Robotics Service Bus

h1. Overview

h2. Problem statement

* easier integration of complex (partially) distributed systems required
** in-house (one middleware)
** adaption (combination with other middlewares)
* targeting at technical architecture (only supporting functional concepts)
* performance requirements
* platform requirements
* programming language requirements
** desired set of languages
** natural feeling in each language by using native mechanisms
* basis for describing and analyzing systems required
* accept heterogenity: open architecture required
** specifications for data format and transport protocols
** use of existing (and open) naming services

h2. Sebastian's Aims

* XCF support ends
** better solution required
* limited client interface that converts to domain data as soon as possible to prevent dependency on middleware throughout the whole client code
* variable dispatching mechanisms not forced by the framework

h2. Lessons learned (from other frameworks, e.g. XCF, YARP, ROS, NaoQi)

identified problems:

* Footprint too large for embedded systems
* Architectural erosion
* Unmaintainable code base
* important parameters and implicit assumptions should not be hidden in the code for analysis
* missing introspection functionalities
* bad error handling strategies, e.g. missing transparency for the client (i.e. swallowed exceptions)
* sending schema-free messages was too easy

lessons learned:

* layered architecture with improved functional decomposition in distinct libraries, layers:
** core (bus realization across different transports)
** tools
** XML compatibility layer
** XCF API adapters
* clear dependency statements:
** core must survive with the least amount of external libraries
** different language implementation should not depend on a single base implementation (reduce development and deployment complexity, increase usability of the framework)
* development process
** bigger developer team > 1 experienced person
** clear testing strategies
** continuous integration
* allow more flexibility through defined interfaces and protocols
** allow different transports, e.g. in-process
** encoding flexibility for user data to meet domain and performance requirements
* provide additional system structures like services to aggregate coherent functionality
* avoid centralized nameservice
* possibility to send schema-free messages is still possible, but using well-defined schemas is much more enforced

h2. What is and what it is not

* Soft-RT but no hard real-time
* in-process, shared-mem, ipc
* no component architecture

h2. Functional properties (so far):

* Event-driven middleware with broadcast semantics
** Additional communication patterns implemented on top of this base functionality
* logically unified bus across multiple transports
** currently a spread-based and an in-process transport are available
** extensible to other transports -> compatibility to existing frameworks
* Bus is structured by hierarchical channels
** / receives all messages of sub-scopes like /a/test
* Support for filtering events:
** content-based
** on meta-data
* URI-based

Adopted concepts (from XCF):
* event-driven
* broadcast semantics
* content-based filtering
* introspection

h2. Non-functional properties (so far):

* lightweight (will remain [for the core]):
** small dependency footprint for the core layer
** Targeting at embedded systems (e.g. Nao)
* Languages: cpp, java, python, common lisp
** real implementations, no bindings to provide natural interfaces in each language
* No single wire type like XML:
** instead arbitrary client data is serialized to different wire types using (user-installed) converters
*** default set of converters exists (e.g. source:/trunk/cpp/core/src/rsb/converter/StringConverter.cpp)
** provide flexible and fast serialization mechanisms
** XML support layer as additional library
* performance requirements on network-based (UDP) and in-process transport
** no hard real-time
** avoid serialization when possible
* No hard dependency on name service

h2. Planned functionality:

* introspection tools
* model-based setup
* Naming, Service discovery
** Distributed (e.g. mdns, upnp, etc.)
* Sychronization primitives
** E.g., AV Fusion
* Tools, in particular
** Introspection
** Logging, Replay
* Platforms
** Web integration (WebServices)
** XMPP
** Mobile devices
** DNLA (streaming, etc.)
* Timing
** Time-informer
*** Clock source
*** Synchronization
** Associated Patterns
*** Clock-driven behavior
*** Clock-based filtering
* Model Layer/Component Model
** Role of @Service@ class
* Framework Interoperability
* Plugin Mechanism (for C++)
* Content-based filtering
** ProtocolBuffer layer
** Ideally, without unpacking