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From "Alejandro Abdelnur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5303) Hadoop Workflow System (HWS)
Date Mon, 23 Feb 2009 23:52:03 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5303?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12676130#action_12676130

Alejandro Abdelnur commented on HADOOP-5303:

h4. Regarding the EL resolution,

Resolution is well defined, how and when.

{{${something}}} expressions are resolved from workflow job properties and EL functions at
them time that a workflow job starts a workflow action (enters the node). Values from the
HWS configuration (hws-default.xml & hws-site.xml) are not used to resolve workflow job
properties (this is different from Hadoop).

The EL language HWS uses is JSP EL (and we use commons-el implementation). It allows you to
support variables, functions and complex expressions. What inconsistency you (Steve) refer
to? probably is a mistake on the spec.

h4. Regarding the choice of SQL DB for job store,

If you need HA/failover for the DB you can get it, granted you need hardware, but it is zero
to minimal effort from HWS side.

We did not use HDFS because we need frequent workflow job context updates (ie every time that
an action starts, ends). 

HWS keeps zero state in memory when workflow job is not doing a transition (this allows HWS
to scale big), because of this we need index access (ie by job ID, action ID, user ID). A
SQL DB gives very good read/write access times. Finally the transaction support makes very
straight forward keeping things consistent in case of failure.

HWS uses SQL standard with no extensions from any implementation, thus it can run on any SQL
DB (we use HSQL for unitests and MySQL when deployed).

h4. Regarding allowing uses to plug in new actions without editing the codebase and schema,

No need to modify HWS codebase for new action.

Your suggestion about using something like {{<action type="email">}} instead {{<email>}}
makes sense. This would also remove the need of tweaking the XML-schema. Something like:

<action name="myPigjob">
  <pig xmlns="hws:pig:1">

By doing this schema validation of the action type can also be performed. Plus you could support
multiple versions of an action type if needed.

h4. Regarding testing, using HSQL

Yes, we already do, testcases fly.

h4. Regarding Why XML and not JSON?

HWS uses JSON for all WS API responses.

HWS uses XML for the workflow job conf (leveraging Hadoop Configuration).

HWS uses XML for the workflow definition (PDL).

h4. Regarding Cascading comments not strictly true

I'm not a Cascading expert so I may have missed something, but I think I got things mostly
right. Corrections please...

> Hadoop Workflow System (HWS)
> ----------------------------
>                 Key: HADOOP-5303
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5303
>             Project: Hadoop Core
>          Issue Type: New Feature
>            Reporter: Alejandro Abdelnur
>            Assignee: Alejandro Abdelnur
>         Attachments: hws-preso-v1_0_2009FEB22.pdf, hws-v1_0_2009FEB22.pdf
> This is a proposal for a system specialized in running Hadoop/Pig jobs in a control dependency
DAG (Direct Acyclic Graph), a Hadoop workflow application.
> Attached there is a complete specification and a high level overview presentation.
> ----
> *Highlights* 
> A Workflow application is DAG that coordinates the following types of actions: Hadoop,
Pig, Ssh, Http, Email and sub-workflows. 
> Flow control operations within the workflow applications can be done using decision,
fork and join nodes. Cycles in workflows are not supported.
> Actions and decisions can be parameterized with job properties, actions output (i.e.
Hadoop counters, Ssh key/value pairs output) and file information (file exists, file size,
etc). Formal parameters are expressed in the workflow definition as {{${VAR}}} variables.
> A Workflow application is a ZIP file that contains the workflow definition (an XML file),
all the necessary files to run all the actions: JAR files for Map/Reduce jobs, shells for
streaming Map/Reduce jobs, native libraries, Pig scripts, and other resource files.
> Before running a workflow job, the corresponding workflow application must be deployed
in HWS.
> Deploying workflow application and running workflow jobs can be done via command line
tools, a WS API and a Java API.
> Monitoring the system and workflow jobs can be done via a web console, command line tools,
a WS API and a Java API.
> When submitting a workflow job, a set of properties resolving all the formal parameters
in the workflow definitions must be provided. This set of properties is a Hadoop configuration.
> Possible states for a workflow jobs are: {{CREATED}}, {{RUNNING}}, {{SUSPENDED}}, {{SUCCEEDED}},
{{KILLED}} and {{FAILED}}.
> In the case of a action failure in a workflow job, depending on the type of failure,
HWS will attempt automatic retries, it will request a manual retry or it will fail the workflow
> HWS can make HTTP callback notifications on action start/end/failure events and workflow
end/failure events.
> In the case of workflow job failure, the workflow job can be resubmitted skipping previously
completed actions. Before doing a resubmission the workflow application could be updated with
a patch to fix a problem in the workflow application code.
> ----

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