Watermarks in a RichParallelSourceFunction - apache-flink

Question

I am implementing a SourceFunction, which reads Data from a Database.
The job should be able to be resumed if stopped or crushed (i.e savepoints and checkpoints) with the data being processed exactly once.
What I have so far:
#SerialVersionUID(1L)
class JDBCSource(private val waitTimeMs: Long) extends
RichParallelSourceFunction[Event] with StoppableFunction with LazyLogging{
#transient var client: PostGreClient = _
#volatile var isRunning: Boolean = true
val DEFAULT_WAIT_TIME_MS = 1000
def this(clientConfig: Serializable) =
this(clientConfig, DEFAULT_WAIT_TIME_MS)
override def stop(): Unit = {
this.isRunning = false
}
override def open(parameters: Configuration): Unit = {
super.open(parameters)
client = new JDBCClient
}
override def run(ctx: SourceFunction.SourceContext[Event]): Unit = {
while (isRunning){
val statement = client.getConnection.createStatement()
val resultSet = statement.executeQuery("SELECT name, timestamp FROM MYTABLE")
while (resultSet.next()) {
val event: String = resultSet.getString("name")
val timestamp: Long = resultSet.getLong("timestamp")
ctx.collectWithTimestamp(new Event(name, timestamp), timestamp)
}
}
}
override def cancel(): Unit = {
isRunning = false
}
}
How can I make sure to only get the rows of the database which aren't processed yet?
I assumed the ctx variable would have some information about the current watermark so that I could change my query to something like:
select name, timestamp from myTable where timestamp > ctx.getCurrentWaterMark
But it doesn't have any relevant methods for me. Any Ideas how to solve this problem would be appreciated

Answer 1

You have to implement CheckpointedFunction so that you can manage checkpointing by yourself. The documentation of the interface is pretty comprehensive but if you need an example I advise you to take a look at an example.
In essence, your function must implement CheckpointedFunction#snapshotState to store the state you need using Flink's managed state and then, when performing a restore, it will read that same state in CheckpointedFunction#initializeState.