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Today, you are going to implement your own process manager from scratch using C and the Process APIs presented by POSIX, such as fork, exec, and wait.

Run source install_deno to install the Deno runtime. This is a JavaScript/TypeScript runtime that is evolved from Node.js.

Running the following

deno --version

Should print results similar to the following:

$ deno --version
deno 1.4.1
v8 8.7.75
typescript 4.0.2

Our buggy app is one of the most mission-critical things on the web: Gabe Parmer's website. If the website goes down, potential PhDs are unable to learn about Gabe's research and consider applying to join his lab.

Open the www directory and look at the contents. This contains the HTML, CSS, and image for Gabe's site.

These static assets need to be served by a web server, and the buggy web server is written in TypeScript

Open the web.ts script, and notice the following:

The port is configurable via an argument to the script, defaulting to 8080 if not provided.

let port: number = parseInt(Deno.args[0], 10) || 8080;

The request handler logic takes a random number from 0 to 19, crashing the program if it is 0. This gives us our 5% failure rate.

// 5% chance of spontaneous failure, causing server to crash
if (Math.floor(Math.random() * Math.floor(20)) == 0) {
  console.log("Chaos Monkey strikes again!");
  Deno.exit(-1);
}

Let's try to run the webserver. By using make web, we tell deno to run the web.ts script with 8081 passed as an argument. As we saw above, this is used to set the port the web server will serve Gabe's website from.

make web

should yield the following

$ make web
deno run --allow-read --allow-net web.ts 8081
port: 8081
HTTP webserver running.  Access it at:  http://localhost:8081/

Note: If you are running from the VSCode terminal, the Remote extensions automatically proxy from the WSL backend / Multipass VM. This means that you can hold down control and left click on the URL in the console as if it were a hyperlink, and you'll open Gabe's webpage in your default browser.

ALSO Note: If you are running WSL, the actual port that the link open may be slightly different. Be sure to click the link in the console rather than copy/pasting text into your URL bar.

Your default browser should load Gabe's web page. Hit refresh until the website doesn't load properly.

Go back to your terminal. You should see the following:

<snip>
gp.png
index.html
index.css
gp.png
Chaos Monkey strikes again!
Makefile:11: recipe for target 'web' failed
make: *** [web] Error 255

The first objective is to create a process manager that launches the Deno web server and relaunches it whenever the web server exits!

You should implement this in main.c. The program should invoke the deno script just as in the web rule in the Makefile.

Running make build will compile your process manager.

Running make run will compile and execute your process manager.

Hints:

• Read the manual pages for fork, exec, and wait
• The arguments probably need to be split into an array of NULL-terminated tokens. The first element should be the relative path used to call the runtime. The last element should be NULL. This exact syntax might vary depending on the exec variant you use

  char *args[] = {"./deno", "run", "--allow-read", "--allow-net", "web.ts", "8081", NULL};

• If you are printing to console from a child process, you might need to use fflush to make sure the text is printed before the process terminates

__________________________
|       deno script      |
|         :8081          |
''''''''''''''''''''''''''
__________________________
|           PM           |
''''''''''''''''''''''''''

Success Criteria: If the chaos monkey strikes and your process terminates, your process manager should automatically restart the process. This should be at the same IP address, such that one can refresh hitting the website again. Specific requests might fail, but the website is never fully down.

Enhance your process to spawn a pool of three workers. This allows requests to automatically fail over to another worker if one fails. That makes the error transparent to the client.

We provide a proxy server called web.ts to help redirect traffic across your pool of workers.

Open this file and notice the following:

1. The script is hardcoded to balance between webservers running on 8080, 8081, and 8082. This means that you need to make sure that these URLs are used exactly:

   let servers = [8080, 8081, 8082];

2. The proxy round robins between the three web servers

   let target_idx = Math.floor(Math.random() * servers.length);
   while (true) {
       target_idx = (target_idx + 1) % servers.length;
       let target_port = servers[target_idx];
       <snip>
   }

3. If the proxy is successful and request.respond is executed, the proxy breaks the loop. Otherwise, it round robins to the next port until successful.

   request.respond({ status: 200, body: text, headers });
   break;

You can run the proxy with your process manager.

Run make run to start your process manager.

Open a second tab and run make proxy.

Notice that the proxy attempts to load balance between ports 8080, 8081, and 8082, but because you're only running a server on 8081, it ends up failing over to 8081 on each request.

$ make proxy
deno run --allow-net proxy.ts
HTTP webserver running.  Access it at:  http://localhost:1337/
GET localhost:1337/
PROXY to localhost:8082/ FAIL
PROXY to localhost:8080/ FAIL
PROXY to localhost:8081/ SUCCESS
GET localhost:1337/index.css
PROXY to localhost:8082/index.css FAIL
PROXY to localhost:8080/index.css FAIL
PROXY to localhost:8081/index.css SUCCESS
GET localhost:1337/gp.png
PROXY to localhost:8082/gp.png FAIL
PROXY to localhost:8080/gp.png FAIL
PROXY to localhost:8081/gp.png SUCCESS

You now have all the pieces that you need to enhance your process manager to spawn and monitor a pool of workers.

                _______________
                |    Proxy    |
                |    :1337    |
                '''''''''''''''
                       |
        v--------------v---------------v
_______________ _______________ _______________
| deno script | | deno script | | deno script |
|    :8080    | |    :8081    | |    :8082    |
''''''''''''''' ''''''''''''''' '''''''''''''''
_______________________________________________
|                     PM                      |
'''''''''''''''''''''''''''''''''''''''''''''''

Hints:

• You need a way to know which the IP address of the node that failed. This probably needs to be maintained in some sort of mapping from pid to IP
• You thus need to know the pid of the child process that failed. One of the members of the wait family of APIs is better for this than others.
• Is the wait call you chose blocking or nonblocking? Which should it be?
• What does WNOHANG do?

If you want to remove deno, run the following script

./remove_deno.sh

And then open ~/.bash_profile and remove the following lines:

export DENO_INSTALL="$HOME/.deno"
export PATH="$PATH:$DENO_INSTALL/bin"