Cloud Platform

Volumes

A volume is a persistent storage device that keeps data across restarts.

In this guide you will create a volume, attach it to a web server instance, and write to it. Then you will detach it, remove that instance, attach it to a new instance, and confirm the data persisted. Finally, the guide covers how to share volumes across instances (read-only at present).

Setting up the volume

To start, create the volume via kraft cloud volume, with size in MBs and name my-volume:

(bash)
 
kraft cloud volume create --size 100 --name my-volume

The command should return the volume's UUID, and you can check the operation worked via:

(bash)
 
kraft cloud volume list

which should output something like:

(ansi)
 
NAME       CREATED AT      SIZE     ATTACHED TO  STATE      PERSISTENT
my-volume  15 seconds ago  100 MiB               available  true

The ATTACHED TO field is empty because the platform hasn't attached it to any instance yet.

Populating the volume with local data (Optional)

If you'd like to populate your empty volume with local data, you can use the kraft cloud volume import command. For example, assuming the volume's name is my-volume and that the data you want to import are in your my-data directory, you would run:

(bash)
 
kraft cloud volume import --volume my-volume --source my-data

You should see output like:

(ansi)
 
[+] Packaging source as a CPIO archive... done!                             [0.0s]
[+] Spawning temporary volume data import instance... done!                 [0.1s]
[+] Importing data (256 B) ••••••••••••••••••••••••••••••••••••••••••• 100% [0.1s]

[●] Import complete
 │
 ├─── volume: my-volume
 ├─ imported: 256 B
 └─ capacity: 100 MiB

Setting up the web server

Use a Flask web server to write to the volume:

(bash)
 
git clone https://github.com/unikraft-cloud/examples
cd examples/http-python3.12-flask3.0

Replace the contents of server.pywith:

server.py(py)
 
from flask import Flask, send_from_directory
from datetime import datetime
import os

app = Flask(__name__)

file_path = "/mnt/log.txt"

def initialize_file():
  if not os.path.exists(file_path):
      with open(file_path, 'w') as log_file:
          log_file.write("Log file created.\n")

@app.route('/')
def write_to_file():
  initialize_file()

  current_datetime = datetime.now().strftime("%Y-%m-%d %H:%M:%S")

  # Write to file
  with open(file_path, 'a') as log_file:
      log_file.write(f"{current_datetime}\n")

  # Read contents of the file
  with open(file_path, 'r') as log_file:
      file_contents = log_file.read()

  return file_contents

if __name__ == '__main__':
  app.run(host='0.0.0.0', port=8080)

On every request, this simple server will write a timestamp to a file on the mounted persistent volume and print out the current contents of the file.

Start the Flask web server, create a service for it via the -p flag, and mount the my-volume volume at /mnt:

(bash)
 
kraft cloud deploy -M 512 -p 443:8080 --volume my-volume:/mnt .

You should see output like:

(ansi)
 
[●] Deployed successfully!
 │
 ├────────── name: http-python312-flask30-2h608
 ├────────── uuid: dcaf899e-6831-4641-86a6-e2b39e9f0d98
 ├───────── state: running
 ├─────────── url: https://holy-bonobo-px345skl.fra.unikraft.app
 ├───────── image: http-python312-flask30@sha256:3ca3e773d5bc76e1231347e5345e19d0293ba05e502e387956ecba39a4c9372c
 ├───── boot time: 219.61 ms
 ├──────── memory: 512 MiB
 ├─────── service: holy-bonobo-px345skl
 ├── private fqdn: http-python312-flask30-2h608.internal
 ├──── private ip: 172.16.6.4
 └────────── args: /usr/bin/python3 /app/server.py

To confirm that the platform attached the volume, run:

(bash)
 
kraft cloud volume get my-volume

You should see output like:

(text)
 
NAME       CREATED AT      SIZE     ATTACHED TO                   STATE    PERSISTENT
my-volume  34 minutes ago  100 MiB  http-python312-flask30-2h608  mounted  true

Testing it

The Flask server writes the time and date to /mnt/log.txt for each request. Test it by running curl several times. Example output:

(bash)
 
$ curl https://holy-bonobo-px345skl.fra.unikraft.app
Log file created.
2024-02-24 08:15:32

$ curl https://holy-bonobo-px345skl.fra.unikraft.app
Log file created.
2024-02-24 08:15:32
2024-02-24 08:15:34

$ curl https://holy-bonobo-px345skl.fra.unikraft.app
Log file created.
2024-02-24 08:15:32
2024-02-24 08:15:34
2024-02-24 08:15:35

To test data persistence, first stop the instance, detach the volume, and remove the instance:

(bash)
 
kraft cloud instance stop http-python312-flask30-2h608
kraft cloud volumes detach my-volume
kraft cloud instance rm http-python312-flask30-2h608

Now start another instance and, like before, mount the same volume:

(bash)
 
kraft cloud deploy -M 512 -p 443:8080 --volume my-volume:/mnt .

This should output something like:

(ansi)
 
[●] Deployed successfully!
 │
 ├────────── name: http-python312-flask30-0s9c7
 ├────────── uuid: 59d06415-84b8-4bf5-85c7-997960382011
 ├───────── state: running
 ├─────────── url: https://winter-field-v6m37jgs.fra.unikraft.app
 ├───────── image: http-python312-flask30@sha256:9f441af88df8968b368e7b658737c804fc8be87c864fc2d695e6adcda9a56acf
 ├───── boot time: 202.11 ms
 ├──────── memory: 512 MiB
 ├─────── service: winter-field-v6m37jgs
 ├── private fqdn: http-python312-flask30-0s9c7.internal
 ├──── private ip: 172.16.6.7
 └────────── args: /usr/bin/python3 /app/server.py

Run one final curl to this new address. You should see the previous contents plus a new entry at the bottom:

(bash)
 
$ curl https://holy-bonobo-px345skl.fra.unikraft.app
Log file created.
2024-02-24 08:15:32
2024-02-24 08:15:34
2024-02-24 08:15:35
2024-02-24 08:20:58

Cleaning up

To clean up, first detach the volume from all instances and then remove it:

(bash)
 
kraft cloud volume detach <volume-name>
kraft cloud volume remove <volume-name>

Learn more

• The kraft cloud CLI reference, in particular the deploy and volumes subcommands
• Unikraft Cloud's REST API reference, in particular the section on volumes