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                            LAB 09 QUESTIONS
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- Name: Michael Zhang
- NetID: zhan4854

Answer the questions below according to the lab specification. Write
your answers directly in this text file and submit it to complete Lab01.


PROBLEM 1 `em_server_mq.c' and `em_server_mq.c'
===============================================

A
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  Examine the source code for `em_server_mq.c' and
  `em_client_mq.c'. Describe the lines of code which are used to
  establish the message queues used for communication. How many message
  queues are there?

There's one message queue for the server, and one for the client. These are
created by msgget(key, 0644 | IPC_CREAT) where key was obtained by ftok using
the identity of the party that called it.

B
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  Describe the sending and receiving functions used in the server to put
  messages onto the queue. Focus your attention on the server side
  first. Describe each argument of the send/receive functions and its
  purpose. You may wish to consult the textbook or manual page for the
  functions to get more information.

To put a message onto the message queue you would use msgsnd, and send the
object containing the request information that you need.

C
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  Describe the protocol that appears to be used for communication. What
  steps are taken by a client to make a request for an email address
  lookup? How does the server respond? You may find it useful to use the
  letters associated with print statements like "A" and "D". Note any
  operations that might happen in an arbitrary order.

The protocol looks like this:

 - The client first sends { name = argv[1], mtype = pid }.
 - The server msgrcvs this, reads the name and name and mtype from the request
   object and looks up the email associated with the name.
 - The server then sends back { name = request.name, mtype = request.mtype,
   email = the email that was looked up. }
 - The email client makes sure that the pid that the mtype returned is the same
   as the pid that it requested with.

D
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  Recall that the previous email lookup server/client that used FIFOs
  would answer requests on a Data FIFO. The Server would only allow one
  Client to access the Data FIFO at a time.  This was to prevent the
  client from retrieving data from the queue that was not intended for
  them.

  In the Message Queue server/client, potentially many clients are
  accessing the To-Client queue simultaneously. Describe why there is
  not a problem in this setting. Look carefully at the arguments used in
  the `msgrcv()' call in the client.

This isn't a problem because msgrcv takes in a flag msgtype which in this case
is the pid of the client. This way, msgrcv will only return the messages
intended for the correct client. Of course, this isn't a valid security measure
but it produces the correct result.

E
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  In the FIFO server/client, sleeping and signaling was used to avoid
  busily waiting for availability of the data. This complicated the
  protocol somewhat in order to get more efficient resource utilization.

  There is NO explicit sleeping in the Message Queue server/client which
  might indicate it is less efficient as processes busily wait for
  messages. Do some research on `msgsnd() / msgrcv()' calls to determine
  if they are busy waiting (polling) or interrupt driven operations.

  Confirm these answers by running `run_simulations_mq.sh' on a large
  number of client and timing the results. A sample run might be
  ,----
  | > time ./run_simulation_mq.sh 1000 > ~/tmp/xxx
  `----

According to MSGOP(2), "If insufficient space is available in the queue, then
the default behavior of msgsnd() is to block until space becomes available.  If
IPC_NOWAIT is specified in msgflg, then the call instead fails with the error
EAGAIN." This means they will wait for the message queue to become available
(after the other clients have read from it) before it sends.