Observability

Arc provides three observability subsystems: Prometheus metrics, structured NDJSON access logs, and W3C distributed tracing.

Metrics

Configuration

observability:
  metrics_bind: "127.0.0.1:9090"
  metrics_enabled: true

Endpoints

Endpoint	Method	Description
`/metrics`	`GET`	Prometheus text format
`/healthz`	`GET`	Returns `ok`

Access control: if no auth_token is configured, only loopback addresses (127.x, ::1) are allowed. If auth_token is set, the Authorization: Bearer <token> header is required from any IP.

Prometheus metric reference

Connection metrics:

Metric	Type	Description
`arc_accepted_total`	counter	Total accepted TCP connections
`arc_accept_rejected_total`	counter	Connections rejected at accept
`arc_active_current`	gauge	Currently active connections
`arc_closed_total`	counter	Total closed connections

Request/response metrics:

Metric	Type	Description
`arc_requests_total`	counter	Total HTTP requests received
`arc_responses_total`	counter	Total HTTP responses sent

Bandwidth:

Metric	Type	Description
`arc_bytes_client_in_total`	counter	Bytes received from clients
`arc_bytes_client_out_total`	counter	Bytes sent to clients
`arc_bytes_upstream_in_total`	counter	Bytes received from upstreams
`arc_bytes_upstream_out_total`	counter	Bytes sent to upstreams

Phase timing (for each of cli_read, up_conn, up_write, up_read, cli_write):

Metric	Type	Description
`arc_phase_time_sum_ns_<phase>`	counter	Cumulative nanoseconds in phase
`arc_phase_count_<phase>`	counter	Phase observations
`arc_phase_timeouts_<phase>`	counter	Phase timeout count

io_uring health:

Metric	Type	Description
`arc_ring_sq_dropped_total`	counter	Submission queue drops
`arc_ring_cq_overflow_total`	counter	Completion queue overflows

Non-zero values in arc_ring_sq_dropped_total or arc_ring_cq_overflow_total indicate the io_uring ring is too small. Increase io_uring.uring_entries in your configuration.

Upstream pool:

Metric	Type	Description
`arc_upstream_pool_open_current`	gauge	Total open upstream connections
`arc_upstream_pool_idle_current`	gauge	Idle upstream connections
`arc_upstream_pool_busy_current`	gauge	In-use upstream connections

Traffic mirroring:

Metric	Type	Description
`arc_mirror_submitted_total`	counter	Mirror tasks enqueued
`arc_mirror_sent_total`	counter	Mirror requests sent
`arc_mirror_queue_full_total`	counter	Mirror tasks dropped (queue full)
`arc_mirror_timeout_total`	counter	Mirror tasks dropped (timeout)
`arc_mirror_status_2xx_total`	counter	Shadow 2xx responses
`arc_mirror_latency_sum_ns`	counter	Total shadow latency

Logging subsystem:

Metric	Type	Description
`arc_log_written_total{kind=...}`	counter	Written log records by kind
`arc_log_dropped_total{reason=...}`	counter	Dropped records by reason
`arc_log_write_errors_total`	counter	Write failures
`arc_log_buffer_depth`	gauge	Ring buffer occupancy
`arc_log_write_duration_seconds`	histogram	io_uring batch write latency

Rate limiting:

Metric	Type	Description
`arc_ratelimit_rejected_total`	counter	Requests rejected by rate limiter
`arc_ratelimit_circuit_open`	gauge	`1` when Redis backend circuit breaker is open

Config and routing:

Metric	Type	Description
`arc_config_reload_total`	counter	Successful hot reloads applied
`arc_config_reload_error_total`	counter	Hot reloads rejected
`arc_route_requests_total{route="..."}`	counter	Requests matched per named route

TLS and plugins:

Metric	Type	Description
`arc_tls_cert_expiry_seconds`	gauge	Seconds until the nearest certificate expiry
`arc_plugin_timeout_total`	counter	Plugin invocations interrupted by budget timeout

XDP:

Metric	Type	Description
`arc_xdp_blacklisted_total`	counter	IPs added to the XDP blacklist (dynamic + manual)

Design

Workers write AtomicU64::fetch_add with Ordering::Relaxed — one instruction, no locking. WorkerMetrics is #[repr(C, align(64))] so each worker’s struct occupies its own cache line. The admin server reads metric snapshots from a background thread that refreshes every 250ms.

Access logs

Arc writes structured NDJSON access logs. There is no text-format option.

Configuration

logging:
  output:
    file: /var/log/arc/access.log
    stdout: false
    rotation:
      max_size: 500mb
      max_files: 30
      compress: true

  access:
    sample: 0.01             # log 1% of requests
    force_on_status: [401, 403, 429, 500, 502, 503, 504]
    force_on_slow: 500       # always log requests > 500ms

  redact:
    headers: [Authorization, Cookie, X-Api-Key]
    query_params: [token, secret, password]

  writer:
    ring_capacity: 8192      # SPSC ring buffer size per worker
    batch_bytes: 262144      # 256 KB — flush when batch reaches this size
    flush_interval: 50       # ms — time-based flush

Log record fields

Each access log line is a single JSON object:

Field	Type	Description
`ts`	string (RFC 3339, nanoseconds)	Request timestamp
`level`	string	`info`, `warn`, or `error`
`kind`	string	`"access"`
`trace_id`	string	W3C trace ID (32 hex chars)
`span_id`	string	W3C span ID (16 hex chars)
`request_id`	string	Per-request unique ID
`method`	string	HTTP method
`path`	string	Request path
`query`	string	Query string (redaction applied)
`host`	string	Host header
`status`	number	HTTP response status
`route`	string	Matched route name
`upstream`	string	Upstream group name
`upstream_addr`	string	Resolved upstream address
`client_ip`	string	Client remote IP
`client_port`	number	Client remote port
`bytes_sent`	number	Response bytes sent
`bytes_received`	number	Request bytes received
`duration_ms`	number	Total request duration
`upstream_connect_ms`	number?	Time to upstream TCP connect
`upstream_response_ms`	number?	Time to first upstream byte
`attempt`	number	Retry attempt number
`tls`	boolean	Whether connection used TLS
`http_version`	string	`HTTP/1.1` or `HTTP/2.0`

Example:

{
  "ts": "2026-03-02T10:45:23.123456789Z",
  "level": "info",
  "kind": "access",
  "trace_id": "4bf92f3577b34da6a3ce929d0e0e4736",
  "span_id": "00f067aa0ba902b7",
  "request_id": "01HRXYZ...",
  "method": "GET",
  "path": "/api/users/42",
  "query": "",
  "host": "api.example.com",
  "status": 200,
  "route": "api",
  "upstream": "app",
  "upstream_addr": "10.0.1.1:3000",
  "client_ip": "203.0.113.5",
  "client_port": 54321,
  "bytes_sent": 1024,
  "bytes_received": 256,
  "duration_ms": 12,
  "upstream_connect_ms": 1,
  "upstream_response_ms": 8,
  "attempt": 1,
  "tls": true,
  "http_version": "HTTP/2.0"
}

Redaction

Arc redacts sensitive values before writing logs:

Headers — matched case-insensitively; values replaced with [REDACTED]
Query parameters — matched case-insensitively; values replaced with [REDACTED]
Body fields — JSONPath-style ($.field.subfield); values replaced with "[REDACTED]"

Redaction rules are rebuilt automatically on hot reload.

File rotation

Rotation is triggered when the active log file exceeds max_size. The steps are:

Rename the active file to a timestamped archive name (near-instant metadata operation)
Reopen the active path immediately so writes continue uninterrupted
Compress the archive with gzip in a background thread (if compress: true)
Delete the oldest archives to keep only max_files retained

Design — no backpressure on workers

Workers push log events into a bounded SPSC ring buffer. If the ring is full, the event is dropped and a counter is incremented. Workers never block. The writer thread drains all rings, encodes NDJSON manually (no serde at write time), and flushes via io_uring Write operations in batches.

Distributed tracing

Arc implements W3C Trace Context. Every request gets a trace_id and span_id that appear in access logs and are propagated to upstreams.

How trace context is resolved

Situation	Behavior
Incoming request has a valid `traceparent`	Parse and reuse `trace_id`; use incoming `span_id` as the parent
`traceparent` is missing or malformed	Generate a fresh 128-bit `trace_id` and 64-bit `span_id`
Forwarding to an upstream	Keep the same `trace_id`; generate a new `span_id` for the child span
Logging	Emit `trace_id` and `span_id` as lowercase hex

`traceparent` format

Arc parses and emits the standard W3C format:

00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01

2-char version (00)
32-char trace ID (must not be all zeros)
16-char span ID (must not be all zeros)
2-char flags (01 = sampled)

Upstream propagation

When Arc forwards a request to an upstream, it injects a traceparent header with the same trace_id but a freshly generated span_id. This creates a parent-child span relationship between the Arc hop and the upstream.

Client → Arc:     traceparent: 00-TTTT-SSSS-01
Arc → Upstream:   traceparent: 00-TTTT-SNEW-01   (new span ID)
Access log:       trace_id=TTTT, span_id=SSSS     (the Arc span)

Searching logs by trace ID

# Using the arc CLI
arc logs query --trace-id 4bf92f3577b34da6a3ce929d0e0e4736

# Or with jq on the log file
grep "4bf92f3577b34da6a3ce929d0e0e4736" /var/log/arc/access.log | jq .

OTLP export

To export traces to an OpenTelemetry collector:

observability:
  tracing:
    endpoint: "http://otel-collector:4317"
    insecure: true    # disable TLS for the OTLP connection

Troubleshooting

/metrics returns 404 or connection refused

Check that observability.metrics_bind is set and that Arc is running. The default is 127.0.0.1:9090 — requests from other hosts will be refused unless you bind to 0.0.0.0:9090. Confirm with: curl http://127.0.0.1:9090/metrics.

Access logs are not appearing

Verify observability.access_log.enabled: true and that logging.output.file is set to a writable path. Also check that sample is not set to 0.0. Logs are written asynchronously via SPSC ring buffer — if the writer thread is behind, a short delay is normal.

Log file is not rotating

Rotation is triggered when FileState.offset >= RotationConfig.max_size_bytes. If the file never reaches that size, no rotation occurs. Check arc_log_written_total in /metrics to confirm bytes are being written. If compression is enabled and arc_log_compress_dropped_total is incrementing, the compression queue is full — reduce write rate or increase queue capacity via ARC_LOG_COMPRESS_QUEUE_CAPACITY.

OTLP traces are not appearing in the backend

Check that observability.tracing.endpoint is reachable from Arc. Set insecure: true if the collector does not have TLS. The trace_id field in access logs can be used to confirm Arc is generating trace context even if OTLP export is failing.

arc_worker_phase_*_seconds histograms are missing

Phase timing metrics are only emitted for requests that go through the full pipeline. Check that Arc is receiving and proxying traffic. If metrics appear after the first request, this is expected behavior.

Getting started

Operations

Reference

Observability

Metrics

Configuration

Endpoints

Prometheus metric reference

Design

Access logs

Configuration

Log record fields

Redaction

File rotation

Design — no backpressure on workers

Distributed tracing

How trace context is resolved

`traceparent` format

Upstream propagation

Searching logs by trace ID

OTLP export

Troubleshooting

Getting started

Operations

Reference

​Metrics

​Configuration

​Endpoints

​Prometheus metric reference

​Design

​Access logs

​Configuration

​Log record fields

​Redaction

​File rotation

​Design — no backpressure on workers

​Distributed tracing

​How trace context is resolved

​traceparent format

​Upstream propagation

​Searching logs by trace ID

​OTLP export

​Troubleshooting

Metrics

Configuration

Endpoints

Prometheus metric reference

Design

Access logs

Configuration

Log record fields

Redaction

File rotation

Design — no backpressure on workers

Distributed tracing

How trace context is resolved

`traceparent` format

Upstream propagation

Searching logs by trace ID

OTLP export

Troubleshooting