MarkSfik commented on a change in pull request #361:
URL: https://github.com/apache/flink-web/pull/361#discussion_r457518958
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File path: _posts/2020-07-21-catalogs.md
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+---
+layout: post
+title: "Sharing is caring - Catalogs in Flink SQL"
+date: 2020-07-21T08:00:00.000Z
+categories: news
+authors:
+- dawid:
+ name: "Dawid Wysakowicz"
+ twitter: "dwysakowicz"
+---
+
+It's not a surprise that, in an era of digitalization, data is the most
valuable asset in many companies: it's always the base for and product of any
analysis or business logic. With an ever growing number of people working with
data, it's a common practice for companies to build self-service platforms with
the goal of democratising its access across different teams and — especially —
to enable users from any background to be independent in their data needs. In
such environments, metadata management becomes a crucial aspect. Without it,
users often work blindly, spending too much time searching for datasets and
their location, figuring out data formats and similar cumbersome tasks.
+
+It is a common practice for companies to start building a data platform with a
metastore, catalog, or schema registries of some sort in place. Those let you
clearly separate making the data available from consuming it. That separation
has a few benefits:
+* improved productivity - The most obvious one. Making data reusable and
shifting the focus on building new models/pipelines rather than data cleansing
and discovery.
+* security - You can control the access to certain features of the data. For
example, you can make the schema of dataset publicly available, but limit the
actual access to the underlying data to only particular teams.
+* compliance - If you have all the metadata in a central entity, it's much
easier to ensure compliance with GDPR and other similar laws.
+
+## What is stored in a catalog?
+
+Almost all data sets can be described by certain properties that must be known
in order to consume them. Those include:
+* schema - It describes the actual contents of the data, what columns it has,
what are the constraints (e.g. keys) on which the updates should be performed,
which fields can act as time attributes, what are the rules for watermark
generation and so on.
+* location - Does the data come from Kafka or a file in a filesystem? How do
you connect to the external system? Which topic or file name do you use?
+* format - Is the data serialized as json, csv, or maybe avro records?
+* statistics - We can also store additional information that can be useful
when creating an execution plan of our query. For example, we can choose the
best join algorithm, based on the number of rows in joined datasets.
+
+Catalogs don’t have to be limited to the metadata of datasets. You can usually
store other objects that can be reused in different scenarios, such as:
+* functions - It's very common to have domain specific functions that can be
helpful in different use cases. Instead of having to create them in each place
separately, you can just create them once and share them with others.
+* queries - Those can be useful when you don’t want to persist a data set, but
want to provide a recipe for creating it from other sources instead.
+
+## Catalogs support in Flink SQL
+Starting from version 1.9, Flink has a set of Catalog APIs that allow to
integrate it with various catalog implementations. With the help of those APIs,
you can query tables in Flink that were created in your external catalogs (e.g.
Hive Metastore). Additionally, depending on the catalog implementation, you can
create new objects such as tables or views from Flink, reuse them across
different jobs, and possibly even use them in other tools compatible with that
catalog. As of Flink 1.11, there are two catalog implementations supported by
the community:
+* a comprehensive Hive catalog
+* a Postgres catalog (preview, read-only, for now)
+
+**Important:** Flink does not store data at rest; it is a compute engine and
requires other systems to consume input from and write its output. This means
that Flink does not own the lifecycle of the data. Integration with Catalogs
does not change that. Flink uses catalogs for metadata management only.
+
+All you need to do to start querying your tables defined in either of these
metastores is to create corresponding catalogs with connection parameters. Once
this is done, you can use them the way you would in any relational database
management system.
+
+```
+--- create a catalog which gives access to the backing Postgres installation
+CREATE CATALOG postgres WITH (
+ 'type'='jdbc',
+ 'property-version'='1',
+ 'base-url'='jdbc:postgresql://postgres:5432/',
+ 'default-database'='postgres',
+ 'username'='postgres',
+ 'password'='example'
+);
+
+--- create a catalog which gives access to the backing Hive installation
+CREATE CATALOG hive WITH (
+ 'type'='hive',
+ 'property-version'='1',
+ 'hive-version'='2.3.6',
+ 'hive-conf-dir'='/opt/hive-conf'
+);
+```
+
+After creating the catalogs, you can confirm that they're available to Flink
and also list the databases or tables in each of these catalogs:
+
+```
+> show catalogs;
+default_catalog
+hive
+postgres
+
+-- switch the default catalog to Hive
+> use catalog hive;
+> show databases;
+default -- hive's default database
+
+> show tables;
+dev_orders
+
+> use catalog postgres;
+> show tables;
+prod_customer
+prod_nation
+prod_rates
+prod_region
+region_stats
+
+-- desribe a schema of a table in Postgres, the Postgres types are
automatically mapped to
+-- Flink's type system
+> describe prod_customer
+root
+ |-- c_custkey: INT NOT NULL
+ |-- c_name: VARCHAR(25) NOT NULL
+ |-- c_address: VARCHAR(40) NOT NULL
+ |-- c_nationkey: INT NOT NULL
+ |-- c_phone: CHAR(15) NOT NULL
+ |-- c_acctbal: DOUBLE NOT NULL
+ |-- c_mktsegment: CHAR(10) NOT NULL
+ |-- c_comment: VARCHAR(117) NOT NULL
+```
+
+Now that you know which tables are available, you can write your first query.
+In this scenario, we keep customer orders in Hive (``dev_orders``) because of
their volume, and reference customer data in Postgres (``prod_customer``) to be
able to easily update it. Let’s write a query that shows customers and their
orders by region and order priority for a specific day.
+
+```
+USE CATALOG postgres;
+SELECT
+ r_name AS `region`,
+ o_orderpriority AS `priority`,
+ COUNT(DISTINCT c_custkey) AS `number_of_customers`,
+ COUNT(o_orderkey) AS `number_of_orders`
+FROM `hive`.`default`.dev_orders -- we need to fully qualify the table in hive
because we set the
+ -- current catalog to Postgres
+JOIN prod_customer ON o_custkey = c_custkey
+JOIN prod_nation ON c_nationkey = n_nationkey
+JOIN prod_region ON n_regionkey = r_regionkey
+WHERE
+ FLOOR(o_ordertime TO DAY) = TIMESTAMP '2020-04-01 0:00:00.000'
+ AND NOT o_orderpriority = '4-NOT SPECIFIED'
+GROUP BY r_name, o_orderpriority
+ORDER BY r_name, o_orderpriority;
+```
+
+Flink's catalog support also covers storing Flink-specific objects in external
catalogs that might not be fully usable by the corresponding external tools.
The most notable use case for this is, for example, storing a table that
describes a Kafka topic in a Hive catalog. Take the following DDL statement,
that contains a watermark declaration as well as a set of connector properties
that are not recognizable by Hive. You won't be able to query the table with
Hive, but it will be persisted and can be reused by different Flink jobs.
+
+```
+USE CATALOG hive;
+CREATE TABLE prod_lineitem (
+ l_orderkey INTEGER,
+ l_partkey INTEGER,
+ l_suppkey INTEGER,
+ l_linenumber INTEGER,
+ l_quantity DOUBLE,
+ l_extendedprice DOUBLE,
+ l_discount DOUBLE,
+ l_tax DOUBLE,
+ l_currency STRING,
+ l_returnflag STRING,
+ l_linestatus STRING,
+ l_ordertime TIMESTAMP(3),
+ l_shipinstruct STRING,
+ l_shipmode STRING,
+ l_comment STRING,
+ l_proctime AS PROCTIME(),
+ WATERMARK FOR l_ordertime AS l_ordertime - INTERVAL '5' SECONDS
+) WITH (
+ 'connector'='kafka',
+ 'topic'='lineitem',
+ 'scan.startup.mode'='earliest-offset',
+ 'properties.bootstrap.servers'='kafka:9092',
+ 'properties.group.id'='testGroup',
+ 'format'='csv',
+ 'csv.field-delimiter'='|'
+);
+```
+
+With ``prod_lineitem`` stored in Hive, you can now write a query that will
enrich the incoming stream with static data kept in Postgres. To illustrate how
this works, let's calculate prices of items based on the current currency rates:
+
+```
+USE CATALOG postgres;
+SELECT
+ l_proctime AS `querytime`,
+ l_orderkey AS `order`,
+ l_linenumber AS `linenumber`,
+ l_currency AS `currency`,
+ rs_rate AS `cur_rate`,
+ (l_extendedprice * (1 - l_discount) * (1 + l_tax)) / rs_rate AS
`open_in_euro`
+FROM hive.`default`.prod_lineitem
+JOIN prod_rates FOR SYSTEM_TIME AS OF l_proctime ON rs_symbol = l_currency
+WHERE
+ l_linestatus = 'O';
+```
+
+The query above uses a `SYSTEM AS OF` [clause]({{ site.DOCS_BASE_URL
}}/dev/table/streaming/temporal_tables.html#temporal-table) for executing a
temporal join. If you'd like to learn more about the different kind of joins
you can do in Flink I highly encourage you to check [this documentation
page]({{ site.DOCS_BASE_URL }}/dev/table/sql/queries.html#joins).
+
+## Conclusion
+
+Catalogs can be a powerful tool for building a platform where work of
different teams can be made reusable. Centralizing the metadata is a common
practice for improving the productivity, security, and compliance when working
with data.
Review comment:
```suggestion
Catalogs can be extremely powerful when building data platforms aimed at
reusing the work of different teams in an organization. Centralizing the
metadata is a common practice for improving productivity, security, and
compliance when working with data.
```
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