Metrics & Monitoring

3270Connect exposes Prometheus metrics for fleet-scale monitoring of workflow performance, concurrency, and outcome distribution. The metrics endpoint is opt-in and runs on a dedicated HTTP listener so it does not collide with the dashboard or API ports.

Enabling the /metrics endpoint

Use the -promListen flag with any host:port address. Disabled when empty (the default).

3270Connect -config workflow.json -concurrent 10 -runtime 300 -promListen :9091

Once running, the metrics endpoint is available at:

http://localhost:9091/metrics

The listener uses a 5-second read-header timeout and runs in its own goroutine — failures are logged but never abort the workflow runner.

Collectors

Metric	Type	Labels	Meaning
tn3270_connect_seconds	Histogram	—	Wall-clock time to establish a TN3270 session (exponential buckets starting at 50 ms).
tn3270_step_seconds	Histogram	action	Wall-clock time per workflow step, partitioned by step action (Connect, FillString, CheckValue, PressEnter, etc.).
tn3270_workflow_total	Counter	result	Workflows that have terminated, partitioned by outcome: success, failure, connect_failed.
tn3270_concurrent_workers	Gauge	—	Active workflow worker count. Useful for confirming that -concurrent ramps cleanly and never overshoots.

Source: internal/metrics/metrics.go.

Sample scrape config

Add a scrape job to prometheus.yml:

scrape_configs:
  - job_name: '3270connect'
    scrape_interval: 15s
    static_configs:
      - targets: ['runner-01:9091', 'runner-02:9091']

Useful queries

# p95 connect time over the last 5 minutes
histogram_quantile(0.95, sum(rate(tn3270_connect_seconds_bucket[5m])) by (le))

# Failure rate by outcome
sum(rate(tn3270_workflow_total[1m])) by (result)

# Slowest step actions (p95)
histogram_quantile(0.95,
  sum(rate(tn3270_step_seconds_bucket[5m])) by (le, action)
)

# Live worker count vs. configured concurrency
tn3270_concurrent_workers

Pairing with the host compatibility profiler

For one-shot host fingerprinting (rather than continuous timing), use the Host Compatibility Profiler — the resulting CompatibilityProfile JSON is comparable to the one produced by 3270Web and can be diffed across environments.