1. Hello, All

Write an Emerald program that writes the message

Hello, All

at each active node in the network.

Let the Emerald-file be called helloall.m, and compile it as helloall.x.

To start up several nodes on your local machine, open a couple terminals. In the first, type emx -R. In the second, third, etc. type emx -Rlocalhost. In the last one, type emx -Rlocalhost helloall.x.

The string “localhost” is the hostname of the Emerald server to connect the node to.

Hint: A Node object has a method getStdOut, which allows you to issue writes on the node's standard output stream.

2. Echo

Write an Emerald program that echo's the received on stdin to all active nodes. Let the program end, if it has received the keyword “exit” on stdin.

Let the Emerald-file be called echo.m, and compile it as echo.x.

Hint: You can read a string from stdin using stdin.getstring, however, that might come with a trailing line break due to the user pressing Enter. For the purposes of this exercise, you may assume that the user always does so. Hence, you might appreciate the following auxiliary functions:

function stripLast [ i : String ] -> [ o : String ]
  o <- i.getSlice[ 0, i.length - 1 ]
end stripLast
function readline -> [ o : String ]
  o <- self.stripLast [ stdin.getstring ]
end readline

What happens if some of the nodes become unreachable? Would your program crash? Test this. Try and mitigate for this using the unavailable construction.

3. The Moving Man

Create a program where an object visits each node and collects their Node identification. “Node identification” here consists of a concatenation of the node Name and IncarnationTime (both properties of a Node object).

Let the Emerald-file be called movingman.m, and compile it as movingman.x.

Hint: Begin with a moving man that doesn't actually move. The NodeList yielded by the activenodes property of the current node, is indeed already a list of Node objects.

Again, what happens if some of the nodes become unreachable? Would your program crash? Test this. Try and mitigate for this using the unavailable construction.

4. Watchdog

Watchdog should check the Emerald nodes if they have gone down or is up again. Create a program where there are more machines that are up, and crash some of them to see if you get notified which node crashed (printout a node id as above).

Let the Emerald-file be called watchdog.m, and compile it as watchdog.x.

5. Keywords

After completing these exercises, you should be familiar with the following keywords. If not, read up on them in the language report.

• Node
• NodeList
• fix
• unfix
• refix
• unavailable
• delay
• Time

6. Tips

You can use the $ as syntactic sugar for .get.

For instance, you can write eric$name in place of eric.getname above.