1## Common Errors
2 
3### "Logs not appearing"
4 
5**Cause:** Observability disabled, Worker not redeployed, no traffic, low sampling rate, or log size exceeds 256 KB
6**Solution:** 
7```bash
8# Verify config
9cat wrangler.jsonc | jq '.observability'
10 
11# Check deployment
12wrangler deployments list <WORKER_NAME>
13 
14# Test with curl
15curl https://your-worker.workers.dev
16```
17Ensure `observability.enabled = true`, redeploy Worker, check `head_sampling_rate`, verify traffic
18 
19### "Traces not being captured"
20 
21**Cause:** Traces not enabled, incorrect sampling rate, Worker not redeployed, or destination unavailable
22**Solution:**
23```jsonc
24// Temporarily set to 100% sampling for debugging
25{
26  "observability": {
27    "enabled": true,
28    "head_sampling_rate": 1.0,
29    "traces": {
30      "enabled": true
31    }
32  }
33}
34```
35Ensure `observability.traces.enabled = true`, set `head_sampling_rate` to 1.0 for testing, redeploy, check destination status
36 
37## Limits
38 
39| Resource/Limit | Value | Notes |
40|----------------|-------|-------|
41| Max log size | 256 KB | Logs exceeding this are truncated |
42| Default sampling rate | 1.0 (100%) | Reduce for high-traffic Workers |
43| Max destinations | Varies by plan | Check dashboard |
44| Trace context propagation | 100 spans max | Deep call chains may lose spans |
45| Analytics Engine write rate | 25 writes/request | Excess writes dropped silently |
46 
47## Performance Gotchas
48 
49### Spectre Mitigation Timing
50 
51**Problem:** `Date.now()` and `performance.now()` have reduced precision (coarsened to 100μs)
52**Cause:** Spectre vulnerability mitigation in V8
53**Solution:** Accept reduced precision or use Workers Traces for accurate timing
54```typescript
55// Date.now() is coarsened - trace spans are accurate
56export default {
57  async fetch(request: Request, env: Env, ctx: ExecutionContext): Promise<Response> {
58    // For user-facing timing, Date.now() is fine
59    const start = Date.now();
60    const response = await processRequest(request);
61    const duration = Date.now() - start;
62    
63    // For detailed performance analysis, use Workers Traces instead
64    return response;
65  }
66}
67```
68 
69### Analytics Engine _sample_interval Aggregation
70 
71**Problem:** Queries return incorrect totals when not multiplying by `_sample_interval`
72**Cause:** Analytics Engine stores sampled data points, each representing multiple events
73**Solution:** Always multiply counts/sums by `_sample_interval` in aggregations
74```sql
75-- WRONG: Undercounts actual events
76SELECT blob1 AS customer_id, COUNT(*) AS total_calls
77FROM api_usage GROUP BY customer_id;
78 
79-- CORRECT: Accounts for sampling
80SELECT blob1 AS customer_id, SUM(_sample_interval) AS total_calls
81FROM api_usage GROUP BY customer_id;
82```
83 
84### Trace Context Propagation Limits
85 
86**Problem:** Deep call chains lose trace context after 100 spans
87**Cause:** Cloudflare limits trace depth to prevent performance impact
88**Solution:** Design for flatter architectures or use custom correlation IDs for deep chains
89```typescript
90// For deep call chains, add custom correlation ID
91const correlationId = crypto.randomUUID();
92console.log({ correlationId, event: 'request_start' });
93 
94// Pass correlationId through headers to downstream services
95await fetch('https://api.example.com', {
96  headers: { 'X-Correlation-ID': correlationId }
97});
98```
99 
100## Pricing (2026)
101 
102### Workers Traces
103- **GA Pricing (starts March 1, 2026):**
104  - $0.10 per 1M trace spans captured
105  - Retention: 14 days included
106- **Free tier:** 10M trace spans/month
107- **Note:** Beta usage (before March 1, 2026) is free
108 
109### Workers Logs
110- **Included:** Free for all Workers
111- **Logpush:** Requires Business/Enterprise plan
112 
113### Analytics Engine
114- **Included:** 10M writes/month on Paid Workers plan
115- **Additional:** $0.25 per 1M writes beyond included quota
116