Run Multiple Process Groups in an App

Process groups are a way to configure a single Fly App to run multiple different programs.

You define processes in your fly.toml, giving each process group a name and a command to run at boot. Every defined process runs in its own Fly Machine(s) within the app, which means they don’t compete with each other for VM resources, and they can be scaled individually.

The default process group

If you don’t explicitly define any processes, then the Machines in a Fly App belong to the default app process group, and on boot they run whatever entrypoint process the app’s Docker image has. (Apps are shipped to Fly.io in Docker images, even though we run VMs, not containers.)

When the fly launch command creates a fly.toml file for your app, the default service defined in the http_service section applies to the app process group.

Run multiple processes

To run multiple processes, first make sure all the things you want to run are installed in your app’s Docker image.

Define a processes section in your app’s fly.toml, pairing process group names with the commands they should run. Here’s an example:

[processes]
  web = "bin/rails fly:server"
  cron = "supercronic /app/crontab"

Once there’s a [processes] section in your config, flyctl assumes this is a complete list of your processes. If you want an app process group alongside others, add it to the config explicitly.

Process group commands in a Fly App correspond to CMD in Docker; they don’t replace the ENTRYPOINT of your app image, but will supersede CMD.

Processes and services

Chances are, you only want user requests to hit one of your processes; in the above example, that process is web, so you would specify the web process in the http_service or in the [[services]] section for the app’s HTTP service:

...
[[services]]
  http_checks = []
  internal_port = 8080
  processes = ["web"]
  ...

You can define distinct services for each process that needs to accept connections via Fly Proxy (whether publicly or via Flycast), by creating multiple [[services]] sections in fly.toml.

Note: Make sure processes handle connections on different external ports. Fly Proxy doesn’t know about process groups; it load-balances requests among all Machines with a service configured on the requested port.

Deployment

fly deploy creates at least one Machine for each process group, and destroys all the Machines that belong to any process group that isn’t defined, in your app’s fly.toml file. It also updates the command, services, and health checks for each fly deploy-managed Machine on the app; and creates a new app release.

So on the first deployment (either at the end of fly launch or at the first explicit fly deploy), flyctl creates and starts at least one Machine for each process group in fly.toml.

If you add new process groups in an app’s [processes] block, then the next fly deploy spins up at least one new Machine to run each new process.

If you remove any process groups from an app’s [processes] block, then the next fly deploy destroys the Machines that belong to the deleted process.

For more information about how many Machines are created by fly launch and fly deploy, refer to App Availability and Resiliency.

Scale a process group horizontally

There are two ways to scale the number of Machines in an app. This section provides a summary of horizontal scaling. For more information, refer to Scale the Number of Machines.

Scale a process group horizontally with fly scale count

You can scale the number of Machines (up or down) per process group with the fly scale count command. The following example scales the web process group to 8 Machines, and the cron process group to 2 Machines:

fly scale count web=8 cron=2

You can scale by region with the --region option. Specify multiple regions and the Machines will be distributed as evenly as possible between them. The following command would create 2 Machines in gru and 2 Machines in bog:

fly scale count web=4 --region gru,bog

Scale a process group horizontally with fly machine clone

You can also use the fly machine clone and fly machine destroy commands to scale the number of Machines individually within a process group. When you clone a Machine that belongs to a process group, the new Machine is created with the command, services, and checks that are configured on the app for that process.

You can add a region to your app by cloning a Machine into a new region.

First, run fly status to get a list of Machines with IDs and process groups.

The following example clones a Machine into the gru (São Paulo) region:

fly machine clone --region gru e2865641be9786

Run the following commands to destroy a Machine:

fly machine stop e2865641be9786
fly machine destroy e2865641be9786

Scale a process group vertically

You can scale Machine CPU and memory settings for an entire process group using the fly scale vm and fly scale memory commands. This section provides a summary of vertical scaling. For more information, refer to Scale Machine CPU and RAM.

The following example changes the CPU/RAM of Machines in the web process group to a different preset combination:

fly scale vm performance-2x --process-group web

Run fly platform vm-sizes for a list of the valid CPU/RAM preset combinations.

This example changes only the RAM of Machines in the web process group:

fly scale memory 4096 --process-group web

Move a Machine between process groups

You can change the process group of an existing Machine by updating its metadata:

fly machine update --metadata fly_process_group=app 21781973f03e89

Then deploy the app to apply the configuration for that process group to the Machine:

fly deploy