bulkhead

Cap concurrent calls so one slow dependency cannot take every thread, connection or task you have.

from nopanic import bulkhead

reports = bulkhead(20, max_wait=0, name="reports")   # at most 20 in flight, reject the 21st

@reports
async def render_report(report_id): ...

• max_wait=None (default) waits for a slot indefinitely; 0 fails fast; N waits up to N seconds. On expiry: BulkheadFull.
• Inside an active deadline, a call never waits past it.
• reports.in_flight reports slots held right now.
• with reports.guard(): / async with reports.guard(): holds a slot for a block; a stream holds its slot until it finishes.

Capacity per flavour

Sync callers share one thread semaphore; async callers share one semaphore per event loop (or Trio run). These capacities are separate: a bulkhead used from sync code and from one event loop admits up to twice max_concurrent in total. Use one instance per flavour if that matters.

Under untrusted load, always set max_wait: backpressure should become fast failure, not an unbounded queue.

Events

bulkhead.rejected (max_concurrent, max_wait).