cFS Software Bus

Software Bus Overview

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).

	   +----------+                         +----------+
       |  App A   |                         |  App B   |
       +----|-----+                         +----^-----+
            |                                    |
            | (Push)                             | (Pull)
            v                                    |
   __________________________________________________________
  |                                                          |
  |  [ DATA PIPE ]  =====>  [ MESSAGE ]  =====>  [ DATA ]    |  <-- SOFTWARE BUS
  |__________________________________________________________|
            |                                    ^
            |                                    |
            +------------------------------------+
                   (Asynchronous Flow)

Creation

  • The ownership of **a pipe is given to the app the creates it. This app has permissions to unsubscribe/subscribe/delete the pipe. Subscriptions are simply a mapping between a MSGID and a pipeID. The software bus will then route messages with ID MSGID to the subscribed pipe. Below all messages with MID: SAMPLE_CMDID_1 will get routed to this pipe. It just so happens that the sample app will poll for messages on SAMPLE_Pipe_1.

Status = CFE_SB_SubscribeEX(
	SAMPLE_CMDID_1,               /* Msg Id to Receive */
	SAMPLE_AppData.SAMPLE_Pipe_1, /* Pipe Msg is to be Rcvd on */
	CFE_SB_DEFAULT_QOS,           /* Quality of Service */
	SAMPLE_CMDID_1_LIMIT         /* Max Number to Queue */
);
Status = CFE_SB_SubscribeEX(
	SAMPLE_CMDID_2,               /* Msg Id to Receive */
	SAMPLE_AppData.SAMPLE_Pipe_1, /* Pipe Msg is to be Rcvd on */
	CFE_SB_DEFAULT_QOS,           /* Quality of Service */
	SAMPLE_CMDID_1_LIMIT         /* Max Number to Queue */
);
Status = CFE_SB_SubscribeEX(
	SAMPLE_CMDID_1,               /* Msg Id to Receive */
	SAMPLE_AppData.SAMPLE_Pipe_2, /* Pipe Msg is to be Rcvd on */
	CFE_SB_DEFAULT_QOS,           /* Quality of Service */
	SAMPLE_CMDID_1_LIMIT         /* Max Number to Queue */
);

  • Implications are that multiple MSGID`s can route to the same pipe, AND a single `MSGID can be routed to multiple different pipes.

Sending Messages

typedef struct CFE_TBL_HousekeepingTlm
{
    CFE_MSG_TelemetryHeader_t         TelemetryHeader; /**< \brief Telemetry header */
    CFE_TBL_HousekeepingTlm_Payload_t Payload;         /**< \brief Telemetry payload */
} CFE_TBL_HousekeepingTlm_t;

// ...

void SendHousekeeping(void)
{
    CFE_TBL_HousekeepingTlm_t HkPacket;  /* stack allocation */

    CFE_MSG_Init(CFE_MSG_PTR(HkPacket.TelemetryHeader),
                 CFE_SB_ValueToMsgId(CFE_TBL_HK_TLM_MID),
                 sizeof(HkPacket));
    CFE_TBL_GetHkData();  /* would need to fill HkPacket, not global */
    CFE_SB_TimeStampMsg(CFE_MSG_PTR(HkPacket.TelemetryHeader));
    CFE_SB_TransmitMsg(CFE_MSG_PTR(HkPacket.TelemetryHeader), true);
}

Important Detail: CFE_SB_TransmitMSG copies the data into the SB’s own struct. So it’s completely find to create a local (stack) variable. This isn’t the case when using allocate_buffer().

Message IDs

  • Must be globally unique

Utility Commands:

  • CFE_SB_ValueToMsgId(integer) — Turn an integer constant into the opaque CFE_SB_MsgId_t for SB APIs.

  • CFE_SB_MsgIdToValue(msgid) — Turn a MsgId back to an integer for switch or logging.

  • CFE_SB_MsgId_Equal(a, b) — Compare MsgIds without direct == on opaque types.

MID Conventions (Make sure they don’t conflict):

  1. Commands: 0x18xx (bit 12 set)

  2. Telemetry: 0x08xx (bit 11 set)

Recommendations

  • If you need data from another app, use the SB

  • Never directly modify Message Headers (there’s always a cFS one to use)

  • Every application needs at least one pipe (usually the CMD pipe).

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