Simpler streaming responses

March 27, 2013

GravatarMichael Snoyman

Yesod is built on top of WAI, which has always provided a means of creating efficient, streaming responses. Throughout Yesod's development, this functionality has always been present in one form or another. In Yesod 1.2, the goal is to make it as simple as possible to leverage this functionality.

Let's kick off with a simple example, and then drill into the details:

{-# LANGUAGE OverloadedStrings, TemplateHaskell, QuasiQuotes, TypeFamilies #-}
import Yesod.Core
import Data.Conduit
import qualified Data.Conduit.Binary as CB
import Control.Concurrent.Lifted (threadDelay)
import Data.Monoid ((<>))
import qualified Data.Text as T
import Control.Monad (forM_)

data App = App

mkYesod "App" [parseRoutes|
/ HomeR GET
|]

instance Yesod App

fibs :: [Int]
fibs = 1 : 1 : zipWith (+) fibs (tail fibs)

getHomeR :: Handler TypedContent
getHomeR = do
    value <- lookupGetParam "x"
    case value of
        Just "file" -> respondSource typePlain $ do
            sendChunkText "Going to read a file\n\n"
            CB.sourceFile "streaming.hs" $= awaitForever sendChunkBS
            sendChunkText "Finished reading the file\n"
        Just "fibs" -> respondSource typePlain $ do
            forM_ fibs $ \fib -> do
                $logError $ "Got fib: " <> T.pack (show fib)
                sendChunkText $ "Next fib is: " <> T.pack (show fib) <> "\n"
                yield Flush
                sendFlush
                threadDelay 1000000
        _ -> fmap toTypedContent $ defaultLayout $ do
            setTitle "Streaming"
            [whamlet|
                <p>Notice how in the code above we perform selection before starting the stream.
                <p>Anyway, choose one of the options below.
                <ul>
                    <li>
                        <a href=?x=file>Read a file
                    <li>
                        <a href=?x=fibs>See the fibs
            |]

main = warp 3000 App

Start simple: a standard response

Consider the following handler:

getHomeR :: Handler Text
getHomeR = return "Hello World!"

What exactly does Yesod do to make this into a response the client can see? The important bit is the ToTypedContent typeclass. Every handler function has toTypedContent applied to its result. So let's look at the relevant classes and types.

type ContentType = ByteString
data Content = ContentBuilder !Blaze.Builder !(Maybe Int)
               -- ^ The content and optional content length.
             | ContentSource !(Source (ResourceT IO) (Flush Blaze.Builder))
             | ContentFile !FilePath !(Maybe FilePart)
             | ContentDontEvaluate !Content

data TypedContent = TypedContent !ContentType !Content
class ToTypedContent a where
    toTypedContent :: a -> TypedContent

-- Relevant instance
instance ToTypedContent Text where
    toTypedContent t = TypedContent
        "text/plain; charset=utf-8"
        (\t -> ContentBuilder (Blaze.fromText t) Nothing)

So every response has to be convertible to a TypedContent, which is two pieces of information: the value for the Content-Type response header, and the body fo the response. In our case, we use the ContentBuilder constructor, which lets us leverage blaze-builder.

Use the Source

ContentBuilder isn't our only option. We could serve a file with ContentFile. ContentDontEvaluate is a modifier to deal with exceptions; we'll discuss that a bit later. But for our streaming discussion, the most interesting constructor is ContentSource. This uses a conduit Source for creating streaming data. Let's try out a minimal example:

getHomeR :: Handler TypedContent
getHomeR = return $ TypedContent "text/plain" $ ContentSource $ do
    yield $ Chunk $ Blaze.fromText "Hello World!"

We can use the TypedContent and ContentSource constructors directly. The result isn't really anything more impressive than what we had previously. Let's improve that, by streaming two files consecutively:

getHomeR :: Handler TypedContent
getHomeR = return $ TypedContent "text/plain" $ ContentSource $ do
    mapOutput (Chunk . Blaze.fromByteString) $ sourceFile "file1.txt"
    mapOutput (Chunk . Blaze.fromByteString) $ sourceFile "file2.txt"

We're guaranteed that our response will live in constant memory and will properly free resources. We have to play with mapOutput, Chunk and fromByteString to convert a stream of ByteStrings to a stream of flushable Builders.

Make it prettier

Having to muck around with those lower-level details isn't fun. Let's bump it up a level:

getHomeR :: Handler TypedContent
getHomeR = respondSource "text/html" $ do
    sendChunk ("Some Text" :: Text)
    sendChunk ("Hello & Goodbye" :: Html)

respondSource wraps up the tedium of dealing with the constructors directly. sendChunk will send a chunk of content to the user, and can take as an argument most common textual types (String, strict/lazy Text, strict/lazy ByteString, and Html). But this doesn't play very nicely with overloaded strings, since you need to provide explicit annotations. So we also have simple type-specified wrappers as well:

getHomeR :: Handler TypedContent
getHomeR = respondSource "text/html" $ do
    sendChunkText "Some Text"
    sendChunkHtml "Hello & Goodbye"

We can also use sendFlush to flush the buffer to the client immediately. And we have the ability to use all common conduit concepts to build up our Source.

And one final but important point: the base monad for the Source is Handler, so you can perform arbitrary Handler operations inside your Source, such as looking up query string parameters.

Exceptions

Let's go back to non-streaming responses. Consider the following:

getHomeR :: Handler Html
getHomeR =
    return $ "Hello " <> name <> "!"
  where
    name = error "Oops, forgot to set the name"

We have an exception being sent from pure code. Let's see what Yesod does with this:

Pure exception

This is the result we want: the user receives a 500 status code to indicate that there was an error on the server. But how does this work? The pure exception should only be discovered after we already send our 200 status code and response headers, right?

In fact, Yesod does some fancy footwork behind the scenes, and fully evaluates pure response bodies before sending any data to the user, specifically to ensure that the user gets proper response codes. And this is also the purpose of the above-mentioned ContentDontEvaluate constructor: to give the user a chance to override this behavior (e.g., for efficiency). For example, we can modify our above code to read:

getHomeR :: Handler (DontFullyEvaluate Html)
getHomeR =
    return $ DontFullyEvaluate $ "Hello " <> name <> "!"
  where
    name = error "Oops, forgot to set the name"

When run like this, the client receives an empty response from the server instead.

"All very interesting," you might be saying, "but what does this have to do with streaming responses?" Quite a bit, actually, as the same reasoning applies. When using streaming responses, there's no way for Yesod to fully evaluate your response body before sending them to the client. So if you throw an exception in your Source, the client will get a corrupted response. This isn't to say you shouldn't use streaming responses, but you have to be careful.

Logic before streaming

Exceptions aren't the only issue. You can't modify the status code or response headers at all once you're inside the Source. That means you can't perform redirects, can't modify the session, or can't switch from a 200 OK response to a 403 Forbidden response. The important point here is to perform your logic before streaming.

getHomeR :: Handler TypedContent
getHomeR = do
    maybeFoo <- lookupGetParam "foo"
    case maybeFoo of
        Just "yesod" -> redirect ("http://www.yesodweb.com" :: Text)
        _ -> return ()
    respondSource "text/plain" $ do
        sendChunkText "You didn't go to yesodweb.com"

We check our query string parameter and perform the redirect before calling respondSource. Once we know that we're returning a normal response, we then use respondSource to create the body.

Database

Making it easy to create streaming database responses was probably my original motivation here. I was never happy with the current recommended approach, so I'm happy to offer something simpler. Basically, we follow the exact same approach as with normal streaming responses, but use the respondSourceDB function instead of respondSource. Take the following example, which just returns a list of people from a database.

{-# LANGUAGE EmptyDataDecls    #-}
{-# LANGUAGE FlexibleContexts  #-}
{-# LANGUAGE GADTs             #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes       #-}
{-# LANGUAGE TemplateHaskell   #-}
{-# LANGUAGE TypeFamilies      #-}
import           Control.Monad.Logger    (runNoLoggingT)
import           Data.Conduit            (awaitForever, runResourceT, ($=))
import           Data.Text               (Text)
import           Database.Persist.Sqlite (ConnectionPool, SqlPersist,
                                          SqliteConf (..), runMigration,
                                          runSqlPool)
import           Database.Persist.Store  (createPoolConfig)
import           Yesod.Core
import           Yesod.Persist

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persist|
Person
    name Text
|]

data App = App
    { appConfig :: SqliteConf
    , appPool   :: ConnectionPool
    }

mkYesod "App" [parseRoutes|
/ HomeR GET
|]

instance Yesod App
instance YesodPersist App where
    type YesodPersistBackend App = SqlPersist
    runDB = defaultRunDB appConfig appPool
instance YesodPersistRunner App where
    getDBRunner = defaultGetDBRunner appPool

getHomeR :: Handler TypedContent
getHomeR =
    respondSourceDB typePlain $ selectSource [] [Asc PersonName] $= awaitForever toBuilder
  where
    toBuilder (Entity _ (Person name)) = do
        sendChunkText name
        sendChunkText "\n"
        sendFlush

main :: IO ()
main = do
    let config = SqliteConf ":memory:" 1
    pool <- createPoolConfig config
    runNoLoggingT $ runResourceT $ flip runSqlPool pool $ do
        runMigration migrateAll
        deleteWhere ([] :: [Filter Person])
        insert_ $ Person "Charlie"
        insert_ $ Person "Alice"
        insert_ $ Person "Bob"
    warp 3000 App
        { appConfig = config
        , appPool = pool
        }

Obviously for our specific case, loading up the three names into memory would be acceptable. But for more complicated responses, some form of streaming is essential. This approach works very well in concert with the new streaming API for yesod-sitemap, allowing us to create a streaming XML response body from a database. The following is some real-life code from the School of Haskell:

getSitemapR :: Handler TypedContent
getSitemapR = do
    AppContent {..} <- getYesod >>= readIORef . appContent
    sitemap $ runDBSource $ do
        yield $ SitemapUrl HomeR Nothing (Just Daily) (Just 1.0)
        mapM_ (yield . goPage) $ unpack acPageMap
        mapM_ (yield . goPost) acPosts
        yield $ SitemapUrl UsersR Nothing (Just Daily) (Just 0.6)
        yield $ SitemapUrl RecentContentR Nothing (Just Daily) (Just 0.6)
        selectSource [] [] $= CL.mapMaybeM (\(Entity _ Profile {..}) -> do
            mus <- getBy $ UniqueUserSummary profileHandle
            case mus of
                Just (Entity _ us) | userSummaryTutcount us > 0 -> return $ Just $
                    SitemapUrl (UserR profileHandle) Nothing (Just Weekly) (Just 0.5)
                _ -> return Nothing
                )
        selectKeys [] [] $= CL.mapMaybeM (fmap (fmap goTutorial) . getCanonicalRoute)
  where
    goPage (pn, PageInfo {..}) = SitemapUrl (PageR pn) Nothing (Just Monthly) (Just 0.8)
    goPost Post {..} =
        SitemapUrl (BlogPostR y m s) (Just postDate) (Just Never) (Just 0.7)
      where
        PostKey y m s = postKey
    goTutorial route = SitemapUrl route Nothing (Just Monthly) (Just 0.6)

As the number of users and tutorials grows considerably, we want to avoid loading all of that information into memory. The above code runs in constant space, dealing with each individual user, and then each individual tutorial. Under the surface, each SitemapUrl value is converted into a stream of xml-types Events, and xml-conduit converts that stream into a stream of ByteStrings.

We have to be careful that our database queries are guaranteed to succeed. If we use functions like get404 inappropriately, we could generate incorrect response bodies.

And yes, that means that the School of Haskell is currently running on Yesod 1.2.

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