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Concepts

.. seealso::

   [Wienke2011-AMC]_
     For a concise introduction to the basic concepts of |project| in
     publication form

The Robotics Service Bus (|project|) is a message-oriented, event-driven middleware aiming at scalable integration of robotics systems in diverse environments. Being fundamentally a bus architecture, |project| structures heterogeneous systems of service providers and consumers using broadcast communication over a hierarchy of logically unified channels instead of a large number of point-to-point connections. Nevertheless |project| comes with a collection of communication patterns and other tools for structuring communication, but does not require a particular functional or decomposition style.

|project| is implemented as a flexible, lightweight toolkit. Based on previous experiences, the design and implementation try to avoid centralization and complicated dependency structures. Instead, functionality is organized in layers with minimal, clearly stated dependencies. The core library merely supplies the abstractions and machinery for building communication systems subject to varying requirements. This core is implemented in multiple programming languages with implementations trying to be in the spirit of the respective language as much as possible. As a result, |project| is not tied to a particular network transport, serialization mechanism or programming language. In addition, much effort is put into systematic testing and continuous integration.

These conceptual and implementation properties hopefully allow |project| to scale across a wider range of diverse functional requirements, heterogeneous hardware platforms and varying reliability and timing requirements than other robotics middlewares. Additionally, |project|'s open architecture and project structure and lightweight implementation enable its use with small entry barriers and little framework lock-in.

Event

.. seealso::

   :ref:`specification-event`
     Detailed technical specification of :term:`events <event>`

An :term:`event` is the basic unit of exchanged data in |project|. Hence, all information required to fully specify and trace the condition it represents need to be present in the :term:`event`. To fulfill these requirements, our :term:`event` model consists of the following components:

Payload

The payload of an :term:`event` is a user-defined object of the respective programming language which contains the major information specifying the condition the :term:`event` represents.

It can be of an arbitrary domain type which reduces the framework lock-in by means of an early transition from framework types to domain objects for technical realization.

ID

A unique ID for each :term:`event` in an |project|-based system to make :term:`events <event>` addressable and foster traceability.

Meta Data

Each :term:`event` is supplemented by meta data.

It consist of the :term:`event` the ID of the sending :term:`participant` and several timestamps that

  • specify timing information relevant to the condition represented by the :term:`event` (user-extensible).
  • make the processing of the :term:`event` within |project| traceable.

Besides these framework-supplied items, a key-value store for string-based additional meta data items is available for the client and user-defined timestamps can be added.

Causal Vector

This vector allows to represent the causing :term:`events <event>` of a given :term:`event`, as proposed in [Luckham2001PEI]_. It facilitates automatic system analysis and debugging.

Destination :term:`Scope`

Specifies the recipients of the event notification by restricting the visibility of :term:`event` notifications [Muehl2006-DEB]_.

The next section explains this concept in greater detail.

Scope

.. seealso::

   :ref:`specification-scope`
     Detailed technical specification of :term:`scopes <scope>`

|project| forms a logically unified bus across different :term:`transport` mechanisms. Different :term:`participants <participant>` connect to this bus. :term:`Informers <informer>` send :term:`events <event>`, whereas :term:`listeners <listener>` receive :term:`events <event>`. From a logical perspective, no point-to-point connections are established.

In order to structure the communication via the bus – or stated differently, restrict the visibility of :term:`events <event>` for :term:`participants <participant>` – |project| utilizes a hierarchical channelization scheme. This scheme is best explained by its declarative representation as a :term:`scope`, which is represented in |project| with a hierarchical notation compatible with the path component of URIs [RFC2396-URI]_. E.g. sending an :term:`event` with destination :term:`scope` /robot/camera/left/ will make this :term:`event` visible in the :term:`channels <channel>` represented by :term:`scopes <scope>` /robot/camera/left/, /robot/camera/, /robot/, and /. Consequently, / represents a :term:`channel` where all :term:`events <event>` of the system are visible. Each :term:`participant` is associated to one :term:`channel`, but multiple :term:`participants <participant>` can participate in the same :term:`channel` (m : n semantics).

The chosen hierarchical :term:`channel` layout provides benefits for logging purposes and provides a first-class means of the framework to structure the data space, e.g. with :term:`subscopes <subscope>` for different services. However, it also increases the chance that a :term:`listener` receives unexpected data, because a new :term:`informer` appeared on a :term:`subscope` of the :term:`listener’s <listener>` :term:`scope`. |project|’s :term:`filter` mechanism allows clients to efficiently specify which :term:`events <event>` they want to receive.

Filter

TODO

Types

|project| is concerned with two kinds of types:

Mapping between Wire Schema and Programming Language Types

This section documents the mapping between :term:`wire schemas <wire schema>`, designators of which are included in |project| :term:`notifications <notification>`, and corresponding programming language types. The values that are actually contained in :term:`notifications <notification>` are called "String Designators" of :term:`wire schemas <wire schema>` here.

Fundamental Types

Wire Schema String Designator C++ Python Java Common Lisp
No value "void" void None null nil
Double precision float "double" double   double double-float
Single precision float "float" float float float single-float
32 bit signed integer "int32" int32   int (signed-byte 32)
64 bit signed integer "int64" int64   long (signed-byte 64)
32 bit unsigned integer "uint32" uint32   int (unsigned-byte 32)
64 bit unsigned integer "uint64" uint64   long (unsigned-byte 64)
bool "bool" bool bool boolean boolean
ASCII string "ascii-string" std::string str String string
UTF-8 string "utf-8-string" std::string unicode String string
Sequence of Bytes "bytes" std::string   ByteString (simple-array (unsigned-byte 8) (*))
:term:`Scope` "scope" rsb::Scope rsb.scope rsb.Scope rsb:scope

Note

In C++, support for the ASCII string and UTF-8 string schemas is limited in the following ways:

  • When decoding data in either schema, invalid strings will be accepted without signaling an error.
  • In both schemas, string values are represented as std::string objects which know nothing about the respective encodings.
    • In particular, UTF-8 multi-byte sequences appear as multiple char s.

Structured Data

TODO

Connector

URIs

.. seealso::

   :ref:`specification-uris`
     Specification for handling of URIs in |project|.

URIs or URLs are used in the following situations

Configuration

.. seealso::

   :ref:`specification-config`
     Specification for the configuration of |project|.

Quality of Service

For :term:`listeners <listener>`, any guarantee applies to the stream of :term:`events <event>` received from the bus (not to the entire processing of a given :term:`event`). In particular, it is possibly that the effective guarantees are weaker than those specified for the :term:`listener` (if the :term:`informer` has weaker guarantees than the :term:`listener`).

For :term:`informer`, any guarantee applies to the submitting of :term:`events <event>` to the bus. Guarantees at the receiving end may effectively be weakened depending on the :term:`listener` configuration.

Note

In the following lists of guarantees, subsequent items include all guarantees given by preceding items.

Ordering

Unordered

:term:`Events <event>` are delivered in (potentially) arbitrary order.

Ordered

Every :term:`listener` receives the :term:`events <event>` of one :term:`informer` in the order the :term:`informer` sent the :term:`events <event>`. No guarantees are given for :term:`events <event>` of multiple :term:`informers <informer>`.

Independently of the requested ordering, no relations are guaranteed between :term:`events <event>` arriving at distinct :term:`listeners <listener>`.

Reliability

Unreliable

:term:`Events <event>` may be dropped and not be visible to a :term:`listener`.

Reliable

:term:`Events <event>` are guaranteed to be delivered. An error is signaled when :term:`events <event>` cannot be delivered.

Threading

:term:`Informers <informer>` are thread-safe.

:term:`Listener` are thread-safe. This implies: