ROS2 Persistent Parameter Server

ROS 2 Persistent Parameter Server, that resides in the ROS 2 system to serve the parameter daemon. The other nodes(e.g the client demo provided in the code) can write/read the parameter in Parameter Server, and Parameter Server is able to store the parameter into the persistent storage which user can specify such as tmpfs, nfs, or disk.

See overview slide deck for general information.

• ROS2 Persistent Parameter Server
  • Background
  • Overview
    • Persistent Parameter Registration
      • Persistent Prefix
      • Scope Overview
    • Configurable Options
  • Sequence
  • Getting Started
    • Supported Distribution
      • Docker Container
    • Dependent Packages
    • Prerequisites
    • Build
    • Run
  • Test
    • Run
  • Known Issues
  • Authors
  • License

Background

The discussion is opened here, and centralized parameter server is not a good affinity to ROS 2 distributed system architecture. One of the most valuable things about ROS APIs is that we make sure that the messages have specific semantic meaning so that they can’t be misinterpreted. As we develop the ROS 2 tools and best practices we should make sure to bring that same level of rigor to parameters too for greater reusability and correctness.

Although, it is expected to be the following requirement.

• Global configuration that many nodes share (e.g. RTOS priorities, vehicle dimensions, …)
• Generic ROS 2 system property server.
• Persistent storage support to re-initialize the system. parameters are modified in runtime and cache it into persistent volume as well. and next boot or next re-spawn, modified parameter will be loaded at initialization. (parameter lifetime is dependent on use case, sometimes system lifetime, sometimes node lifetime.)
• Using ROS1 based application with Parameter Server.

Overview

Generally ROS 2 Parameter Server is simple blackboard to write/read parameters on that. The other nodes can write/read the parameter on the server to share them in the ROS 2 system. there is a new concept for “Persistent Parameter” which is described later.

ROS 2 Parameter Server is constructed on ROS parameter API’s, nothing specific API’s are provided to connect to the server from the client. Also, about the security it just relies on ROS 2 security aspect.

Persistent Parameter Registration

Persistent Prefix

persistent parameter must have prefix “persistent”

Scope Overview

parameter server has the following scope for persistent parameter. since parameter server is built on top of ROS 2 Parameter API, parameter server supports “persistent” parameter based on /parameter_events topic.

Configurable Options

• Node Name Since ROS 2 parameter is owned by node, node name will be needed to access the parameters, this is designed to clarify semantics for the parameters and owners. Node name will be “parameter_server” if node name is not specifies. so the other nodes can use “parameter_server” as well to access in the same system Parameter Server. If there must exist multiple parameter servers, these parameter servers need to specify a different node name, such as “parameter_server_[special_string]”, please notice that ROS 2 node name can only contains alphanumerics and ‘_’.
• Persistent Volume Definition of “Persistent” is different from user and use cases, so it should be configurable to set the path to store the persistent –file-path FILE_PATH parameter. Expecting if the parameter’s lifespan is system boot, path would be “/tmp” because user wants a fresh start via reboot. Or maybe physical persistent volume is chosen if users want to keep the parameter into the hardware storage. At the initialization time, Parameter Server will load the parameters from the storage which is specified by user.
• Node Options there are two important options, allow_undeclared_parameters: (default true) automatically_declare_parameters_from_overrides: (default true)

all of the configuration options will be passed via arguments as following.

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