cFS Software Bus

What is the Software Bus?

The Software Bus (SB) is an inter-application, message-based communication service in cFE. It decouples senders and receivers via publish/subscribe so applications can exchange packets without knowing each other’s location or implementation details.

It supports one-to-one, one-to-many, and many-to-one routing. Multiple message types can be delivered to a single pipe (commonly for ground command processing).

Software Bus Terminology

Messages

Messages are self-contained data units identified by an abstract Message ID (MsgID). The Message API hides header details so applications remain portable across message-header implementations.

Pipes

Pipes are per-application queues that hold incoming messages until read. Each pipe has a single reader, but an application may own multiple pipes.

Read modes:

  • Poll (non-blocking)

  • Pend (blocking)

  • Pend with timeout

Message Limits and Flow Control

Limits help isolate slow consumers and protect system throughput:

  • Pipe depth: maximum total messages allowed at a pipe (including in-process). If reached, further deliveries to that pipe are rejected (overflow).

  • Per-MsgID limit: maximum messages of a specific MsgID at a pipe. If reached, deliveries of that MsgID are rejected (overrun).

Choose limits based on producer/consumer timing characteristics.

Routing

The SB looks up routes for a MsgID and delivers to all subscribed pipes.

Routing implementations (SBR):

  • DIRECT: array indexed by MsgID (CFE_PLATFORM_SB_HIGHEST_VALID_MSGID) — simplest, larger memory footprint

  • HASH: hash map (4 * CFE_PLATFORM_SB_MAX_MSG_IDS) — smaller, manage collisions reported at subscribe

Missions may replace SBR with a custom router where warranted.

Sending Applications

Any application can send messages. Do not send from interrupt/exception handlers (SB may invoke blocking OS calls).

Receiving Applications

Any application can receive (not from interrupt/exception handlers). Messages arrive FIFO per pipe and remain accessible until the next receive call.

The Software Bus (SB) and the Software Bus Network (SBN)

  • SB: intra-instance publish/subscribe between applications on one computer

  • SBN: bridges SB between separate cFS instances (partitions/processors/networks)

What SBN does:

  • Mirrors subscriptions so selected MsgIDs are forwarded across network links

  • Transports messages over a network interface (e.g., Ethernet)

  • Preserves the SB model so apps remain unaware of remote origins

Resources