将字节流式传输到网络 websocket

Question

我有一个代码使用文件句柄来模拟来自源 (AWS S3) 的流 Bytestring 的接收器。如果我们想使用 Network.Websocket as the sink, would it suffice to swap LBS.writeFile in the code below with sendBinaryData（带连接句柄）？

{-# LANGUAGE OverloadedStrings,ScopedTypeVariables #-}

import qualified Aws
import qualified Aws.S3 as S3
import           Data.Conduit (($$+-))
import qualified Data.Conduit.List as CL (mapM_)
import qualified Data.ByteString.Streaming.HTTP as SP
import qualified Data.ByteString.Lazy as LBS
import Streaming as S
import Streaming.Prelude as S hiding (show,print)
import Control.Concurrent.Async (async,waitCatch)
import Data.Text as T (Text)

data AwsConfig a = AwsConfig { _aws_cfg :: Aws.Configuration, _aws_s3cfg :: S3.S3Configuration a, _aws_httpmgr :: SP.Manager }

getObject :: AwsConfig Aws.NormalQuery -> T.Text -> T.Text ->  IO Int
getObject cfg bucket key = do
  req <- waitCatch =<< async (runResourceT $ do
    {- Create a request object with S3.getObject and run the request with pureAws. -}
    S3.GetObjectResponse { S3.gorResponse = rsp, S3.gorMetadata = mdata } <- 
      Aws.pureAws (_aws_cfg cfg) (_aws_s3cfg cfg) (_aws_httpmgr cfg) $
        S3.getObject bucket key
    {- Stream the response to a lazy bytestring -}
    liftIO $ LBS.writeFile "testaws" LBS.empty -- this will be replaced by content-length of the bytes 
    let obj = (($$+- CL.mapM_ S.yield) . hoist lift ) (SP.responseBody rsp)
    S.mapM_ (liftIO . (LBS.appendFile "testaws") . LBS.fromStrict) obj
    return $ lookup "content-length" (S3.omUserMetadata mdata))
  case req of
    Left _ -> return 2 -- perhaps, we could use this to send an error message over websocket
    Right _ -> return 0

让我困惑的是如何确定流的终止？对于文件，这由 writeFile API 处理。 sendBinaryData 呢？它是否以与 writeFile 类似的方式处理终止？还是由客户端的数据解析器决定？

更新

这个问题是关于如何将数据流式传输到 websocket 句柄（让我们假设已经提供了句柄），就像我们在上面的示例中处理文件句柄一样，而不是真正关于如何在 resourceT。 conduit 似乎确实采用 mapM_ 方法来接收数据。所以，看来这确实是要走的路。

termination问题是因为我有这样的思路：如果我们有一个函数监听Websocket句柄另一端的数据，然后确定消息结束似乎在流媒体环境中很重要。给出如下函数：

f :: LBS.ByteString -> a

如果我们 S.mapM_ 将数据流式传输到 websocket 句柄，它是否负责添加某种 end of stream 标记以便 f 在另一端监听可以停止处理惰性字节串。否则 f 将不知道消息何时完成。

Answer 1

你认为手柄需要额外的技巧是正确的。但是，由于您已经在使用 ResourceT monad 转换器，因此这是 delightfully simple to do with allocate。 allocate 允许您在资源 monad 中创建一个句柄并注册一个清理操作（在您的情况下只是关闭连接）。

ok <- runResourceT $ do
  (releaseKey, handle) <-
    allocate (WebSockets.acceptRequest request) 
             (`WebSockets.sendClose` closeMessage)
  WebSockets.sendBinaryData handle data
  return ok
where
  request = ...
  closeMessage = ...
  data = ...
  ok = ...

通过使用 allocate，句柄保证在 runResourceT return 秒 ok.

之前关闭

不过，我不完全确定这就是您想要的。在我看来 getObject 应该不知道如何接受和关闭 WS 连接；也许它应该将 WS 连接句柄作为参数然后写入它。如果您将其 return 类型升级为 ResourceT 那么您可以将 caller 收费为 getObject 并负责调用 runResourceT 和分配WS 句柄等等。但希望上面的示例足以让您继续前进。

Answer 2

（警告 - 代码未经测试。）

您的代码会重新打开输出文件并在每次传入数据包时附加到它。显然，更好的解决方案是使用 LBS.hPutStr 使用已打开的文件句柄写入文件.

也就是说，而不是：

S.mapM_ (liftIO . (LBS.appendFile "testaws") . LBS.fromStrict) obj

您想使用：

S.mapM_ (liftIO . (LBS.hPutStr h) . LBS.fromStrict) obj

当然，这引用了句柄 h，那是从哪里来的？

一个解决方案是将它传递给 getObject 或者在调用 getObject 的主体之前创建它，例如：

getObject cfg bucket key = withFile "output" $ \h -> do
    req <- ...
    ...
    S.mapM_ (liftIO . (LBS.hPutStr h) . LBS.fromStrict) obj
    ...

或者您可能必须在 runResourceT 中创建...我不确定。

更新 - 请参阅@haoformayor 的回答，了解如何让 ResourceT 为您管理文件句柄。

Answer 3

这里有一些细节可以使事情更容易理解。首先，对于第一个小演示，修改你的 getObject，我使用 Streaming.ByteString.writeFile，无论如何都在 ResourceT 中，以减少惰性字节串的绕路。

{-# LANGUAGE OverloadedStrings,ScopedTypeVariables #-}

import qualified Aws
import qualified Aws.S3 as S3
import           Data.Conduit 
import qualified Data.Conduit.List as CL (mapM_)
import qualified Data.ByteString.Streaming.HTTP as HTTP
import qualified Data.ByteString.Streaming as SB
import qualified Data.ByteString.Streaming.Internal as SB
import qualified Data.ByteString as B
import qualified Data.ByteString.Lazy as BL
import           Streaming as S
import           Streaming.Prelude as S hiding (show,print)
import           Control.Concurrent.Async (async,waitCatch)
import           Data.Text as T (Text) 
import qualified Network.WebSockets as WebSockets
import           Control.Monad.Trans.Resource

data AwsConfig a = AwsConfig { _aws_cfg :: Aws.Configuration
                             , _aws_s3cfg :: S3.S3Configuration a
                             , _aws_httpmgr :: HTTP.Manager }

getObject :: AwsConfig Aws.NormalQuery -> FilePath -> T.Text -> T.Text ->  IO Int
getObject cfg file bucket key = do
  req <- waitCatch =<< async (runResourceT $ do
    S3.GetObjectResponse { S3.gorResponse = rsp, S3.gorMetadata = mdata } <- 
      Aws.pureAws (_aws_cfg cfg) (_aws_s3cfg cfg) (_aws_httpmgr cfg) $
        S3.getObject bucket key
    let bytestream = do 
         -- lookup "content-length" (S3.omUserMetadata mdata))
         SB.chunk B.empty -- this will be replaced by content-length 
         hoist lift (HTTP.responseBody rsp)  $$+- CL.mapM_ SB.chunk 
    SB.writeFile file bytestream ) -- this is in ResourceT 
  case req of
    Left _ -> return 2
    Right _ -> return 0

我们可以或多或少地从中抽象出你用 SB.writeFile:

做的事情

getObjectAbstracted
      :: (SB.ByteString (ResourceT IO) () -> ResourceT IO b)
         -> AwsConfig Aws.NormalQuery -> S3.Bucket -> Text -> ResourceT IO b
getObjectAbstracted action cfg bucket key = do
    S3.GetObjectResponse { S3.gorResponse = rsp, S3.gorMetadata = mdata } <- 
      Aws.pureAws (_aws_cfg cfg) 
                  (_aws_s3cfg cfg) 
                  (_aws_httpmgr cfg) 
                  (S3.getObject bucket key)

    action (hoist lift (HTTP.responseBody rsp)  $$+- CL.mapM_ SB.chunk)

现在，我们需要一个流字节串库中没有的小帮手

mapMChunks_ :: Monad m => (B.ByteString -> m ()) -> SB.ByteString m r -> m r
mapMChunks_ act bytestream = do
  (a S.:> r) <- SB.foldlChunksM (\_ bs -> act bs) (return ()) bytestream
  return r

并且可以或多或少地按照@haoformayor 的计划进行，使用流字节串

writeConnection :: MonadIO m => WebSockets.Connection -> SB.ByteString m r -> m r
writeConnection connection  = 
  mapMChunks_ (liftIO . WebSockets.sendBinaryData connection)

-- following `haoformayor`
connectWrite
    :: (MonadResource m, WebSockets.WebSocketsData a) 
    => WebSockets.PendingConnection 
    -> a                  -- closing  message
    -> SB.ByteString m r  -- stream from aws
    -> m r
connectWrite request closeMessage bytestream = do
    (releaseKey, connection) <- allocate (WebSockets.acceptRequest request)
                                         (`WebSockets.sendClose` closeMessage)
    writeConnection connection bytestream

getObjectWS :: WebSockets.WebSocketsData a =>
       WebSockets.PendingConnection
       -> a
       -> AwsConfig Aws.NormalQuery
       -> S3.Bucket
       -> Text
       -> ResourceT IO ()
getObjectWS request closeMessage = getObjectAbstracted (connectWrite request closeMessage)

当然 none 到此为止利用了 conduit 和 streaming/streaming-bytestring.

之间的任何区别

将字节流式传输到网络 websocket

Streaming bytes to network websocket

haskell

haskell-pipes