Model of Patroni Passive Failure

RTO Timeline

Failure Model

Key Difference Between Passive and Active Failover:

During passive failure, Patroni stops with the node and cannot release the Leader Key. An election waits until the DCS lease TTL expires.

Timeline Analysis

Phase 1: Lease Expiration

Primary Patroni renews the Leader Key each loop_wait cycle and resets its TTL.

Timeline:
     t-loop        t          t+ttl-loop    t+ttl
       |           |              |           |
    Last Refresh  Failure      Best Case   Worst Case
       |←── loop ──→|              |           |
       |←──────────── ttl ─────────────────────→|

• Best case: Failure occurs just before lease refresh (elapsed loop since last refresh), remaining TTL = ttl - loop
• Worst case: Failure occurs right after lease refresh, must wait full ttl
• Average case: ttl - loop/2

Phase 2: Replica Detection

Each loop_wait cycle, replicas check the DCS Leader Key.

Timeline:
    Lease Expired   Replica Wakes
       |            |
       |←── 0~loop ─→|

• Best case: Replica happens to wake when lease expires, wait 0
• Worst case: Replica just entered sleep when lease expires, wait loop
• Average case: loop/2

Phase 3: Lock Contest & Promote

After detecting Leader Key expiration, replicas begin an election. The winner acquires the key and runs pg_ctl promote.

1. Query replica replication positions in parallel through REST APIs. Typical response time is 10ms; the hardcoded timeout is 2s.
2. Compare WAL positions and select the best candidate. Replicas attempt atomic CAS creation of the Leader Key.
3. Run pg_ctl promote. Its duration is usually negligible.

Election Flow:
  ReplicaA ──→ Query replication position ──→ Compare ──→ Contest lock ──→ Success
  ReplicaB ──→ Query replication position ──→ Compare ──→ Contest lock ──→ Fail

• Best case: Single replica or immediate lock acquisition and promotion, constant overhead 0.1s
• Worst case: DCS API call timeout: 2s
• Average case: 1s constant overhead

Phase 4: Health Check

HAProxy requires rise consecutive successful checks before routing traffic to the new primary.

Detection Timeline:
  New Primary    First Check   Second Check  Third Check (UP)
     |          |           |           |
     |←─ 0~inter ─→|←─ fast ─→|←─ fast ─→|

• Best case: New primary promoted just before check, (rise-1) × fastinter
• Worst case: New primary promoted right after check, (rise-1) × fastinter + inter
• Average case: (rise-1) × fastinter + inter/2

RTO Formula

Add the phase durations to calculate total RTO:

Best Case

Average Case

Worst Case

Model Calculation

Apply the four RTO presets to these formulas:

pg_rto_plan:  # [ttl, loop, retry, start, margin, inter, fastinter, downinter, rise, fall]
  fast: [ 20  ,5  ,5  ,15 ,5  ,'1s' ,'500ms'  ,'1s' ,3 ,3 ]  # rto < 30s
  norm: [ 30  ,5  ,10 ,25 ,5  ,'2s' ,'1s'     ,'2s' ,3 ,3 ]  # rto < 45s
  safe: [ 60  ,10 ,20 ,45 ,10 ,'3s' ,'1500ms' ,'3s' ,3 ,3 ]  # rto < 90s
  wide: [ 120 ,20 ,30 ,95 ,15 ,'4s' ,'2s'     ,'4s' ,3 ,3 ]  # rto < 150s

Four Mode Calculation Results (unit: seconds, format: min / avg / max)