Spvirit

/ˈspɪrɪt/ of the Machine

Spvirit is a Rust library for working with EPICS PVAccess protocol, including encoding/decoding and connection state tracking. It also includes tools for monitoring and testing PVAccess connections. These are not yet production ready , but they are available for anyone to use and contribute to.

Key areas of development in the near future include:

• More complete support for EPICS Normative Types (NT) and their associated metadata.
• Expanding spvirit-server with more complete softIOC behaviours and record processing.

Why Rust?

Because why not, admittedly I just wanted to learn Rust and this seemed like a fun project with a moderately useful outcome.

Project Structure

The project is structured as a Cargo workspace with five crates:

• spvirit-types: Shared data model types for PVAccess Normative Types (NT).
• spvirit-codec: PVAccess protocol encoding/decoding logic and connection state tracking.
• spvirit-client: Client library — search, connect, get, put, monitor.
• spvirit-server: Server library — db parsing, PV store trait, PVAccess server runtime.
• spvirit-tools: Command-line tools (CLI binaries) for monitoring and testing PVAccess connections.

Key Concepts

If you're new to EPICS or PVAccess, the terminology can be confusing. This section explains the key concepts and how they map to Spvirit's Rust types.

PVs, Records, and .db files

In EPICS, a Process Variable (PV) is a named data point — a temperature reading, a motor position, or a shutter state. PVs are the things clients read and write over the network.

On the server side, each PV is backed by a Record. A record has a type (ai, ao, bi, bo, waveform, etc.) that determines its behaviour: whether it's read-only or writable, what data shape it holds, and what processing it does.

Records are usually declared in .db files — plain-text configuration files using the EPICS database syntax:

record(ai, "SIM:TEMPERATURE") {
    field(DESC, "Simulated sensor")
    field(EGU,  "degC")
    field(PREC, "2")
    field(HOPR, "100")
    field(LOPR, "-20")
}

record(ao, "SIM:SETPOINT") {
    field(DESC, "Target temperature")
    field(EGU,  "degC")
    field(DRVH, "100")
    field(DRVL, "0")
}

In Spvirit, a RecordInstance holds all of this — the record type, the current value (as a Normative Type), and the common fields. You can either build records in code with PvaServer::builder().ai(...) or load them from .db files with .db_file("path.db").

flowchart LR
    DB[".db file"] -->|parse_db| RI["RecordInstance"]
    Code["builder.ai(...)"] --> RI
    RI --> Store["SimplePvStore"]
    Store --> Server["PvaServer"]
    Server -->|PVAccess protocol| Client["PvaClient"]

Record Types at a Glance

Record Type	Rust Builder	Direction	Data Shape	Typical Use
ai	.ai(name, f64)	Input (read-only)	Scalar	Sensor readings
ao	.ao(name, f64)	Output (writable)	Scalar	Setpoints, commands
bi	.bi(name, bool)	Input (read-only)	Boolean	Status bits
bo	.bo(name, bool)	Output (writable)	Boolean	On/off switches
stringin	.string_in(name, str)	Input (read-only)	String	Status messages
stringout	.string_out(name, str)	Output (writable)	String	Text commands
waveform	.waveform(name, data)	Writable	Array	Spectra, traces

Input records are read-only from the client's perspective — values are produced by the server (scan callbacks, hardware, simulation). Output records accept writes from PVAccess clients (PUT operations).

Normative Types (NT)

The PVAccess protocol doesn't send plain numbers — it sends structured payloads called Normative Types. These wrap the actual value with rich metadata (alarm state, timestamp, display limits, engineering units, etc.).

flowchart TD
    NTP["NtPayload"]
    NTP --> NTS["NtScalar"]
    NTP --> NTSA["NtScalarArray"]
    NTP --> NTT["NtTable"]
    NTP --> NTNA["NtNdArray"]

    NTS --> V1["value: ScalarValue"]
    NTS --> A1["alarm severity/status/message"]
    NTS --> D1["display: limits, units, precision"]
    NTS --> C1["control: limits, min_step"]
    NTS --> VA1["valueAlarm: thresholds"]

    NTSA --> V2["value: ScalarArrayValue"]
    NTSA --> A2["alarm"]
    NTSA --> D2["display"]

    NTT --> L["labels + columns"]
    NTNA --> DIM["dimensions + codec + attributes"]

The four Normative Types in Spvirit:

Normative Type	Rust Type	Backed by	Used for
NTScalar	NtScalar	ScalarValue (f64, i32, bool, String, …)	Single-value PVs (ai, ao, bi, bo, etc.)
NTScalarArray	NtScalarArray	ScalarArrayValue (Vec<f64>, Vec<i32>, …)	Array PVs (waveform, aai, aao)
NTTable	NtTable	Named columns of ScalarArrayValue	Tabular data
NTNDArray	NtNdArray	ScalarArrayValue + dimensions + attributes	Image / detector data (areaDetector)

Enums in EPICS (bi/bo and ZNAM/ONAM)

EPICS doesn't have a first-class enum type like Rust. Instead, binary records (bi/bo) use two string labels — ZNAM (the "zero" name) and ONAM (the "one" name) — to map a boolean value to human-readable choices:

record(bo, "SHUTTER:CTRL") {
    field(ZNAM, "Closed")
    field(ONAM, "Open")
}

When a client reads this PV, the NTScalar's value is the integer index (0 or 1), and the display.form.choices field carries ["Closed", "Open"] so the UI can show a dropdown. In Spvirit, bi/bo records store the underlying value as ScalarValue::Bool and the labels in the znam/onam fields of RecordData::Bi / RecordData::Bo.

How it all fits together

flowchart TD
    subgraph Server Side
        DB[".db file"] -->|load_db / parse_db| Records["HashMap<String, RecordInstance>"]
        Builder["PvaServer::builder()
        .ai() .ao() .bo() ..."] --> Records
        Records --> Store["SimplePvStore
        (implements PvStore trait)"]
        Store --> Runtime["PvaServer::run()
        UDP search + TCP handler + beacons"]
        Scan["scan callbacks"] -->|periodic timer| Store
        OnPut["on_put callbacks"] -.->|fired after PUT| Store
    end

    subgraph Client Side
        PC["PvaClient::builder().build()"]
        PC -->|pvget| Runtime
        PC -->|pvput| Runtime
        PC -->|pvmonitor| Runtime
        PC -->|pvinfo| Runtime
    end

    subgraph Wire Format
        Runtime <-->|"PVAccess TCP/UDP
        (spvirit-codec)"| PC
    end

    style Server Side fill:#1a1a2e,stroke:#16213e,color:#e0e0e0
    style Client Side fill:#1a1a2e,stroke:#16213e,color:#e0e0e0
    style Wire Format fill:#0f3460,stroke:#16213e,color:#e0e0e0

Getting Started

Install Rust

# Linux

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

clone the repo

git clone https://github.com/ISISNeutronMuon/spvirit

Build the project

cd spvirit

cargo build --release

Run the tools

cargo run --bin spmonitor my:pv:name

# or

./target/release/spmonitor my:pv:name

# or if installed 

cargo install spvirit-tools


spmonitor my:pv:name

Using the library in your own Rust project

Add the crates you need to your Cargo.toml:

[dependencies]

spvirit-client = "0.1"   # client library: search, connect, get, put, monitor

spvirit-server = "0.1"   # server library: db parsing, PvStore trait, PVA server

spvirit-codec  = "0.1"   # low-level PVA protocol encode/decode

spvirit-types  = "0.1"   # shared Normative Type data model

spvirit-tools  = "0.1"   # all of the above + CLI tool helpers

Fetching a PV value (pvget)

use spvirit_client::PvaClient;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = PvaClient::builder().build();
    let result = client.pvget("MY:PV:NAME").await?;
    println!("{}: {}", result.pv_name, result.value);
    Ok(())
}

cargo run -p spvirit-client --example pvget

Writing a value to a PV (pvput)

use spvirit_client::PvaClient;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = PvaClient::builder().build();
    client.pvput("MY:PV:NAME", 42.0).await?;
    println!("OK");
    Ok(())
}

cargo run -p spvirit-client --example pvput

Monitoring a PV for live updates (pvmonitor)

use std::ops::ControlFlow;
use spvirit_client::PvaClient;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = PvaClient::builder().build();
    client.pvmonitor("MY:PV:NAME", |value| {
        println!("{value}");
        ControlFlow::Continue(())   // return Break(()) to stop
    }).await?;
    Ok(())
}

cargo run -p spvirit-client --example pvmonitor

Running a PVAccess server

use spvirit_server::PvaServer;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let server = PvaServer::builder()
        .ai("SIM:TEMPERATURE", 22.5)
        .ao("SIM:SETPOINT", 25.0)
        .bo("SIM:ENABLE", false)
        .build();

    server.run().await
}

cargo run -p spvirit-server --example simple_server

Reacting to client writes (on_put)

Register a callback that fires whenever a PVAccess client writes to a PV:

use spvirit_server::PvaServer;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let server = PvaServer::builder()
        .ao("SIM:SETPOINT", 25.0)
        .on_put("SIM:SETPOINT", |pv, val| {
            // `val` is a DecodedValue — the raw structure sent by the client
            println!("{pv} was set to {val:?}");
        })
        .build();

    server.run().await
}

cargo run -p spvirit-server --example on_put

Reading and writing PVs at runtime (store())

The store() handle lets your own code read and write PV values while the server is running — useful for simulation loops, hardware I/O, or responding to external events:

use spvirit_server::PvaServer;
use spvirit_types::ScalarValue;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let server = PvaServer::builder()
        .ai("SIM:TEMPERATURE", 22.5)
        .ao("SIM:SETPOINT", 25.0)
        .build();

    let store = server.store().clone();

    // Spawn a task that reads/writes PVs independently
    tokio::spawn(async move {
        loop {
            // Read the current setpoint (like an on_get)
            if let Some(sp) = store.get_value("SIM:SETPOINT").await {
                println!("Current setpoint: {sp:?}");
            }

            // Push a new temperature reading
            store.set_value("SIM:TEMPERATURE", ScalarValue::F64(23.1)).await;

            tokio::time::sleep(std::time::Duration::from_secs(1)).await;
        }
    });

    server.run().await
}

cargo run -p spvirit-server --example store_runtime

Periodic scan callbacks

Use .scan() to produce new values on a timer — the server pushes updates to any monitoring clients automatically:

use spvirit_server::PvaServer;
use spvirit_types::ScalarValue;
use std::sync::atomic::{AtomicU64, Ordering};
use std::time::Duration;

static TICK: AtomicU64 = AtomicU64::new(0);

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let server = PvaServer::builder()
        .ai("SIM:TEMPERATURE", 22.5)
        .scan("SIM:TEMPERATURE", Duration::from_secs(1), |_pv| {
            let t = TICK.fetch_add(1, Ordering::Relaxed) as f64;
            ScalarValue::F64(22.5 + (t * 0.1).sin())
        })
        .build();

    server.run().await
}

cargo run -p spvirit-server --example scan_callback

Serving a waveform (array PV)

use spvirit_server::PvaServer;
use spvirit_types::{ScalarArrayValue, ScalarValue};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let server = PvaServer::builder()
        .waveform("SIM:SPECTRUM", ScalarArrayValue::F64(vec![0.0; 1024]))
        .build();

    let store = server.store().clone();
    tokio::spawn(async move {
        let mut tick = 0u64;
        loop {
            let first_val = ((tick as f64) * 0.01).sin();
            store.set_value("SIM:SPECTRUM", ScalarValue::F64(first_val)).await;
            tick += 1;
            tokio::time::sleep(std::time::Duration::from_millis(100)).await;
        }
    });

    server.run().await
}

cargo run -p spvirit-server --example waveform

Building a custom record by hand

For record types not covered by the builder helpers, construct a RecordInstance directly and insert it into the store:

use std::collections::HashMap;
use spvirit_server::{PvaServer, RecordInstance, RecordType, RecordData, DbCommonState};
use spvirit_types::{NtScalar, ScalarValue};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Create a custom ai record with pre-populated fields
    let custom = RecordInstance {
        name: "CUSTOM:SENSOR".into(),
        record_type: RecordType::Ai,
        common: DbCommonState {
            desc: "My custom sensor".into(),
            ..DbCommonState::default()
        },
        data: RecordData::Ai {
            nt: NtScalar::from_value(ScalarValue::F64(21.0))
                .with_units("degC".into())
                .with_precision(2)
                .with_limits(-20.0, 100.0),
            inp: None,
            siml: None,
            siol: None,
            simm: false,
        },
        raw_fields: HashMap::new(),
    };

    let server = PvaServer::builder()
        .ao("CUSTOM:SETPOINT", 25.0)
        .build();

    // Insert the hand-built record into the running store
    server.store().insert("CUSTOM:SENSOR".into(), custom).await;

    server.run().await
}

cargo run -p spvirit-server --example custom_record

Running the examples

All examples live in spvirit-client/examples/ and spvirit-server/examples/ and can be run directly from the repo:

# Client examples (need a running PVAccess server on the network)

cargo run -p spvirit-client --example pvget            # fetch a PV

cargo run -p spvirit-client --example pvput            # write a PV

cargo run -p spvirit-client --example pvmonitor         # subscribe to updates


# Server examples (start a PVAccess server on localhost:5075)

cargo run -p spvirit-server --example simple_server     # minimal server

cargo run -p spvirit-server --example on_put            # on_put callback

cargo run -p spvirit-server --example store_runtime     # runtime get/set via store()

cargo run -p spvirit-server --example scan_callback     # periodic scan

cargo run -p spvirit-server --example waveform          # array PV

cargo run -p spvirit-server --example custom_record     # hand-built RecordInstance

Quick demo — server + client in two terminals:

# Terminal 1: start a server

cargo run -p spvirit-server --example simple_server


# Terminal 2: read a PV from it

cargo run -p spvirit-client --example pvget -- SIM:TEMPERATURE

Tools available

spvirit tool	EPICS Base equivalent	Description
spget	pvget	Fetch the current value of a PV
spput	pvput	Write a value to a PV
spmonitor	pvmonitor	Subscribe to a PV and print value changes
spinfo	pvinfo	Display field/metadata information for a PV
splist	pvlist	List all available PVs on discovered servers
spserver	softIoc	Not fully one-to-one - just a demo, it does parse some db file vocab
spexplore		Interactive TUI to browse servers, select PVs, and monitor values
spsearch		TUI showing PV search network traffic for diagnostics
spsine		Continuously write a sine wave to a PV (demo/testing)
spdodeca		Server publishing a rotating 3D dodecahedron as an NTNDArray PV

Server (softIOC-like experiment)

The spvirit-server crate provides a reusable PVAccess server runtime at two levels:

• High-level: Use PvaServer::builder() to declare typed records (ai, ao, bi, bo, string_in, string_out, waveform), register on_put callbacks, attach periodic scan callbacks, load .db files, and call .run(). See the "Running a PVAccess server" example above.
• Low-level: Implement the [PvStore] trait to supply your own PV data source, then call run_pva_server to serve PVs over PVAccess. The bundled spserver CLI tool demonstrates this by parsing a limited subset of EPICS .db file syntax to serve static PVs.

Both levels prove that the encoding/decoding and connection handling logic in spvirit-codec is sufficient to implement a server, and they can be used as a starting point for a more full-featured softIOC in the future. (hint hint PRs welcome :))

Integration test matrix

I have tested the tools in this repo against the following EPICS PVAccess servers:

• EPICS
• p4p (pvxs under the hood)
• PvAccessJava

Related Projects

• spvirit-scry — A Rust tool for capturing and analyzing pvAccess EPICS packets.

References

I used the following libraries and repos as refernce materials for PVAccess protocol:

• pvxs
• pvAccess Protocol Specification
• EPICS Base
• PVAshark

GenAI Usage Log

Section / Area	What Was Done With AI	Plans Ahead
spvirit-types	Hand coded, few types completed with AI, the prettified with AI	keep the same, fairly complete
spvirit-codec	Most was hand-coded, some restructuring and prettifying was done with AI.	keep the same, bring in any common helpers, maybe write a siplified API for users
spvirit-tools	Mostly AI generated, manually coded parts of Put and Get then let the Agents build on top. Client and server logic has been split out into spvirit-client and spvirit-server crates.	The APIs are now split idiomatically. Continued refinement of high-level convenience functions for put and monitor.
PvaClient / PvaServer	High-level builder-pattern APIs (PvaClient::builder(), PvaServer::builder()) designed with AI assistance. Wraps protocol-level operations into ergonomic one-liners for get, put, monitor, info, and typed server records.	Extend with more record types, structured put payloads, and TLS support.
Testing	I wrote some basic tests, then used GenAI agents to generate more tests and test cases, which I then manually curated and edited.	Suite is fairly comprehensive so I will keep it as is.