epics-bridge 3.2.0

A generic bridge between EPICS IOCs and Python logic.

EPICS Bridge

EPICS Bridge is a high-availability Python framework designed for implementing a robust EPICS-Python interface. It provides a structured environment for bridging external control logic with the EPICS control system, emphasizing synchronous execution, fault tolerance, and strict process monitoring.

This library addresses the common reliability challenges like preventing silent stalls ("zombie processes") and handling network IO failures deterministically.

Documentation

Comprehensive project documentation lives in docs/README.md.

System Architecture

The core of epics-bridge relies on a Twin-Thread Architecture that decouples the control logic from the monitoring signal.

1. Synchronous Control Loop (Main Thread)

The primary thread executes the user-defined logic in a strict, synchronous cycle:

1. Trigger: Waits for an input event or timer.
2. Run Task: Executes user-defined task
3. Acknowledge: Updates the task status and completes the handshake.

2. Isolated Heartbeat Monitor (Daemon Thread)

A separate, isolated thread acts as an internal watchdog. It monitors the activity timestamp of the Main Thread.

• Operational: Pulses the Heartbeat PV as long as the Main Thread is active.
• Stalled (Zombie Protection): If the Main Thread hangs (e.g., infinite loop, deadlocked IO) for longer than the defined tolerance, the Heartbeat thread ceases pulsing immediately. This alerts external watchdogs (e.g., the IOC or alarm handler) that the process is unresponsive.

3. Automatic Recovery ("Suicide Pact")

To support containerized environments (Docker, Kubernetes) or systemd supervisors, the daemon implements a fail-fast mechanism. If network connectivity is lost or IO errors persist beyond a configurable threshold (max_stuck_cycles), the watchdog performs a hard-kill of the process (os._exit(1)). This allows the external supervisor to perform a clean restart of the service.

4. Logger

Output important messages in the daemon shell to a configured log file.

Installation

# Install the package
pip install .

# Install test dependencies
pip install -r requirements-test.txt

Conda environment (recommended for integration tests)

Integration tests run a real IOC and require EPICS tooling. A working reference environment is provided in environment.yml.

conda env create -f environment.yml
conda activate epics-bridge
pip install -e .

Project Structure

• epics_bridge.daemon Main control loop, heartbeat logic, and failure handling

• epics_bridge.io Synchronous P4P client wrapper with strict error handling

• epics_bridge.base_pv_interface PV template definitions and prefix validation

• epics_bridge.utils Small utilities (for example, the Timer context manager)

• epics_bridge.io Synchronous P4P client wrapper and P4P-to-Python value unwrapping

---

Quick Start

1. EPICS Interface

There should be a standard epics db to handle the basic functionalities of the daemon and any amount of specialized dbs to fulfill the intended functionality.

The standard db should always be loaded by the IOC that interfaces with the daemon. These are its contents:

record(bo, "$(P)Trigger") {
    field(DESC, "Start Task")
    field(ZNAM, "Idle")
    field(ONAM, "Run")
}

record(bi, "$(P)Busy") {
    field(DESC, "Task Running Status")
    field(ZNAM, "Idle")
    field(ONAM, "Busy")
}

record(bi, "$(P)Heartbeat") {
    field(DESC, "Daemon Heartbeat")
}

record(mbbi, "$(P)TaskStatus") {
    field(DESC, "Last Cycle Result")
    field(DTYP, "Raw Soft Channel")

    # State 0: Success (Green)
    field(ZRVL, "0")
    field(ZRST, "Success")
    field(ZRSV, "NO_ALARM")

    # State 1: Logic Failure (Yellow - e.g. Interlock)
    field(ONVL, "1")
    field(ONST, "Task Fail")
    field(ONSV, "MINOR")

    # State 2: EPICS IO Failure (Yellow - e.g. PV Read/Write Error)
    field(TWVL, "2")
    field(TWST, "IO Failure")
    field(TWSV, "MINOR")

    # State 3: Exception (Red - Software/Hardware Crash)
    field(THVL, "3")
    field(THST, "Code Crash")
    field(THSV, "MAJOR")
    # State 4: Skipped (e.g. trigger=False)
    field(FRVL, "4")
    field(FRST, "Skipped")
    field(FRSV, "NO_ALARM")
}

record(ai, "$(P)TaskDuration") {
    field(DESC, "Task duration")
    field(PREC, "2")
    field(EGU,  "s")
}

2. Define a Python PV Interface

Subclass BasePVInterface and create PV instances in your constructor. Call super().__init__(prefixes=...), add your PVs; placeholder resolution runs automatically. Placeholders use Python format syntax {key} (e.g. {main}, {p1}) and are replaced from prefixes. Standard PVs (trigger, busy, heartbeat, task_status, task_duration) are created by the base.

from epics_bridge import BasePVInterface, PV

class MotorInterface(BasePVInterface):
    def __init__(self, prefixes: dict | None = None) -> None:
        super().__init__(prefixes=prefixes)
        self.position_rbv = PV("{main}Pos:RBV")
        self.velocity_sp = PV("{main}Vel:SP")
        self.temperature = PV("{sys}Temp:Mon")

3. Implement Control Logic

Subclass BridgeDaemon and implement the synchronous run_task() method. Use pvget(PV or list of PVs) to read (mutates each PV’s .val and .raw), then read pv.val. Use pvput(list of PVs) to write (each PV’s .val is written to its channel). Let exceptions from run_task() bubble up; the base class guarantees cleanup and logs failures.

from epics_bridge import BridgeDaemon, TaskStatus

class MotorControlDaemon(BridgeDaemon):
    def run_task(self) -> TaskStatus:
        self.io.pvget(self.iface.velocity_sp)
        velocity = self.iface.velocity_sp.val

        if velocity is None:
            return TaskStatus.IO_FAILURE

        self.iface.position_rbv.val = velocity * 0.5
        self.io.pvput([self.iface.position_rbv])

        return TaskStatus.SUCCESS

4. Run the Daemon

def main():

    prefixes = {
        "main": "IOC:MOTOR:01:",
        "sys": "IOC:SYS:"
    }

    interface = MotorInterface(prefixes=prefixes)

    daemon = MotorControlDaemon(
        iface=interface,
    )

    daemon.start()

if __name__ == "__main__":
    main()

Example: Echo daemon (IOC + daemon)

This repository includes a complete example under examples/echo_daemon/:

• st.cmd: IOC startup script (loads examples/base_interface.db + echo-specific DBs)
• echo_interface.py: PV interface (PVs created in constructor)
• echo_daemon.py: example BridgeDaemon subclass
• main.py: runnable entrypoint which sets up logging and starts the daemon

Typical workflow (requires EPICS + pvxs tooling; easiest via environment.yml):

# Terminal A: start IOC
E3_CMD_TOP="$(pwd)/examples/echo_daemon" run-iocsh examples/echo_daemon/st.cmd

# Terminal B: start daemon (logs under --log-dir)
python examples/echo_daemon/main.py --log-dir /tmp --log-level INFO

Testing

# Unit tests (pure Python)
pytest -m "not integration" -v
# or
conda run -n epics-bridge pytest -m "not integration" -v

# Integration tests (IOC + daemon)
pytest -m integration -v