A Lazy Sequence

Configuration Middleware

When you are developing a web application in Clojure, there are likely to be libraries (such as database access libs) that require configuration such as connection information to be provided. If the library provides a way to dynamically bind the configuration data, we can use Ring middleware to simplify our applications. I assume you already know about Ring. I’m going to use Moustache for my routes here, by the principle applies to Compojure – and presumably noir – as well. For a database library I’ll show an example using Clutch (the Clojure CouchDB api), again this also applies to other libraries.

Lets imagine a simple moustache app:

(def my-app 
    [] (delegate home)
    [slug] (delegate page slug))

In this example, home and page do some lookups into a CouchDB database, and return some html, for example:

(use '[ring.util.response :only [response]]
    '[com.ashafa.clutch :only [with-db get-view]])

(defn get-page-from-db
    [slug]
    (-> (with-db "example-db" (get-view "site" :page {:key slug}))
                 first
                 :content))    

(defn get-page-from-db-2
    [slug]
    (-> (get-view "example-db" "site" :page {:key slug})
        first
        :content))

(defn page 
    [req slug]
    (response (get-page-from-db slug)))

This simplified handler and database access function (and its variant) highlight the problem: the connection information (in this case just the string "example-db") is coupled with the code the does the request. Newer Clojure programmers may try hoisting the (with-db …) above the defns but this doesn’t work due to the binding semantics of dynamic scopes.

Ring middleware presents an answer to this problem. If we create a middleware that will set the (with-db …) for us on each request, then hoist the definition out of the data access code and specify it only once. Here is an example:

(defn clutch-with-db
  "Wraps the routes in a clutch with-db binding"
  [app database]
  (ƒ [req]
    (com.ashafa.clutch/with-db database (app req))))

(def my-app2
   (app 
     (clutch-with-db "example-db")

     [] (delegate home)
     [slug] (delegate page slug)))

Easy!

In addition to hoisting the configuration out of the data access code, we can now trivially use a different database for two sub apps that use the same app definition but accesses a different database, in this case two blogs: one with serious content, and another with humorous cats:

(def simple-blog (app …))

(def blogs 
    (app 
       ["funny-cats" &] (clutch-with-db simple-blog "cats-blog")
       [&] (clutch-with-db simple-blog "serious-blog")))

Finally, this also means you can write test harnesses that work off separate databases without fear.

Fiction (Read Recently)

A brief summary of some the fiction I have read recently:

• Snuff – Terry Pratchett. The 39th Discworld novel reads like the 39th Discworld novel. Another Vimes story that follows the familiar beats. Fans will probably enjoy it but its not going to blow anyone away. Night Watch remains the high point for this series. Better than Making Money and more consistent than Unseen Academicals.
• Changes – Jim Butcher. The Dresden Files continue to be an enjoyable action heavy fantasy-noir series. Changes lives up to the name, and Butcher again beats his heroes to emotional and physical pulp.
• The Gathering Storm – Brandon Sanderson and Robert Jordan. I haven’t read a Wheel of Time book since 2007; turns out it was a mistake to wait so long, I’ve forgotten so much its become very confusing. I have found this chronological listing of characters invaluable.
• Frankestein: Lost Souls – Dean Koontz. B-grade Stephen King / Tom Clancy. Apparently the fourth in the series. I wont be reading any of the others.
• King Rat – China Miéville. A strange fantasy story set in 90s London. Didn’t sell me on Miéville, but not bad.
• Neverwhere – Neil Gaiman. Another fantasy story set in in London. This one stands up to its reputation.

Stephen King

• Cujo. A relatively short King book without overt supernatural elements. It maintains suspense better than a lot of his other books. This, along with books 4 and 5 of the Dark Tower are my favorite of Kings books that I have read.
• The Tommyknockers. A massive King tome with a lot of his typical small-town-in-Maine-goes-to-hell tropes spun from the stock of B-Movie sci-fi and H.P. Lovecraft’s The Colour out of Space. Enjoyable but nothing amazing.
• The Dark Tower. I finished the last three books of this spaghetti western fantasy. Overall I enjoyed the series, but it is certainly uneven and more than a bit chaotic. I did enjoy the ending.

Tim Powers

I’ve recently discovered Powers, and have really enjoyed the three books I’ve read. He has a fairly clear formula with his secret history approach. He has a couple of tropes that some readers might find annoying, especially with respect to his main protagonist and that characters relationship with the main female character. The stories are very plot and action driven.

• Declare. This is an excellent World War 2 and Cold War spy thriller with magic tangled around the real history and people. Of the three Power’s books I’ve read, this is probably the best. In particular, Andrew Hale is a better developed character than the protagonists of the other two I have read.
• The Anubis Gates. A fantastical timetravelling romp centering on 1800s London that starts slow but doesn’t slow down once its moving.
• Last Call. Powers doing a Gaimen-esq modern fantasy set in Las Vegas.