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'''Robotics middleware''' is [[middleware]] to be used in complex [[robot]] control software systems.
:"robotic middleware is designed to manage the complexity and heterogeneity of the hardware and applications, promote the integration of new technologies, simplify software design, hide the complexity of low-level communication and the sensor heterogeneity of the sensors, improve software quality, reuse robotic software infrastructure across multiple research efforts, and to reduce production costs."<ref>{{cite journal|last1=Elkady|first1=Ayssam|title=Robotics Middleware: A Comprehensive Literature Survey and Attribute-Based Bibliography|journal=Journal of Robotics|date=29 January 2012|volume=2012|doi=http://dx.doi.org/10.1155/2012/959013|url=https://www.hindawi.com/journals/jr/2012/959013/|ref=959013}}</ref>
==General concepts==
As ''glue'' code, the middleware should be invisible, and introduce no overhead or extra constraints on the components. This is of course an unreachable [[non-functional requirement|(non-functional)]] design requirement, so compromises have to be made. Different middleware projects mostly differ in which compromises are made (implicitly, most often!) and in which robotics applications are being targeted.
As glue software, middleware should support the ''coupling'' of subsystems, which is a fundamentally different software development skill than the ''[[separation of concerns|decoupling]]'' [[Requirements analysis|design requirements]] that most [[software engineer]]s are educated in. Indeed, decoupling is the major focus of class library development: one should make a library as independent from other libraries as possible. Middleware, on the other hand, must provide optimal support for ''coupling'': allowing to couple multiple, decoupledly designed components together in a way that satisfies [[systems engineering|system-level]] requirements.
The ''[[Principle of compositionality|composition]]'' of sub-systems into a new system is often a difficult task: designing the ''[[Systems architecture|architecture]]'' of the system is hard, since it requires to find the optimal trade-offs between ''all'' system requirements and to realise the optimal cooperation between all system components. There are currently close to no [[software tool]]s, or internationally accepted standards and [[workflow]]s, to support the job of the system designer.
Some of the problems to be solved when designing a composite system are:
* the composed system should have an interface that is not (much) more complex than the combination of all composing subsystems. Otherwise, the composite system offers no real design advantages to the human developer. In practice, this means that the composite system developer makes some design decisions that restrict the use of each of the components to only a part of its potential ___domain.
* the composed system should act to its users as one consistent, monolithic system in itself.
* building a system from reusable components is challenging with respect to the balance between ''performance'' (it (seems to be) easier to optimize performance if one is not restricted to using only pre-built components) and ''ease of reuse''.
At a conceptual level, a complex robot controller has components that each ''serve'' one of the following four [[separation of concerns|concerns]]:
* '''Communication''': components must exchange information (data, events, commands,…), and ''how'' this exchange is done is an important property of the composite system.
* '''Computation''': each component performs certain computations that are necessary to provide the functionality that is expected from the system.
* '''Configuration''': components should be usable in more than one possible configuration (i.e., concrete settings for each of their variable parameters), but the amount of configuration is an important aspect of the design and the implementation of components and systems. Configuration is required at various moments in the lifetime of a software system: [[compile time]], [[software deployment|deployment]] time, [[Run time (program lifecycle phase)|run time]],…
* '''Coordination''': the activities in components have to be coordinated by ''something'' at the system level, in order to guarantee the expected behaviour and performance of the composed system. Coordination involves: decision making, scheduling, (de)activating subsystems and/or their interconnections,…
Whether these four above-mentioned primitive concepts are really ''minimal'' (i.e., one needs only these four concepts to cover all relevant system design aspects) and/or ''complete'' (i.e., these concepts cover ''all'' possible systems) is not so important in this discussion; the important thing is that, at systems level, the designer should benefit from a level of abstraction that is an appropriate trade-off between complexity (the fewer concepts are needed, the better) and flexibility (the more diverse systems can be represented by the conceptual primitives, the better). Again, the ''appropriate'' trade-off is not an absolute concept, so it will depend on many (non-functional) design requirements. As such, both the number and the nature of the primitive concepts, and the particular trade-off, are discriminating factors between different middleware projects.
Composing two or more components that each belong to one of these categories is an [[software architecture|architectural]] design activity. It is often complex, in that it has to balance a large amount of functional and non-functional requirements (performance, [[compositionality]],…). The robotics research community has not yet come up with fully satisfying software frameworks, architectures, or methodologies to deal with the composition problem, but a large number of ([[open source]]) projects exist already, and they all claim to provide good solutions to this component composition problem, at least to (implicitly described) parts of it. Anyway, many fundamental questions are still unsolved, or rather, are still unnoticed within the robotics research community. This article presents an overview of some of the relevant issues to be considered in the design and use of such middleware, and also provides an annotated list of middleware projects with an evaluation of which design constraints they took (or did not take) into account, and about how well they perform.
==Composition of subsystems==
How to ''optimally'' compose subsystems into a larger system is the core activity of system developers, but is remains more of an art than of a science. The major challenge is to develop subsystems that are ''stable'' on their own, while still very ''willing'' to be part of a larger system. There are four different ways of composing software components:
* linking object classes by providing explicit references to each other:
* composing object classes without them knowing about each other
* composing [[component-based programming|components]]
* composing [[software service]]s:
A composed system is ''stable'' if it can be used without the user having to know that it is a composed system in itself. Examples of commonly used compositions that are not stable are:
* [[Simulink]] blocks in feedback controllers: one often has to introduce explicit delay blocks; one cannot predict the overall performance on the basis of the performance of the individual blocks.
* [[Real-time computing|Realtime]] aspects at the system level: only one of the components can really have the highest priority; schedulability of the activities in all components becomes exponentially harder to analyse, let alone to guarantee, when the number of components grows; [[Inter-process communication|IPC]] [[deadlock]]s become more likely, and more difficult to trace; [[formal verification]] becomes more difficult, since [[jitter]] and [[latency (engineering)|latency]] deteriorate in less predictable ways, compromising the ideal, abstract model of states with atomic and infinitely fast transitions and condition checks.
* Adding sensor processing or control blocks to a control loop: each new sensor can bring with it a [[device driver]] that requires a different [[sampling frequency]], that provides a different [[spatial resolution]], …
==Robotics middleware projects==
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