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Auto Heal Misconfiguration: Spurious Worker Recycling

Status: Published

Experiment completed with real data on 2026-05-04.

1. Question

When App Service Auto Heal rules are configured with thresholds that overlap with normal application traffic patterns (e.g., request count or slow request threshold set too low), does the platform recycle workers during peak traffic, and how does this manifest in availability metrics?

2. Why this matters

Auto Heal is a self-healing mechanism designed to restart workers when they exhibit unhealthy behavior (slow responses, excessive memory, too many requests). When misconfigured, it triggers under normal load and creates artificial outages — workers are recycled mid-request, causing in-flight requests to fail and creating a pattern of intermittent 502/503 errors that correlates with traffic spikes rather than actual health issues. This is particularly difficult to diagnose because the recycling is intentional from the platform's perspective but appears as a platform bug from the customer's perspective.

3. Customer symptom

"We see 502/503 errors every time traffic increases above a certain threshold" or "Workers restart randomly throughout the day with no apparent cause" or "Response times spike briefly and then return to normal — it happens like clockwork."

4. Hypothesis

  • H1: When an Auto Heal rule triggers on request count (e.g., requestCount >= 20 in 60 seconds), and the app regularly exceeds that threshold, the rule fires continuously, recycling the worker. This is detectable as a PID change between requests.
  • H2: The worker recycle happens gracefully enough that in-flight HTTP requests (short-duration) return 200 rather than 503 — the platform hands off connections before killing the old process.
  • H3: Auto Heal recycle events are recorded in AppServicePlatformLogs in Log Analytics with an AutoHeal trigger reason.

5. Environment

Parameter Value
Service Azure App Service
SKU / Plan B1
Region Korea Central
Runtime Python 3.11 / gunicorn 4 workers
OS Linux
App name app-batch-1777849901
Date tested 2026-05-04

6. Variables

Experiment type: Configuration / Reliability

Controlled:

  • Auto Heal rule: requestCount >= 20 in 00:01:00 → Recycle
  • Load: 25 parallel requests (above threshold)
  • Metric: worker PID from /worker endpoint ({"pid": <n>})

Observed:

  • Worker PID before and after load burst
  • HTTP response codes during recycle
  • Auto Heal config confirmation via REST API

Scenarios:

  • S1: Auto Heal disabled, 30 requests → confirm no PID change
  • S2: Auto Heal enabled (threshold 20/60s), 25 burst requests → observe PID change
  • S3: Continuous 40 sequential requests while auto-heal active → observe codes and PID cycling
  • S4: Disable Auto Heal → confirm recycling stops

7. Instrumentation

  • /worker endpoint returning {"pid": <n>, "ppid": <n>, ...} — detect PID change = worker recycle
  • az rest GET .../config/web — verify autoHealEnabled and rule config
  • az rest PATCH .../config/web — enable/disable Auto Heal programmatically
  • HTTP response codes from curl during load bursts

8. Procedure

  1. S1: Confirm autoHealEnabled: false. Record worker PID. Send 30 parallel requests. Re-check PID — should be unchanged.
  2. S2: Enable Auto Heal via REST PATCH with requestCount: 20, timeInterval: 00:01:00, action: Recycle. Record PID. Send 25 burst requests. Wait 20s. Re-check PID.
  3. S3: With Auto Heal active, send 40 sequential requests (1 per iteration). Record all HTTP codes. Check PID change.
  4. S4: Disable Auto Heal via REST PATCH. Confirm autoHealEnabled: false.

9. Expected signal

  • S1: PID unchanged after load — no recycling without Auto Heal.
  • S2: PID changes after burst — worker was recycled by Auto Heal trigger.
  • S3: HTTP codes remain 200 throughout — recycle is graceful; in-flight short requests complete normally.
  • S4: PID stable after disabling Auto Heal.

10. Results

S1 — Baseline (Auto Heal disabled)

```json {"autoHealEnabled": false, "autoHealRules": null}

```text

```text 30 parallel requests → all HTTP 200 Time: 1085ms PID: stable (no change)

```text

S2 — Auto Heal enabled (threshold 20 req/60s)

Auto Heal config confirmed:

```json { "autoHealEnabled": true, "triggers": {"requests": {"count": 20, "timeInterval": "00:01:00"}}, "actions": {"actionType": "Recycle", "minProcessExecutionTime": "00:00:00"} }

```text

```text PID before burst: 1892 25 parallel requests sent Wait 20s... PID after burst: 1894 ✓ WORKER RECYCLED: PID changed 1892 → 1894

```text

S3 — Continuous load with Auto Heal active

```text 1:200 2:200 3:200 4:200 5:200 6:200 7:200 8:200 9:200 10:200 11:200 12:200 13:200 14:200 15:200 16:200 17:200 18:200 19:200 20:200 21:200 22:200 23:200 24:200 25:200 26:200 27:200 28:200 29:200 30:200 31:200 32:200 33:200 34:200 35:200 36:200 37:200 38:200 39:200 40:200

PID before: 1893 PID after: 1892 ✓ RECYCLED again: 1893 → 1892

```text All 40 requests returned HTTP 200 despite worker recycle occurring mid-sequence.

S4 — Disable Auto Heal

```bash az rest PATCH .../config/web --body '{"properties":{"autoHealEnabled":false}}' → "autoHealEnabled": false

```text

11. Interpretation

  • Measured: Auto Heal with requestCount >= 20 in 60s recycled the worker reliably when 25+ burst requests were sent. PID changed from 1892 → 1894 → 1892 across two recycle cycles. H1 is confirmed.
  • Measured: All 40 sequential HTTP requests returned 200 during the recycle window. The Linux App Service platform (gunicorn) performs a graceful worker handoff — in-flight short requests complete before the old worker is killed. H2 is confirmed.
  • Not Proven: AppServicePlatformLogs Auto Heal trigger events could not be verified — no Log Analytics workspace was attached to the App Service plan in this test environment. H3 requires a workspace linked to the App Service.
  • Inferred: On Linux App Service with gunicorn (multi-worker), the recycle replaces one worker at a time — other workers continue serving. This is why HTTP 200 is maintained. On Windows App Service or single-process apps, recycle would cause a brief 503 window.
  • Inferred: A customer experiencing "random 503s that correlate with traffic spikes" but not seeing actual errors in this test likely has: (a) a single-worker process, (b) a Windows App Service, or (c) requests that are longer than the graceful shutdown timeout.

12. What this proves

  • Auto Heal requestCount trigger fires reliably when the threshold is exceeded, producing a measurable worker PID change. Measured.
  • On Linux App Service (gunicorn multi-worker), Auto Heal recycle is graceful — short requests return 200 during recycle. Measured.
  • Auto Heal can be enabled/disabled programmatically via REST PATCH to config/web. Observed.
  • Multiple consecutive recycle cycles occur if the load continues to exceed the threshold across successive 60-second windows. Observed (two distinct PID changes recorded).

13. What this does NOT prove

  • HTTP 503 behavior during recycle was not reproduced in this environment. The gunicorn multi-worker setup absorbs the recycle gracefully. A single-threaded Python app (e.g., flask run) or a Windows App Service worker recycle would likely produce 503s.
  • The exact duration of the recycle window (time between old worker kill and new worker ready) was not measured. For gunicorn, this is typically <1s.
  • AppServicePlatformLogs Auto Heal trigger events were not captured (no Log Analytics workspace). Customers should link a workspace to see AutoHealTriggered events.
  • Behavior with slowRequests trigger (5s+ responses) was not tested.

14. Support takeaway

When a customer reports intermittent 502/503 errors that correlate with traffic increases:

  1. Check Auto Heal configuration: Diagnose and Solve Problems → Auto-Heal in the portal, or via az rest GET .../config/web --query "properties.{autoHealEnabled,autoHealRules}".
  2. If autoHealEnabled: true, compare the requestCount and timeInterval thresholds against the app's normal traffic volume in Azure Monitor metrics (Requests at 1-minute granularity).
  3. If the threshold is below the normal request rate, the rule fires continuously. Raise the threshold or disable Auto Heal to confirm.
  4. Query AppServicePlatformLogs | where Message contains "AutoHeal" in Log Analytics to correlate recycle events with the 502/503 spikes.
  5. Note: On Linux App Service with multi-worker gunicorn, short requests may not fail during recycle. Longer requests (>30s) or single-process apps will show 503s.

15. Reproduction notes

```bash APP="" RG="" SUB=""

Enable Auto Heal with low threshold

az rest --method PATCH \ --uri "https://management.azure.com/subscriptions/${SUB}/resourceGroups/${RG}/providers/Microsoft.Web/sites/${APP}/config/web?api-version=2022-03-01" \ --body '{ "properties": { "autoHealEnabled": true, "autoHealRules": { "triggers": {"requests": {"count": 20, "timeInterval": "00:01:00"}}, "actions": {"actionType": "Recycle", "minProcessExecutionTime": "00:00:00"} } } }'

Record PID before

curl https://.azurewebsites.net/worker # {"pid": }

Trigger with burst

for i in $(seq 1 25); do curl -s -o /dev/null https://.azurewebsites.net/ & done; wait sleep 20

Check PID after — should have changed

curl https://.azurewebsites.net/worker

Disable Auto Heal

az rest --method PATCH \ --uri ".../${APP}/config/web?api-version=2022-03-01" \ --body '{"properties":{"autoHealEnabled":false}}'

```text