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22 Jan 2012 - San Francisco

Cascalog Testing 2.0

A few months ago I announced Midje-Cascalog, my layer of Midje testing macros over the Cascalog MapReduce DSL. These allow you to write tests for your Cascalog jobs in a style that mimics Cascalog's own query execution syntax. In this post I discuss midje-cascalog's 0.4.0 release, which brings tighter Midje integration and a number of new ways to write tests. I'll start with a refresher on the old syntax before debuting the new. If you're eager, add the following to your project.clj:

[midje-cascalog "0.4.0"]

Midje-Cascalog Refresher

Take the following Cascalog query:

(use 'cascalog.api)

(let [src [["word"]]]
  (?<- (stdout)
       (src ?word)
       (str ?word " up!" :> ?out-word)))

Executing this code at the repl prints a single tuple with the string word up! to standard out.

How would you go about testing that this is true? With midje-cascalog, you would swap out the ?<- form for its testing equivalent: fact?<-. Here's the same Cascalog test alongside a typical Midje test:

(let [src [["word"]]]
  (fact?<- [["word up!"]]
           (src ?word)
           (str ?word " up!" :> ?out-word)))

(fact "+ should add two numbers."
  (+ 2 2) => 4)

I find that fact?<- and fact?- macros can be a bit confusing when you start mixing Cascalog and Midje tests, as they break the Midje pattern of <thing-to-test> => <expected-thing>. The syntax updates fix all of this with a set of checker functions that mimic Midje's excellent set of collection checkers.

The "produces" checker

Midje-cascalog 0.4.0 introduces the produces function, mirroring Midje's just. Let's define a source of tuples and a query to test.

(use 'cascalog.api)
(require '[cascalog.ops :as c])

(def src
  [[1 2] [1 3]
   [3 4] [3 6]
   [5 2] [5 9]])

;; adds the values in each input tuple, sorts the output and returns
;; 2-tuples of the first number and the sum. [1 2] becomes [1 3], for
;; example.
(def query
  (<- [?x ?sum]
      (src ?x ?y)
      (:sort ?x)
      (c/sum ?y :> ?sum)))

You can think of a query as a set of tuples waiting to be generated (through query execution). With Midje, you test sets using the just checker:

  [1 2 3] => (just [1 2 3])    ;; true
  [1 2 3] => (just [1 2 3 4])) ;; false

The cascalog analog to just is the produces checker. produces works like just, but against queries instead of bare collections. Executing the following test shows that the query produces the expected set of pairs, in any order:

  query => (produces [[3 10] [1 5] [5 11]])  ;; true
  query => (produces [[1 5] [3 10] [5 11]])) ;; true

You can read this test as saying "query, when executed, produces [3 10], [1 5] and [5 11]. You can also check that a query doesn't produce a set of tuples by swapping out =not=> for =>:

  query =not=> (produces [["string!" 11] [1 5] [5 11]])) ;; true

Using the :in-order keyword after the expected tuple sequence forces the test to respect ordering:

  query =not=> (produces [[3 10] [5 11] [1 5]] :in-order) ;; true
  query => (produces [[1 5] [3 10] [5 11]] :in-order))    ;; true

(:in-order is really only helpful in cases where output is sorted, like our query above.)


The produces-some checker tests that a query's output contains a subset of tuples:

  query => (produces-some [[5 11] [1 5]])) ;; true

Note that the behaviour of produces-some is similar to the behavior of Midje's contains collection checker.

As with produces, you can use the :in-order keyword to force produces-some to respect ordering. Gaps between tuples are okay.

  query =not=> (produces-some [[5 11] [1 5]] :in-order) ;; true
  query => (produces-some [[1 5] [5 11]] :in-order))    ;; true

Adding the :no-gaps keyword introduces the constraint that tuples must also be contiguous:

  query =not=> (produces-some [[1 5] [5 11]] :in-order :no-gaps) ;; true
  query => (produces-some [[1 5] [3 10]] :in-order :no-gaps))    ;; true

produces-prefix and produces-suffix

produce-prefix mimics the has-prefix collection checker by checking that some set of tuples is produced at the beginning of the query's output. produces-prefix implicitly assumes that tuples will be produced in order with no gaps:

  query => (produces-prefix [[1 5]])         ;; true
  query => (produces-prefix [[1 5] [3 10]])) ;; true

Similarly, produce-suffix mimics the has-suffix collection checker by checking that the supplied set of tuples is produced at the tail end of a query:

  query => (produces-suffix [[5 11]])) ;; true

log-level keywords

In addition to the keyword options supported above, every one of these checkers supports on optional logging-level keyword. For example, the following two facts are equivalent, but the second one produces :info level logging when it runs:

  query => (produces-suffix [[5 11]])        ;; true
  query => (produces-suffix [[5 11]] :info)) ;; true

Log level keywords can be useful when debugging tests, as errors will often only appear in the logging output. Currently supported keywords are :off (the default), :fatal, :warn, :info and :debug. The log level needs to be the first keyword argument if you supply multiple.


The real power of the 0.4.0 update is the way in which the previous query checkers were defined. Each of the above checkers mimics the behavior of one of Midje's built-in collection checkers with slightly different keyword arguments. This makes sense if you think of a query as a collection of tuples waiting to be produced (by query execution). The above checkers will get you quite a ways, but what if you want to test a query against some other Midje collection checker?

The answer is wrap-checker. wrap-checker is a higher-order function that accepts a midje collection checker and wraps it up, turning it into a Cascalog query checker. I'll demonstrate the power of this function by wrapping Midje's has checker.

has is a powerful way to run functions across every value in some sequence:

  [1 3 5 7 9] => (has every? odd?) ;; true
  [1 3 5 6] => (has some even?))   ;; true

If you try to use has against a query it will fail, as it expects to be tested against a sequence, not an unexecuted query. Here's how to get around this:

(defn odd-tuple? [tuple]
  (odd? (first tuple)))

(defn even-tuple? [tuple]
  (even? (first tuple)))

(def has-tuples
  (wrap-checker has))

(def new-query
  (let [src [[1] [3] [5]]]
    (<- [?x] (src ?x))))

  new-query     => (has-tuples every? odd-tuple?) ;; true
  new-query =not=> (has-tuples some even-tuple?)) ;; true

has-tuples will support log-level keywords like any of the predefined query collection checkers.

A few more examples:

(defn id-query [src]
  (<- [?x] (src ?x)))

(let [one-of-tuples (wrap-checker one-of)
      two-of-tuples (wrap-checker two-of)
      src [[1] [3] [4]]]
    src            => (two-of odd-tuple?)           ;; true
    src            => (one-of even-tuple?)          ;; true
    (id-query src) => (two-of-tuples odd-tuple?)    ;; true
    (id-query src) => (one-of-tuples even-tuple?))) ;; true

Backwards Compatibility

All of the collection checkers discussed above can be used with the fact?<- and fact?- macros:

(fact?<- (produces-some [[1 5] [5 11]] :in-order)
         [?x ?sum]
         (src ?x ?y)
         (:sort ?x)
         (c/sum ?y :> ?sum)) ;; true

fact?<- and fact?- are also compatible with all of Midje's unwrapped collection checkers, as discussed here.


Midje is an astonishingly good testing framework; I'm continually surprised by how well its idioms and conventions satisfy Cascalog's needs. In my next post here I'll go over some of the more subtle details of the wrap-checker function. For the curious, here's the code.

If you'd like more information or additional features, please add your thoughts to the midje-cascalog github issues page, or let me know in the comments below (or on twitter! I'm @sritchie09.)

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