Custom message definition in ROS - Deep Dive

Overview - Custom message definition

What is it?

Custom message definition in ROS means creating your own data format to send between different parts of a robot system. Instead of using only built-in message types, you design messages that fit your specific needs. These messages describe the structure and types of data you want to share. This helps different robot components understand each other clearly.

Why it matters

Without custom messages, you would be limited to generic data formats that might not fit your robot's unique sensors or commands. This would make communication confusing or inefficient. Custom messages let you tailor data exchange exactly, improving clarity and performance. It makes building complex robot behaviors easier and more reliable.

Where it fits

Before learning custom messages, you should understand basic ROS concepts like nodes, topics, and standard messages. After mastering custom messages, you can learn about services, actions, and advanced communication patterns in ROS.

Mental Model

Core Idea

A custom message is a user-defined data blueprint that structures how robot parts talk by specifying exactly what data is sent and how.

Think of it like...

It's like designing a custom form to fill out when sending a package, where you decide what information to include so the receiver knows exactly what to expect.

┌─────────────────────────────┐
│ Custom Message Definition    │
├───────────────┬─────────────┤
│ Field Name    │ Data Type   │
├───────────────┼─────────────┤
│ position      │ float64[3]  │
│ velocity      │ float64     │
│ status        │ string      │
└───────────────┴─────────────┘

Build-Up - 7 Steps

1

Foundation

Understanding ROS Messages Basics

Concept: Learn what a ROS message is and how it carries data between nodes.

ROS messages are simple data containers with predefined fields. Each message type has a name and a set of fields with types like integers, floats, or strings. Nodes publish and subscribe to topics using these messages to communicate.

Result

You understand that messages are the language ROS nodes use to talk to each other.

Knowing that messages are structured data packets helps you see why defining their format clearly is essential for communication.

2

Foundation

Standard vs Custom Messages

Concept: Distinguish between built-in ROS messages and user-defined custom messages.

ROS provides many standard messages like geometry_msgs/Pose or std_msgs/String. But sometimes these don't fit your data needs. Custom messages let you create new types with fields tailored to your robot's data.

Result

You realize that custom messages extend ROS communication beyond fixed formats.

Understanding this difference prepares you to create messages that exactly match your robot's data.

3

Intermediate

Writing a Custom Message File

🤔Before reading on: do you think a custom message file uses a programming language or a simple text format? Commit to your answer.

Concept: Learn the syntax and structure of a .msg file to define custom messages.

A custom message is defined in a plain text file with the .msg extension. Each line declares a field with a type and a name, for example: float64 temperature. You can use primitive types, arrays, or other message types.

Result

You can write a valid .msg file describing your data structure.

Knowing the simple syntax of .msg files makes creating custom messages straightforward and error-free.

4

Intermediate

Building and Using Custom Messages

🤔Before reading on: do you think custom messages are ready to use immediately after writing the .msg file? Commit to your answer.

Concept: Understand the build process and how to use custom messages in ROS code.

After creating .msg files, you must update your package configuration (package.xml and CMakeLists.txt) to generate message code. Then, rebuild your package. Your code can import and use the new message types to publish or subscribe.

Result

Your ROS nodes can send and receive your custom messages correctly.

Knowing the build steps prevents confusion when your messages don't appear in code automatically.

5

Intermediate

Using Nested and Array Fields

Concept: Learn how to include other message types and arrays inside your custom message.

Custom messages can have fields that are arrays (fixed or variable length) or other message types. For example, you can include geometry_msgs/Point inside your message or define float64[] values. This allows complex data structures.

Result

You can design rich messages that represent complex robot data.

Understanding nested and array fields unlocks powerful ways to model real-world robot information.

6

Advanced

Handling Message Versioning and Compatibility

🤔Before reading on: do you think changing a custom message breaks all existing nodes immediately? Commit to your answer.

Concept: Learn how to manage changes in custom messages without breaking communication.

Changing a message definition can cause incompatibility between nodes. Best practice is to avoid changing existing fields or to create new message versions. ROS tools help detect mismatches. Planning message evolution is key for stable systems.

Result

You can update messages safely without crashing your robot's communication.

Knowing how to handle message changes prevents costly downtime and debugging in real robots.

7

Expert

Custom Messages in Multi-language Environments

🤔Before reading on: do you think custom messages work the same in Python and C++ ROS nodes without extra steps? Commit to your answer.

Concept: Understand how custom messages are generated and used across different ROS client languages.

ROS generates language-specific code for custom messages during build. Python uses dynamic imports, while C++ uses compiled headers. Some message features behave differently or require extra care in each language. Knowing these differences helps write portable code.

Result

You can write ROS nodes in multiple languages using the same custom messages reliably.

Understanding language-specific message handling avoids subtle bugs and improves cross-language integration.

Under the Hood

When you define a custom message, ROS tools generate source code in your target languages that represent the message structure. This code includes serialization and deserialization logic to convert data to a byte stream for transport over the ROS communication system. At runtime, publishers serialize message data, and subscribers deserialize it back into usable objects.

Why designed this way?

ROS uses code generation to ensure messages are strongly typed and efficient across languages. This approach avoids runtime parsing overhead and enforces consistency. Alternatives like dynamic typing would be slower and error-prone. The design balances flexibility with performance and safety.

┌───────────────┐       ┌───────────────┐       ┌───────────────┐
│ .msg File    │──────▶│ Code Generator│──────▶│ Language Code │
│ (Definition) │       │ (Build Step)  │       │ (C++, Python) │
└───────────────┘       └───────────────┘       └───────────────┘
        │                                               │
        ▼                                               ▼
┌───────────────┐                               ┌───────────────┐
│ Publisher Node│                               │ Subscriber Node│
│ Serializes    │                               │ Deserializes   │
│ Message Data  │                               │ Message Data   │
└───────────────┘                               └───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Do you think you can use a custom message immediately after writing the .msg file without rebuilding? Commit to yes or no.

Common Belief: Once you write a custom message file, you can use it right away in your code.

Reality: You must update build files and rebuild your package to generate message code before using it.

Why it matters: Skipping the build step causes errors and confusion because the message type won't exist in code.

Quick: Do you think changing a custom message definition is always safe and backward compatible? Commit to yes or no.

Common Belief: You can freely change custom messages anytime without affecting running nodes.

Reality: Changing message fields can break communication between nodes expecting the old format.

Why it matters: Unplanned changes cause runtime errors and lost data, disrupting robot operation.

Quick: Do you think custom messages behave identically in Python and C++ ROS nodes? Commit to yes or no.

Common Belief: Custom messages work exactly the same in all ROS client languages without extra care.

Reality: Different languages have different message code generation and usage patterns that can cause subtle bugs.

Why it matters: Ignoring language differences leads to integration problems in multi-language robot systems.

Quick: Do you think you can define any data type you want in a custom message? Commit to yes or no.

Common Belief: You can use any programming language type or complex object inside a custom message.

Reality: Custom messages support only a fixed set of primitive types, arrays, and other message types.

Why it matters: Trying unsupported types causes build failures and communication errors.

Expert Zone

1

Custom messages must be carefully versioned and documented to avoid silent communication failures in large systems.

2

The order of fields in a message affects serialization and must remain consistent across versions.

3

Using nested messages can increase complexity but improves modularity and reuse of common data structures.

When NOT to use

Avoid custom messages when standard messages fully meet your needs to reduce maintenance overhead. For dynamic or loosely structured data, consider using ROS parameters or dynamic reconfigure instead.

Production Patterns

In production, teams often create message packages shared across projects to ensure consistency. They use semantic versioning for messages and automated tests to verify compatibility. Messages are designed to be backward compatible to allow rolling updates.

Connections

Data Serialization

Custom messages rely on serialization to convert structured data into byte streams for transport.

Understanding serialization helps grasp how messages are efficiently sent and received in ROS.

API Design

Designing custom messages is similar to designing APIs where clear data contracts are essential.

Good message design improves system modularity and reduces bugs, just like good API design.

Human Language Grammar

Custom messages define a strict grammar for robot communication, like grammar rules in human languages.

Knowing this analogy highlights why precise message definitions prevent misunderstandings between nodes.

Common Pitfalls

#1 Trying to use a custom message without rebuilding the package.

Wrong approach:

import my_package.msg
msg = my_package.msg.MyCustomMessage() # Error: message not found

Correct approach:

Run catkin_make or colcon build after adding .msg files, then import and use the message.

Root cause:

Not understanding that message code generation happens during the build process.

#2 Changing a message field type after deployment without versioning.

Wrong approach:

float64 temperature # changed to int32 temperature in new version without updating nodes

Correct approach:

Create a new message version or add new fields instead of changing existing ones.

Root cause:

Ignoring backward compatibility and the impact on running nodes.

#3 Using unsupported data types in message definitions.

Wrong approach:

object custom_data # invalid type in .msg file

Correct approach:

Use only supported primitive types, arrays, or other message types.

Root cause:

Misunderstanding the limited set of allowed types in ROS messages.

Key Takeaways

Custom messages let you define exactly what data your robot parts exchange, improving clarity and flexibility.

You must write .msg files, update build configurations, and rebuild your package to use custom messages.

Message changes require careful versioning to avoid breaking communication between nodes.

Custom messages generate language-specific code that handles serialization for efficient data transport.

Understanding custom messages deeply helps build robust, maintainable, and scalable robot systems.

Practice

1. What is the main purpose of defining a custom message in ROS?

easy

A. To send specific data types tailored to your robot's needs

B. To create new robot hardware components

C. To write Python scripts for robot control

D. To install ROS packages automatically

Solution

1. Step 1: Understand the role of custom messages

   Custom messages allow you to define exactly what data your robot nodes exchange.

2. Step 2: Compare with other options

   Options B, C, and D relate to hardware, scripting, or package management, not message definition.

3. Final Answer:

   To send specific data types tailored to your robot's needs -> Option A

4. Quick Check:

   Custom messages = tailored data [OK]

Hint: Custom messages define data, not hardware or scripts [OK]

Common Mistakes:

• Confusing message definition with hardware setup
• Thinking custom messages are scripts
• Assuming messages install packages

2. Which of the following is the correct syntax to define a field named speed of type float32 in a ROS .msg file?

easy

A. speed float32

B. float32 speed

C. float speed32

D. float speed

Solution

1. Step 1: Recall ROS message field syntax

   In ROS .msg files, the type comes first, then the field name, e.g., float32 speed.

2. Step 2: Check each option

   float32 speed matches the correct syntax. Options B, C, and D have incorrect order or wrong types.

3. Final Answer:

   float32 speed -> Option B

4. Quick Check:

   Type before name in .msg files [OK]

Hint: Type first, then field name in .msg files [OK]

Common Mistakes:

• Swapping field name and type order
• Using incorrect type names
• Omitting the type or name

3. Given this custom message definition in RobotStatus.msg:

bool is_active
float32 battery_level
string status_message

What will be the default value of is_active when a new message is created in a ROS node?

medium

A. false

B. 1

C. null

D. true

Solution

1. Step 1: Understand default values in ROS messages

   Boolean fields default to false when a message is created without explicit initialization.

2. Step 2: Evaluate options

   false is false, which matches the default. Options A and B represent true values, and C is invalid for bool.

3. Final Answer:

   false -> Option A

4. Quick Check:

   Boolean default = false [OK]

Hint: Boolean fields default to false unless set [OK]

Common Mistakes:

• Assuming booleans default to true
• Confusing null with default values
• Using numeric 1 instead of boolean true

4. You wrote this in your SensorData.msg file:

int32 temperature
float64 humidity
string location

But when building your ROS package, you get an error. What is the likely cause?

medium

A. Using float64 instead of float32 is invalid in ROS messages

B. Field names cannot be lowercase

C. Missing blank line at the end of the file

D. No error; the message definition is correct

Solution

1. Step 1: Check ROS message type validity

   ROS supports float64 as a valid type, so Using float64 instead of float32 is invalid in ROS messages is incorrect.

2. Step 2: Verify field naming and formatting

   Lowercase field names are allowed, and no blank line is required at the end.

3. Final Answer:

   No error; the message definition is correct -> Option D

4. Quick Check:

   Valid types and names mean no error [OK]

Hint: ROS supports float64 and lowercase names [OK]

Common Mistakes:

• Thinking float64 is invalid
• Assuming field names must be uppercase
• Expecting blank line at file end

5. You want to define a custom message RobotCommand.msg that includes a nested message Header from the standard ROS messages and a custom field command of type string. Which is the correct way to define this?

hard

A. Header header
string command

B. header Header
command string

C. std_msgs/Header header
string command

D. Header: std_msgs/Header
string command

Solution

1. Step 1: Understand nested message syntax in ROS

   To use a message from another package, you must prefix the type with the package name, e.g., std_msgs/Header.

2. Step 2: Check each option

   std_msgs/Header header string command correctly uses std_msgs/Header header. Header header string command misses the package prefix, C reverses type and name, D uses invalid syntax.

3. Final Answer:

   std_msgs/Header header string command -> Option C

4. Quick Check:

   Use package/type for nested messages [OK]

Hint: Prefix nested types with package name [OK]

Common Mistakes:

• Omitting package name for nested messages
• Swapping field name and type
• Using colon syntax incorrectly