NetworkTables Core (ntcore) API

NetworkTables Core (ntcore) provides a high-performance, real-time publish-subscribe protocol for communication between robot components, driver stations, and coprocessors.

Overview

NetworkTables is WPILib’s primary communication protocol, enabling:

Automatic data synchronization across network
Publisher-subscriber pattern for loose coupling
Type-safe data exchange
Dashboard integration (Shuffleboard, SmartDashboard)
Vision coprocessor communication
Robot-to-robot communication

Architecture

Core Concepts

Topics

Named data channels for publishing/subscribing

Publishers

Send data to a topic

Subscribers

Receive data from a topic

Entries

Combined publisher and subscriber

Basic Usage

Java API

import edu.wpi.first.networktables.*;

// Get default instance
NetworkTableInstance inst = NetworkTableInstance.getDefault();

// Get a table
NetworkTable table = inst.getTable("datatable");

// Publish values
DoublePublisher xPub = table.getDoubleTopic("x").publish();
xPub.set(1.0);

// Subscribe to values
DoubleSubscriber xSub = table.getDoubleTopic("x").subscribe(0.0);
double value = xSub.get();

// Use entries (publish + subscribe)
DoubleEntry xEntry = table.getDoubleTopic("x").getEntry(0.0);
xEntry.set(2.0);
double x = xEntry.get();

C++ API

#include <networktables/NetworkTableInstance.h>
#include <networktables/NetworkTable.h>
#include <networktables/DoubleTopic.h>

// Get default instance
nt::NetworkTableInstance inst = nt::NetworkTableInstance::GetDefault();

// Get a table
std::shared_ptr<nt::NetworkTable> table = inst.GetTable("datatable");

// Publish values
nt::DoublePublisher xPub = table->GetDoubleTopic("x").Publish();
xPub.Set(1.0);

// Subscribe to values
nt::DoubleSubscriber xSub = table->GetDoubleTopic("x").Subscribe(0.0);
double value = xSub.Get();

// Use entries
nt::DoubleEntry xEntry = table->GetDoubleTopic("x").GetEntry(0.0);
xEntry.Set(2.0);
double x = xEntry.Get();

Topics

Topics are named data channels. Each topic has a specific data type.

Topic Types

Type	Java	C++	Description
Boolean	`BooleanTopic`	`BooleanTopic`	true/false values
Integer	`IntegerTopic`	`IntegerTopic`	64-bit integers
Float	`FloatTopic`	`FloatTopic`	32-bit floating point
Double	`DoubleTopic`	`DoubleTopic`	64-bit floating point
String	`StringTopic`	`StringTopic`	Text strings
Boolean Array	`BooleanArrayTopic`	`BooleanArrayTopic`	Array of booleans
Integer Array	`IntegerArrayTopic`	`IntegerArrayTopic`	Array of integers
Float Array	`FloatArrayTopic`	`FloatArrayTopic`	Array of floats
Double Array	`DoubleArrayTopic`	`DoubleArrayTopic`	Array of doubles
String Array	`StringArrayTopic`	`StringArrayTopic`	Array of strings
Raw	`RawTopic`	`RawTopic`	Binary data
Struct	`StructTopic<T>`	`StructTopic<T>`	Serialized structs
Protobuf	`ProtobufTopic<T>`	`ProtobufTopic<T>`	Protocol buffers

Getting Topics

// From table
NetworkTable table = inst.getTable("mytable");
DoubleTopic topic = table.getDoubleTopic("value");

// Directly from instance
DoubleTopic topic2 = inst.getDoubleTopic("/mytable/value");

// Topic info
String name = topic.getName();  // "/mytable/value"
String type = topic.getType();  // "double"

Publishers

Publishers send data to a topic.

Publishing Data

// Create publisher
DoublePublisher pub = topic.publish();

// Publish with options
PubSubOption[] options = {
    PubSubOption.periodic(0.01),      // Publish every 10ms
    PubSubOption.sendAll(true),       // Send all value changes
    PubSubOption.keepDuplicates(true) // Keep duplicate values
};
DoublePublisher pub2 = topic.publish(options);

// Set values
pub.set(1.5);
pub.set(2.5, timestampMicros);

// Close publisher
pub.close();

// Create publisher
nt::DoublePublisher pub = topic.Publish();

// Publish with options
nt::DoublePublisher pub2 = topic.Publish(
    {{"periodic", 0.01}});

// Set values
pub.Set(1.5);
pub.Set(2.5, timestampMicros);

Array Publishers

// Double array
DoubleArrayPublisher arrayPub = topic.publish();
arrayPub.set(new double[]{1.0, 2.0, 3.0});

// String array
StringArrayPublisher stringPub = stringTopic.publish();
stringPub.set(new String[]{"a", "b", "c"});

Subscribers

Subscribers receive data from a topic.

Subscribing to Data

// Create subscriber with default value
DoubleSubscriber sub = topic.subscribe(0.0);

// Subscribe with options
PubSubOption[] options = {
    PubSubOption.pollStorage(10),     // Buffer size
    PubSubOption.periodic(0.02)       // Update rate
};
DoubleSubscriber sub2 = topic.subscribe(0.0, options);

// Get latest value
double value = sub.get();

// Get with default if no value
double value2 = sub.get(99.9);

// Check if value exists
boolean exists = sub.exists();

// Get timestamped value
TimestampedDouble tValue = sub.getAtomic();
double val = tValue.value;
long timestamp = tValue.timestamp;
long serverTime = tValue.serverTime;

// Close subscriber
sub.close();

Reading All Updates

// Get all value changes since last read
TimestampedDouble[] values = sub.readQueue();
for (TimestampedDouble val : values) {
    System.out.println(val.value + " at " + val.timestamp);
}

Entries

Entries combine publisher and subscriber functionality.

// Create entry
DoubleEntry entry = topic.getEntry(0.0);

// Read value
double value = entry.get();

// Write value  
entry.set(2.5);

// Set default (published if topic doesn't exist)
entry.setDefault(1.0);

// Atomic get and set
entry.set(entry.get() + 1.0);

Struct Topics

Publish custom structs with type safety.

import edu.wpi.first.math.geometry.Pose2d;
import edu.wpi.first.util.struct.Struct;

// Publish Pose2d (has built-in struct support)
StructTopic<Pose2d> poseTopic = 
    inst.getStructTopic("pose", Pose2d.struct);
StructPublisher<Pose2d> posePub = poseTopic.publish();
posePub.set(new Pose2d(1.0, 2.0, new Rotation2d()));

// Subscribe to Pose2d
StructSubscriber<Pose2d> poseSub = 
    poseTopic.subscribe(new Pose2d());
Pose2d pose = poseSub.get();

#include <frc/geometry/Pose2d.h>

// Publish Pose2d
nt::StructTopic<frc::Pose2d> poseTopic = 
    inst.GetStructTopic<frc::Pose2d>("pose");
nt::StructPublisher<frc::Pose2d> posePub = poseTopic.Publish();
posePub.Set(frc::Pose2d{1.0_m, 2.0_m, frc::Rotation2d{}});

// Subscribe to Pose2d
nt::StructSubscriber<frc::Pose2d> poseSub = poseTopic.Subscribe({});
frc::Pose2d pose = poseSub.Get();

NetworkTableInstance

Manages NetworkTables connections and configuration.

Client/Server Mode

NetworkTableInstance inst = NetworkTableInstance.getDefault();

// Start as client (coprocessor, dashboard)
inst.startClient4("myclient");
inst.setServerTeam(1234);  // Team number
// or
inst.setServer("10.12.34.2");  // IP address

// Start as server (robot)
inst.startServer();

// Stop
inst.stopClient();
inst.stopServer();

nt::NetworkTableInstance inst = nt::NetworkTableInstance::GetDefault();

// Start as client
inst.StartClient4("myclient");
inst.SetServerTeam(1234);
// or
inst.SetServer("10.12.34.2");

// Start as server
inst.StartServer();

Connection Listeners

int listenerId = inst.addConnectionListener(true, event -> {
    if (event.is(NetworkTableEvent.Kind.kConnected)) {
        System.out.println("Connected to server");
    } else if (event.is(NetworkTableEvent.Kind.kDisconnected)) {
        System.out.println("Disconnected from server");
    }
});

// Remove listener
inst.removeListener(listenerId);

Listeners

Value Change Listeners

// Listen to value changes
int listenerId = sub.addListener(event -> {
    System.out.println("New value: " + event.value.getDouble());
}, EnumSet.of(NetworkTableEvent.Kind.kValueAll));

// Remove listener
sub.removeListener(listenerId);

Topic Listeners

// Listen for new topics
int listenerId = inst.addListener(
    new String[]{"/vision/"},  // Topic prefix
    EnumSet.of(NetworkTableEvent.Kind.kPublish),
    event -> {
        System.out.println("New topic: " + event.topicInfo.name);
    }
);

Querying Topics

// Get all topics
TopicInfo[] topics = inst.getTopics();
for (TopicInfo info : topics) {
    System.out.println(info.name + ": " + info.type);
}

// Get topics by prefix
TopicInfo[] visionTopics = inst.getTopics("/vision/");

// Get topic info
Topic topic = inst.getTopic("/mytable/value");
TopicInfo info = topic.getInfo();
String name = info.name;
String type = info.type;
boolean retained = info.retained;

Properties

Topic Properties

// Set properties when publishing
PubSubOption[] options = {
    PubSubOption.retained(true),      // Retain value across connections
    PubSubOption.persistent(true)     // Save to persistent storage
};
DoublePublisher pub = topic.publish(options);

// Set properties on topic
topic.setProperty("units", "meters");
topic.setProperty("min", -10.0);
topic.setProperty("max", 10.0);

// Get properties
String units = topic.getProperty("units");

Data Logging

NetworkTables can log data to DataLog files.

import edu.wpi.first.util.datalog.DataLog;
import edu.wpi.first.networktables.DataLogManager;

// Start logging all NetworkTables data
DataLogManager.start();

// Custom log directory
DataLogManager.start("/home/lvuser/logs");

// Log specific entry
DataLog log = DataLogManager.getLog();
DoubleLogEntry logEntry = new DoubleLogEntry(log, "/drive/velocity");
logEntry.append(velocity);

Best Practices

Naming Conventions

// Use hierarchical naming
NetworkTable subsystem = inst.getTable("drivetrain");
DoublePublisher leftVelocity = subsystem.getDoubleTopic("left/velocity").publish();
DoublePublisher rightVelocity = subsystem.getDoubleTopic("right/velocity").publish();

// Results in topics:
// /drivetrain/left/velocity
// /drivetrain/right/velocity

Performance Tips

// Cache publishers/subscribers
private final DoublePublisher velocityPub = 
    table.getDoubleTopic("velocity").publish();

// Don't create publishers in loops
public void periodic() {
    velocityPub.set(getVelocity());  // Good
    // table.getDoubleTopic("velocity").publish().set(getVelocity());  // Bad!
}

// Use struct topics for complex data instead of multiple topics
StructPublisher<Pose2d> posePub = 
    inst.getStructTopic("pose", Pose2d.struct).publish();
posePub.set(pose);  // One publish instead of 3

Resource Management

// Close publishers/subscribers when done
public void close() {
    velocityPub.close();
    poseSub.close();
}

// Or use try-with-resources
try (DoublePublisher pub = topic.publish()) {
    pub.set(1.0);
}

Integration with Dashboards

Shuffleboard

import edu.wpi.first.wpilibj.shuffleboard.*;

// Automatically publishes to NetworkTables
ShuffleboardTab tab = Shuffleboard.getTab("Drive");
tab.add("Velocity", 0.0);
tab.add("Enabled", false);

SmartDashboard

import edu.wpi.first.wpilibj.smartdashboard.SmartDashboard;

// Legacy API (uses NetworkTables internally)
SmartDashboard.putNumber("Velocity", velocity);
double value = SmartDashboard.getNumber("Velocity", 0.0);

Vision Processing

// Robot side
NetworkTable visionTable = inst.getTable("vision");
DoubleArraySubscriber targetsSub = 
    visionTable.getDoubleArrayTopic("targets").subscribe(new double[0]);

// Coprocessor side
NetworkTableInstance.getDefault().startClient4("vision");
NetworkTableInstance.getDefault().setServerTeam(1234);

NetworkTable visionTable = inst.getTable("vision");
DoubleArrayPublisher targetsPub = 
    visionTable.getDoubleArrayTopic("targets").publish();
targetsPub.set(new double[]{x, y, distance});

C API

For C programs, ntcore provides a C API.

#include <ntcore_c.h>

// Get default instance
NT_Inst inst = NT_GetDefaultInstance();

// Publish value
NT_Topic topic = NT_GetTopic(inst, "/datatable/x");
NT_Publisher pub = NT_Publish(topic, NT_DOUBLE, "double", NULL, 0);
NT_SetDouble(pub, 0, 1.5);

// Subscribe to value
NT_Subscriber sub = NT_Subscribe(topic, NT_DOUBLE, "double", NULL, 0);
double value = NT_GetDouble(sub, 0.0);

Source Code

View the full source code on GitHub:

WPIUtil

Utilities including data logging

WPILibJ

Java robot API using NetworkTables

WPILibC

C++ robot API using NetworkTables

Core Libraries

Math & Utilities

Networking & Vision

Command Framework

Simulation

Documentation Index

​Overview

​Architecture

​Core Concepts

Topics

Publishers

Subscribers

Entries

​Basic Usage

​Java API

​C++ API

​Topics

​Topic Types

​Getting Topics

​Publishers

​Publishing Data

​Array Publishers

​Subscribers

​Subscribing to Data

​Reading All Updates

​Entries

​Struct Topics

​NetworkTableInstance

​Client/Server Mode

​Connection Listeners

​Listeners

​Value Change Listeners

​Topic Listeners

​Querying Topics

​Properties

​Topic Properties

​Data Logging

​Best Practices

​Naming Conventions

​Performance Tips

​Resource Management

​Integration with Dashboards

​Shuffleboard

​SmartDashboard

​Vision Processing

​C API

​Source Code

​Related Documentation

WPIUtil

WPILibJ

WPILibC

Overview

Architecture

Core Concepts

Basic Usage

Java API

C++ API

Topics

Topic Types

Getting Topics

Publishers

Publishing Data

Array Publishers

Subscribers

Subscribing to Data

Reading All Updates

Entries

Struct Topics

NetworkTableInstance

Client/Server Mode

Connection Listeners

Listeners

Value Change Listeners

Topic Listeners

Querying Topics

Properties

Topic Properties

Data Logging

Best Practices

Naming Conventions

Performance Tips

Resource Management

Integration with Dashboards

Shuffleboard

SmartDashboard

Vision Processing

C API

Source Code

Related Documentation