Health check

Pings a list of URLs in parallel and aggregates the OK/FAIL counts. Demonstrates each ... in parallel(workers=N), HTTP fetch, and structured log() output.

Source: examples/02_healthcheck.c4

# Parallel "health check" over a list of URLs.
# Demonstrates: each ... in parallel { ... } as an expression returning a list,
# safe_attr (?.) and default (??) operators, structured logging.

hosts = [
    "https://www.google.com",
    "https://www.python.org",
    "https://www.invalid-host-doesnotexist.local",
]

fn check(url) {
    try {
        resp = read(url)
        return {"url": url, "ok": True, "size": len(resp)}
    } catch Exception as e {
        return {"url": url, "ok": False, "error": str(e)}
    }
}

results = each url in hosts in parallel(workers=8) { check(url) }

ok_count = 0
for r in results {
    if r["ok"] {
        ok_count = ok_count + 1
        log("up", url=r["url"], size=r["size"])
    } else {
        log.warn("down", url=r["url"], error=r["error"])
    }
}

log("summary", ok=ok_count, total=len(results))

Run it

c4 run examples/02_healthcheck.c4