Work with cluster data on Fly.io

With your Corrosion cluster deployed, you can work with the example database.

To get started, shell into each Corrosion node, in separate terminals.

fly ssh console --pty --app <your-app-name> --select

If you run the command from the directory containing your Corrosion app's fly.toml configuration file, you can omit the --app flag.

We'll call one Machine "Node A" and the other "Node B". Every node is read-write, so it doesn't matter which is which.

The example schema, todo.sql, specifies a single table called todos, with id, title, and completed_at columns.

-- /etc/corrosion/schemas/todo.sql

CREATE TABLE todos (
    id BLOB NOT NULL PRIMARY KEY,
    title TEXT NOT NULL DEFAULT '',
    completed_at INTEGER
);

Inserting and querying data

Insert some data on Node A

From Node A's terminal session:

# corrosion exec --param 'some-id' --param 'Write some Corrosion docs!' 'INSERT INTO todos (id, title) VALUES (?, ?)'
INFO corrosion: Rows affected: 1

Query data on Node A

Via SQLite directly:

# sqlite3 /var/lib/corrosion/state.db 'SELECT * FROM todos;'
some-id|Write some Corrosion docs!|

Using the API, via the CLI:

# corrosion query 'SELECT * FROM todos;' --columns
id|title|completed_at
some-id|Write some Corrosion docs!|

Query data on Node B

From Node B's terminal:

# corrosion query 'SELECT * FROM todos;' --columns
id|title|completed_at
some-id|Write some Corrosion docs!|

Node A's contribution is already present in Node B's database.

Insert data on Node B

# corrosion exec --param 'some-id-2' --param 'Show how broadcasts work' 'INSERT INTO todos (id, title) VALUES (?, ?)'
INFO corrosion: Rows affected: 1

Check the data in Node A's database

# corrosion query 'SELECT * FROM todos;' --columns
id|title|completed_at
some-id|Write some Corrosion docs!|
some-id-2|Show how broadcasts work|

The second update has propagated back to Node A.

Updating a file using a Corrosion template

The example template todos.rhai makes a checklist out of the rows in our todos table.

/* /etc/corrosion/templates/todos.rhai */

<% for todo in sql("SELECT title, completed_at FROM todos") { %>
[<% if todo.completed_at.is_null() { %> <% } else { %>X<% } %>] <%= todo.title %>
<% } %>

Start corrosion template and watch the output file

On Node A, start processing the template.

# corrosion template "/etc/corrosion/templates/todos.rhai:todos.txt" &
[1] 354
root@4d8964eb9d9487:/#  INFO corrosion::command::tpl: Watching and rendering /etc/corrosion/templates/todos.rhai to todos.txt

Whenever there's an update to the results of the template's query (or queries), corrosion template re-renders the output file.

Start watching the output file.

# watch -n 0.5 cat todos.txt
Every 0.5s: cat todos.txt

[ ] Write some Corrosion docs!
[ ] Show how broadcasts work

Add a todo item

On the other Machine (Node B), insert some data.

# corrosion exec --param 'some-id-3' --param 'Hello from a template!' 'INSERT INTO todos (id, title) VALUES (?, ?)'
INFO corrosion: Rows affected: 1

The new todo item gets propagated back to Node A, and your watch should look like this:

Every 0.5s: cat todos.txt

[ ] Write some Corrosion docs!
[ ] Show how broadcasts work
[ ] Hello from a template!

Mark all items as done

Mark all tasks as completed, on either node:

# corrosion exec 'UPDATE todos SET completed_at = 1234567890'
INFO corrosion: Rows affected: 3

$ watch -n 0.5 cat todos.txt
Every 0.5s: cat todos.txt

[X] Write some Corrosion docs!
[X] Show how broadcasts work
[X] Hello from a template!

Now you have a distributed to-do list app! A front end is left as an exercise for the reader ;)