» New Features for Perpubplat and Ruminations on Service APIs for the Web #

Paul R. Brown @ 2008-02-18

I've added some new features to perpubplat, and each one presented a nice exercise in Haskell, working with Haskell libraries, and the design and consumption of web APIs.

Collage of Random Flickr Photos

Flickr Sidebar screenshotThe first feature is the collage of photos that uses the Flickr JSON API. The collage appears at the bottom of the sidebar under the "Photos" heading.

The implementation of the collage (Blog.Widgets.FlickrCollage; source here) uses a polite (i.e., supports conditional GET) HTTP poller (Blog.BackEnd.HttpPoller; source here) to call flickr.people.getPublicPhotos (docs here) every fifteen minutes and pull down the data for my most recent 500 photos. (I'll discuss the HTTP poller below.) To deal with concurrency — many readers (HTTP requests) and one writer (the polling thread) — an MVar holds the list of photos, with the writer taking the old value and putting the new and the reader taking the old value and then putting it right back. The implementation of MVar ensures that waiters are awakened in FIFO order, so this should (and does) work great.

The JSON parser that I've been using uses Haskell's datatype polymorphism to model polymorphism in JSON, and this means that you work with wrapped (JSON Array wrapped around a list, JSON String wrapped around a Haskell String, etc.) primitive values instead of primitive values. To make things a little more ergonomic, I've bundled up some one-line utility functions in Blog.Widgets.JsonUtilities (source here). My favorite of the bunch is </>:

(</>) :: J.Value -> String -> J.Value
(J.Object o) </> s = o M.! s
(J.Array a) </> s = J.Array $ map (flip (</>) $ s) a

This makes it possible to compactly express access to nested JSON objects. For example, from the Flickr integration:

to_photo :: J.Value -> FlickrPhoto
to_photo m = FlickrPhoto { photo_id = uns $ m </> "id"
                         , owner = uns $ m </> "owner"
                         , secret = uns $ m </> "secret"
                         , server = uns $ m </> "server"
                         , photo_title = uns $ m </> "title"
                         , farm = unn $ m </> "farm" }

The uns function pulls the value out of a wrapped JSON String, and the unn function pulls the value out of a wrapped JSON Number. With a bit more thought, someone could probably come up with a nice library for JSON handling along the lines of Jaql or something like Pig Latin.

HTTP Polling

My rough cut at an HTTP polling library built on top of Network.HTTP is Blog.BackEnd.HttpPoller (source here), and it supports the bare minimum of features that I needed:

del.icio.us Bookmarks on an Entry

The second feature is integration with del.icio.us bookmarks pointing to an entry via the del.icio.us JSON API, and it shows up as a trailer on entries in the detail view:

del.icio.us entry trailer screenshot

I've already blogged about most of the interesting stuff from integrating with the del.icio.us JSON API using Network.HTTP; see Haskell, del.icio.us, and JSON (encodings and non-standard JSON) and A Short Adventure with simpleHTTP (unclosed sockets).

The part I didn't cover was how to schedule queries against del.icio.us, and I'll probably go back to both simplify and enhance it. As present, it's a bit convoluted; three threads interact as follows:

  1. The driver triggers the scheduler on a fixed interval.
  2. The scheduler manages an ordered list of scheduled times and entries. In response to a trigger from the driver, if the head of the list is past due, the scheduler pops the head of the list, refreshes the data about bookmarks for that entry, sends it to the controller, and schedules the next refresh for that entry based on its age in days. The scheduler also receives information about new entries and adds them to the schedule.
  3. The controller manages a Data.Map of data about bookmarks per entry and either updates data in response to the scheduler or returns the current data for rendering a response.

The current design is in-memory only, so it gets repopulated each time the service is booted. I intend to add simple file-based persistence along the same lines used for entries and comments. The other major missing features are support for conditional GET and grouping requests into groups of 15, as allowed by the del.icio.us API.

I would have liked to use the delicious API, but Network.HTTP doesn't currently support HTTPS.

Personal Aggregation

StreamOfConsciousness Sidebar screenshotThe third feature is aggregation of my del.icio.us bookmarks (via RSS feed), Google Reader shared items (via Atom feed), and Twitter "tweets" (via JSON API). The aggregated flotsam, jetsam, dross, and detritus shows up in the sidebar under the "Stream of Consciousness" heading in the sidebar.

The feature is a bit like Moveable Type's Action Streams plugin, but the perpubplat implementation benefits from the fact that a Haskell FastCGI application can have background threads (so no crontab hacking).

The implementation is in the Blog.Widgets.StreamOfConsciousness.* modules:

To handle the multiple writers and multiple readers, I implemented a lightweight version of multi-version concurrency control where readers can always get data but writers may have to repeat a computation if someone else updated the data in the meantime. Here's a fragment from B.W.S.Controller (full source here):

commit :: SoCController -> [Thought] -> IO ()
commit socc new_items =
    do { snap <- get_data socc
       ; let items' = take (max_size snap) $ merge new_items $ items snap
       ; let rendered' = thoughts_to_xhtml items' 
       ; let snap' = snap { items = items'
                          , rendered = rendered' }
       ; ok <- update socc snap'
       ; if ok then
             return ()
             do { threadDelay collision_delay
                ; commit socc new_items }

loop :: Chan SoCRequest -> Snapshot -> IO ()
loop ch snap = 
    do { req <- readChan ch
       ; snap' <- case req of
                   GetHtmlFragment c ->
                       do { putMVar c $ rendered snap
                          ; return snap }
                   GetData h ->
                       do { putMVar h snap
                          ; return snap }
                   Update ok snap'' ->
                       if (version snap) == (version snap'') then
                           do { putMVar ok True
                              ; let snap' = snap'' { version = (version snap) + 1 }
                              ; return snap' }
                           do { putMVar ok False
                              ; return snap }
       ; loop ch snap' }

The commit function runs in the HTTP polling thread doing the updating, and it's responsible both for merging the items into the sorted data and for updating the HTML representation that will get handed to the page rendering process.

The other interesting nut to crack was extracting data from XML using Haskell. I could have used the del.icio.us JSON feed and the JSON feed that the Google Reader shared items Javascript widget uses, but those lack the timestamps that I need to fold the streams together.

Extracting Data from RSS and Atom

I followed the standard trail for learning HXT, which involves building from source, reading the gentle introduction, and trying some of the practical examples. The only issue I had was with namespace handling.

Here's a code fragment from B.W.S.DeliciousPosts (source here) to read the RSS feed of my del.icio.us bookmarks:

import Text.XML.HXT.Arrow

handle_posts :: SoCController -> String -> IO ()
handle_posts socc body = do { posts <- runX ( readString parse_opts body >>> getItems )
                            ; commit socc posts }

parse_opts = [(a_validate, v_0), (a_check_namespaces,v_1)]
atElemQName qn = deep (isElem >>> hasQName qn)
text = getChildren >>> getText
textOf qn = atElemQName qn >>> text

rdf_uri = "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
rdf_RDF = QN "rdf" "RDF" rdf_uri

rss_uri = "http://purl.org/rss/1.0/"
rss_item = QN "rss" "item" rss_uri
rss_title = QN "rss" "title" rss_uri
rss_link = QN "rss" "link" rss_uri

dc_uri = "http://purl.org/dc/elements/1.1/"
dc_date = QN "dc" "date" dc_uri

getItem = atElemQName rss_item >>>
          proc i -> do
            t <- textOf rss_title -< i
            u <- textOf rss_link -< i
            d <- textOf dc_date -< i
            returnA -< Thought Delicious d u t

getItems = atElemQName rdf_RDF >>>
           proc r -> do
             items <- getItem -< r
             returnA -< items

HXT uses arrow notation; the quick and dirty explanation is that proc is like λ (but for arrows instead of functions), the <- is the usual monadic "bind" operator, and the -< feeds a value to the expression on the shaft of the arrow.

The first time I ran this against the RSS from del.icio.us, I got nothing back, so after looking at the XML for the RSS, I switched the prefix for the RSS QNames to the empty string to match the input file, and it worked. Grrr... That means that the (==) for QName is broken, and a quick look at the source in Text.XML.HXT.DOM.TypeDefs showed why:

data QName = QN { namePrefix    :: String
ualified name \"namePrefix:localPart\"
                , localPart     :: String
ed name \"namePrefix:localPart\"
                , namespaceUri  :: String
             deriving (Eq, Ord, Show, Read, Typeable)

The derived (==) will just and together the (==) for the three components (prefix, local, uri), but XML QNames are equal if their local parts and URIs (as strings) are equal. It's easy to fix by dropping the derivation of Eq and supplying a good version:

-              deriving (Eq, Ord, Show, Read, Typeable)
+              deriving (Ord, Show, Read, Typeable)
+ instance Eq QName where
+     q1 == q2 = ((localPart q1) == (localPart q2))
+                && ((namespaceUri q1) == (namespaceUri q2))

After which, it works according to my expectations for namespace handling.

Couldn't You Do All That With JavaScript...?

Yes. I could. I didn't. Here are a few of the reasons that I chose not to:

Other Integrations and Aggregations

The other two features that I'd like to add are backlinks to other blogs and backlinks to posts on community sites like Reddit and DZone. (I'm on the fence about implementing trackback support; you could twist my arm.)

Nonetheless, I'm on the fence about directing people to comment threads in other locations, i.e., Reddit. (My reasons are similar to Reg Braithwaite's.) It would be a simple matter to sniff referring URLs, deduce where an entry is posted on Reddit, and then integrate the comments together, but Reddit's draconian User Agreement forbids it:

The content, organization, graphics, text, images, video, design, compilation, advertising and all other material on the Website, including without limitation, the "look and feel" of this website, are protected under applicable copyrights and other proprietary (including but not limited to intellectual property) rights and are the property of Website Provider or its licensors. The copying, rearrangement, redistribution, modification, use or publication by you, directly or indirectly, of any such matters or any part of the website, including but not limited to the removal or alteration of advertising, except for the limited rights of use granted hereunder, is strictly prohibited.

Someone should implement a community hub that integrates discussion threads, followup posts, and blog comments on an original entry in a transparent and open fashion...


My first observation from this experiment is that APIs are preferable to feeds are preferable to widgets when it comes to integration of services on the web. (Note that I didn't say web serivces...) Even listing widgets is somewhat questionable in my opinion, since it's more of a "put my stuff on your page" than a "use my service".

My second observation is nothing new, but I now have experimental evidence — JSON is preferable to XML, whether or not the target client runs in a browser. If I were building a service, I'm not sure that I'd bother with supporting an XML API.

My third observation is that I would use Haskell to build a product or service, and I mean that in the sense that I can see how to train a team and build processes (prototyping, implementation, quality, deployment, support) around Haskell. The language does have a relatively steep learning curve (q.q. Kevin Scaldefarri's post on the subject and the comments that follow or Reg Braithwaite's general ruminations on learning languages), but the real problem is collectively getting through the challenges once. It reminds me of learning spectral sequences as a graduate student; fifteen minutes with my advisor to work an example was better than a week of staring at otherwise incrutable notation. As a measure of the view from my current location on the learning curve, I coded up a working rough cut of the "stream of consciousness" feature in an evening plus an afternoon cup of coffee, and I wouldn't regard myself as being fully around the curve yet (FFI, custom monads/transformers, etc. await).


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