As organizations consolidate analytics workloads to Databricks, they usually must adapt conventional knowledge warehouse methods. This sequence explores how you can implement dimensional modeling—particularly, star schemas—on Databricks. The primary weblog targeted on schema design. This weblog walks by ETL pipelines for dimension tables, together with Slowly Altering Dimensions (SCD) Kind-1 and Kind-2 patterns. The final weblog will present you how you can construct ETL pipelines for reality tables.
Slowly Altering Dimensions (SCD)
Within the final weblog, we outlined our star schema, together with a reality desk and its associated dimensions. We highlighted one dimension desk specifically, DimCustomer, as proven right here (with some attributes eliminated to preserve house):
The final three fields on this desk, i.e., StartDate, EndDate and IsLateArriving, characterize metadata that assists us with versioning data. As a given buyer’s earnings, marital standing, dwelling possession, variety of kids at dwelling, or different traits change, we are going to wish to create new data for that buyer in order that info comparable to our on-line gross sales transactions in FactInternetSales are related to the correct illustration of that buyer. The pure (aka enterprise) key, CustomerAlternateKey, would be the identical throughout these data however the metadata will differ, permitting us to know the interval for which that model of the shopper was legitimate, as will the surrogate key, CustomerKey, permitting our info to hyperlink to the correct model.
NOTE: As a result of the surrogate key’s generally used to hyperlink info and dimensions, dimension tables are sometimes clustered based mostly on this key. Not like conventional relational databases that make the most of b-tree indexes on sorted data, Databricks implements a novel clustering methodology often called liquid clustering. Whereas the specifics of liquid clustering are outdoors the scope of this weblog, we constantly use the CLUSTER BY clause on the surrogate key of our dimension tables throughout their definition to leverage this characteristic successfully.
This sample of versioning dimension data as attributes change is named the Kind-2 Slowly Altering Dimension (or just Kind-2 SCD) sample. The Kind-2 SCD sample is most well-liked for recording dimension knowledge within the basic dimensional methodology. Nevertheless, there are different methods to cope with modifications in dimension data.
Some of the frequent methods to cope with altering dimension values is to replace current data in place. Just one model of the report is ever created, in order that the enterprise key stays the distinctive identifier for the report. For varied causes, not the least of that are efficiency and consistency, we nonetheless implement a surrogate key and hyperlink our reality data to those dimensions on these keys. Nonetheless, the StartDate and EndDate metadata fields that describe the time intervals over which a given dimension report is taken into account energetic should not wanted. This is named the Kind-1 SCD sample. The Promotion dimension in our star schema gives an excellent instance of a Kind-1 dimension desk implementation:
However what in regards to the IsLateArriving metadata subject seen within the Kind-2 Buyer dimension however lacking from the Kind-1 Promotion dimension? This subject is used to flag data as late arriving. A late arriving report is one for which the enterprise key reveals up throughout a reality ETL cycle, however there isn’t a report for that key positioned throughout prior dimension processing. Within the case of the Kind-2 SCDs, this subject is used to indicate that when the info for a late arriving report is first noticed in a dimension ETL cycle, the report must be up to date in place (identical to in a Kind-1 SCD sample) after which versioned from that time ahead. Within the case of the Kind-1 SCDs, this subject isn’t essential as a result of the report shall be up to date in place regardless.
NOTE: The Kimball Group acknowledges extra SCD patterns, most of that are variations and combos of the Kind-1 and Kind-2 patterns. As a result of the Kind-1 and Kind-2 SCDs are probably the most continuously carried out of those patterns and the methods used with the others are carefully associated to what’s employed with these, we’re limiting this weblog to simply these two dimension varieties. For extra details about the eight forms of SCDs acknowledged by the Kimball Group, please see the Slowly Altering Dimension Methods part of this doc.
Implementing the Kind-1 SCD Sample
With knowledge being up to date in place, the Kind-1 SCD workflow sample is probably the most easy of the two-dimensional ETL patterns. To help a lot of these dimensions, we merely:
- Extract the required knowledge from our operational system(s)
- Carry out any required knowledge cleaning operations
- Evaluate our incoming data to these already within the dimension desk
- Replace any current data the place incoming attributes differ from what’s already recorded
- Insert any incoming data that do not need a corresponding report within the dimension desk
For instance a Kind-1 SCD implementation, we’ll outline the ETL for the continuing inhabitants of the DimPromotion desk.
Step 1: Extract knowledge from an operational system
Our first step is to extract the info from our operational system. As our knowledge warehouse is patterned after the AdventureWorksDW pattern database supplied by Microsoft, we’re utilizing the carefully related AdventureWorks (OLTP) pattern database as our supply. This database has been deployed to an Azure SQL Database occasion and made accessible inside our Databricks atmosphere by way of a federated question. Extraction is then facilitated with a easy question (with some fields redacted to preserve house), with the question outcomes continued in a desk in our staging schema (that’s made accessible solely to the info engineers in the environment by permission settings not proven right here). That is however considered one of some ways we will entry supply system knowledge on this atmosphere:
Step 2: Evaluate incoming data to these within the desk
Assuming we now have no extra knowledge cleaning steps to carry out (which we might implement with an UPDATE or one other CREATE TABLE AS assertion), we will then deal with our dimension knowledge replace/insert operations in a single step utilizing a MERGE assertion, matching our staged knowledge and dimension knowledge on the enterprise key:
One essential factor to notice in regards to the assertion, because it’s been written right here, is that we replace any current data when a match is discovered between the staged and printed dimension desk knowledge. We might add extra standards to the WHEN MATCHED clause to restrict updates to these cases when a report in staging has totally different info from what’s discovered within the dimension desk, however given the comparatively small variety of data on this specific desk, we’ve elected to make use of the comparatively leaner logic proven right here. (We are going to use the extra WHEN MATCHED logic with DimCustomer, which accommodates way more knowledge.)
The Kind-2 SCD sample
The Kind-2 SCD sample is a little more complicated. To help a lot of these dimensions, we should:
- Extract the required knowledge from our operational system(s)
- Carry out any required knowledge cleaning operations
- Replace any late-arriving member data within the goal desk
- Expire any current data within the goal desk for which new variations are present in staging
- Insert any new (or new variations) of data into the goal desk
Step 1: Extract and cleanse knowledge from a supply system
As within the Kind-1 SCD sample, our first steps are to extract and cleanse knowledge from the supply system. Utilizing the identical strategy as above, we problem a federated question and persist the extracted knowledge to a desk in our staging schema:
Step 2: Evaluate to a dimension desk
With this knowledge landed, we will now examine it to our dimension desk with a purpose to make any required knowledge modifications. The primary of those is to replace in place any data flagged as late arriving from prior reality desk ETL processes. Please be aware that these updates are restricted to these data flagged as late arriving and the IsLateArriving flag is being reset with the replace in order that these data behave as regular Kind-2 SCDs transferring ahead:
Step 3: Expire versioned data
The following set of knowledge modifications is to run out any data that should be versioned. It’s essential that the EndDate worth we set for these matches the StartDate of the brand new report variations we are going to implement within the subsequent step. For that purpose, we are going to set a timestamp variable for use between these two steps:
NOTE: Relying on the info obtainable to you, it’s possible you’ll elect to make use of an EndDate worth originating from the supply system, at which level you wouldn’t essentially declare a variable as proven right here.
Please be aware the extra standards used within the WHEN MATCHED clause. As a result of we’re solely performing one operation with this assertion, it will be doable to maneuver this logic to the ON clause, however we saved it separated from the core matching logic, the place we’re matching to the present model of the dimension report for readability and maintainability.
As a part of this logic, we’re making heavy use of the equal_null() perform. This perform returns TRUE when the primary and second values are the identical or each NULL; in any other case, it returns FALSE. This gives an environment friendly option to search for modifications on a column-by-column foundation. For extra particulars on how Databricks helps NULL semantics, please check with this doc.
At this stage, any prior variations of data within the dimension desk which have expired have been end-dated.
Step 4: Insert new data
We will now insert new data, each really new and newly versioned:
As earlier than, this might have been carried out utilizing an INSERT assertion, however the consequence is identical. With this assertion, we now have recognized any data within the staging desk that don’t have an unexpired corresponding report within the dimension tables. These data are merely inserted with a StartDate worth per any expired data which will exist on this desk.
Subsequent steps: implementing the actual fact desk ETL
With the size carried out and populated with knowledge, we will now deal with the actual fact tables. Within the subsequent weblog, we are going to exhibit how the ETL for these tables may be carried out.
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