How to write streaming dataframe to PostgreSQL?

Question

I have a streaming dataframe that I am trying to write into a database. There is documentation for writing an rdd or df into Postgres. But, I am unable to find examples or documentation on how it is done in Structured streaming.

I have read the documentation https://spark.apache.org/docs/latest/structured-streaming-programming-guide.html#foreachbatch , but I couldn't understand where I would create a jdbc connection and how I would write it to the database.

def foreach_batch_function(df, epoch_id):
    # what goes in here?
    pass

view_counts_query = windowed_view_counts.writeStream \
    .outputMode("append") \
    .foreachBatch(foreach_batch_function)
    .option("truncate", "false") \
    .trigger(processingTime="5 seconds") \
    .start() \
    .awaitTermination()

This function takes in a regular dataframe and writes into a postgres table

def postgres_sink(config, data_frame):
    config.read('/src/config/config.ini')
    dbname = config.get('dbauth', 'dbname')
    dbuser = config.get('dbauth', 'user')
    dbpass = config.get('dbauth', 'password')
    dbhost = config.get('dbauth', 'host')
    dbport = config.get('dbauth', 'port')

    url = "jdbc:postgresql://"+dbhost+":"+dbport+"/"+dbname
    properties = {
        "driver": "org.postgresql.Driver",
        "user": dbuser,
        "password": dbpass
    }

    data_frame.write.jdbc(url=url, table="metrics", mode="append",
                          properties=properties)

score 5 · Answer 1 · answered Aug 18 '20 at 13:23

Examples of how Postgres ingestion is done using Structured streaming

class PostgreSqlSink(url: String, user: String, pwd: String) extends org.apache.spark.sql.ForeachWriter[org.apache.spark.sql.Row] {
  val driver = "org.postgresql.Driver"
  var connection: java.sql.Connection = _
  var statement: java.sql.PreparedStatement = _
  val v_sql = "insert INTO Table (A,B,C) values ( ?, ?, ?)"

  def open(partitionId: Long, version: Long): Boolean = {
    Class.forName(driver)
    connection = java.sql.DriverManager.getConnection(url, user, pwd)
    connection.setAutoCommit(false)
    statement = connection.prepareStatement(v_sql)
    true
  }
  def process(value: org.apache.spark.sql.Row): Unit = {
    // ignoring value(0) as this is address
    statement.setString(1, value(1).toString)
    statement.setString(2, value(2).toString)
    statement.setString(3, value(3).toString)
    statement.executeUpdate()        
  }
  def close(errorOrNull: Throwable): Unit = {
    connection.commit()
    connection.close
  }
}

val url = "jdbc:postgresql://XX.XX.XX.XX:5432/postgres"
val user = "abc"
val pw = "abc@123"
val jdbcWriter = new PostgreSqlSink(url,user,pw)
val writeData = pg_df.writeStream 
    .foreach(jdbcWriter)
    .outputMode("Append")
    .trigger(ProcessingTime("30 seconds"))
    .option("checkpointLocation", "s3a://bucket/check")
    .start()

writeData.awaitTermination

how to handle a scenario where postgres db is not accessible? Eg: DB fail-over — Don Srinath, Apr 26 '22 at 13:49

score 4 · Accepted Answer · answered Feb 19 '19 at 10:47

There is really little be done here, beyond what you already have. foreachBatch takes a function (DataFrame, Int) => None, so all you need is a small adapter, and everything else should work just fine:

def foreach_batch_for_config(config)
    def _(df, epoch_id):
        postgres_sink(config, df)
   return _

view_counts_query = (windowed_view_counts
    .writeStream
    .outputMode("append") 
    .foreachBatch(foreach_batch_for_config(some_config))
    ...,
    .start()
    .awaitTermination())

though to be honest passing ConfigParser around is a strange idea from the beginning. You could adjust the signature adn initialize it in place

def postgres_sink(data_frame, batch_id):
    config = configparser.ConfigParser()
    ...
    data_frame.write.jdbc(...)

and keep the rest as-is. This way you could use your function directly:

...
.foreachBatch(postgres_sink)
...

how to handle Database fail-over here ? – Don Srinath Apr 28 '22 at 14:56 — Don Srinath, Apr 28 '22 at 14:56

How to write streaming dataframe to PostgreSQL?

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