<p>We have recently published a preprint on arXiv of a JSS paper we wrote with more examples of using RProtoBuf, including sending RPC requests to remote web services.  For more exposition of sharing data between R and other languages with RProtoBuf, see <a href="http://arxiv.org/abs/1401.7372" rel="nofollow">RProtoBuf: Efficient Cross-Language Data Serialization in R</a>.</p>

<p>You can use RProtoBuf with any transport mechanism, as explained in the article -- You can save serialized protocol buffers to files to be read by other applications written in other languages, or you can send them over connections/sockets or other higher level RPC systems.  Protocol Buffers are widely used in everything from Sony Playstations to large scale web services, but they do not include an RPC system -- you use them as your serialization format with whatever transport system you are already using.</p>
Calling Java functions from R using RProtoBuf (Protocol Buffers)
<p>It is not entirely obvious how to go about using RProtoBuf for communicating between R and other languages (Java, in my case).</p>

<p>The RprotoBuf Developers developed something that is still here - <a href="https://r-forge.r-project.org/scm/viewvc.php/java/?root=rprotobuf" rel="nofollow">https://r-forge.r-project.org/scm/viewvc.php/java/?root=rprotobuf</a>, but it seems very outdated. I am not sure if this is the way to go. Here are two conversations between the authors of RProtoBuf that might help with understanding the code -</p>

<p><a href="http://lists.r-forge.r-project.org/pipermail/rprotobuf-yada/2009-December/000116.html" rel="nofollow">http://lists.r-forge.r-project.org/pipermail/rprotobuf-yada/2009-December/000116.html</a></p>

<p><a href="http://lists.r-forge.r-project.org/pipermail/rprotobuf-yada/2009-December/000119.html" rel="nofollow">http://lists.r-forge.r-project.org/pipermail/rprotobuf-yada/2009-December/000119.html</a></p>

<p>It seems that they started work with Java and then abandoned it in C++'s favour!</p>

<p>Is there anyone using R-RProtoBuf-Java combination? How do you do it? Is there a tutorial or example available?</p>

<p>My exposure to Java is very very limited. I want to use a few programs written in Java.</p>

<p>Edit :
To clarify, I suppose I want to see an example of an R rpc client being used with RProtobuf. Pointers towards Java RPC servers would be welcome.</p>

<p>Edit2 :
The first link actually points to some documentation generator code, as Dirk pointed out.</p>