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The following page has been changed by AlanGates:
http://wiki.apache.org/pig/PigMetaData
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information, histograms, etc.
== Pig Interface to File Specific Metadata ==
- Pig should support four options with regard to file specific
metadata:
+ Pig should support four options with regard to reading file
specific
metadata:
1. No file specific metadata available. Pig uses the file as
input
with
no knowledge of its content. All data is assumed to be !
ByteArrays.
2. User provides schema in the script. For example, `A = load
'myfile'
as (a: chararray, b: int);`.
3. Self describing data. Data may be in a format that describes
the
schema, such as JSON. Users may also have other proprietary ways
to
store
information about the data in a file either in the file itself or
in an
associated file. Changes to the !LoadFunc interface made as part
of the
pipeline rework support this for data type and column layout
only. It
will
need to be expanded to support other types of information about
the file.
4. Input from a data catalog. Pig needs to be able to query an
external
data catalog to acquire information about a file. All the same
information
available in option 3 should be available via this interface. This
interface does not yet exist and needs to be designed.
+ It should support options 3 and 4 for writing file specific
metadata as
well.
+
== Pig Interface to Global Metadata ==
- An interface will need to be designed for pig to interface to an
external
data catalog.
+ An interface will need to be designed for pig to read from and
write to
an external data catalog.
== Architecture of Pig Interface to External Data Catalog ==
Pig needs to be able to connect to various types of external data
catalogs
(databases, catalogs stored in flat files, web services, etc.). To
facilitate this
- pig will develop a generic interface that allows it to make
specific
types of queries to a data catalog. Drivers will then need to be
written
to
implement
+ pig will develop a generic interface that allows it to query
and update
a
data catalog. Drivers will then need to be written to implement
that interface and connect to a specific type of data catalog.
== Types of File Specific Metadata Pig Will Use ==
- Pig should be able to acquire the following types of
information about
a
file via either self description or an external data catalog.
This is
not
to say
+ Pig should be able to acquire and record the following types of
information about a file via either self description or an
external data
catalog. This is not to say
that every self describing file or external data catalog must
support
every
one of these items. This is a list of items pig may find useful
and
should
be
- able to query for. If the metadata source cannot provide the
information, pig will simply not make use of it.
+ able to query for and create. If the metadata source cannot
provide or
store the information, pig will simply not make use of it or
record it.
* Field layout (already supported)
* Field types (already supported)
* Sortedness of the data, both key and direction (ascending/
descending)
@@ -52, +54 @@
== Priorities ==
Given that the usage for global metadata is unclear, the priority
will be
placed on supporting file specific metadata. The first step
should be to
define the
- interface changes in !LoadFunc and the interface to external data
catalogs.
+ interface changes in !LoadFunc, !StoreFunc and the interface to
external
data catalogs.
