使用elasticsearch-spark连接器从Spark读取ES：返回所有字段

Question

I've done some experiments in the spark-shell with the elasticsearch-spark connector. Invoking spark:

] $SPARK_HOME/bin/spark-shell --master local[2] --jars ~/spark/jars/elasticsearch-spark-20_2.11-5.1.2.jar

In the scala shell:

scala> import org.elasticsearch.spark._
scala> val es_rdd = sc.esRDD("myindex/mytype",query="myquery")

It works well, the result contains the good records as specified in myquery. The only thing is that I get all the fields, even if I specify a subset of these fields in the query. Example:

myquery = """{"query":..., "fields":["a","b"], "size":10}"""

returns all the fields, not only a and b (BTW, I noticed that size parameter is not taken in account neither : result contains more than 10 records). Maybe it's important to add that fields are nested, a and b are actually doc.a and doc.b.

Is it a bug in the connector or do I have the wrong syntax?

Answer 1

The spark elasticsearch connector uses fields thus you cannot apply projection.

If you wish to use fine-grained control over the mapping, you should be using DataFrame instead which are basically RDDs plus schema.

pushdown predicate should also be enabled to translate (push-down) Spark SQL into Elasticsearch Query DSL.

Now a semi-full example :

myQuery = """{"query":..., """
val df = spark.read.format("org.elasticsearch.spark.sql")
                     .option("query", myQuery)
                     .option("pushdown", "true")
                     .load("myindex/mytype")
                     .limit(10) // instead of size
                     .select("a","b") // instead of fields

Answer 2

怎么打电话：

scala> val es_rdd = sc.esRDD("myindex/mytype",query="myquery", Map[String, String] ("es.read.field.include"->"a,b"))

Answer 3

You want restrict fields returned from elasticsearch _search HTTP API? (I guess to improve download speed).

First of all, use a HTTP proxy to see what the elastic4hadoop plugin is doing (I use on MacOS Apache Zeppelin with Charles proxy). This will help you to understand how pushdown works.

There are several solutions to achieve this:

1. dataframe and pushdown

You specify fields, and the plugin will "forward" to ES (here the _source parameter):

POST ../events/_search?search_type=scan&scroll=5m&size=50&_source=client&preference=_shards%3A3%3B_local

(-) Not fully working for nested fields.

(+) Simple, straightaway, easy to read

2. RDD & query fields

With JavaEsSpark.esRDD , you can specify fields inside the JSON query, like you did. This only work with RDD (with DataFrame, the fields is not sent).

(-) no dataframe -> no Spark way

(+) more flexible, more control