# Pattern: ADB-S High Availability
# Composable architecture block for Autonomous Database Serverless HA

pattern:
  name: "ADB-S High Availability"
  id: database_ha_adb_s
  category: database-ha

description: |
  Autonomous Database Serverless (ADB-S) provides built-in high availability through
  Transparent Application Continuity (TAC) and automated failover. TAC replays in-flight
  transactions transparently after infrastructure failures, masking outages from applications.
  For additional protection, Autonomous Data Guard provides a same-region standby with
  synchronous replication for zero data loss.

variants:
  - id: local_ha
    name: "Built-in Local HA (TAC)"
    topology: "Same-region, ADB-S infrastructure-managed failover"
    rpo: "0"
    rto: "~0 perceived (transparent replay via TAC)"
    use_when:
      - "Standard HA for ADB-S workloads"
      - "Application can tolerate brief transparent replay"
      - "No separate standby database required"

  - id: local_standby
    name: "Same-Region Autonomous Data Guard Standby"
    topology: "Primary + Standby in same region via Autonomous Data Guard"
    rpo: "0 (synchronous replication)"
    rto: "~2 minutes (automatic switchover)"
    use_when:
      - "Zero RPO mandatory"
      - "Protection against full AD or FD failure"
      - "Read-only offload to standby needed"

preconditions:
  - "ADB-S instance provisioned (Shared infrastructure)"
  - "Private endpoint recommended for production workloads"
  - "VCN with Service Gateway for secure OCI service access"
  - "Wallet or mTLS connectivity configured for clients"

implied_services:
  - service: service_gateway
    reason: "Secure access to OCI services without internet traversal"
  - service: oci_vault
    reason: "Customer-managed encryption keys for TDE"
  - service: data_safe
    reason: "Database security posture management, auditing, and masking"

gotchas:
  - "TAC replay does not cover all operations — UTL_HTTP calls, DBMS_PIPE, NOCACHE sequences, and external interactions are not replayed"
  - "Auto-scaling takes 2-3 minutes to add OCPUs; plan base capacity for burst tolerance"
  - "Autonomous Data Guard standby is read-only; cannot run DML on standby"
  - "Switchover requires brief downtime; failover is automatic on primary failure"
  - "TAC requires JDBC Thin driver 18c+ or ODP.NET with TAC enabled in connection string"

cost_impact:
  - "Built-in local HA (TAC): no additional cost, included in ADB-S"
  - "Autonomous Data Guard standby: billed at same compute (OCPU/ECPU) rate as primary"
  - "Standby storage is additional and billed at standard ADB-S storage rates"
  - "Auto-scaling can increase OCPU cost up to 3x base during peaks"

sizing_rules:
  - "Standby must be same shape and OCPU/ECPU count as primary for seamless failover"
  - "Storage on standby mirrors primary automatically"
  - "Base OCPU count should handle normal workload without relying on auto-scaling for HA events"

references:
  - "https://docs.oracle.com/en/cloud/paas/autonomous-database/adbsa/autonomous-data-guard.html"
  - "https://docs.oracle.com/en/cloud/paas/autonomous-database/serverless/adbsb/application-continuity-autonomous.html"