# `mix fly_deploy.hot`

Performs a hot code upgrade without restarting the application.

## Quick Start

    # Zero configuration - everything auto-detected from fly.toml
    mix fly_deploy.hot

    # Use staging config
    mix fly_deploy.hot --config fly-staging.toml

    # Pass Docker build arguments
    mix fly_deploy.hot --build-arg ELIXIR_VERSION=1.18.2 --build-arg OTP_VERSION=27.1.2

    # Preview without executing
    mix fly_deploy.hot --dry-run

## Configuration

Configuration is merged from multiple sources with priority:

    CLI options > Mix config > fly.toml > Auto-detected defaults

### fly.toml (or --config custom fly toml file)

The `[env]` section is automatically read and passed to orchestrator machines:

    [env]
      AWS_ENDPOINT_URL_S3 = "https://t3.storage.dev"
      AWS_REGION = "auto"
      AWS_BUCKET = "my-app-staging"

### Mix Config

In `config/config.exs`:

    config :fly_deploy,
      bucket: "my-releases",
      max_concurrency: 10,
      env: %{
        "CUSTOM_VAR" => "value"
      }

## CLI Options

  * `--config` - Path to fly.toml file (default: "fly.toml")
  * `--skip-build` - Skip building and use existing image (requires --image)
  * `--image` - Use specific pre-built image
  * `--build-arg` - Pass build-time variables to Docker (can be used multiple times)
  * `--dry-run` - Show what would be done without executing
  * `--no-cache` - Do not use previously cached builder docker layers
  * `--force` - Override deployment lock (use with caution)
  * `--lock-timeout` - Lock expiry timeout in seconds (default: 300)
  * `--buildkit` - Use buildkit based Fly builder
  * `--mode` - Upgrade mode: "hot" (default) or "blue_green"

## Required Setup

  * Fly CLI must be authenticated: `fly auth login`
  * App secrets must include `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` for Tigris/S3
  * App secrets must include `FLY_API_TOKEN` (used by orchestrator machine)

## How It Works

1. **Build Phase**: Builds Docker image with `fly deploy --build-only`
2. **Orchestrator Phase**: Spawns temporary machine with new image
3. **Tarball Phase**: Orchestrator creates tarball of all .beam files
4. **Upload Phase**: Uploads tarball to Tigris/S3
5. **Reload Phase**: Each app machine downloads and extracts tarball
6. **Upgrade Phase**: Processes are upgraded using :sys.change_code/4

The orchestrator machine automatically has access to app secrets and
environment variables from the `[env]` section of your fly.toml.

## Safety

- Processes are suspended before code loading (prevents race conditions)
- Only proc_lib processes are upgraded (filters out Tasks)
- Machines can be upgraded concurrently or sequentially
- Each process upgrade is isolated with error handling
- Rollback support (coming soon)

---

*Consult [api-reference.md](api-reference.md) for complete listing*