This is not crawler in the sense that would pull data from data sources
A crawler reads data from data sources ONLY TO determine its data structure / schema
Crawls databases using a connection (actually a connection profile)
Crawls files on S3 without needing a connection
After each crawl, virtual tables are created in Data Catalog, these tables stores meta data like the columns, data types, location, etc. of the data, but not the data itself
Crawler is serverless, you pay by Data Processing Unit (time consumed for crawling), but there is a 10 minute minimum duration for each crawl
You can create tables directly without using crawlers
Crawling Behaviors
For database tables, it is easy to determine data types for columns, but crawler mostly shines on file based data (i.e. data lake)
The crawler determines column and data type by reading data from the files and “guess” the format based on patterns and similarity of the data format for different files
It creates tables based on the prefix / folder name of similarly-formatted files, and create partitions of tables as needed
It uses “classifiers” to parse and understand data, classifiers are sets of pattern matchers
User may create custom classifiers
Classifiers are categorized based on file types, e.g. the Grok classifier is for text based files
There are JSON and CSV classifiers, they are for respected file types
Classifier will only classify file types into their primitive data types, for example, even if a JSON contains ISO 8601 formatted timestamp, the crawler will still see it as a string
Glue Data Catalog
An index to the location, schema and runtime metrics of your data
Connections
This is actually connection configuration for Glue to connect to databases
If you access S3 via a VPCE, then you also need a NETWORK type connection
Creating NETWORK type connection without a VPCE will cause “Cannot create enum from NETWORK value!” error which is very confusing
If you access S3 via its public endpoints then no connection is required
Glue ETL
Glue ETL is EMR made serverless
It runs Spark underneath
User can create Spark jobs and run it directly without provisioning servers
Glue ETL provisions EMR clusters on-the-fly, so expect 5-10 minutes cold start time even for the simplest jobs
Glue has built-in helpers to perform common tasks like casting string to timestamp (you will need this for crawled JSONs)
ETL is useful for
Type conversion for columns
Compress data from plain format (text, log, CSV, JSON) to columnar format (parquet)
Data joining, so data can be scanned more easily
Other data transforming and manipulation
ETL Jobs can be connected to form a Workflow, every Job in the workflow can be Triggered manually or automatically
Dev Endpoint
Glue ETL provisions EMR for every Job run, so it is too slow for development purposes
If you are developing your Spark script and want to have an environment that is in the cloud and has access to the resources, use Dev Endpoint
Basically Glue provisions an EMR cluster that is long-running (and long-charging) for you as a dev environment, remember to delete the endpoint after you are done to prevent bill overflow
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