# `FlyDeploy.Poller`

Polls S3 for hot upgrades and applies them automatically.

This GenServer periodically checks S3 for new hot upgrades and applies them
when detected. It compares the S3 state's `source_image_ref` with the local
marker file to determine if an upgrade is needed.

## Usage

Add to your application's supervision tree:

    def start(_type, _args) do
      children = [
        {FlyDeploy, otp_app: :my_app},
        # ... rest of your children
      ]

      Supervisor.start_link(children, strategy: :one_for_one)
    end

## Options

- `:otp_app` - The OTP application name (required)
- `:poll_interval` - How often to poll S3 in ms (default: 1000)
- `:suspend_timeout` - Timeout for suspending processes during upgrade in ms (default: 10_000)

## Important

Place `{FlyDeploy, otp_app: :my_app}` at the TOP of your children list.
The poller blocks during `init/1` to apply any pending hot upgrades before
the rest of the supervision tree starts.

## Efficiency

Uses ETag/If-None-Match headers to minimize bandwidth. When no upgrade is
available, S3 returns a 304 Not Modified response.

## Version Tracking

The poller maintains a "fingerprint" of the current code version in
`:persistent_term`. Access it via `FlyDeploy.current_vsn/0`.

# `child_spec`

Returns a specification to start this module under a supervisor.

See `Supervisor`.

# `current_vsn`

Returns the current code version fingerprint.

The fingerprint is a map containing:
- `:base_image_ref` - The base Docker image (FLY_IMAGE_REF)
- `:hot_ref` - The hot upgrade source image ref (nil if no hot upgrade)
- `:version` - The hot upgrade version (nil if no hot upgrade)
- `:fingerprint` - A short hash combining base + hot refs

Returns `nil` if not yet initialized.

# `get_state`

Returns the current poller state for debugging.

# `start_link`

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