August 17, 2023

tHDFSExist – Docs for ESB 5.x

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tHDFSExist

thdfsexist_icon32_white.png

Warning

This component will be available in the Palette of
Talend Studio on the condition that you have subscribed to one of
the Talend
solutions with Big Data.

tHDFSExist properties

Component family

Big Data/File

  

Function

This component checks whether a file exists in a specific
directory in HDFS.

Purpose

This component checks the existence of a specific file in
HDFS.

Basic settings

Property type

Either Built-in or Repository

Built-in: No property data stored
centrally.

Repository: Select the repository
file in which the properties are stored. The fields that follow are
completed automatically using the data retrieved.

Since version 5.6, both the Built-In mode and the Repository mode are
available in any of the Talend solutions.

Use an existing connection

Select this check box and in the Component List click the
relevant connection component to reuse the connection details you already defined.

Note

When a Job contains the parent Job and the child Job, Component
list presents only the connection components in the same Job
level.

Version

Distribution

Select the cluster you are using from the drop-down list. The options in the list vary
depending on the component you are using. Among these options, the following ones requires
specific configuration:

  • If available in this Distribution drop-down list, the
    Microsoft HD Insight option allows you to use a
    Microsoft HD Insight cluster. For this purpose, you need to configure the
    connections to the WebHCat service, the HD Insight service and the Windows Azure
    Storage service of that cluster in the areas that are displayed. A demonstration
    video about how to configure this connection is available in the following link:
    https://www.youtube.com/watch?v=A3QTT6VsNoM

  • The Custom option allows you to connect to a
    cluster different from any of the distributions given in this list, that is to
    say, to connect to a cluster not officially supported by Talend.

In order to connect to a custom distribution, once selecting Custom, click the dotbutton.png button to display the dialog box in which you can
alternatively:

  1. Select Import from existing version to import an
    officially supported distribution as base and then add other required jar files
    which the base distribution does not provide.

  2. Select Import from zip to import a custom
    distribution zip that, for example, you can download from http://www.talendforge.org/exchange/index.php.

    Note

    In this dialog box, the active check box must be kept selected so as to import
    the jar files pertinent to the connection to be created between the custom
    distribution and this component.

    For an step-by-step example about how to connect to a custom distribution and
    share this connection, see Connecting to a custom Hadoop distribution.
 

Hadoop version

Select the version of the Hadoop distribution you are using. The available options vary
depending on the component you are using. Along with the evolution of Hadoop, please note
the following changes:

  • If you use Hortonworks Data Platform V2.2, the
    configuration files of your cluster might be using environment variables such as
    ${hdp.version}. If this is your situation, you
    need to set the mapreduce.application.framework.path property in the Hadoop properties table of this component with the path value
    explicitly pointing to the MapReduce framework archive of your cluster. For
    example:

  • If you use Hortonworks Data Platform V2.0.0, the
    type of the operating system for running the distribution and a Talend
    Job must be the same, such as Windows or Linux. Otherwise, you have to use Talend
    Jobserver to execute the Job in the same type of operating system in which the
    Hortonworks Data Platform V2.0.0 distribution you
    are using is run. For further information about Talend Jobserver, see
    Talend Installation
    and Upgrade Guide    .

 Authentication

Use kerberos authentication

If you are accessing the Hadoop cluster running with Kerberos security, select this check
box, then, enter the Kerberos principal name for the NameNode in the field displayed. This
enables you to use your user name to authenticate against the credentials stored in
Kerberos.

This check box is available depending on the Hadoop distribution you are connecting
to.
  Use a keytab to authenticate

Select the Use a keytab to authenticate check box to log
into a Kerberos-enabled Hadoop system using a given keytab file. A keytab file contains
pairs of Kerberos principals and encrypted keys. You need to enter the principal to be used
in the Principal field and the access path to the keytab
file itself in the Keytab field.

Note that the user that executes a keytab-enabled Job is not necessarily the one a
principal designates but must have the right to read the keytab file being used. For
example, the user name you are using to execute a Job is user1 and the principal to be used is guest; in this situation, ensure that user1 has the right to read the keytab file to be used.
 

NameNode URI

Type in the URI of the Hadoop NameNode. The NameNode is the master node of a Hadoop system.
For example, we assume that you have chosen a machine called masternode as the NameNode of an Apache Hadoop distribution, then the
location is hdfs://masternode:portnumber.

 

User name

Enter the user authentication name of HDFS.
 

Group

Enter the membership including the authentication user under which the HDFS instances were
started. This field is available depending on the distribution you are using.

HDFS directory

Browse to, or enter the directory in HDFS where the data you need to use is.

 

File name or relative path

Enter the name of the file you want to check whether this file
exists. Or if needs be, browse to the file or enter the path to the
file, relative to the directory you entered in HDFS directory.
Advanced settings

Hadoop properties

Talend Studio uses a default configuration for its engine to perform
operations in a Hadoop distribution. If you need to use a custom configuration in a specific
situation, complete this table with the property or properties to be customized. Then at
runtime, the customized property or properties will override those default ones.

  • Note that if you are using the centrally stored metadata from the Repository, this table automatically inherits the
    properties defined in that metadata and becomes uneditable unless you change the
    Property type from Repository to Built-in.

For further information about the properties required by Hadoop and its related systems such
as HDFS and Hive, see the documentation of the Hadoop distribution you
are using or see Apache’s Hadoop documentation on http://hadoop.apache.org/docs and then select the version of the documentation you want. For demonstration purposes, the links to some properties are listed below:
 

tStatCatcher Statistics

Select this check box to gather the Job processing
metadata at a Job level as well as at each component level.

Dynamic settings

Click the [+] button to add a row in the table and fill the
Code field with a context variable to choose your HDFS
connection dynamically from multiple connections planned in your Job. This feature is useful
when you need to access files in different HDFS systems or different distributions,
especially when you are working in an environment where you cannot change your Job settings,
for example, when your Job has to be deployed and executed independent of Talend Studio.

The Dynamic settings table is available only when the
Use an existing connection check box is selected in the
Basic settings view. Once a dynamic parameter is
defined, the Component List box in the Basic settings view becomes unusable.

For more information on Dynamic settings and context
variables, see Talend Studio User Guide.

Global Variables

EXISTS: the result of whether a specified file exists.
This is a Flow variable and it returns a boolean.

FILENAME: the name of the file processed. This is an
After variable and it returns a string.

ERROR_MESSAGE: the error message generated by the
component when an error occurs. This is an After variable and it returns a string. This
variable functions only if the Die on error check box is
cleared, if the component has this check box.

A Flow variable functions during the execution of a component while an After variable
functions after the execution of the component.

To fill up a field or expression with a variable, press Ctrl +
Space to access the variable list and choose the variable to use from it.

For further information about variables, see Talend Studio
User Guide.

Usage

tHDFSExist is a standalone
component.

Prerequisites

The Hadoop distribution must be properly installed, so as to guarantee the interaction
with Talend Studio. The following list presents MapR related information for
example.

  • Ensure that you have installed the MapR client in the machine where the Studio is,
    and added the MapR client library to the PATH variable of that machine. According
    to MapR’s documentation, the library or libraries of a MapR client corresponding to
    each OS version can be found under MAPR_INSTALL
    hadoophadoop-VERSIONlib
    ative    . For example, the library for
    Windows is lib
    ativeMapRClient.dll     in the MapR
    client jar file. For further information, see the following link from MapR: http://www.mapr.com/blog/basic-notes-on-configuring-eclipse-as-a-hadoop-development-environment-for-mapr.

    Without adding the specified library or libraries, you may encounter the following
    error: no MapRClient in java.library.path.

  • Set the -Djava.library.path argument, for example, in the Job Run VM arguments area
    of the Run/Debug view in the [Preferences] dialog box. This argument provides to the Studio the
    path to the native library of that MapR client. This allows the subscription-based
    users to make full use of the Data viewer to view
    locally in the Studio the data stored in MapR. For further information about how to
    set this argument, see the section describing how to view data of Talend Big Data Getting Started Guide.

For further information about how to install a Hadoop distribution, see the manuals
corresponding to the Hadoop distribution you are using.

Log4j

The activity of this component can be logged using the log4j feature. For more information on this feature, see Talend Studio User
Guide.

For more information on the log4j logging levels, see the Apache documentation at http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/Level.html.

Limitation

JRE 1.6+ is required.

Scenario: Checking the existence of a file in HDFS

In this scenario, the two-component Job checks whether a specific file exists in HDFS
and returns a message to indicate the result of the verification.

In the real-world practice, you can take further action to process the file checked
according to the verification result, using the other HDFS components provided with the
Studio.

use_case-thdfsexist1.png

Launch the Hadoop distribution in which you want to check the existence of a
particular file. Then, proceed as follows:

Linking the components

  1. In the Integration perspective of
    Talend Studio, create an empty Job, named hdfsexist_file for example, from the
    Job Designs     node in the Repository tree view.

    For further information about how to create a Job, see the Talend Studio User
    Guide    .

  2. Drop tHDFSExist and tMsgBox onto the workspace.

  3. Connect them using the Trigger > Run if
    link.

Configuring the connection to HDFS

  1. Double-click tHDFSExist to open its
    Component view.

    use_case-thdfsexist2.png

  2. In the Version area, select the Hadoop
    distribution you are connecting to and its version.

  3. In the Connection area, enter the values
    of the parameters required to connect to the HDFS.

    In the real-world practice, you may use tHDFSConnection to create a connection and reuse it from the
    current component. For further information, see tHDFSConnection.

  4. In the HDFS Directory field, browse to,
    or enter the path to the folder where the file to be checked is. In this
    example, browse to /user/ychen/data/hdfs/out/dest.

  5. In the File name or relative path field,
    enter the name of the file you want to check the existence. For example,
    output.csv.

Defining the message to be returned

  1. Double-click tMsgBox to open its
    Component view.

    use_case-thdfsexist3.png

  2. In the Title field, enter the title to be
    used for the pop-up message box to be created.

  3. In the Buttons list, select OK. This defines the button to be displayed on
    the message box.

  4. In the Icon list, select Icon information.

  5. In the Message field, enter the message
    you want to displayed once the file checking is done. In this example, enter
    “This file does not exist!”.

Defining the condition

  1. Click the If link to open the Basic settings view, where you are able to define
    the condition for checking the existence of this file.

    use_case-thdfsexist4.png

  2. In the Condition box, press Ctrl+Space to access the variable list and select
    the global variable EXISTS. Type an
    exclamation mark before the variable to negate the meaning of the
    variable.

Executing the Job

  • Press F6 to execute this Job.

Once done, a message box pops up to indicate that this file called output.csv does not exist in the directory you defined
earlier.

use_case-thdfsexist5.png

In the HDFS we check the existence of the file, browse to this directory
specified, you can see that this file does not exist.

use_case-thdfsexist6.png
Document get from Talend https://help.talend.com
Thank you for watching.
Docs 5.x
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