CSCI 4061 HW09: FIFOs servers Server and Clients

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                            HW 09 QUESTIONS
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- Name: (FILL THIS in)
- NetID: (THE kauf0095 IN kauf0095@umn.edu)

Write your answers to the questions below directly in this text file.
HW quiz questions will be related to the questions in this file.


PROBLEM 1 `em_server.c'
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A
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  Examine the source code for `em_server.c'. Describe the
  `requests.fifo' item that it creates.  Resolve theses specific items
  1. What system call is used initially to delete any existing version
     of `requests.fifo'?
  2. What system call is used to create `requests.fifo'?
  3. What system call is used to set up reading from `requests.fifo'?


B
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  In the main `while()' loop of the server, describe how the server
  handles requests from the client.
  1. What C data type represents the requests from clients and what
     parts does it have?
  2. How does the server use the request data to search for a name/email
     association?
  3. How does the server use the request data to communicate back to the
     client?


PROBLEM 2 `em_client.c' and Protocol
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A
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  Examine the code for `em_client.c'. Describe the steps that it
  performs to contact the server to make a request and then receive the
  response.  Describe how system calls are used in this process.


B
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  Describe the overall Protocol of communication of between the server
  and client. Which entity is responsible for creating, reading, and
  writing each FIFO at what time.


C
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  Use the provided script `run_simulation.sh <N>' to launch start a
  server and run a specified number of clients such as 5 as below
  ,----
  | > ./run_simulation.sh 5
  | SERVER 14205: starting up
  | SERVER 14205: created new requests.fifo, now opening it
  | SERVER 14205: opened requests.fifo, listening for requests
  | CLIENT 14208: sending request: {fifo_file='14208.fifo' query_name='Christopher Jonathan' }
  | CLIENT 14208: opening '14208.fifo'
  | ...
  | SERVER 14205: closing connection to fifo '14212.fifo'
  | CLIENT 14211: response for name 'Dan Knights' is email 'knights@cs.umn.edu'
  | CLIENT 14212: response for name 'George Karypis' is email 'karypis@cs.umn.edu'
  | > 
  `----
  Examine the output for this run along with running the simulation
  several more times with different numbers of clients.

  Explain why there never seems to be any problem with multiple clients
  overwriting data associated with each other particularly in the
  `requests.fifo' and the client responses.


PROBLEM 3: Pitfalls
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A
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  Compile the server/client with `make' and run the server via
  ,----
  | > ./em_server
  `----
  Note the output that it prints. Then kill the server, perhaps with a
  C-c interrupt signal.

  Edit the `em_server.c' code to use the line marked `ALTERNATE
  OPEN'. You should comment on the preceding `open()' line to get
  ,----
  | // int requests_fd = open("requests.fifo", O_RDWR);               // open FIFO read/write to avoid blocking
  | int requests_fd = open("requests.fifo", O_RDONLY);             // ALTERNATE OPEN: read only
  `----

  Recompile and re-run `em_server' and explain why the output of the
  server changes in this case. To understand the change in output, you
  may need to review what happens when a process opens only one end of a
  pipe/FIFO.


B
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  With the ALTERNATE OPEN line in use (uncommented), start the
  `em_server' which should appear to be 'waiting' for something. Then in
  a separate terminal, run `em_client'.  Describe if the client and
  server behave differently than when the original `open()' line was in
  use by the server.


C
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  With the ALTERNATE OPEN line in use (uncommented), run the
  `run_simulation.sh 5' script a few times until it appears to stall and
  fail to complete.

  Examine the output of the simulation run carefully and find a line
  involving the server that gives some hints as to why the simulation
  fails. Show that line below.


D
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  Do some careful thinking, reading, and exploration and explain why the
  ALTERNATE OPEN line creates a flaw in the server/client protocol that
  is NOT present in the original version.