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Great Expectations Core Concepts

Great Expectations is all about helping you understand data better, so you can communicate with your team and others about what you've built and what you expect. Great Expectations delivers three key features: expectations validate data quality, tests are docs, and docs are tests, and automatic profiling of data. This guide helps you understand how Great Expectations does that by describing the core concepts used in the tool. The guide aims for precision, which can sometimes make it a bit dense, so we include examples of common use cases to help build intuition.

Below, you'll find a brief introduction to the big ideas you'll need to understand how Great Expectations works, information on some of the defining design decisions in the tool, and links to more detailed documentation on concepts.

Table of Contents

Key ideas#

If you only have time to remember a few key ideas about Great Expectations, make them these:

It all starts with Expectations. An Expectation is how we communicate the way data should appear. It's also how Profilers communicate what they learn about data, and what Data Docs uses to describe data or diagnose problems. When lots of Expectations are grouped together to define a kind of data asset, like "monthly taxi rides", we call it an Expectation Suite.

Datasources are the first thing you'll need to configure to use Great Expectations. A Datasource brings together a way of interacting with data (like a database or Spark cluster) and some specific data (a description of that taxi ride data for last month). With a Datasource, you can get a Batch of data or a Validator that can evaluate expectations on data.

When you're deploying Great Expectations, you'll use a Checkpoint to run a validation, testing whether data meets expectations, and potentially performing other actions like building and saving a Data Docs site, sending a notification, or signaling a pipeline runner.

Great Expectations makes it possible to maintain state about data pipelines using Stores. A Store is a generalized way of keeping Great Expectations objects, like Expectation Suites, Validation Results, Metrics, Checkpoints, or even Data Docs sites. Stores, and other configuration, is managed using a Data Context. The Data Context configuration is usually stored as a YAML file or declared in your pipeline directly, and you should commit the configuration to version control to share it with your team.

Concepts in the codebase#

This section describes links to explanations of the foundational concepts used to integrate Great Expectations into your code. It is a glossary of the main concepts and classes you will encounter while using Great Expectations.

Design decisions#

Great Expectations is designed to help you think and communicate clearly about your data. To do that, we need to rely on some specific ideas about what we're protecting. You usually do not need to think about these nuances to use Great Expectations, and many users never think about what exactly makes a Data Asset or Batch. But we think it can be extremely useful to understand the design decisions that guide Great Expectations.

Great Expectations protects Data Assets. A Data Asset is a logical collection of records. Great Expectations consumes and creates metadata about Data Assets.

  • For example, a Data Asset might be a user table in a database, monthly financial data, a collection of event log data, or anything else that your organization uses.

How do you know when a collection of records is one Data Asset instead of two Data Assets or when two collections of records are really part of the same Data Asset? In Great Expectations, we think the answer lies in the user. Great Expectations opens insights and enhances communication while protecting against pipeline risks and data risks, but that revolves around a purpose in using some data (even if that purpose starts out as "I want to understand what I have here"!).

We recommend that you call a collection of records a Data Asset when you would like to track metadata (and especially, Expectations) about it. A collection of records is a Data Asset when it's worth giving it a name.

Since the purpose is so important for understanding when a collection of records is a Data Asset, it immediately follows that Data Assets are not necessarily disjoint. The same data can be in multiple Data Assets. You may have different Expectations of the same raw data for different purposes or produce documentation tailored to specific analyses and users.

  • Similarly, it may useful to describe subsets of a Data Asset as new Data Assets. For example, if we have a Data Asset called the "user table" in our data warehouse, we might also have a different Data Asset called the "Canadian User Table" that includes data only for some users.

Not all records in a Data Asset need to be available at the same time or place. A Data Asset could be built from streaming data that is never stored, incremental deliveries, analytic queries, incremental updates, replacement deliveries, or from a one-time snapshot.

That implies that a Data Asset is a logical concept. Not all of the records may be accessible at the same time. That highlights a very important and subtle point: no matter where the data comes from originally, Great Expectations validates batches of data. A batch is a discrete subset of a Data Asset that can be identified by a some collection of parameters, like the date of delivery, value of a field, time of validation, or access control permissions.

  • Batches often correspond to deliveries of data or runs of an ETL pipeline, but they do not have to. For example, an analyst studying New York City taxi data might take one logical view into the data where each batch is a month's delivery. But if the analyst selects data from the dataset based on other criteria for her analysis--the time of the ride and number of passengers, for example--then each batch corresponds to the specific query she runs. In that case, the Expectations she creates may have more to do with the analysis she is running than aggregate characteristics of the taxi data.

In some cases the thing that "makes a batch a batch" is the act of attending to it--for example by validating or profiling the data. It's all about your Expectations.

Great Expectations provides a mechanism to automatically generate expectations, using a feature called a Profiler. A Profiler builds an Expectation Suite from one or more Data Assets. It may also validates the data against the newly-generated Expectation Suite to return a Validation Result. There are several Profilers included with Great Expectations.

A Profiler makes it possible to quickly create a starting point for generating expectations about a Dataset. For example, during the init flow, Great Expectations currently uses the UserConfigurableProfiler to demonstrate important features of Expectations by creating and validating an Expectation Suite that has several different kinds of expectations built from a small sample of data. A Profiler is also critical to generating the Expectation Suites used during profiling.