What causes "failed to set otel globals" in denoland/deno?

Ensure the telemetry/global-OTel setup runs exactly once per process (guard it behind your own OnceCell or lazy static)

denoland/deno · error · anyhow::Error

failed to set otel globals

Error message

failed to set otel globals

What it means

Deno stores its initialized telemetry state (processors, meter provider, sampler, limits) in a global OnceCell. Setting that cell fails — yielding 'failed to set otel globals' — if telemetry globals were already populated, i.e. the OTEL setup path executed a second time in the same process. This is an internal double-initialization guard, not an environment-configuration problem.

Solutions

Ensure the telemetry/global-OTel setup runs exactly once per process (guard it behind your own OnceCell or lazy static)
If you embed Deno, don't call the OTel init API yourself and also pass --otel; pick one owner
Update to the latest Deno patch release — double-init in standard `deno` CLI flows is a bug worth reporting at https://github.com/denoland/deno/issues
As a workaround, drop the custom OTel env/flags in the second init site so only the first initializes globals

Defensive patterns

Strategy: validation

Validate before calling

// Embedders: guard so OTel globals init at most once per process
use std::sync::Once;
static OTEL_INIT: Once = Once::new();
fn init_telemetry_once(sys: &impl TelemetrySys) -> anyhow::Result<()> {
    // First caller initializes; later callers are no-ops, avoiding the
    // OnceCell double-set that produces "failed to set otel globals".
    let mut first_err: Option<anyhow::Error> = None;
    OTEL_INIT.call_once(|| {
        if let Err(e) = deno_telemetry::init(sys) { first_err = Some(e); }
    });
    first_err.map_or(Ok(()), Err)
}

Try / catch

// In embedder bootstrap code
match deno_telemetry::init(&sys) {
    Err(e) if e.to_string().contains("failed to set otel globals") => {
        // Globals already installed by an earlier init: log and continue
        log::warn!("otel globals already set; skipping re-init");
    }
    other => other?,
}

Prevention

Never call the telemetry init API from two places (CLI --otel path and embedder bootstrap)
Wrap global installs in Once/OnceCell at the call site, since the internal set is not idempotent
For multi-runtime test harnesses, run each --otel runtime in its own process

When it happens

Trigger: Embedding deno_core/deno_runtime and calling the telemetry init (configure_opentelemetry-style entry point) twice; a Deno version regression where a worker or snapshot path re-runs OTel setup; runtime customization code that installs OTel globals before Deno's own --otel startup path runs.

Common situations: Custom embedders (runtime builders on top of deno_core) that add their own OTel bootstrap; upgrading Deno versions where init ordering changed; tests that boot multiple JsRuntimes in one process with --otel semantics each time.

Related errors

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/bd632989637d482b. Report an issue: GitHub.

Appendix: source

Thrown at ext/telemetry/lib.rs:1423

  let span_attribute_count_limit = span_attribute_count_limit_from_env(sys);
  let span_attribute_value_length_limit =
    span_attribute_value_length_limit_from_env(sys);
  let sampler = Sampler::from_env(sys)?;

  OTEL_GLOBALS
    .set(OtelGlobals {
      log_processor,
      span_processor,
      id_generator,
      meter_provider,
      builtin_instrumentation_scope,
      span_event_count_limit,
      span_attribute_count_limit,
      span_attribute_value_length_limit,
      sampler,
      config,
    })
    .map_err(|_| deno_core::anyhow::anyhow!("failed to set otel globals"))?;

  deno_signals::before_exit(before_exit);
  deno_net::tunnel::disable_before_exit();

  Ok(())
}

fn before_exit() {
  log::trace!("deno_telemetry::before_exit");

  let Some(OtelGlobals {
    span_processor: spans,
    log_processor: logs,
    meter_provider,
    ..
  }) = OTEL_GLOBALS.get()
  else {
    return;

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