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System Modes

NSB supports two system modes that control how messages are delivered to clients: PULL and PUSH. This page owns the conceptual explanation of each mode. To set the mode field in your YAML config, see Configuration → System Modes.

PULL Mode (mode: 0) — Default​

In PULL mode, clients poll the daemon to check for messages. No message is sent unless explicitly requested.

  • NSBAppClient.receive() sends a RECEIVE request to the daemon, which responds with either a payload (MESSAGE) or nothing (NO_MESSAGE)
  • NSBSimClient.fetch() sends a FETCH request to the daemon, which responds similarly
  • Clients must explicitly request messages — no message is delivered unless asked for

Best for / Recommended for: Most configurations. Easier to manage, no persistent connection requirements, and simpler to reason about and debug.

PUSH Mode (mode: 1)​

In PUSH mode, the daemon automatically forwards payloads to clients as soon as they are available.

  • When NSBAppClient.send() is called, the daemon immediately forwards the payload to the simulator client
  • When NSBSimClient.post() is called, the daemon immediately forwards the payload to the destination application client
  • Clients must maintain persistent connections throughout the simulation

Best for / Recommended for: Latency-sensitive applications. Requires stable network and connection management — be aware that this may not work in all network configurations (e.g., NAT, firewalls, unstable connections).

Behavior of receive() and fetch() by Mode​

MethodPULL ModePUSH Mode
NSBAppClient.receive()Sends a RECEIVE request to the daemon. Daemon responds with MESSAGE or NO_MESSAGE.Waits on the RECV channel using select with the given timeout. Use timeout=0 for polling, timeout=None for blocking.
NSBSimClient.fetch()Sends a FETCH request to the daemon. Daemon responds with MESSAGE or NO_MESSAGE.Waits for forwarded payloads from the daemon/broker.

Go Deeper​