Lookup in Transforms

Requirements

1. Operations
   1. Perform single + batch read on single + multiple dataset from script transform
   2. Perform single + batch read on single + multiple files from script transform
2. Supported tables for lookup
   1. KeyValueTable dataset
   2. ObjectMappedTable dataset
   3. CSV files treated as a list of key-value pairs
3. Optional caching with time-based expiration

Design

1.  Lookup interface 

   interface Lookup<T> {
     T lookup(String key);
     Map<String, T> lookup(String... keys);
     Map<String, T> lookup(Set<String> keys);
   }

2. Implement Lookup in KeyValueTable and ObjectMappedTable
   1. KeyValueTable implements Lookup<String>
   2. ObjectMappedTable implements Lookup<StructuredRecord>
3. DatasetConfigurer changes
   1. Add method: void useDataset(String datasetName);
4. ScriptTransform changes

   1. Add configuration property for declaring lookup tables to use, properties for each table (e.g. dataset properties)

      "tables": [
        {
          "name":"purchases",
          "type":"dataset",
          "properties": {
            "dataset":"purchases",
            "properties":{.. dataset properties ..},
            "enableCache":"true",
            "cacheExpiry":1234
          }
        },
        {"name":"ip2geo", "type":"file", "properties":{"file":"/data/ip2geo.csv"}}
      ]

   2. configure(): verify tables (datasets and files) exist by calling DatasetConfigurer.useDataset()
   3. transform(): execute lookup methods in a transaction, provide Lookup instance to script

      1. Options for lookup usage: 

         var result = context.getLookup("purchases").lookup(user);

      2. Options for batch lookup usage:

         var result = context.getLookup("purchases").lookup(["alice", "bob"]);
         // do something with result["alice"]
         // do something with result["bob"]