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From "Alan Gates (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-143) Proposal for refactoring of parsing logic in Pig
Date Tue, 11 Mar 2008 23:40:46 GMT

    [ https://issues.apache.org/jira/browse/PIG-143?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12577649#action_12577649
] 

Alan Gates commented on PIG-143:
--------------------------------

Pi,

I am totally in agreement with your suggestions.  I agree that performance is not a huge issue
as most plans will be represented by graphs of 1000 nodes or less.

In the types branch we have already made some progress in this direction.  I suspect therefore
it would be easiest for you to implement these changes (if you are interested in doing so)
in the types branch.  We're reworking the logical plan to be a generic graph and have implemented
visitation methods that traverse the graph depth first and in dependency order.  My previous
experience has been that these two together (depth first for speed, dependency order when
you need to propagate state (such as in a type checker)) are sufficient for the needs of all
types of visitors.  I haven't checked in the changes to the logical plan yet, but under pig/impl/plan/OperatorPlan.java
and associated classes you can see the generic graph package we wrote.  The logical plan will
extend this.  I hope to have changes to the logical plan that match this checked in within
a couple of days.  Take a look and let me know what you think.



> Proposal for refactoring of parsing logic in Pig
> ------------------------------------------------
>
>                 Key: PIG-143
>                 URL: https://issues.apache.org/jira/browse/PIG-143
>             Project: Pig
>          Issue Type: Improvement
>          Components: impl
>            Reporter: Pi Song
>            Assignee: Pi Song
>         Attachments: ParserDrawing.png
>
>
> h2. Pig Script Parser Refactor Proposal 
> This is my initial proposal on pig script parser refactor work. Please note that I need
your opinions for improvements.
> *Problem*
> The basic concept is around the fact that currently we do validation logics in parsing
stage (for example, file existence checking) which I think is not clean and difficult to add
new validation rules. In the future, we will need to add more and more validation logics to
improve usability.
> *My proposal:-*  (see [^ParserDrawing.png])
> - Only keep parsing logic in the parser and leave output of parsing logic being unchecked
logical plans. (Therefore the parser only does syntactic checking)
> - Create a new class called LogicalPlanValidationManager which is responsible for validations
of the AST from the parser.
> - A new validation logic will be subclassing LogicalPlanValidator 
> - We can chain a set of LogicalPlanValidators inside LogicalPlanValidationManager to
do validation work. This allows a new LogicalPlanValidator to be added easily like a plug-in.

> - This greatly promotes modularity of the validation logics which  is +particularly good
when we have a lot of people working on different things+ (eg. streaming may require a special
validation logic)
> - We can set the execution order of validators
> - There might be some backend specific validations needed when we implement new execution
engines (For example a logical operation that one backend can do but others can't).  We can
plug-in this kind of validations on-the-fly based on the backend in use.
> *List of LogicalPlanValidators extracted from the current parser logic:-*
> - File existence validator
> - Alias existence validator
> *Logics possibly be added in the very near future:-*
> - Streaming script test execution
> - Type checking + casting promotion + type inference
> - Untyped plan test execution
> - Logic to prevent reading and writing from/to the same file
> The common way to implement a LogicalPlanValidator will be based on Visitor pattern.

> *Cons:-*
>  - By having every validation logic traversing AST from the root node every time, there
is a performance hit. However I think this is neglectable due to the fact that Pig is very
expressive and normally queries aren't too big (99% of queries contain no more than 1000 AST
nodes).
> *Next Step:-*
> LogicalPlanFinalizer which is also a pipeline except that each stage can modify the input
AST. This component will generally do a kind of global optimizations.
> *Further ideas:-*
> - Composite visitor can make validations more efficient in some cases but I don't think
we need
> - ASTs within the pipeline never change (read-only) so validations can be done in parallel
to improve responsiveness. But again I don't think we need this unless we have so many I/O
bound logics.
> - The same pipeline concept can also be applied in physical plan validation/optimization.

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