Performance Profiling

Investigate and resolve performance issues using .NET Aspire distributed tracing. Analyzes trace span patterns to identify N+1 queries, excessive aggregate rehydration, and missing projections.

When to Use

• An endpoint feels slow or returns high latency
• You suspect N+1 database/storage reads
• After adding new data access code and want to verify efficiency
• Comparing before/after performance of an optimization

Prerequisites

• Aspire AppHost running (all resources healthy)
• Aspire CLI 13.2+ — use ~/.dotnet/tools/aspire (13.1.1 at ~/.aspire/bin/ may shadow it on PATH)

Instructions for Claude

Phase 1: Verify Environment

Check the Aspire CLI and running AppHost:

~/.dotnet/tools/aspire ps --format Json

Optionally run ~/.dotnet/tools/aspire doctor (note: --format Json is not supported for doctor — it renders a terminal UI).

If no AppHost is running, tell the user and stop.

List resources and confirm the target (usually api) is Running/Healthy:

~/.dotnet/tools/aspire describe --format Json

If unhealthy, check console logs:

~/.dotnet/tools/aspire logs <resource> --format Json -n 50

Phase 2: Generate Traces

If the user provided a specific endpoint or URL, exercise it to generate a trace:

# Login (if auth required) and hit the endpoint
curl -s -c /tmp/perf-cookies.txt -X POST 'http://localhost:5132/api/auth/dev-login' \
  -H 'Content-Type: application/json' -d '{"isAdmin":true}'

curl -s -b /tmp/perf-cookies.txt -D /tmp/perf-headers.txt \
  'http://localhost:5132/api/<endpoint>' \
  -o /dev/null -w "Status: %{http_code}, Time: %{time_total}s\n"

Extract the trace ID from the response headers:

grep -i traceparent /tmp/perf-headers.txt
# Format: 00-{traceId}-{spanId}-{flags}

If no specific endpoint was given, list recent traces and pick the slowest:

~/.dotnet/tools/aspire otel traces api --format Json -n 10

Phase 3: Analyze Trace

Drill into the trace — spans and logs separately:

~/.dotnet/tools/aspire otel spans api --trace-id <trace-id> --format Json
~/.dotnet/tools/aspire otel logs api --trace-id <trace-id> --format Json

Count and categorize the spans. See trace-patterns.md for the pattern reference.

Key metrics to extract:

• Total span count
• Total duration
• Number of storage reads (blob GET/HEAD operations)
• Number of unique aggregates loaded
• Number of projection reads

Phase 4: Diagnose

Map spans back to code. Common patterns to look for:

Span pattern	Diagnosis	Fix
3 spans per aggregate (HEAD + GET doc + GET events)	Aggregate rehydration	Use projection if only reading
Same aggregate loaded multiple times	Duplicate rehydration	Cache or restructure the call chain
30+ spans for a single request	N+1 — loading aggregates in a loop	Replace with projection or batch query
Many GetAll* or GetBy* factory calls	Iterating all streams	Add a projection with an index
2 spans (HEAD + GET on projections/*.json)	Projection read (efficient)	This is good — no action needed
1 span (GET tags/document/*.json)	Tag-based lookup (efficient)	This is good — no action needed

Read the relevant endpoint code to confirm which calls produce the excess spans.

Phase 5: Recommend

Present findings to the user:

PERFORMANCE ANALYSIS — {endpoint}
==================================

Request:    {method} {url}
Duration:   {time}s
Spans:      {count} ({breakdown})

Diagnosis:
  {description of the bottleneck}

Bottleneck Code:
  {file}:{line} — {description of the problematic call}

Recommendation:
  {specific fix — e.g., "Replace factory.GetByXAsync() with projection lookup"}

Expected Improvement:
  Spans: {current} → ~{expected}
  Reason: {why this reduces spans}

Phase 6: Verify Fix (if user applies the fix)

After code changes:

1. Restart the resource and wait for healthy state:

   ~/.dotnet/tools/aspire resource api restart
   ~/.dotnet/tools/aspire wait api --status healthy
   

2. Re-exercise the same endpoint (Phase 2).

3. Pull the new trace and compare:

   Before: {X} spans, {Y}s
   After:  {X} spans, {Y}s
   Improvement: {reduction}
   

4. If spans are still high, repeat from Phase 3.

Flags

Flag	Behavior
(no flag)	Full profiling workflow — trace, analyze, recommend
--compare	Re-run a previous trace comparison after a fix

Alternative: Browser-Driven Trace Generation

If you need to exercise the full frontend→API flow (e.g., to catch middleware, auth, or CORS spans that don't fire from curl), use playwright-cli to drive a real browser session:

playwright-cli open <frontend-url>
# Navigate to login page and authenticate if needed
playwright-cli goto <frontend-url>/<target-page>
playwright-cli close

Then pull the trace from Aspire as in Phase 3.

Tips

• zsh URL quoting: Always single-quote URLs with ? in curl to avoid zsh glob expansion:

  curl -s -b /tmp/perf-cookies.txt 'http://localhost:5132/api/events?distance=50'
  

Supporting Files

• trace-patterns.md — Detailed span pattern reference for event sourcing projects