Mounts per process group for Apps v2

Fresh Produce
1

Since flyctl v0.0.522, it is possible to have different mounts per process group on V2 applications. This means your web app and your background jobs can have attached volumes from the same pool, different pools, or none at all. This was possible with Nomad but was missing for V2 apps until now.

This is useful when an app is composed of different process groups (like web, background jobs, stateless components) that have different volume requirements (size, count, encryption, …).

For example: let’s say we want to build a video converter app that keeps metadata in sqlite (backed by litefs cluster). The app has a background job for converting uploaded videos that requires 50GB of disk space. Once converted, the videos are uploaded to S3 and removed to free up space. As a plus, there is a process that consumes messages from the task queue and sends notifications when conversion are done, and it doesn’t need any volume to work.

This is the example fly.toml:

[processes]
app = "/start webserver"
video = "/start video-converter"
notifier = "/start notifier"

[http_service]
  internal_port = 8080
  force_https = true
  processes = ["app"]
 
[[mounts]]
source = "litefs"
destination = "/.litefs"
processes = ["app"]

[[mounts]]
source = "stage"
destination = "/stage"
processes = ["video"]

And here how to launch this app:

$ fly launch --force-machines -o personal --region iad --copy-config --now --name mounts-per-group
...
Creating 1GB volume 'litefs' for process group 'app'. See `fly vol extend` to increase its size
Creating 1GB volume 'stage' for process group 'video'. See `fly vol extend` to increase its size
Process groups have changed. This will:
 * create 1 "notifier" machine
 * create 1 "video" machine
 * create 1 "app" machine

No machines in group 'app', launching one new machine
  Machine 148ed590a04258 [app] update finished: success
No machines in group 'notifier', launching one new machine
  Machine 148edd77f75548 [notifier] update finished: success
No machines in group 'video', launching one new machine
  Machine e2865649c53328 [video] update finished: success
Finished launching new machines
Updating existing machines in 'mounts-per-group' with rolling strategy
  Finished deploying

I removed the uninteresting parts from above log.

See how fly launch is creating 2 volumes (one per group) and launching 3 machines, two of which have mounts.

$ fly status
App
  Name     = mounts-per-group
  Owner    = personal
  Hostname = mounts-per-group.fly.dev
  Image    = mounts-per-group:deployment-01GY8HXKB7A6FG17JF8W0AYPA3
  Platform = machines

Machines
ID              PROCESS         VERSION REGION  STATE   HEALTH CHECKS   LAST UPDATED
148ed590a04258  app             1       iad     started                 2023-04-17T21:29:28Z
148edd77f75548  notifier        1       iad     started                 2023-04-17T21:29:32Z
e2865649c53328  video           1       iad     started                 2023-04-17T21:29:47Z

p$ fly vol list
ID                      STATE   NAME            SIZE    REGION  ZONE    ENCRYPTED       ATTACHED VM     CREATED AT
vol_ez1nvxk6zm8rmxl7    created stage         1GB     iad     7806    true            e2865649c53328  2 minutes ago
vol_3q80vd3qnekrgzy6    created litefs        1GB     iad     7806    true            148ed590a04258  2 minutes ago

Note: the only thing left is extending the volume to 50GB. I will leave that as a reader’s exercise :house:

Anything missing? something you would like to see?
let me know!
thanks

5 Likes
2

Awesome!

Are you able to mount multiple volumes to the same process group?
e.g.

[[mounts]]
source = "data1"
destination = "/data1"
processes = ["app"]

[[mounts]]
source = "data2"
destination = "/data2"
processes = ["app"]
1 Like
4

Aren’t those processes just different fly apps ? Since A single VM can only mount one volume at a time Volumes · Fly Docs how would they manage it ? By deploying for the same process app a VM A with a data 1 volume and a VM B with a data 2 volume ?

5

Not yet! The file format is certainly ready to accept multiple volumes for the same group but it is not supported by the backend.

6

The format leads me to believe I can attach the same volume to multiple processes, is that correct?

7

Not quite. Currently each volume can be mounted to only 1 machine at a time.

1 Like
8

@dangra appreciate the work and info here!

I’m sure I’m echo’ing a few others before, but it would be great to share one volume between two processes.

The main use case I’m thinking about is being able to point ActiveStorage (Rails) to a singular fly mounted volume, where images/data can be accessed in both web and worker processes.

ActiveStorage can only be pointed to one source at a time per environment, therefore distinct volumes between processes undermines the ability to use the library.

It’d also be great to not have to use a third party storage solution, like S3.