# Down

Down is a utility tool for streaming, flexible and safe downloading of remote
files. It can use [open-uri] + `Net::HTTP`, [HTTP.rb] or `wget` as the backend
HTTP library.

## Installation

```rb
gem "down", "~> 4.4"
```

## Downloading

The primary method is `Down.download`, which downloads the remote file into a
Tempfile:

```rb
require "down"

tempfile = Down.download("http://example.com/nature.jpg")
tempfile #=> #<Tempfile:/var/folders/k7/6zx6dx6x7ys3rv3srh0nyfj00000gn/T/20150925-55456-z7vxqz.jpg>
```

### Metadata

The returned Tempfile has `#content_type` and `#original_filename` attributes
determined from the response headers:

```rb
tempfile.content_type      #=> "text/plain"
tempfile.original_filename #=> "document.txt"
tempfile.charset           #=> "utf-8"
```

### Maximum size

When you're accepting URLs from an outside source, it's a good idea to limit
the filesize (because attackers want to give a lot of work to your servers).
Down allows you to pass a `:max_size` option:

```rb
Down.download("http://example.com/image.jpg", max_size: 5 * 1024 * 1024) # 5 MB
# Down::TooLarge: file is too large (max is 5MB)
```

What is the advantage over simply checking size after downloading? Well, Down
terminates the download very early, as soon as it gets the `Content-Length`
header. And if the `Content-Length` header is missing, Down will terminate the
download as soon as the downloaded content surpasses the maximum size.

### Destination

By default the remote file will be downloaded into a temporary location and
returned as a `Tempfile`. If you would like the file to be downloaded to a
specific location on disk, you can specify the `:destination` option:

```rb
Down.download("http://example.com/image.jpg", destination: "/path/to/destination")
```

### Basic authentication

`Down.download` and `Down.open` will automatically detect and apply HTTP basic
authentication from the URL:

```rb
Down.download("http://user:password@example.org")
Down.open("http://user:password@example.org")
```

### Progress

`Down.download` supports `:content_length_proc`, which gets called with the
value of the `Content-Length` header as soon as it's received, and
`:progress_proc`, which gets called with current filesize whenever a new chunk
is downloaded.

```rb
Down.download "http://example.com/movie.mp4",
  content_length_proc: -> (content_length) { ... },
  progress_proc:       -> (progress)       { ... }
```

## Streaming

Down has the ability to retrieve content of the remote file *as it is being
downloaded*. The `Down.open` method returns a `Down::ChunkedIO` object which
represents the remote file on the given URL. When you read from it, Down
internally downloads chunks of the remote file, but only how much is needed.

```rb
remote_file = Down.open("http://example.com/image.jpg")
remote_file.size # read from the "Content-Length" header

remote_file.read(1024) # downloads and returns first 1 KB
remote_file.read(1024) # downloads and returns next 1 KB

remote_file.eof? #=> false
remote_file.read # downloads and returns the rest of the file content
remote_file.eof? #=> true

remote_file.close # closes the HTTP connection and deletes the internal Tempfile
```

### Caching

By default the downloaded content is internally cached into a `Tempfile`, so
that when you rewind the `Down::ChunkedIO`, it continues reading the cached
content that it had already retrieved.

```rb
remote_file = Down.open("http://example.com/image.jpg")
remote_file.read(1*1024*1024) # downloads, caches, and returns first 1MB
remote_file.rewind
remote_file.read(1*1024*1024) # reads the cached content
remote_file.read(1*1024*1024) # downloads the next 1MB
```

If you want to save on IO calls and on disk usage, and don't need to be able to
rewind the `Down::ChunkedIO`, you can disable caching downloaded content:

```rb
Down.open("http://example.com/image.jpg", rewindable: false)
```

### Yielding chunks

You can also yield chunks directly as they're downloaded via `#each_chunk`, in
which case the downloaded content is not cached into a file regardless of the
`:rewindable` option.

```rb
remote_file = Down.open("http://example.com/image.jpg")
remote_file.each_chunk { |chunk| ... }
remote_file.close
```

### Data

You can access the response status and headers of the HTTP request that was made:

```rb
remote_file = Down.open("http://example.com/image.jpg")
remote_file.data[:status]   #=> 200
remote_file.data[:headers]  #=> { ... }
remote_file.data[:response] # returns the response object
```

Note that `Down::NotFound` error will automatically be raised if response
status was 4xx or 5xx.

### `Down::ChunkedIO`

The `Down.open` performs HTTP logic and returns an instance of
`Down::ChunkedIO`. However, `Down::ChunkedIO` is a generic class that can wrap
any kind of streaming. It accepts an `Enumerator` that yields chunks of
content, and provides IO-like interface over that enumerator, calling it
whenever more content is needed.

```rb
require "down/chunked_io"

Down::ChunkedIO.new(...)
```

* `:chunks` – `Enumerator` that yields chunks of content
* `:size` – size of the file if it's known (returned by `#size`)
* `:on_close` – called when streaming finishes or IO is closed
* `:data` - custom data that you want to store (returned by `#data`)
* `:rewindable` - whether to cache retrieved data into a file (defaults to `true`)
* `:encoding` - force content to be returned in specified encoding (defaults to `Encoding::BINARY`)

Here is an example of creating a streaming IO of a MongoDB GridFS file:

```rb
require "down/chunked_io"

mongo = Mongo::Client.new(...)
bucket = mongo.database.fs

content_length = bucket.find(_id: id).first[:length]
stream = bucket.open_download_stream(id)

io = Down::ChunkedIO.new(
  size: content_length,
  chunks: stream.enum_for(:each),
  on_close: -> { stream.close },
)
```

### Exceptions

Down tries to recognize various types of exceptions and re-raise them as one of
the `Down::Error` subclasses. This is Down's exception hierarchy:

* `Down::Error`
  * `Down::TooLarge`
  * `Down::NotFound`
    * `Down::InvalidUrl`
    * `Down::TooManyRedirects`
    * `Down::ResponseError`
      * `Down::ClientError`
      * `Down::ServerError`
    * `Down::ConnectionError`
    * `Down::TimeoutError`
    * `Down::SSLError`

## Backends

By default Down implements `Down.download` and `Down.open` using the built-in
[open-uri] + [Net::HTTP] Ruby standard libraries. However, there are other
backends as well, see the sections below.

You can use the backend directly:

```rb
require "down/net_http"

Down::NetHttp.download("...")
Down::NetHttp.open("...")
```

Or you can set the backend globally (default is `:net_http`):

```rb
require "down"

Down.backend :http # use the Down::Http backend

Down.download("...")
Down.open("...")
```

### open-uri + Net::HTTP

```rb
gem "down", "~> 4.4"
```
```rb
require "down/net_http"

tempfile = Down::NetHttp.download("http://nature.com/forest.jpg")
tempfile #=> #<Tempfile:/var/folders/k7/6zx6dx6x7ys3rv3srh0nyfj00000gn/T/20150925-55456-z7vxqz.jpg>

io = Down::NetHttp.open("http://nature.com/forest.jpg")
io #=> #<Down::ChunkedIO ...>
```

`Down::NetHttp.download` is implemented as a wrapper around open-uri, and fixes
some of open-uri's undesired behaviours:

* uses `URI::HTTP#open` or `URI::HTTPS#open` directly for [security](https://sakurity.com/blog/2015/02/28/openuri.html)
* always returns a `Tempfile` object, whereas open-uri returns `StringIO`
  when file is smaller than 10KB
* gives the extension to the `Tempfile` object from the URL
* allows you to limit maximum number of redirects

On the other hand `Down::NetHttp.open` is implemented using Net::HTTP directly,
as open-uri doesn't support downloading on-demand.

#### Redirects

`Down.download` turns off open-uri's following redirects, as open-uri doesn't
have a way to limit the maximum number of hops, and implements its own. By
default maximum of 2 redirects will be followed, but you can change it via the
`:max_redirects` option:

```rb
Down::NetHttp.download("http://example.com/image.jpg")                   # 2 redirects allowed
Down::NetHttp.download("http://example.com/image.jpg", max_redirects: 5) # 5 redirects allowed
Down::NetHttp.download("http://example.com/image.jpg", max_redirects: 0) # 0 redirects allowed

Down::NetHttp.open("http://example.com/image.jpg")                       # 2 redirects allowed
Down::NetHttp.open("http://example.com/image.jpg", max_redirects: 5)     # 5 redirects allowed
Down::NetHttp.open("http://example.com/image.jpg", max_redirects: 0)     # 0 redirects allowed
```

#### Proxy

Both `Down.download` and `Down.open` support a `:proxy` option, where you can
specify a URL to an HTTP proxy which should be used when downloading.

```rb
Down::NetHttp.download("http://example.com/image.jpg", proxy: "http://proxy.org")
Down::NetHttp.open("http://example.com/image.jpg",     proxy: "http://user:password@proxy.org")
```

#### Timeouts

Both `Down.download` and `Down.open` support `:read_timeout` and `:open_timeout`
options, which are forwarded to `Net::HTTP`:

```rb
Down::NetHttp.download("http://example.com/image.jpg", open_timeout: 5)
Down::NetHttp.open("http://example.com/image.jpg", read_timeout: 10)
```

#### Additional options

Any additional options passed to `Down.download` will be forwarded to
[open-uri], so you can for example add basic authentication or a timeout:

```rb
Down::NetHttp.download "http://example.com/image.jpg",
  http_basic_authentication: ['john', 'secret'],
  read_timeout: 5
```

`Down.open` accepts `:ssl_verify_mode` and `:ssl_ca_cert` options with the same
semantics as in open-uri, and any options with String keys will be interpreted
as request headers, like with open-uri.

```rb
Down::NetHttp.open("http://example.com/image.jpg", {"Authorization" => "..."})
```

You can also initialize the backend with default options:

```rb
net_http = Down::NetHttp.new(open_timeout: 3)

net_http.download("http://example.com/image.jpg")
net_http.open("http://example.com/image.jpg")
```

### HTTP.rb

```rb
gem "down", "~> 4.4"
gem "http", "~> 3.2"
```
```rb
require "down/http"

tempfile = Down::Http.download("http://nature.com/forest.jpg")
tempfile #=> #<Tempfile:/var/folders/k7/6zx6dx6x7ys3rv3srh0nyfj00000gn/T/20150925-55456-z7vxqz.jpg>

io = Down::Http.open("http://nature.com/forest.jpg")
io #=> #<Down::ChunkedIO ...>
```

Some features that give the HTTP.rb backend an advantage over `open-uri` +
`Net::HTTP` include:

* Low memory usage (**10x less** than `open-uri`/`Net::HTTP`)
* Correct URI parsing with [Addressable::URI]
* Proper support for streaming downloads (`#download` and now reuse `#open`)
* Proper support for SSL
* Chaninable HTTP client builder API for setting default options
* Support for persistent connections

#### Additional options

All additional options will be forwarded to `HTTP::Client#request`:

```rb
Down::Http.download("http://example.org/image.jpg", headers: { "Foo" => "Bar" })
Down::Http.open("http://example.org/image.jpg", follow: { max_hops: 0 })
```

If you prefer to add options using the chainable API, you can pass a block:

```rb
Down::Http.open("http://example.org/image.jpg") do |client|
  client.timeout(connect: 3)
end
```

You can also initialize the backend with default options:

```rb
http = Down::Http.new(headers: { "Foo" => "Bar" })
# or
http = Down::Http.new { |client| client.timeout(connect: 3) }

http.download("http://example.com/image.jpg")
http.open("http://example.com/image.jpg")
```

#### Request method

By default `Down::Http` makes a `GET` request to the specified endpoint, but you
can specify a different request method using the `:method` option:

```rb
Down::Http.download("http://example.org/image.jpg", method: :post)
Down::Http.open("http://example.org/image.jpg", method: :post)

down = Down::Http.new(method: :post)
down.download("http://example.org/image.jpg")
```

### Wget (experimental)

```rb
gem "down", "~> 4.4"
gem "posix-spawn" # omit if on JRuby
gem "http_parser.rb"
```
```rb
require "down/wget"

tempfile = Down::Wget.download("http://nature.com/forest.jpg")
tempfile #=> #<Tempfile:/var/folders/k7/6zx6dx6x7ys3rv3srh0nyfj00000gn/T/20150925-55456-z7vxqz.jpg>

io = Down::Wget.open("http://nature.com/forest.jpg")
io #=> #<Down::ChunkedIO ...>
```

The Wget backend uses the `wget` command line utility for downloading. One
major advantage of `wget` is that it automatically resumes downloads that were
interrupted due to network failures, which is very useful when you're
downloading large files.

However, the Wget backend should still be considered experimental, as it wasn't
easy to implement a CLI wrapper that streams output, so it's possible that I've
made mistakes. Let me know how it's working out for you 😉.

#### Additional arguments

You can pass additional arguments to the underlying `wget` commmand via symbols:

```rb
Down::Wget.download("http://nature.com/forest.jpg", :no_proxy, connect_timeout: 3)
Down::Wget.open("http://nature.com/forest.jpg", user: "janko", password: "secret")
```

You can also initialize the backend with default arguments:

```rb
wget = Down::Wget.new(:no_proxy, connect_timeout: 3)

wget.download("http://nature.com/forest.jpg")
wget.open("http://nature.com/forest.jpg")
```

## Supported Ruby versions

* MRI 2.2
* MRI 2.3
* MRI 2.4
* JRuby

## Development

You can run tests with

```
$ bundle exec rake test
```

The test suite pulls and runs [kennethreitz/httpbin] as a Docker container, so
you'll need to have Docker installed and running.

## License

[MIT](LICENSE.txt)

[open-uri]: http://ruby-doc.org/stdlib-2.3.0/libdoc/open-uri/rdoc/OpenURI.html
[Net::HTTP]: https://ruby-doc.org/stdlib-2.4.1/libdoc/net/http/rdoc/Net/HTTP.html
[HTTP.rb]: https://github.com/httprb/http
[Addressable::URI]: https://github.com/sporkmonger/addressable
[kennethreitz/httpbin]: https://github.com/kennethreitz/httpbin