Azure Bicep Patterns

Azure Bicep Patterns Skill

Reusable infrastructure patterns for Azure Bicep templates. Complements iac-bicep-best-practices.instructions.md (style) and azure-defaults skill (naming, tags, regions).

Canonical sources — the security baseline, AVM-first mandate, naming conventions, required tags, and unique-suffix rule live in azure-defaults/SKILL.md and iac-policy-compliance.md. This skill restates the rules tersely below for IaC-output convenience only; in conflict, the canonical sources win.

Quick Reference

Canonical Example — Module Interface

Every Bicep module in this repo follows the same input/output contract:

• Inputs (required): name, location, tags, logAnalyticsWorkspaceName
• Outputs (required): resourceId, resourceName, principalId (use .?principalId ?? '' so modules without managed identity still expose the output)

Full code sample and rationale: references/module-interface.md.

Steps

Applying a pattern in a Bicep template:

1. Identify the pattern — match your need to a row in Quick Reference (hub-spoke, private endpoint, diagnostics, conditional, identity, budget)
2. Load the reference — read the linked references/*.md for the chosen pattern; do not load all at once
3. Compose the module — follow the Module Interface contract above (name/location/tags/logAnalyticsWorkspaceName in; resourceId/resourceName/principalId out)
4. Pin AVM versions — query MCR at plan time and follow the azure-defaults live-resolution and exception policy
5. Add diagnostics + budget — every deployed resource gets diagnostics; every deployment implements the governed cost-monitoring contract
6. What-if before deploy — run az deployment group what-if and review for unexpected deletes, SKU downgrades, or auth changes
7. Validate — bicep build + bicep lint + npm run validate:iac-security-baseline

Rules

• Hub-Spoke: Hub holds shared infra; spokes peer to hub only; NSGs per subnet
• Private Endpoints: Always wire PE + DNS Zone Group + DNS Zone; see group ID table in reference
• Diagnostics: categoryGroup: 'allLogs' + AllMetrics; pass workspace name not ID
• Conditional: bool params with defaults; guard outputs with ternary
• Identity: guid() for idempotent role names; principalType: 'ServicePrincipal'; scope narrowly
• Budget: Follow the canonical cost-monitoring contract; amount and notification inputs MUST be parameters
• What-If: Run before every deploy; watch for unexpected deletes and SKU downgrades
• AVM Version Source of Truth: MCR tag listing (mcr.microsoft.com/v2/bicep/{module}/tags/list) is authoritative. Helpers and doc tables are NOT — they go stale. Validator: npm run validate:avm-versions.

Gotchas

• AVM output shapes vary across modules — Different AVM modules expose different outputs. Always check the module README before referencing outputs.
• Tag merging in AVM modules — Some AVM modules merge tags internally. Verify deployed tags include all required policy tags after deployment.
• What-If red flags — Watch for unexpected deletes, SKU downgrades, public access changes, authentication mode changes, or identity removal. Always run what-if before deploy.
• MCR version discovery — When AVM version helpers are incomplete, query mcr.microsoft.com/v2/bicep/{module}/tags/list for authoritative published versions.
• Cross-RG module scope: ARM ID split indexes — Splitting a full resource ID (/subscriptions/{sub}/resourceGroups/{rg}/providers/...) on / yields ['', 'subscriptions', '{sub}', 'resourceGroups', '{rg}', ...]. Subscription is at index 2, RG name is at index 4. Use: scope: resourceGroup(split(resId, '/')[2], split(resId, '/')[4]). Indexes [1]/[3] are the literals 'subscriptions'/'resourceGroups' and only fail at az deployment ... validate time.
• AVM insights/metric-alert:0.4.1+ requires criteria.allof[].name — Each entry in criteria.allOf[] needs a name field; omission passes bicep build but fails az deployment ... validate:

  allOf: [{
    name: 'HighCpu'        // REQUIRED
    metricName: 'Percentage CPU'
    operator: 'GreaterThan'
    threshold: 80
    timeAggregation: 'Average'
  }]
  

• bicep build poisons tree-hash folders — Running bicep build main.bicep emits main.json next to the source. If the folder is hashed by validate-iac-handoff.mjs, that compiled output drifts the hash. The validator now excludes <stem>.json siblings of <stem>.bicep, but if you must build manually inside a handoff tree, rm -f main.json immediately after.
• AKS outboundType must match the egress topology — a NAT Gateway on the node subnet requires userAssignedNATGateway, NOT userDefinedRouting (UDR needs a 0.0.0.0/0 route table and is rejected when public network access is restricted). BYO-VNet clusters also need Network Contributor on the VNet, and fixed-size node pools must set minCount/maxCount to null. See avm-pitfalls § AKS.
• MySQL Flexible Server: PE not delegated subnet; engine version 8.4 — on a shared PE subnet use privateEndpoints[] (not delegatedSubnetResourceId); pin version: '8.4' (latest GA LTS → 8.4.7), never the stale 8.0 or an innovation 9.x; aad_auth_only is read-only (enforce post-deploy). See avm-pitfalls § MySQL.

Reference Index

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