Block, drop, or spill — when_full=block paired with a synchronous logger

1def write_log(line):
2    # request-handler thread is the caller
3    if sync_logger:
4        agent.enqueue(line)   # blocks if queue full
5        return
6    # async logger: hand to in-process queue, return now
7    inproc_queue.put_nowait(line)

1def enqueue(line):
2    if len(queue.mem) < queue.mem.capacity:
3        queue.mem.append(line)        # 3200 events default
4        return
5    # buffer is full — backend is not draining fast enough
6    if when_full == 'block':
7        wait_until_room()             # caller's thread parks
8    elif when_full == 'drop':
9        metrics.lines_lost += 1       # silently discarded
10    elif when_full == 'spill':
11        spill_to_disk(line)           # queue.disk / WAL

1def spill_to_disk(line):
2    # queue.disk = 10 GB on Filebeat, storage.type=filesystem
3    # on Fluent Bit, WAL on Promtail, disk buffer on Vector
4    disk_buffer.append(line)
5 
6def replay_loop():           # runs when backend recovers
7    while disk_buffer and backend.healthy():
8        batch = disk_buffer.read_batch()
9        backend.send(batch)  # at-least-once
10        disk_buffer.advance(batch)