Data Warehousing concepts

 

Data warehousing concepts:

1) What is Data warehouse?

Data warehouse is relational database used for query analysis and reporting. By definition data

warehouse is subject-oriented, integrated, non-volatile, time variant.
Subject oriented: Data warehouse is maintained particular subject.
Integrated: Data collected from multiple sources integrated into a user readable unique format.
Non volatile: Maintain Historical date.
Time variant: data display the weekly, monthly, yearly.

2) What is Data mart?

A subset of data warehouse is called Data mart.

3) Different between data warehouse an data mart?

Data warehouse is maintaining the total organization of data. Multiple data

marts used in data

warehouse. Where as data mart is maintained only particular subject.

4) Difference between OLTP and OLAP?

OLTP is online transaction processing.This maintains current transactional data. That means insert,

update and delete must be fast.

5) Explain ODS?

Operational data store is a part of data warehouse. This is maintained only current transactional

data.ODS is subject oriented, integrated, volatile, current data.

6) Difference between power center and power mart?

Power center receive all product functionality including ability to multiple register servers and metadata
across the repository and partition data. One repository multiple informatica servers. Power mart
received all features expect multiple register servers and partition data.

7) What is a staging area?

Staging area is a temporary storage area used for transactions integrated and rather than transaction

processing when ever your data put in data warehouse you need to clean and process your data.

8) Explain additive, semi-additive, non-additive facts?

Additive fact: Additive Fact can be aggregated by simple arithmetical additions.

Semi-Additive fact: semi additive fact can be aggregated simple arithmetical

Incremental Aggregation in informatica

Implementing Informatica Incremental Aggregation

Using incremental aggregation, we apply captured changes in the source data (CDC part) to aggregate calculations in a session. If the source changes incrementally and we can capture the changes, then we can configure the session to process those changes. This allows the Integration Service to update the target incrementally, rather than forcing it to delete previous loads data, process the entire source data and recalculate the same data each time you run the session.

Incremental Aggregation

When the session runs with incremental aggregation enabled for the first time say 1st week of Jan, we will use the entire source. This allows the Integration Service to read and store the necessary aggregate data information. On 2nd week of Jan, when we run the session again, we will filter out the CDC records from the source  i. e the records loaded after the initial load. The Integration Service then processes these new data and updates the target accordingly.

Use incremental aggregation when the changes do not significantly change the target.If processing the incrementally changed source alters more than half the existing target, the session may not benefit from using incremental aggregation. In this case, drop the table and recreate the target with entire source data and recalculate the same aggregation formula .

INCREMENTAL AGGREGATION, may be helpful in cases when we need to load data in monthly facts in a weekly basis.

Sample Mapping

Let us see a sample mapping to implement incremental aggregation:

Look at the Source Qualifier query to fetch the CDC part using a BATCH_LOAD_CONTROL table that saves the last successful load date for the particular mapping.

Look at the ports tab of Expression transformation.

Look at the ports tab of Aggregator Transformation.

Now the most important session properties configuration to implement incremental Aggregation

If we want to reinitialize the aggregate cache suppose during first week of every month we will configure the same session in a new workflow at workflow level with the Reinitialize aggregate cache property checked in.

Example with Data

Now have a look at the source table data:

CUSTOMER_KEY	INVOICE_KEY	AMOUNT	LOAD_DATE
1111	5001	100	01/01/2010
2222	5002	250	01/01/2010
3333	5003	300	01/01/2010
1111	6007	200	07/01/2010
1111	6008	150	07/01/2010
2222	6009	250	07/01/2010
4444	1234	350	07/01/2010
5555	6157	500	07/01/2010

After the first Load on 1st week of Jan 2010, the data in the target is as follows:

CUSTOMER_KEY	INVOICE_KEY	MON_KEY	AMOUNT
1111	5001	201001	100
2222	5002	201001	250
3333	5003	201001	300

Now during the 2nd week load it will process only the incremental data in the source i.e those records having load date greater than the last session run date. After the 2nd weeks load after incremental aggregation of the incremental source data with the aggregate cache file data will update the target table with the following dataset:

CUSTOMER_KEY	INVOICE_KEY	MON_KEY	AMOUNT	Remarks/Operation
1111	6008	201001	450	The cache file updated after aggretation
2222	6009	201001	500	The cache file updated after aggretation
3333	5003	201001	300	The cache file remains the same as before
4444	1234	201001	350	New group row inserted in cache file
5555	6157	201001	500	New group row inserted in cache file

Understanding Incremental Aggregation Process

The first time we run an incremental aggregation session, the Integration Service processes the entire source. At the end of the session, the Integration Service stores aggregate data for that session run in two files, the index file and the data file. The Integration Service creates the files in the cache directory specified in the Aggregator transformation properties.

Each subsequent time we run the session with incremental aggregation, we use the incremental source changes in the session. For each input record, the Integration Service checks historical information in the index file for a corresponding group. If it finds a corresponding group, the Integration Service performs the aggregate operation incrementally, using the aggregate data for that group, and saves the incremental change. If it does not find a corresponding group, the Integration Service creates a new group and saves the record data.

When writing to the target, the Integration Service applies the changes to the existing target. It saves modified aggregate data in the index and data files to be used as historical data the next time you run the session.

Each subsequent time we run a session with incremental aggregation, the Integration Service creates a backup of the incremental aggregation files. The cache directory for the Aggregator transformation must contain enough disk space for two sets of the files.

The Integration Service creates new aggregate data, instead of using historical data, when we configure the session to reinitialize the aggregate cache, Delete cache files etc.

When the Integration Service rebuilds incremental aggregation files, the data in the previous files is lost.

Note: To protect the incremental aggregation files from file corruption or disk failure, periodically back up the files.

Type of tasks in Informatica:

Tasks are used to create workflows and worklets. Tasks are designed in workflow manager and represent an executable set of actions. Which shows the run time behavior of ETL process. There are several types of tasks available.

1.Session Task

• It is used to run the mapping by using set of instructions.

2.Command task

• Allows to execute operating system command
• In unix we can specify one or more unix commands or a shell script.
• In windows we can specify one or more dos commands or batch files.

3.Decision Task 

• It allows to define own condition that determines the execution of the workflow.
• Integration service evaluates the decision condition specified in a decision task and returns true or false.
• Predefined variable called $Decision_task_name.condition

4.Event Wait/Raise Task

• Event Raise Task: Event Raise task raises the event while running the workflow. It represents the location of a user-defined event. 
• Event Wait Task: The Event Wait task waits for an event to occur.
• There are two types of event

                           Predefined event: A predefined event  is a file-watch event.
                           User defined event: A user defined event is a sequence of tasks in the workflow.

• When all the tasks in the branch from the start task to the Event raise task complete, the Event raise task triggers the event. The Event wait task waits for the Event-Raise task to trigger the event before continuing with the rest of the tasks in its branch. 

5.Assignment task

• It allows to assign a value to a user-defined workflow variable.

6.Timer 

• It allows to set the time between the tasks to set dependencies in the workflow properly.

                            Absolute time: We can specify the time to start a particular task.

                            Relative time: Instruct the integration service to wait for specified period of time                                  after timer starts.

7.Email Tasks

• Send Email during workflow.
• Email addresses, a subject line and the email message text can be defined. When called from within a Session task, the message text can contain variable session-related metadata.

8.Control Task

• It is used to stop or abort the workflow.