Semalt Review – Running A Scraping Script

23.05.2018

Semalt Review – Running A Scraping
Script

Air ow is a scheduler libraries for Python used to con gure multi-system work ows executed in parallel across any
number of users. A single Air ow pipeline comprises of SQL, bash, and Python operations. The tool works by
specifying on dependencies between tasks, a critical element that helps determine the tasks to be run in parallel
and which ones to be executed after the other functions are complete.

Why Air ow?

Air ow tool is written in Python, giving you the advantage to add your operators to the already set custom
functionality. This tool allows you to scrape data through transformations from a website to a well-structured
datasheet. Air ow uses Directed Acyclic Graphs (DAG) to represent a speci c work ow. In this case, a work ow
refers to a collection of tasks that comprises of directional dependencies.

How Apache Air ow works

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Air ow is a Warehouse Management System that works to de ne tasks
as their ultimate dependencies as the code executes the functions on a
schedule and distributes the task execution across all the worker
processes. This tool offers a user interface that displays the state of both
running and past tasks.

Air ow displays diagnostic information to users regarding the task
execution process and allows the end-user to manage execution of tasks
manually. Note that a directed acyclic graph is only used to set the
execution context and to organize tasks. In Air ow, tasks are the crucial
elements that run a scraping script. In scraping, tasks comprise of two

avors that include:

Operator

In some cases, tasks work as operators where they execute operations as speci ed by the end users. Operators are
designed to run scraping script and other functions that can be performed in Python programming language.

Sensor

Tasks are also developed to work as sensors. In such a case, execution of tasks that depend on each other can be
paused until a criterion where a work ow runs smoothly has been met.

Air ow is used in different elds to run a scraping script. Below is a guide on how to use Air ow.

Open your browser and check your user interface

Check the work ow that failed and clicks on it to see the tasks that went wrong

Click on "View log" to check the cause of failure. In many cases, password authentication failure causes the
work ow failure

Go to the admin section and click on "Connections." Edit the Postgres connection to retrieve the new
password and click "Save."

Re-visit your browser and click on the task that had failed. Click on the task and tap "Clear" so that the task
runs successfully next time.

Other Python schedulers to consider

Cron

Cron is a Unix-based OS used to run scraping scripts periodically at xed intervals, dates, and times. This library is
mostly used to maintain and set up software environments.

Luigi

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Luigi is a Python module that will allow you to handle visualization and
dependency resolution. Luigi is used for creating complex pipelines of
jobs collection.

Air ow is a scheduler library for Python used to handle dependency
management projects. In Air ow, running tasks depends on each other. To
obtain consistent results, you can set your Air ow script to run
automatically after every an hour or two.

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