[jira] [Commented] (SPARK-2094) Ensure exactly once semantics for DDL / Commands

["Cheng Lian (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-2094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14026989#comment-14026989 ] 

Cheng Lian commented on SPARK-2094:
-----------------------------------

The basic idea is illustrated in {{CacheCommandPhysical}}: https://github.com/apache/spark/pull/1038/files#diff-726d84ece1e6f6197b98a5868c881ac7R68

> 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
>             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.



--
This message was sent by Atlassian JIRA
(v6.2#6252)