• Sender must have zero or more packages.
• Package must have one and only one sender.
• Package must have zero or one courier.
• Package must have one and only one source machine.
• Package must have one and only one destination machine.
• Post machine must have zero or more packages to hold.
• Courier must have zero or more packages to deliver.

Post machine interface is available at /post_machines.

Peter wants to send a package to Chloe. He signs up to our Shipping service, entering his username and password. The username and password hash is sent to DB. Having signed up, he logs in and then registers a shipment to be sent. He enters shipment title, description picks up a post machine (he picks an address, post machines have unique addresses) from which he wants to send the package (source post machine) and a destination post machine. After Peter confirms the package registration, system generates a unique link to track the package. Also it generates source and destination post machines’ unlocking codes [the codes are stored in shipment entity]. The tracking link does not contain the codes (for security reasons), instead the codes are generated and shown to Peter along with the tracking link when he registers the shipment for sending. Chloe (package receiver) does not need the account to receive a package, instead, Peter sends destination post machine unlocking code and the tracking link to Chloe. When Chloe (or anybody else since links don’t require authentication, although are difficult to guess) enters the link, the system checks if there is such one. Since the shipment tracking link can be generated from the entity's ID, it is easy to verify the link's existence [link is (shipment ID).concat(shipmentID hash) so no need to store it, can be easily calculated and checked]. If the link is confirmed to exist, package status and destination post machine information are sent as a response. When ready to send out the package, Peter goes to the source post machine, enters its unlocking code [the machine checks if one of it's shipments has the unlocking code] and places the shipment inside. Once Peter shuts the shipment compartment, he cannot open it again since the unlocking code becomes invalid. Shipment status changes.

To send a shipment to the destination machine, the courier is needed. Courier can register himself just like the sender. Courier’s name is William. Shipments can have no couriers for some time, William is shown a list of these shipments [the list consists of shipments with status: inSourcePostMachine and for which courier is null (not set)]. Peter's shipment is one of them. Source and destination machines’ addresses of shipments are shown. If William is willing to deliver the shipment to the endpoint, he can assign it to himself (press assign button). Once William does that, a source and destination machines’ unlocking codes are generated given to him. The shipment itself is given a courier and is no longer shown in available shipments list for other couriers to consider delivering (if William changes his mind and does not want to deliver his shipment, he can cancel his assigment if he has not yet taken the shipment from source post machine. The courier will be unnasigned and shipment will once again appear in available shipment list for other couriers to consider delivering). William goes to the source machine [the page of post machines and selects the needed machine], enters the unlocking code [the machine checks if one of its packages has the unlocking code] and picks up the package. The status of the shipment changes.

After some time, William arrives at the endpoint [the page of post machines and selects the needed machine] and enters destination machine unlocking code [the machine checks if one of its packages has the unlocking code.] and places the package [presses place package]. The status of the shipment changes. Courier cannot unlock the machine once it shuts the doors [after the place package button press]. William has done his work for this shipment.

Since Peter sent the shipment tracking link to Chloe, she sees that the package has arrived. She goes to the destination post machine [the page of post machines and selects the needed machine] and enters the unlocking code [the machine checks if one of its packages has the unlocking code.] Peter has given to her. Chloe takes the package [presses pick up package]. Shipment status changes. Peter has successfully delivered the package to Chloe.

Database system used is PostgreSQL.

1. install postgresql
2. sudo mkdir /run/postgresql
3. sudo chown -R $USER:$USER /run/postgresql
4. initdb --locale $LANG -E UTF8 -D ~/postgres/data/
5. pg_ctl -D ~/postgres/data -l ~/postgres/logfile start
6. createuser -d -P admin and enter password as password

Execute steps 2,3 and 5 on each startup to run the DB.

To delete the database, run dropdb shipping_service_db.

If database directory is deleted, steps 2-6 need to be repeated.

1. download newest PostgreSQL from https://www.enterprisedb.com/downloads/postgres-postgresql-downloads
2. launch installer
3. select at least "PostgreSQL Server", "pgAdmin4" and "Command Line Tools" in the installer
4. choose user postgres password and remember it, don't change the default port
5. install
6. launch pgAdmin
7. choose pgAdmin password
8. in the servers panel (upper left corner) select first and connect using password from step 4
9. create a new role, named "admin" with password "password" and privileges "Can login" and "Create databases"
10. run the application, the database will be automatically created and will be visible in pgAdmin

Note: on closing, pgAdmin will show message saying that changes might not be saved, just choose leave, everything is ok.

execute dotnet ef migrations add create_db -o Persistence/Migrations in project directory