chapter_20

Exercises for Chapter 20

1. Build a database for a website. Database needs to have three tables: users, tips and abuse.

erlang_tips.erl is stored in exercise_1/. Example usage:

1> erlang_tips:module_info(exports).
[{all_users,0},
 {get_user,1},
 {add_user,3},
 {all_tips,0},
 {get_tip,1},
 {add_tip,3},
 {all_abuse,0},
 {get_abuse,1},
 {add_abuse,2},
 {start_database,0},
 {create_database,0},
 {reset_tables,0},
 {module_info,0},
 {module_info,1}]
2> erlang_tips:create_database().
ok
3> erlang_tips:add_tip("google.com", "Use this site to find stuff", undefined).
{atomic,ok}
4> erlang_tips:add_tip("google.com", "Use this site to find stuff", undefined).
{atomic,ok}
5> erlang_tips:add_tip("stratus3d.com", "This site has tons of info on Erlang!", undefined).
{atomic,ok}
6> erlang_tips:all_tips().
[{tip,"stratus3d.com",
      "This site has tons of info on Erlang!",undefined},
       {tip,"google.com","This site is a fake",undefined}]
       5>

2. Take the previous exercise and get it run with ram and disk copies on two nodes.

erlang_tips.erl is stored in exercise_2/. To run the erlang_tips database on two nodes open up two terminal sessions and start two Erlang nodes. The first:

cd exercise_2/
erl -sname node1@localhost -setcookie cookie

The second:

cd exercise_2/
erl -sname node2@localhost -setcookie cookie

Note that the node names are hardcoded at the top of the exercise_2/erlang_tips.erl module. If you need to change them alter the NODE_NAMES macro (as well as the commands shown above). Once both nodes are running create the database and run a few commands on the first node:

(node1@localhost)1> erlang_tips:create_database().
ok
(node1@localhost)2> erlang_tips:add_tip("stratus3d.com", "This site contains a lot of great articles on Erlang!", undefined).
{atomic,ok}
(node1@localhost)3> erlang_tips:add_tip("google.com", "Use this site to find stuff", undefined).
{atomic,ok}
(node1@localhost)4> erlang_tips:add_tip("google.com", "Use this site to find stuff", undefined).
{atomic,ok}
(node1@localhost)5>

Then run the all_tips function to retrieve the tips on node 2:

(node2@localhost)1> erlang_tips:all_tips().
[{tip,"stratus3d.com",
    "This site contains a lot of great articles on Erlang!",
    undefined},
{tip,"google.com","Use this site to find stuff",undefined}]
(node2@localhost)2>

3. Write function to reject tip submissions if the user has submitted more than 10 tips in a one-day period. Time queries.

erlang_tips.erl for exercise 3 is stored in exercise_3/. To run this version of the erlang_tips you only need to run one node.