8th Light Blog

Background

When building a software system composed of multiple decoupled components, the need typically arises for interprocess coordination and communication. As an example, say that we have a customer requirement that allows emails to be sent based on certain events or conditions that occur while the user is interacting with a web application. We build an email component that takes care of the details of actually sending of the email. The web application could use the email component directly, but that would introduce a dependency that we’d rather avoid. How can the two processes communicate without creating a dependency between the two?

If you’re using Ruby, there’s an option that is built right into the standard library. The Rinda module implements the Linda distributed computing paradigm in Ruby. That’s great, but what is Linda? Linda is a coordination language developed by a couple of Yale researchers in the early 1990s. Linda itself is built on the concept of a tuplespace. A tuple is an ordered list of objects, and a tuplespace contains a set of tuples which can be accessed concurrently. A basic set of operations were defined by the language to allow reads, writes and takes.

Getting back to the example, how can Rinda be utilized to enable communication between the web application and the email component? First, a tuplespace will need to be setup that both components can access. When a condition arises within the web application that requires an email to be sent on its behalf, a specific tuple will be written to the tuplespace. The email component is looking for tuples that match a specific pattern. When it finds the tuple written by the web application that matches this pattern, it processes the request and reports back to the tuplespace when finished. In this way, we end up with only data coupling between the two processes, which is a much looser coupling than a direct dependency. This type of approach bears some resemblance to a blackboard system.

Now that we know a little bit about what Rinda can buy us, let’s dive in and try it out.

Hello World

Let’s put together a simple server and client to demonstrate how each works.

Here’s a simple server (server.rb):

require "rinda/tuplespace"

port = 4000
ts = Rinda::TupleSpace.new
DRb.start_service("druby://:#{port}", ts)
puts "Rinda listening on #{DRb.uri}..."
DRb.thread.join

We start by setting the default port to 4000. We then create a new TupleSpace object. The call to start_service starts a local dRuby server listening on port 4000. Passing the TupleSpace as the second parameter sets the server’s front object to the TupleSpace. The URI of the server is output, and the program waits for a kill signal, and we have a Rinda server ready to be accessed by a client. To start the server:

$ ruby server.rb

Here’s a simple interactive client (client.rb). It allows input to be gathered on standard input, with the form .

require "rinda/rinda"
include Rinda

port = 4000
ts = DRbObject.new(nil, "druby://:#{port}")

while message = gets
  begin
    args = message.split(" ")
    method = args.shift.to_sym
    topic = args.shift.to_sym
    message = args.shift
    message = nil if message == "nil"
    tuple = [topic, message]

    puts ts.send(method, tuple)
  rescue Exception => e
    puts e.message
  end
end

We’re using the send method in order to invoke the method specified on standard input. There’s also a conversion between nil as a string into nil proper. This is necessary because of wildcards, which we’ll get to in a minute. With the server running, run the client to start a new interactive session:

$ ruby client.rb

Below is a sample session (output is indented for clarity):

write message hello
    Rinda::TupleEntry:0x4f998
write message world
    Rinda::TupleEntry:0x4cfa4
read message nil
    message
    hello
take message nil
    message
    hello
take message nil
    message
    world

The inputs above translate to these method calls:

ts.write([:message, "hello"]) => Rinda::TupleEntry:0x4f998
ts.write([:message, "world"]) => Rinda::TupleEntry:0x4cfa4
ts.read([:message, nil]) => "hello"
ts.take([:message, nil]) => "hello"
ts.take([:message, nil]) => "world"

Note that reads do not remove the tuple from the tuplespace. The tuple is left untouched such that other clients can access the same data. This is an important concept when dealing with synchronization that we’ll return to in a future post.

The structure of the patterns used in the read and take methods deserve some additional attention. When the tuplespace is queried through a read or a take, there are two criteria used to determine if a tuple matches. First, the length of the tuple must match the pattern’s length. In the example above, all patterns are pairs (2-tuples), so only tuples that are pairs will qualify. Second, the tuple must match the pattern specified. When specifying a pattern, nil is used as a wildcard, which will match anything.

Assume that our tuplespace has the following tuples:

1) [:message]
2) [:message, "hello"]
3) [:message, "hello", "world"]

If the pattern [nil] is specified, e.g. ts.read([nil]), only #1 will match.

If the pattern [:message, nil] is specified, only #2 will match. Of course, [:message, “hello”] would also result in #2 matching as well, while [:message, “world”] would not.

Using [:message, nil, nil], [:message, “hello”, nil], or [:message, nil, “world”] would all result in #3 matching. So it is important to remember that patterns are matched on both length and content of the tuple.

Conclusion

We haven’t even scratched the surface of what can be done with Rinda. This introduction only gets the simplest client and server talking to each other. Stay tuned for more.