prometheus-d ~main

Prometheus D

Prometheus metrics library for the D programming language. Zero external dependencies — only the D standard library and runtime.

See here for a comparison with other D Prometheus implementations.

Example

import prometheus;
import core.thread;

void main()
{
    auto reg = new Registry();

    // Counter — monotonically increasing value
    auto requests = reg.add(new Counter("http_requests_total", "Total HTTP requests"));
    requests(["method": "GET", "status": "200"]).inc();

    // Gauge — value that can go up and down
    auto cpu = reg.add(new Gauge("cpu_usage_percent", "CPU usage"));
    cpu.set(42.5);

    // Histogram — observations bucketed by configurable thresholds
    auto latency = reg.add(new Histogram("request_latency_seconds",
        "Request latency", [0.1, 0.5, 1.0, 2.0, 5.0]));
    latency(["endpoint": "/api"]).observe(0.34);

    // Summary — observations with streaming quantile estimates
    auto summary = reg.add(new Summary("response_size_bytes",
        "Response size", [0.5, 0.9, 0.99]));
    summary.observe(1024);

    // Start HTTP server (blocking — run in its own thread)
    new Thread({ serveMetrics(reg, 8080); }).start();

    // Update metrics in your application loop
    while (true) {
        requests.inc();
        Thread.sleep(1.seconds);
    }
}

Run the included sample:

cd examples/sample && dub run
curl http://localhost:8081/metrics

Install

Add to your dub.json:

{
    "dependencies": {
        "prometheus-d": "~>0.0.1"
    }
}

Or use the git dependency:

{
    "dependencies": {
        "prometheus-d": { "path": "path/to/prometheus-d" }
    }
}

Build & Test

dub build -c lib          # static library
dub build -c shared       # dynamic library
dub build -c asan         # static library with AddressSanitizer
dub test                  # run all unit tests
dub test --coverage       # run tests with coverage (.lst files)

Implemented functionality

Metric types

Labels

All metric types support label-based dimensionality via opCall:

auto c = reg.add(new Counter("requests", "Help text", ["app": "myapp"]));
c(["host": "server1", "method": "GET"]).inc();  // per-label-set value
c.inc();  // uses default labels ["app": "myapp"]

Labels are immutable string[string] associative arrays. Default labels passed to the constructor are merged into all rendered output automatically.

Registry

• `add(metric)` — register a metric (duplicate names are warned and skipped)
• `counter(name)` / `gauge(name)` — typed lookup by name
• `get!(T)(name)` — template-based typed lookup for any metric type
• `renderAll()` — produces full Prometheus text-format output

HTTP server

serveMetrics(registry, port, host) starts a blocking HTTP server:

Built-in GC metrics

The server automatically exposes D runtime GC statistics as gauges:

• gc_free_size_bytes / gc_used_size_bytes — heap memory
• gc_num_collections / gc_total_collection_time / gc_max_collection_time — collection stats
• gc_total_pause_time / gc_max_pause_time — pause time stats in microseconds

Updated on every /metrics scrape via GC.stats() and GC.profileStats().

Thread safety

Metrics are safe for concurrent use:

• Hot path (inc, set, observe): core.atomic operations on shared fields — @nogc nothrow, no heap allocation
• Container access (label creation, rendering): synchronized(this) mutual exclusion
• Registry add(), get(), and renderAll() are all synchronized

Prometheus format compliance

Output follows the Prometheus exposition format:

# HELP http_requests_total Total HTTP requests
# TYPE http_requests_total counter
http_requests_total{method="GET",status="200"} 42

# HELP request_latency_seconds Request latency
# TYPE request_latency_seconds histogram
request_latency_seconds_bucket{le="0.1"} 0
request_latency_seconds_bucket{le="0.5"} 3
request_latency_seconds_bucket{le="1"} 7
request_latency_seconds_bucket{le="+Inf"} 7
request_latency_seconds_sum 1.23
request_latency_seconds_count 7

TODO

• [ ] @nogc rendering path (currently format() allocates)
• [ ] Metric removal / unregister from Registry
• [ ] Configurable GC metrics (opt-in/opt-out per gauge)
• [ ] Support for Prometheus protobuf format
• [ ] Push-gateway client
• [ ] Exemplar support
• [ ] Native histogram support

History

• v0.0.1 — First release. Counter, Gauge, Histogram, Summary. Labels. HTTP server. GC stats.