prometheus-ada

Prometheus text-format metrics client library for Ada. Zero dependencies, text exposition only (v0.0.4).

What / Why

Observability-ready services need metrics. This library provides:

• Counter, Gauge, and Histogram metric types
• Labels for dimensional metrics
• Prometheus text exposition format v0.0.4 (the wire format Prometheus scrapes)
• Deterministic output (reproducible for testing)
• Zero dependencies (only Ada standard library)

Status

Text-format client only — no HTTP server, no OpenMetrics. This is the foundation; an HTTP endpoint will be layered on top (via a simple TCP listener or integration with an Ada web framework).

Available in the Alire index — the first Prometheus client library published for Ada:

alr with prometheus

Usage

Counter

with Prometheus; use Prometheus;

Reg : Registry_Type;
Cnt : Counter_Access;
Labels : Label_Set;

Labels.Append (Label'(Name => To_Unbounded_String ("method"),
                      Value => To_Unbounded_String ("GET")));
Cnt := Register_Counter (Reg, "http_requests_total",
                         "Total HTTP requests", Labels);
Inc (Cnt.all);  --  Increment by 1.0 (default)
Inc (Cnt.all, 100.0);  --  Custom increment

Gauge

Gauge := Register_Gauge (Reg, "temperature_celsius", "Current temperature", Empty_Vector);
Set (Gauge.all, 23.5);
Inc (Gauge.all, 2.0);
Dec (Gauge.all, 0.5);

Histogram

Hist := Register_Histogram (Reg, "request_duration_seconds",
                            "Request duration distribution", Empty_Vector);
--  Default buckets: .005, .01, .025, .05, .075, .1, .25, .5, .75, 1.0, 2.5, 5.0, 7.5, 10.0, +Inf
Observe (Hist.all, 0.123);
Observe (Hist.all, 0.456);

Exposition

Output : constant String := Collect (Reg);
--  Returns Prometheus text format v0.0.4:
--  # HELP http_requests_total Total HTTP requests
--  # TYPE http_requests_total counter
--  http_requests_total{method="GET"} 101

Design

Thread safety

Metric types (Counter_Type, Gauge_Type, Histogram_Type) are limited records (non-copyable). The registry is also limited. This design keeps the API simple: the caller owns concurrency. To instrument a multi-task service, either:

• Share one registry across tasks and wrap metric operations in a protected object, or
• Give each task its own registry and aggregate at collection time.

Labels

Label sets are vectors of (name, value) pairs. Registration validates each label name ([a-zA-Z_][a-zA-Z0-9_]*, ASCII only) and rejects duplicates; the set is sorted by name so exposition output is deterministic.

Float formatting

Values are rendered as plain decimals (1027, 0.05, -0.15) rather than Ada's native scientific notation (1.02700E+03). Non-finite values use the spec tokens NaN, +Inf, -Inf.

Roadmap

• Core metric types (Counter, Gauge, Histogram)
• Text exposition format v0.0.4
• Label escaping (backslash, quote, newline)
• HELP escaping
• Validation (metric/label name format)
• Tests (74 checks, all PASS)
• Publish to alire-index (alr with prometheus)
• HTTP server integration (simple TCP listener)
• OpenMetrics format (for native histograms, exemplars)
• Push gateway client (for batch jobs)

Testing

gprbuild -p -P prometheus_tests.gpr
./bin/prometheus-tests

Expected: All tests passed! (74 checks).

License

MIT License — see LICENSE

Contributing

PRs welcome. For major changes, open an issue first to discuss the design. Follow Ada Quality and Style Guide conventions.