Tutorial: Chatter Backend

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DUE Wed, 09/10, 2 pm

We will build on the backend server you created in the previous tutorial. If you haven’t created a server, please create one according to the instructions in the previous tutorial. All subsequent tutorials and projects in this course require access to a backend server as set up in these two tutorials. Instead of setting a separate server for each assignment, you may want to keep your server up and running for the duration of the term.

Install updates

Remember to install updates available to your Ubuntu back end. If N in the following notice you see when you ssh to your back-end server is not 0,

N updates can be applied immediately.

run the following:

server$ sudo apt update
server$ sudo apt upgrade

server$ sync
server$ sudo reboot

PostgreSQL

We will be using the PostgreSQL relational database management system (RDBMS) to store chatts posted by the frontend and, later, to store responses returned by LLM as history/context for subsequent prompts.

First install PostgreSQL:

server$ sudo apt install libpq-dev postgresql postgresql-contrib

Once PostgreSQL is installed:,

• Log into an interactive Postgres (psql) session as user postgres:

  server$ sudo -u postgres psql
  

  You may receive the message: “could not change directory to “/root”: Permission denied”. You can safely ignore this message.

  Your command-line prompt should now change to postgres=#.

• Check the version of your PostgreSQL:

  SELECT version();
  

  It should say version 13 or higher. Consult the teaching staff if it doesn’t.

• Create a database user for your project. Make sure to select a secure password.

  CREATE USER chatter WITH PASSWORD 'chattchatt';
  

  TIP: Forgetting to do this is a common cause of getting HTTP error code 500 Internal Server Error.

  All SQL commands must end with a ;.

• Create a database for your project and change its owner to chatter:

  CREATE DATABASE chatterdb;
  ALTER DATABASE chatterdb OWNER TO chatter;
  

• Connect to the database you just created:

  \connect chatterdb
  

  \connect may be shortened to \c.

  Your command-line prompt should now change to chatterdb=#.

• Next use SQL command to create a chatts table in the chatterdb database. The table consists of four columns: username, message, id, and time:

  CREATE TABLE chatts (username VARCHAR(100) NOT NULL, message TEXT NOT NULL, id UUID NOT NULL, time TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP(0));
  

  We will generate a random UUID as the id of each entry; time will be automatically filled in by the database when an entry is inserted.

• Give user chatter access to administer the new database, including querying and inserting new data:

  GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA public TO chatter;
  

• Insert and view a sample entry:

  INSERT INTO chatts  VALUES ('testuser1', 'Hello world', gen_random_uuid());
  SELECT * from chatts;
  

  You should see something like:

   username  |   message   |                  id                  |            time            |
  -----------+-------------+--------------------------------------+----------------------------|
   testuser1 | Hello world | 20f713af-015d-48c8-8b09-3036cf104134 | 2025-01-03 10:54:58.362108 |
  (1 row)
  

  TIP: Trying to send a username longer than 100 characters will also result in HTTP error code 500 Internal Server Error.

• To delete all entries from a table:

  TRUNCATE TABLE chatts; 
  

• You can issue the \dt command to list all tables in your database:

  \dt
  
           List of relations
   Schema |  Name  | Type  |  Owner   
  --------+--------+-------+----------
   public | chatts | table | postgres
  (1 rows)
  

  and \d <tablename> to list the schema of a table:

  \d chatts
  
                                Table "public.chatts"
    Column  |           Type           | Collation | Nullable |       Default
  ----------+--------------------------+-----------+----------+----------------------
  username  | character varying(100)   |           | not null |
  message   | text                     |           | not null |
  id        | uuid                     |           | not null |
  time      | timestamp with time zone |           |          | CURRENT_TIMESTAMP(0)
  

• When you are finished, exit PostgreSQL:

  \q
  

  or hit Ctl-d (^d).

Add PostgreSQL as chatterd’s dependency

To ensure PostgreSQL is running when chatterd runs, edit chatterd’s service configuration file:

server$ sudo vi /etc/systemd/system/chatterd.service

Update the [Unit] block to say:

[Unit]
Description=EECS Reactive chatterd
Requires=postgresql.service
After=network.target
StartLimitIntervalSec=0

Then reload chatterd’s configuration to systemctl and restart chatterd:

server$ sudo systemctl daemon-reload
server$ sudo systemctl restart chatterd

chatterd

Please click the relevant link to setup the chatterd server with the Chatter APIs using the web framework of your choice:

Go | Python | Rust | TypeScript

and return here to resume the server setup once you have your web framework set up.

  server$ sudo systemctl daemon-reload
  server$ sudo systemctl restart chatterd

Testing Chatter APIs

with Postman on desktop

Assuming you have set up your Postman for HTTP/2 with disabled SSL certificate validation as in the previous tutorial:

• Create a new request in Postman to POST to https://YOUR_SERVER_IP/postchatt/ with Body > raw > JSON:

  {
      "username": "Postman",
      "message": "Ring! Ring!"
  }
  

  and click the big blue Send button.

  If everything works as expected, the bottom pane of Postman should say, to the right of its menu line, Status: 200 OK and the pane should simply display {}.

• You can create a new request in Postman to do GET from https://YOUR_SERVER_IP/getchatts and click the big blue Send button. It should return something like:

  [
    [
      "Postman",
      "Ring! Ring!",
      "9249b958-6e46-44b2-8004-9bccf0e8f1c1", 
      "2022-07-22T17:33:25.947"
    ]
  ]
  

with a command-line tool

laptop$ curl -X POST -d '{ "username": "Curly", "message": "Hello World" }' --insecure https://YOUR_SERVER_IP/postchatt/

laptop$ curl --insecure https://YOUR_SERVER_IP/getchatts/

laptop$ echo '{ "username": "weepie", "message": "Yummy!" }' | http --verify=no POST https://YOUR_SERVER_IP/postchatt/
# output:
HTTP/1.1 200 OK
Connection: keep-alive
Content-Length: 2
Content-Type: application/json
Date: Wed, 22 Jul 2022 17:45:53 GMT
Server: nginx/1.14.0 (Ubuntu)
X-Content-Type-Options: nosniff
X-Frame-Options: DENY

{}

laptop$ http --verify=no https://YOUR_SERVER_IP/getchatts/
# output:
HTTP/1.1 200 OK
Connection: keep-alive
Content-Length: 116
Content-Type: application/json
Date: Wed, 22 Jul 2022 17:46:32 GMT
Server: nginx/1.14.0 (Ubuntu)
X-Content-Type-Options: nosniff
X-Frame-Options: DENY

[
  [
    "weepie",
    "Yummy!",
    "d6e5f8a6-e3cc-4501-9359-517af9f64cda",
    "20242-07-22T17:45:53.177"
  ]
]

server$ sudo systemctl restart chatterd

server$ sudo systemctl status chatterd

That’s all we need to do to prepare the back end. Before you return to work on your front end, wrap up your work here by submitting your files to GitHub.

Submitting your back end

We will only grade files committed to the main branch. If you use multiple branches, please merge them all to the main branch for submission.

Navigate to your reactive folder:

server$ cd ~/reactive/

Commit changes to the local repo:

server$ git commit -am "chatter back end"

and push your chatterd folder to the remote GitHub repo:

server$ git push

If git push failed due to changes made to the remote repo by your tutorial partner, you must run git pull first. Then you may have to resolve any conflicts before you can git push again.

Go to the GitHub website to confirm that your back-end files have been uploaded to your GitHub repo.

References

• PostgreSQL tutorial
• Unexpected downsides of UUID keys in PostgreSQL