Robotics Service Bus

h1. SpreadProtocol

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This wiki page describes the protocol used by the Spread-based connectors.

h2. Data Format

Data exchanged on the wire by the Spread-based transport is encoded using "Google protocol buffers":http://code.google.com/apis/protocolbuffers/. A Spread message always contains a fragment of a notification (source:/trunk/protocol/rsb/protocol/FragmentedNotification.proto) as elementary communication unit. Descriptions of the Notification contents are given as comments on the descriptor files.

h2. Fragmentation

Because Spread has a message size limit, a single @Notification@ may not be sufficient to transport a whole event with a huge amount of user data. Hence, events may be encoded in several @FragmentedNotification@ s which are sent sequentially. Multiple @FragmentedNotification@ objects are constructed according to the following rules:

* The size of individual fragments (i.e. serialized size of the @FragmentedNotification@ objects) must not exceed 100,000 octets

* Fragment numbers are in the range [0, NUMBER-OF-FRAGMENTS - 1]

* The fields which differ among @FragmentedNotification@ s for one event are:

** The number of the fragment (@FragmentedNotification.data_part@ field)

** The paylod data (@FragmentedNotification.notification.data@ field)

** Which fields of the @Notification@ embedded in @FragmentedNotification.notification@ are present (see below)

* Each of the @FragmentedNotification@ objects contains a @Notification@ object

** The id of the event (@Notification.event_id@ field) is always present (to specify the event to which a fragment belongs)

** For the initial fragment (fragment number 0), all fields of the embedded @Notification@ object are present

** For subsequent fragments (fragment number >= 1), only the following fields of the embedded @Notification@ object are present:

*** @Notification.event_id@

*** @Notification.data@

h2. Hierarchical Bus

The hierarchical bus is created by sending each message to a group corresponding to its scope as well as groups corresponding to all super scopes including "/" (multigroup mulitcast).

_Example_

<pre>

super-scopes(/foo/bar/, include-self? = yes) = /, /foo/, /foo/bar/

</pre>

h2. Group Names

Group names are created by applying the following steps to the fully formal scope string representation (*including trailing slash*):

# Hash the scope string using "MD5":http://en.wikipedia.org/wiki/MD5.

# Convert the 16 bytes of output to a 32 character string by concatenating the *zero-padded* hex (base-16) representations of the individual bytes.

  Letters of the hex representation have to be lower case.

# Remove the final character of the hex representation of the hash.

  (Since spread group names can only be 32 bytes long _including the 0-terminator_)

_Example_

<pre>

/         -> "6666cd76f96956469e7be39d750cc7d\0"

/foo/     -> "4f87be8f6e593d167f5fd1ab238cfc2\0"

/foo/bar/ -> "1c184f3891344400380281315d9e738\0"

</pre>

h2. Quality of Service

The following table explains how the 2D RSB QoS settings are mapped to spread message types.

|             |_.UNRELIABLE     |_.RELIABLE     |

|_.UNORDERED  | UNRELIABLE_MESS | RELIABLE_MESS |

|_.ORDERED    | FIFO_MESS       | FIFO_MESS     |

h2. Implementations

|_.Language   |_.File(s)/Directory                                   |

| C++         | source:trunk/cpp/core/src/rsb/transport/spread       |

| Java        | source:trunk/java/core/src/rsb/transport/spread      |

| Python      | source:"trunk/python/core/rsb/rsbspread/__init__.py" |

| Common Lisp | source:trunk/cl/cl-rsb/src/transport/spread          |