Cascalog + Hadoop Counters, Finally!

I've just merged a Cascalog pull request of mine that gives Cascalog operations access to the statistics that Cascading generates at the end of each job. I've also added global inc! and inc-by! functions that let you increment custom Hadoop counters from within your functions and operations without having to deal with all that prepfn nastiness we introduced in Cascalog 2.0. Here's a link to the code. If you want to follow along, or just want to get the hell away from this blog and start playing with the code now, get yourself a copy of the new snapshot:

[cascalog/cascalog-core "3.0.0-SNAPSHOT"]

Or, for those of you who are like me and hate SNAPSHOT builds:

[cascalog/cascalog-core "3.0.0-20150220.233712-1"]

Counters

With the new interface you can increment Hadoop's counters inside your functions:

(defn square [x]
  (stats/inc-by! "tuples" x)
  (* x x))

If you call that definition of square outside of a Cascalog query - in a test, for example - the inc-by! call is a no-op. Inside of a running query you'll increment the relevant Hadoop counter.

Stats Access

I've also added a new option predicate, :stats-fn, that lets you access these stats at the end of a job run. :stats-fn takes a function that accepts an instance of cascalog.cascading.StatsMap. (Oh, yeah - I'm obsessed with Prismatic's Schema. I'm holding out hope that the fantastic documentation that library provides will finally entice more contributors into the murky swamps of Cascalog's codebase.)

There are three default stats functions in the cascalog.cascading.stats namespace:

  • (stdout) prints out any custom counters you've used in tab-separated format to stdout. You can also supply a custom group name, if you've namespaced your stats.
  • (clojure-file <filename>) prints the entire stats object in Clojure format to the supplied path.
  • (json-file <filename>) does the same, but formats the output as JSON.

Here's an example query using the new option:

(??<- [?x ?y]
      ([1 2 3] ?x)
      (:stats-fn (stats/stdout))
      (square ?x :> ?y))

(Check out square above again to see how I'm incrementing the counter.) Executing that query returns the following:

user>  
(??<- [?x ?y]
      ([1 2 3] ?x)
      (:name "StatsTestJob")
      (:stats-fn (stats/stdout))
      (square ?x :> ?y))
Custom counters for group CascalogStats:  
tuples    6  
;;=> ([1 1] [2 4] [3 9])

Boom! Custom counters printed out for your pleasure. The full stats object looks more like this:

The actual stats object is a lot more detailed, so you'll have to play around yourself. Oh, what the hell, I'll print it. It looks like this:

{:name "StatsTestJob",
 :start-time 1424474244707,
 :run-time 1424474244807,
 :duration 101,
 :counters
 {"org.apache.hadoop.mapred.FileInputFormat$Counter" {"BYTES_READ" 0},
  "cascading.flow.SliceCounters"
  {"Write_Duration" 0,
   "Process_End_Time" 1424474244790,
   "Read_Duration" 1,
   "Tuples_Read" 3,
   "Tuples_Written" 3,
   "Process_Begin_Time" 1424474244775},
  "org.apache.hadoop.mapred.FileOutputFormat$Counter"
  {"BYTES_WRITTEN" 124},
  "CascalogStats" {"counter" 6},
  "FileSystemCounters"
  {"FILE_BYTES_READ" 4619437, "FILE_BYTES_WRITTEN" 5918762},
  "cascading.flow.StepCounters" {"Tuples_Read" 3, "Tuples_Written" 3},
  "org.apache.hadoop.mapred.Task$Counter"
  {"SPILLED_RECORDS" 0,
   "MAP_INPUT_BYTES" 3,
   "MAP_INPUT_RECORDS" 3,
   "MAP_OUTPUT_RECORDS" 3,
   "SPLIT_RAW_BYTES" 294,
   "COMMITTED_HEAP_BYTES" 344981504}},
 :finished-time 1424474244808,
 :successful? true,
 :submit-time 1424474244739,
 :id "D9AA6A80E4AB4BFDBB260C66957D981D",
 :skipped? false,
 :failed? false,
 :stopped? false}

This feature has been a long time coming. Huge thanks to the Factual guys for supporting Cascalog development and giving me a chance to work on this in my spare time. Check out 3.0.0-SNAPSHOT and let me know what you think, in the comments below or on the Cascalog mailing list.

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