com.bazaarvoice.jolt

Class Shiftr

java.lang.Object

com.bazaarvoice.jolt.Shiftr

All Implemented Interfaces:

JoltTransform, SpecDriven, Transform

public class Shiftr
extends Object
implements SpecDriven, Transform

Shiftr is a kind of JOLT transform that specifies where "data" from the input JSON should be placed in the output JSON, aka how the input JSON/data should be shifted around to make the output JSON/data. At a base level, a single Shiftr "command" is a mapping from an input path to an output path, similar to the "mv" command in Unix, "mv /var/data/mysql/data /media/backup/mysql". In Shiftr, the input path is a JSON tree structure, and the output path is flattened "dot notation" path notation. The idea is that you can start with a copy your JSon input data data and modify it into a Shiftr spec by supplying a "dot notation" output path for each piece of data that you care about. For example, given this simple input JSON :

 {
   "rating": {
       "quality": {
           "value": 3,
           "max": 5
       }
    }
 }
 

A simple Shiftr spec could be constructed by coping of that input, and modifying it to supply an output path for each piece of data :

 {
   "rating": {
     "quality": {
         "value": "SecondaryRatings.quality.Value",     // copy 3 to "SecondaryRatings.quality.Value"
         "max": "SecondaryRatings.quality.RatingRange"  // copy 5 to "SecondaryRatings.quality.RatingRange"
     }
 }
 

would product the following output JSON :

 {
   "SecondaryRatings" : {
     "quality" : {
       "Value" : 3,
       "RatingRange" : 5
     }
   }
 }
 

As shown above, Shiftr specs can be entirely made up of literal string values, but it's real power comes from its wildcards. Using wildcards, you can leverage the fact that you know, not just the data and it's immediate key, but the whole input path to that data. Expanding the example above, say we have the following expanded Input JSON:

 {
   "rating": {
       "primary": {
           "value": 3,   // want this value to goto output path "Rating"
           "max": 5      // want this value to goto output path "RatingRange"
       },
       "quality": {      // want output path "SecondaryRatings.quality.Id" = "quality", aka we want the value of the key to be used
           "value": 3,   // want this value to goto output path "SecondaryRatings.quality.Value"
           "max": 5      // want this value to goto output path "SecondaryRatings.quality.Range"
       },
       "sharpness" : {   // want output path "SecondaryRatings.sharpness.Id" = "sharpness"
           "value" : 7,  // want this value to goto output path "SecondaryRatings.sharpness.Value"
           "max" : 10    // want this value to goto output path "SecondaryRatings.sharpness.Range"
       }
   }
 }
 

The Spec would be :

 {
   "rating": {
     "primary": {
         "value": "Rating",                       // output -> "Rating" : 3
         "max": "RatingRange"                     // output -> "RatingRange" : 5
     },
     "*": {                                       // match input data like "rating.[anything-other-than-primary]"
         "value": "SecondaryRatings.&1.Value",    // the data at "rating.*.value" goes to "SecondaryRatings.*.Value"
                                                  // the "&1" means use the value one level up the tree ( "quality" or "sharpness" )
                                                  // output -> "SecondaryRatings.quality.Value" : 3 AND
                                                  //           "SecondaryRatings.sharpness.Value" : 7

         "max": "SecondaryRatings.&1.Range",      // the data at "rating.*.max" goes to "SecondaryRatings.*.Range"
                                                  // the "&1" means use the value one level up the tree ( "quality" or "sharpness" )
                                                  // output -> "SecondaryRatings.quality.Range" : 5 AND
                                                  //           "SecondaryRatings.sharpness.Range" : 10

         "$": "SecondaryRatings.&1.Id"            // Special operator $ means, use the value of the input key itself as the data
                                                  // output -> "SecondaryRatings.quality.Id" : "quality"
                                                  // output -> "SecondaryRatings.sharpness.Id" : "sharpness"
     }
   }
 }
 

Yielding the following output:

 {
   "Rating": 3,
   "RatingRange": 5,
   "SecondaryRatings": {
      "quality": {
         "Range": 5,
         "Value": 3,
         "Id": "quality"     // the special $ operator allows us to use input key the text value of "quality", as the "Id" of the output
      },
      "sharpness": {
         "Range": 10,
         "Value": 7,
         "Id": "sharpness"   // the special $ operator allows us to use input key the text value of "sharpness", as the "Id" of the output
      }
   }
 }
 

Shiftr Wildcards '*' Wildcard Valid only on the LHS ( input JSON keys ) side of a Shiftr Spec The '*' wildcard can be used by itself or to match part of a key. '*' wildcard by itself : As illustrated in the example above, the '*' wildcard by itself is useful for "templating" JSON maps, where each key / value has the same "format".

    // example input
    {
      "rating" : {
        "quality": {
          "value": 3,
          "max": 5
        },
        "sharpness" : {
          "value" : 7,
          "max" : 10
        }
    }
    

In this example, "rating.quality" and "rating.sharpness" both have the same structure/format, and thus we can use the '*' to allow use to write more compact rules and avoid having to to explicitly write very similar rules for both "quality" and "sharpness". '*' wildcard as part of a key : This is useful for working with input JSON with keys that are "prefixed". Ex : if you had an input document like

    {
       "tag-Pro" : "Awesome",
       "tag-Con" : "Bogus"
    }
    

A 'tag-*' would match both keys, and make the whole key and "matched" part of the key available. Ex, input key of "tag-Pro" with LHS spec "tag-*", would "tag-Pro" and "Pro" available to reference. Note the '*' wildcard is as non-greedy as possible, hence you can use more than one '*' in a key. For example, "tag-*-*" would match "tag-Foo-Bar", making "tag-Foo-Bar", "Foo", and "Bar" all available to reference. '&' Wildcard Valid on the LHS (left hand side - input JSON keys) and RHS (output data path) Means, dereference against a "path" to get a value and use that value as if were a literal key. The canonical form of the wildcard is "&(0,0)". The first parameter is where in the input path to look for a value, and the second parameter is which part of the key to use (used with * key). There are syntactic sugar versions of the wildcard, all of the following mean the same thing. Sugar : '&' = '&0' = '&(0)' = '&(0,0) The syntactic sugar versions are nice, as there are a set of data transforms that do not need to use the canonical form, eg if your input data does not have any "prefixed" keys. '&' Path lookup As Shiftr processes data and walks down the spec, it maintains a data structure describing the path it has walked. The '&' wildcard can access data from that path in a 0 major, upward oriented way. Example :

    {
        "foo" : {
            "bar" : {
                "baz" :  // &0 = baz, &1 = bar, &2 = foo
            }
        }
    }
    

'&' Subkey lookup '&' subkey lookup allows us to referece the values captured by the '*' wildcard. Example, "tag-*-*" would match "tag-Foo-Bar", making &(0,0) = "tag-Foo-Bar" &(0,1) = "Foo" &(0,2) = "Bar" '$' Wildcard Valid only on the LHS of the spec. The existence of this wildcard is a reflection of the fact that the "data" of the input JSON, can be both in the "values" and the "keys" of the input JSON The base case operation of Shiftr is to copy input JSON "values", thus we need a way to specify that we want to copy the input JSON "key" instead. Thus '$' specifies that we want to use an input key, or input key derived value, as the data to be placed in the output JSON. '$' has the same syntax as the '&' wildcard, and can be read as, dereference to get a value, and then use that value as the data to be output. There are two cases where this is useful 1) when a "key" in the input JSON needs to be a "id" value in the output JSON, see the ' "$": "SecondaryRatings.&1.Id" ' example above. 2) you want to make a list of all the input keys. Example of "a list of the input keys" :

   // input
   {
     "rating": {
       "primary": {
         "value": 3,
         "max": 5
       },
       "quality": {
         "value": 3,
         "max": 7
       }
     }
   }

   // desired output
   {
     "ratings" : [ "primary", "quality" ]    // Aside : this is an example of implicit JSON array creation in the output which is detailed further down.
                                             // For now just observe that the input keys "primary" and "quality" have both made it to the output.
   }

   // spec
   {
     "rating": {
       "*": {               // match all keys below "rating"
         "$": "ratings"     // output each of the "keys" to "ratings" in the output
       }
     }
   }
   

'#' Wildcard Valid only on the RHS of the spec, nested in an array, like "[#2]" This wildcard is useful if you want to take a JSON map and turn it into a JSON array, and you do not care about the order of the array. While Shiftr is doing its parallel tree walk of the input data and the spec, it tracks how many matched it has processed at each level of the spec tree. '|' Wildcard Valid only on the LHS of the spec. This 'or' wildcard allows you to match multiple input keys. Useful if you don't always know exactly what your input data will be. Example Spec :

   {
     "rating|Rating" : "rating-primary"   // match "rating" or "Rating" copy the data to "rating-primary"
   }
   

This is really just syntactic sugar, as the implementation really just treats the key "rating|Rating" as two keys when processing. '@' Wildcard Valid only on the LHS of the spec. This wildcard is necessary if you want to do put both the input value and the input key somewhere in the output JSON. Example '@' wildcard usage :

  // Say we have a spec that just operates on the value of the input key "rating"
  {
     "foo" : "place.to.put.value",  // leveraging the implicit operation of Shiftr which is to operate on input JSON values
  }

  // if we want to do something with the "key" as well as the value
  {
     "foo" : {
       "$" : "place.to.put.key",
       "@" : "place.to.put.value"    // '@' explicitly tell Shiftr to operate on the input JSON value of the parent key "foo"
     }
  }
  

Thus the '@' wildcard is the mean "copy the value of the data at this level in the tree, to the output". JSON Arrays : Reading from (input) and writing to (output) JSON Arrays is fully supported. 1) Handling Arrays in the input JSON Shiftr treats JSON arrays in the input data as Maps with numeric keys. Example :

    // input
    {
       "Photos": [ "AAA.jpg", "BBB.jpg" ]
    }

    // spec
    {
       "Photos" :
       {
         "1" : "photo-&-url"      // Specify that we only want to operate on the 1-th index of the "Photos" input array
       }
    }

   // output
   {
       "photo-1-url": "BBB.jpg"
   }
  

2) Handling Arrays in the output JSON Traditional array brackets, [ ], are used to specify array index in the output JSON. []'s are only valid on the RHS of the Shiftr spec. Example :

    // input
    {
      "photo-1-id": "327704",
      "photo-1-url": "http://bob.com/0001/327704/photo.jpg"
    }

    // spec
    {
      "photo-1-id": "Photos[1].Id",   // Declare the "Photos" in the output to be an array,
      "photo-1-url": "Photos[1].Url"  // that the 1-th array location should have data

      // same as above but more powerful
      // note '&' logic can be used inside the '[ ]' notation
      "photo-*-url": "Photos[&(0,1)].Url"
    }

    // output
    {
      "Photos": [
        null ,                // note Photos[0] is null, because no data was pushed to it
        {
          "Id":"327704",
          "Url":"http://bob.com/0001/327704/photo.jpg"
        }
      ]
    }
  

3) JSON arrays in the spec file JSON Arrays in Shiftr spec are used to to specify that piece of input data should be copied to two places in the output JSON. Example :

   // input
   { "foo" : 3 }

   // spec
   { "foo" : [ "bar", "baz" ] }    // push the 3, to both the of the output paths

   // output
   {
     "bar" : 3,
     "baz" : 3
   }
 

4) Implicit Array creation in the output JSON If a spec file is configured to output multiple pieces of data to the same output location, the output location will be turned into a JSON array. Example :

    // input
    {
        "foo" : "bar",
        "tuna" : "marlin"
    }

    // spec
    {
        "foo"  : "baz",
        "tuna" : "baz"
    }

    // output
    {
        "baz" : [ "bar", "marlin" ]     // Note the order of this Array should not be relied upon
    }
  

Algorithm High Level Walk the input data, and Shiftr spec simultaneously, and execute the Shiftr command/mapping each time there is a match. Algorithm Low Level - Simultaneously walk of the spec and input JSon, and maintain a walked "input" path data structure. - Determine a match between input JSON key and LHS spec, by matching LHS spec keys in the following order : -- Note that '|' keys are are split into their subkeys, eg "literal", '*', or '&' LHS keys 1) Try to match the input key with "literal" spec key values 2) If no literal match is found, try to match against LHS '&' computed values. 2.1) For deterministic behavior, if there is more than one '&' LHS key, they are applied/matched in alphabetical order, after the '&' syntactic sugar is replaced with its canonical form. 3) If no match is found, try to match against LHS keys with '*' wildcard values. 3.1) For deterministic behavior, '*' wildcard keys are sorted and applied/matched in alphabetical order. Note, processing of the '@' and '$' LHS keys always occur if their parent's match, and do not block any other matching. Implementation Instances of this class execute Shiftr transformations given a transform spec of Jackson-style maps of maps and a Jackson-style map-of-maps input.

Field Summary

Fields

Modifier and Type	Field and Description
static String	ROOT_KEY

Constructor Summary

Constructors

Constructor and Description
Shiftr(Object spec) Initialize a Shiftr transform with a Spec.

Method Summary

Methods

Modifier and Type	Method and Description
Object	transform(Object input) Applies the Shiftr transform.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

ROOT_KEY

public static final String ROOT_KEY

See Also:

Constant Field Values

Constructor Detail

Shiftr

@Inject
public Shiftr(Object spec)

Initialize a Shiftr transform with a Spec.

Throws:

SpecException - for a malformed spec

Method Detail

transform

public Object transform(Object input)

Applies the Shiftr transform.

Specified by:

transform in interface Transform

Parameters:

input - the JSON object to transform

Returns:

the output object with data shifted to it

Throws:

TransformException - for a malformed spec or if there are issues during the transform