Robotics Service Bus

h1. SpreadProtocol

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

h2. Data Format

Data exchanged on spread is encoded using Google protocol buffers. A spread message always contains a notification (source:/trunk/RSBProtocol/rsbprotocol/Notification.proto) as elementary communication unit. Descriptions of the Notification contents are given as comments on the descriptor file.

h2. Sequencing

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 notification sent subsequently. In this case the multiple Notification objects are constructed according to the following rules:

* The size of individual fragments (i.e. the values of @Attachment.binary@) must not exceed 100.000 octets

* Most field values are repeated in all Notifications, in particular:

** The id of the event (@id@ field)

** The number of Notifications into which the event has been split (@num_data_parts@ field)

* The two fields which differ among Notifications for one event are:

** The index of the data chunk encoded in the Notification (@data_part@ field).

** The user data (@binary@ field of the @Attachment@ object) are split. 

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|