTANALYSIS.md and INVARIANTSANDNONGOALS.md when present
• the service, endpoint, or backend feature to design, named in the task or the prompt
• the expected scale signal when known (users, request rate, data volume, read-to-write ratio); mark unknowns rather than inventing them
• relevant external references already captured in projects/__/REFERENCES.md (read-only grounding)

Task repository files to create or update:

• BACKEND_SYSTEM_DESIGN.md in the active task folder: the 12-section RFC.

Operating rules:

• Do not implement product code. This command produces a design document, not a slice plan and not an implementation.
• Handoff: end with the adaptive ### Handoff block per WORKFLOW_OPERATING_SYSTEM.md ## Global output contract (Mode A compact or Mode B full).
• Capability-routed, not stack-locked. Design for the stack the task actually uses (read it from SOURCEOFTRUTH.md or DECISIONS.md). Name a specific database, queue, or runtime only when the task already chose it; never assume one. The structure below holds for any backend stack.
• Scale-honest, not scale-cargo-culted. Design for the task's real scale, not FAANG scale. A single Postgres instance is the correct answer for most solo and small-team work; reach for sharding, multi-region, or a message bus only when a stated requirement forces it. Do not import distributed-systems machinery a solo builder will never operate. When a number (users, request rate, data volume) is not known, mark it unknown and design for the plausible near-term, not an imagined peak.
• The 12 sections. Produce every section; mark a section not applicable with a one-line reason rather than dropping it:

1. Problem statement and context: the user or business problem this backend serves, scope boundaries, who calls it.
2. Requirements: functional and non-functional, split explicitly; core versus nice-to-have; success metrics; the expected scale (users, request rate, data volume, read-to-write ratio) or an explicit unknown.
3. High-level architecture: the services or components and their relationships (entry point, application layer, data layer, background workers), and the synchronous-versus-asynchronous boundaries. Name the trade-off pairs at play (see wos/architecture-tradeoffs.md).
4. Data model and storage: entities, the schema shape, the storage engine choice and why, indexing, and the consistency model. Compose with db-context-postgres or db-context-supabase when a real schema exists.
5. API and interface contract: transport (REST, RPC, GraphQL, or an event), payload shape, pagination, error and retry semantics, idempotency, and versioning. Compose with api-contract-review for the endpoint-level audit rather than duplicating it.
6. Caching and data access: the read and write paths, and the cache update strategy when a cache is in play, named from wos/cache-update-strategies.md (cache-aside, write-through, write-behind, refresh-ahead) with its failure mode guarded.
7. Scaling and bottlenecks: the likely bottleneck under the stated scale (the database, a hot path, an external call), whether the design scales vertically or horizontally, and what statelessness or partitioning that requires. Name the one bottleneck you would hit first and how you would relieve it.
8. Reliability and SLOs: the availability and latency targets, the failure modes, and the resilience posture (timeouts, retries with backoff, circuit breakers, idempotent retries). Compose with slo-define for the target artifact rather than restating it.
9. Security: authentication and authorization model, secret handling, input validation, data protection at rest and in transit, and multi-tenant isolation when relevant. Route deep review to security-review or rls-auth-boundary-auditor.
10. Observability and operations: the logs, metrics, and traces that make this service debuggable in production, the key alerts, and the one dashboard you would watch on launch day.
11. Rollout and migration: deploy strategy, feature flags, backward compatibility, and schema-migration safety. Compose with release-plan for the rollout and migration-safety-steward for any DDL rather than duplicating them.
12. Trade-offs and alternatives: the options considered and why the chosen design wins, citing the named axes in wos/architecture-tradeoffs.md. This is the section that separates a design from a policy statement; never leave it empty.

• Ground external contracts. When the design commits to an external library, SDK, datastore, or protocol, ground it in a captured REFERENCES.md entry; when it is not captured, name the gap and route to capture-references rather than designing the contract from memory. The captured entry wins over recollection (per WORKFLOW_OPERATING_SYSTEM.md ## Evidence priority).
• Respect locked decisions. Read DECISIONS.md; the design must not silently reopen a locked decision. When the design needs a decision that is not yet made, label it PROPOSED and route to decision-interview instead of asserting it.
• No invented metrics. Scale numbers, latency targets, availability SLOs, and cost figures must cite a source (a measured baseline, a published standard, or a user-supplied target) or be marked PROPOSED-pending-baseline.
• Treat task-memory write policy per WORKFLOW_OPERATING_SYSTEM.md: PROPOSED in Ask mode, APPLIED only in Agent mode (the design artifact follows the same policy).
• Self-review before emit. Before writing the file, check it for placeholders, contradictions, an empty trade-offs section, distributed-systems machinery unjustified by a stated requirement, and any section asserting a number without a source; fix them inline.

Required output:

1. The service, endpoint, or feature being designed and the stack read from task memory
2. The 12-section design, every section present
3. Exact BACKEND_SYSTEM_DESIGN.md content, marked PROPOSED or APPLIED per editor mode
4. Any PROPOSED decision the design surfaced, with the upstream command to lock it
5. Recommended next command
6. Recommended editor mode
7. Why that is the correct next step

Standard output layout (required)

Produce the command output using this structure (English only):

Artifact changes

Follow ## Global output contract in WORKFLOW_OPERATING_SYSTEM.md for APPLIED / PROPOSED / SKIP rules.

Command transcript

Brief audit trail (max 4 lines; max 3 in no-op runs with NO_OP_TRACE).

Handoff

Use the adaptive ending format from WORKFLOW_OPERATING_SYSTEM.md ## Global output contract (Mode A compact or Mode B full per session state).

Definition of done (command output)

• All 12 sections are present; a dropped section is invalid output unless marked not applicable with a reason.
• The trade-offs and alternatives section is non-empty and names the rejected options, citing the relevant axes in wos/architecture-tradeoffs.md.
• No scale, latency, or SLO number is asserted without a cited source or a PROPOSED-pending-baseline mark.
• The design is scale-honest: no sharding, multi-region, or message-bus machinery appears without a stated requirement forcing it.
• The design respects DECISIONS.md; any decision it needs but does not have is marked PROPOSED and routed to decision-interview, not asserted.
• The artifact is marked PROPOSED (Ask) or APPLIED (Agent).
• Output ends with a complete ### Handoff block per the adaptive format in WORKFLOW_OPERATING_SYSTEM.md ## Global output contract.
• Before declaring this output done, confirm it satisfies the shared Definition of done (command outputs) and Gate conditions in WORKFLOWOPERATINGSYSTEM.md.

Quality bar: A reviewer can read the document and understand the backend architecture, the data model, the contracts, the failure modes, and why this design beats the alternatives, with no number asserted on no evidence, no locked decision silently reopened, and no scale machinery the task does not need.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Mozurok
• Source: Mozurok/fhorja.dev
• License: MIT
• Homepage: https://fhorja.dev

Install and usage instructions live in the source repository linked above.