[Spark MOOC note] Lec5. Semi-structured Data

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KEY DATA MANAGEMENT CONCEPTS

data model: collection of concepts for describing data schema: a description of a particular collection of data using a given data model

structure spectrum:
semi-structured data: apply schema after creating data.

FILES

files: named collection of bytes, in hierarchical namespace (but: In a Content-Addressable Storage system files are stored, arranged, and accessed based on their content or metadata, not in hierarchy)

SEMI-STRUCTURED TABULAR DATA

table: a collection of rows and columns, each row has an index, each column has a name. cell: by a pair (row, col), values can be missing, types are inffered from content

CSV:

PDB:(filed name can be repeated on multuple lines)

CHALLENGES WITH TABULAR DATA

challenges:

challenges for tabular data from multiple source:

challenges for tabular data from sensors:

PANDAS AND SEMI-STRUCTURED DATA IN PYSPARK

pandas DataFrame: represented as python dict (colname → series) pandas Series: 1D labeled array capable of holding any data type

spark DataFrame: Distributed collection of data organized into named columns. types of columns are inferred from values.

Using dataframes can be 5 times faster than using RDDs:

SEMI-STRUCTURED LOG FILES

ex. Apache web server log format

EXPLORING A WEB SERVER ACCESS LOG

NASA http server access log
http://ita.ee.lbl.gov/html/contrib/NASA-HTTP.html

DATA MINING LOG FILES

Data mining log files is a data exploration process that often involves searching through the data for unusual events, a task that can be done using dashboards for visualizing anomalies. The data being analyzed usually includes machine resource usage data and application queue information.

FILE PERFORMANCE

binary/text performance benchmark:



  • read and write times are comparable
  • binary files are mach faster than palin text files

compression performance benchmark:



  • write times are much larger than read times
  • small range of compressed file size
  • binary still much faster than text
  • LZ4 compression ~= raw IO speed
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