如何使用scala将postgreSQL数据库连接到Apache Spark？

Question

I want to know how can I do following things in scala?

1. Connect to a postgreSQL database using Spark scala.
2. Write SQL queries like SELECT , UPDATE etc. to modify a table in that database.

I know to do it using scala but how to import the connector jar of psql scala into sbt while packaging it?

Answer 1

Our goal is to run parallel SQL queries from the Spark workers.

Build setup

Add the connector and JDBC to the libraryDependencies in build.sbt . I've only tried this with MySQL, so I'll use that in my examples, but Postgres should be much the same.

libraryDependencies ++= Seq(
  jdbc,
  "mysql" % "mysql-connector-java" % "5.1.29",
  "org.apache.spark" %% "spark-core" % "1.0.1",
  // etc
)

Code

When you create the SparkContext you tell it which jars to copy to the executors. Include the connector jar. A good-looking way to do this:

val classes = Seq(
  getClass,                   // To get the jar with our own code.
  classOf[mysql.jdbc.Driver]  // To get the connector.
)
val jars = classes.map(_.getProtectionDomain().getCodeSource().getLocation().getPath())
val conf = new SparkConf().setJars(jars)

Now Spark is ready to connect to the database. Each executor will run part of the query, so that the results are ready for distributed computation.

There are two options for this. The older approach is to use org.apache.spark.rdd.JdbcRDD :

val rdd = new org.apache.spark.rdd.JdbcRDD(
  sc,
  () => {
    sql.DriverManager.getConnection("jdbc:mysql://mysql.example.com/?user=batman&password=alfred")
  },
  "SELECT * FROM BOOKS WHERE ? <= KEY AND KEY <= ?",
  0, 1000, 10,
  row => row.getString("BOOK_TITLE")
)

Check out the documentation for the parameters. Briefly:

• You have the SparkContext .
• Then a function that creates the connection. This will be called on each worker to connect to the database.
• Then the SQL query. This has to be similar to the example, and contain placeholders for the starting and ending key.
• Then you specify the range of keys (0 to 1000 in my example) and the number of partitions. The range will be divided among the partitions. So one executor thread will end up executing SELECT * FROM FOO WHERE 0 <= KEY AND KEY <= 100 in the example.
• And at last we have a function that converts the ResultSet into something. In the example we convert it into a String , so you end up with an RDD[String] .

Since Apache Spark version 1.3.0 another method is available through the DataFrame API. Instead of the JdbcRDD you would create an org.apache.spark.sql.DataFrame :

val df = sqlContext.load("jdbc", Map(
  "url" -> "jdbc:mysql://mysql.example.com/?user=batman&password=alfred",
  "dbtable" -> "BOOKS"))

See https://spark.apache.org/docs/1.3.1/sql-programming-guide.html#jdbc-to-other-databases for the full list of options (the key range and number of partitions can be set just like with JdbcRDD ).

Updates

JdbcRDD does not support updates. But you can simply do them in a foreachPartition .

rdd.foreachPartition { it =>
  val conn = sql.DriverManager.getConnection("jdbc:mysql://mysql.example.com/?user=batman&password=alfred")
  val del = conn.prepareStatement("DELETE FROM BOOKS WHERE BOOK_TITLE = ?")
  for (bookTitle <- it) {
    del.setString(1, bookTitle)
    del.executeUpdate
  }
}

(This creates one connection per partition. If that is a concern, use a connection pool!)

DataFrame s support updates through the createJDBCTable and insertIntoJDBC methods.