Official Google Cloud Certified Professional Data Engineer Study Guide. Dan Sullivan

typically used for batch uploads. It may also be used in conjunction with Cloud Transfer Service and Transfer Appliance when uploading large volumes of data.

Once data enters the GCP platform through ingestion, it can be stored for longer-term access by other applications or services.

      Store

      The focus of the storage stage of the data lifecycle is to make data available for transformation and analysis. Several factors influence the choice of storage system, including

       How the data is accessed—by individual record (row) or by an aggregation of columns across many records (rows)

       The way access controls need to be implemented, at the schema or database level or finer-grained level

       How long the data will be stored

      These three characteristics are the minimum that should be considered when choosing a storage system; there may be additional criteria for some use cases. (Structure is another factor and is discussed later in this chapter.)

      Data Access Patterns

      Data is accessed in different ways. Online transaction processing systems often query for specific records using a set of filtering parameters. For example, an e-commerce application may need to look up a customer shipping address from a data store table that holds tens of thousands of addresses. Databases, like Cloud SQL and Cloud Datastore, provide that kind of query functionality.

      In another example, a machine learning pipeline might begin by accessing files with thousands of rows of data that is used for training the model. Since machine learning models are often trained in batch mode, all of the training data is needed. Cloud Storage is a good option for storing data that is accessed in bulk.

      If you need to access files using filesystem operations, then Cloud Filestore is a good option.

      Access Controls

      Security and access control in particular also influence how data is stored.

      Relational databases, like Cloud SQL and Cloud Spanner, provide mechanisms to restrict access to tables and views. Some users can be granted permission to update data, whereas others can only view data, and still others are not allowed any direct access to data in the database. Fine-grained security can be implemented at the application level or by creating views that limit the data available to some users.

      Some access controls are coarse grained. For example, Cloud Storage can limit access based on bucket permissions and access control lists on objects stored in a bucket. If a user has access to a file in the bucket, then they will have access to all the data in that file. Cloud Storage treats files as atomic objects; there is no concept of a row of data, for example, in Cloud Storage as there is in a relational database.

In some cases, you may be able to use other security features of a service along with access controls. BigQuery, for example, is an analytical database used for data warehousing, data analytics, and machine learning. Data is organized into datasets, which are groups of tables and views. At the current time, BigQuery supports dataset-level access controls but not access controls on tables or views directly. One way to work around these limitations is to create authorized views in one dataset that reference tables in another dataset. The dataset with the authorized views can have one set of access controls whereas the dataset with the source tables can have more restrictive access controls.

      When choosing a data store, it is important to consider access control requirements and how well a storage system supports those requirements.

      Time to Store

      Consider how long data will be stored when choosing a data store. Some data is transient. For example, data that is needed only temporarily by an application running on a Compute Engine instance could be stored on a local solid-state drive (SSD) on the instance. As long as the data can be lost when the instance shuts down, this could be a reasonable option.

      Data is often needed longer than the lifetime of a virtual machine instance, so other options are better fits for those cases. Cloud Storage is a good option for long-term storage, especially if you can make use of storage lifecycle policies to migrate older data to Nearline or Coldline storage. For long-lived analytics data, Cloud Storage or BigQuery are good options, since the costs are similar.

      

Nearline storage is used for data that is accessed less than once per 30 days. Coldline storage is used to store data accesses less than once per year.

      Data that is frequently accessed is often well suited for either relational or NoSQL databases. As data ages, it may not be as likely to be accessed. In those cases, data can be deleted or exported and archived. If the data is not likely to be used for other purposes, such as machine learning, and there are no regulations that require you to keep the older data, then deleting it may be the best option. In cases where the data can be useful for other purposes or you are required to retain data, then exporting and storing it in Cloud Storage is an option. Then, if the data needs to be accessed, it can be imported to the database and queried there.

      Process and Analyze

      During the process and analyze stage, data is transformed into forms that make the data readily available to ad hoc querying or other forms of analysis.

      Data Transformations

Transformations include data cleansing, which is the process of detecting erroneous data and correcting it. Some cleansing operations are based on the data type of expected data. For example, a column of data containing only numeric data should not have alphabetic characters in the column. The cleansing process could delete rows of data that have alphabetic characters in that column. It could alternatively keep the row and substitute another value, such as a zero, or treat the value as NULL.

      In other cases, business logic is applied to determine incorrect data. Some business logic rules may be simple, such as that an order date cannot be earlier than the date that the business began accepting orders. An example of a more complex rule is not allowing an order total to be greater than the credit limit assigned to a customer.

      The decision to keep the row or delete it will depend on the particular use case. A set of telemetry data arriving at one-minute intervals may include an invalid value. In that case, the invalid value may be dropped without significantly affecting hour-level aggregates. A customer order that violates a business rule, however, might be kept because orders are significant business events. In this case, the order should be processed by an exception-handling process.

      Transformations also include normalizing or standardizing data. For example, an application may expect phone numbers in North America to include a three-digit area code. If a phone number is missing an area code, the area code can be looked up based on the associated address. In another case, an application may expect country names specified using the International Organization for Standardization (ISO) 3166 alpha-3 country code, in which case data specifying Canada would be changed to CAN.

      Cloud Dataflow is well suited to transforming both stream and batch data. Once data has been transformed, it is available for analysis.

      Data Analysis

      In the analyze stage, a variety of techniques may be used to extract useful information from data. Statistical techniques are often used with numeric data to do the following:

       Describe characteristics of a dataset, such as a mean and standard deviation of the dataset.

       Generate histograms to understand the distribution of values of an attribute.

       Find correlations between variables, such as customer type and average revenue per sales order.