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Is Replica Of

Definition

Asserts that the dataset created by the targeted field(s) is replicated by the referred field(s).

In-Depth Overview

The IsReplicaOf rule ensures that data integrity is maintained when data is replicated from one source to another. This involves checking not only the data values themselves but also ensuring that the structure and relationships are preserved.

In a distributed data ecosystem, replication often occurs to maintain high availability, create backups, or feed data into analytical systems. However, discrepancies might arise due to various reasons such as network glitches, software bugs, or human errors. The IsReplicaOf rule serves as a safeguard against these issues by:

  1. Preserving Data Structure: Ensuring that the structure of the replicated data matches the original.
  2. Checking Data Values: Ensuring that every piece of data in the source exists in the replica.

Field Scope

Multi: The rule evaluates multiple specified fields.

Accepted Types

Type
Date
Timestamp
Integral
Fractional
String
Boolean

General Properties

Name Supported
Filter
Allows the targeting of specific data based on conditions
Coverage Customization
Allows adjusting the percentage of records that must meet the rule's conditions

The filter allows you to define a subset of data upon which the rule will operate.

It requires a valid Spark SQL expression that determines the criteria rows in the DataFrame should meet. This means the expression specifies which rows the DataFrame should include based on those criteria. Since it's applied directly to the Spark DataFrame, traditional SQL constructs like WHERE clauses are not supported.

Examples

Direct Conditions

Simply specify the condition you want to be met.

Correct usage
O_TOTALPRICE > 1000
C_MKTSEGMENT = 'BUILDING'
Incorrect usage
WHERE O_TOTALPRICE > 1000
WHERE C_MKTSEGMENT = 'BUILDING'

Combining Conditions

Combine multiple conditions using logical operators like AND and OR.

Correct usage
O_ORDERPRIORITY = '1-URGENT' AND O_ORDERSTATUS = 'O'
(L_SHIPDATE = '1998-09-02' OR L_RECEIPTDATE = '1998-09-01') AND L_RETURNFLAG = 'R'
Incorrect usage
WHERE O_ORDERPRIORITY = '1-URGENT' AND O_ORDERSTATUS = 'O'
O_TOTALPRICE > 1000, O_ORDERSTATUS = 'O'

Utilizing Functions

Leverage Spark SQL functions to refine and enhance your conditions.

Correct usage
RIGHT(
    O_ORDERPRIORITY,
    LENGTH(O_ORDERPRIORITY) - INSTR('-', O_ORDERPRIORITY)
) = 'URGENT'
LEVENSHTEIN(C_NAME, 'Supplier#000000001') < 7
Incorrect usage
RIGHT(
    O_ORDERPRIORITY,
    LENGTH(O_ORDERPRIORITY) - CHARINDEX('-', O_ORDERPRIORITY)
) = 'URGENT'
EDITDISTANCE(C_NAME, 'Supplier#000000001') < 7

Using scan-time variables

To refer to the current dataframe being analyzed, use the reserved dynamic variable {{ _qualytics_self }}.

Correct usage
O_ORDERSTATUS IN (
    SELECT DISTINCT O_ORDERSTATUS
    FROM {{ _qualytics_self }}
    WHERE O_TOTALPRICE > 1000
)
Incorrect usage
O_ORDERSTATUS IN (
    SELECT DISTINCT O_ORDERSTATUS
    FROM ORDERS
    WHERE O_TOTALPRICE > 1000
)

While subqueries can be useful, their application within filters in our context has limitations. For example, directly referencing other containers or the broader target container in such subqueries is not supported. Attempting to do so will result in an error.

Important Note on {{ _qualytics_self }}

The {{ _qualytics_self }} keyword refers to the dataframe that's currently under examination. In the context of a full scan, this variable represents the entire target container. However, during incremental scans, it only reflects a subset of the target container, capturing just the incremental data. It's crucial to recognize that in such scenarios, using {{ _qualytics_self }} may not encompass all entries from the target container.

Specific Properties

Specify the datastore and table/file where the replica of the targeted fields is located for comparison.

Name Description
Row Identifiers
The list of fields defining the compound key to identify rows in the comparison analysis.
Datastore
The source datastore where the replica of the targeted field(s) is located.
Table/file
The table, view or file in the source datastore that should serve as the replica.
Comparators
Specifies how variations are handled, allowing for slight deviations within a defined margin of error.

Details

Row Identifiers

This optional input allows row comparison analysis by defining a list of fields as row identifiers, it enables a more detailed comparison between tables/files, where each row compound key is used to identify its presence or abscence in the reference table/file compared to the target table/file. Qualytics can inform if the row exists or not and distinguish which field values differ in each row present in the reference table/file, helping to determine if it is a replica.

Info

Anomalies produced by a IsReplicaOf quality check making use of Row Identifiers have their source records presented in a different visualization.

See more at: Comparison Source Records

Comparators

The Comparators allow you to set margins of error, accommodating slight variations in data validation. This flexibility is crucial for maintaining data integrity, especially when working with different data types such as numeric values, durations, and strings. Here's an overview of how each type of comparator can be beneficial for you:

Numeric

Numeric comparators enable you to compare numbers with a specified margin, which can be a fixed absolute value or a percentage. This allows for minor numerical differences that are often acceptable in real-world data.

Comparison Type
  • Absolute Value: Uses a fixed threshold for determining equality. It's ideal when you need consistent precision across measurements.
  • Percentage Value: Uses a percentage of the original value as the threshold for equality comparisons. It's suitable for floating point numbers where precision varies.
Threshold

The threshold is the value you set to define the margin of error:

  • When using Absolute Value, the threshold represents the maximum allowable difference between two values for them to be considered equal.
  • For Percentage Value, the threshold is the percentage that describes how much a value can deviate from a reference value and still be considered equal.
Illustration using Absolute Value

In this example, it is being compared Value A and Value B according to the defined Threshold of 50.

Value A Value B Difference Are equal?
100 150 50 True
100 90 10 True
100 155 55
False
100 49 51
False
Illustration using Percentage Value

In this example, it is being compared Value A and Value B according to the defined Threshold of 10%.

Percentage Change Formula: [ (Value B - Value A) / Value A ] * 100

Value A Value B Percentage Change Are equal?
120 132 10% True
150 135 10% True
200 180 10% True
160 150 6.25% True
180 200 11.11%
False

Duration

Duration comparators support time-based comparisons, allowing for flexibility in how duration differences are managed. This flexibility is crucial for datasets where time measurements are essential but can vary slightly.

Unit

The unit of time you select determines how granular the comparison is:

  • Millis: Measures time in milliseconds, ideal for high-precision needs.
  • Seconds: Suitable for most general purposes where precision is important but doesn't need to be to the millisecond.
  • Days: Best for longer durations.
Value

Value sets the maximum acceptable difference in time to consider two values as equal. It serves to define the margin of error, accommodating small discrepancies that naturally occur over time.

Illustration using Duration Comparator
Unit Value A Value B Difference Threshold Are equal?
Millis 500 ms 520 ms 20 ms 25 ms True
Seconds 30 sec 31 sec 1 sec 2 sec True
Days 5 days 7 days 2 days 1 day
False
Millis 1000 ms 1040 ms 40 ms 25 ms
False
Seconds 45 sec 48 sec 3 sec 2 sec
False

String

String comparators facilitate comparisons of textual data by allowing variations in spacing. This capability is essential for ensuring data consistency, particularly where minor text inconsistencies may occur.

Ignore Whitespace

When enabled, this setting allows the comparator to ignore differences in whitespace. This means sequences of whitespace are collapsed into a single space, and any leading or trailing spaces are removed. This can be particularly useful in environments where data entry may vary in formatting but where those differences are not relevant to the data's integrity.

Illustration

TBD

Thresholds: Min Value = 100, Max Value = 300

Scan Current Value Anomaly Detected
#1 150 No
#2
90
Yes
#3 250 No
#4
310
Yes

Anomaly Types

Type Supported
Record
Flag inconsistencies at the row level
Shape
Flag inconsistencies in the overall patterns and distributions of a field

Example

Scenario: Consider that the fields N_NATIONKEY and N_NATIONNAME in the NATION table are being replicated to a backup database for disaster recovery purposes. The data engineering team wants to ensure that both fields in the replica in the backup accurately reflect the original.

Objective: Ensure that N_NATIONKEY and N_NATIONNAME from the NATION table are replicas in the NATION_BACKUP table.

Sample Data from NATION

N_NATIONKEY N_NATIONNAME
1 Australia
2 United States
3 Uruguay

Replica Sample Data from NATION_BACKUP

N_NATIONKEY N_NATIONNAME
1 Australia
2 USA
3 Uruguay
{
    "description": "Ensure that N_NATIONKEY and N_NATIONNAME from the NATION table are replicas in the NATION_BACKUP table",
    "coverage": 1,
    "properties": {
        "ref_container_id": {ref_container_id},
        "ref_datastore_id": {ref_datastore_id}
    },
    "tags": [],
    "fields": ["N_NATIONKEY", "N_NATIONNAME"],
    "additional_metadata": {"key 1": "value 1", "key 2": "value 2"},
    "rule": "isReplicaOf",
    "container_id": {container_id},
    "template_id": {template_id},
    "filter": "1=1"
}

Anomaly Explanation

The datasets representing the fields N_NATIONKEY and N_NATIONNAME in the original and the replica are not completely identical, indicating a possible discrepancy in the replication process or an unintended change.

graph TD
A[Start] --> B[Retrieve Original Data]
B --> C[Retrieve Replica Data]
C --> D{Do datasets match for both fields?}
D -->|Yes| E[End]
D -->|No| F[Mark as Anomalous]
F --> E
-- An illustrative SQL query comparing original to replica for both fields.
select
    orig.n_nationkey as original_key,
    orig.n_nationname as original_name,
    replica.n_nationkey as replica_key,
    replica.n_nationname as replica_name
from nation as orig
left join nation_backup as replica on orig.n_nationkey = replica.n_nationkey
where
    orig.n_nationname <> replica.n_nationname
or
    orig.n_nationkey <> replica.n_nationkey

Potential Violation Messages

Shape Anomaly

There is 1 record that differ between NATION_BACKUP (3 records) and NATION (3 records) in <datastore_name>


Last update: May 30, 2024