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https://issues.apache.org/jira/browse/SPARK-2094?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Michael Armbrust updated SPARK-2094:
------------------------------------
Assignee: Cheng Lian
> Ensure exactly once semantics for DDL / Commands
> ------------------------------------------------
>
> Key: SPARK-2094
> URL: https://issues.apache.org/jira/browse/SPARK-2094
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Reporter: Michael Armbrust
> Assignee: Cheng Lian
> Fix For: 1.1.0
>
>
> From [~lian cheng]...
> The constraints presented here are:
> * The side effect of a command SchemaRDD should take place eagerly;
> * The side effect of a command SchemaRDD should take place once and only
> once;
> * When .collect() method is called, something meaningful, usually the output
> message lines of the command, should be presented.
> Then how about adding a lazy field inside all the physical command nodes to
> wrap up the side effect and hold the command output? Take the
> SetCommandPhysical as an example:
> {code}
> trait PhysicalCommand(@transient context: SQLContext) {
> lazy val commandOutput: Any
> }
> case class SetCommandPhysical(
> key: Option[String], value: Option[String], output: Seq[Attribute])(
> @transient context: SQLContext)
> extends PhysicalCommand(context)
> with PhysicalCommand {
> override lazy val commandOutput = {
> // Perform the side effect, and record appropriate output
> ???
> }
> def execute(): RDD[Row] = {
> val row = new GenericRow(Array[Any](commandOutput))
> context.sparkContext.parallelize(row, 1)
> }
> }
> {code}
> In this way, all the constraints are met:
> * Eager evaluation: done by the toRdd call in SchemaRDDLike (PR #948),
> * Side effect should take place once and only once: ensured by the lazy
> commandOutput field,
> * Present meaningful output as RDD contents: command output is held by
> commandOutput and returned in execute().
> An additional benefit is that, side effect logic of all the commands can be
> implemented within their own physical command nodes, instead of adding
> special cases inside SQLContext.toRdd and/or HiveContext.toRdd.
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