infoverload commented on a change in pull request #460:
URL: https://github.com/apache/flink-web/pull/460#discussion_r691890439
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File path: _posts/2021-08-16-connector-table-sql-api-part1.md
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+---
+layout: post
+title: "Implementing a custom source connector for Table API and SQL - Part
One "
+date: 2021-08-18T00:00:00.000Z
+authors:
+- Ingo Buerk:
+ name: "Ingo Buerk"
+excerpt:
+---
+
+{% toc %}
+
+# Introduction
+
+Apache Flink is a data processing engine that keeps
[state](https://ci.apache.org/projects/flink/flink-docs-release-1.13/docs/ops/state/state_backends/)
locally in order to do computations but does not store data. This means that
it does not include its own fault-tolerant storage component by default and
relies on external systems to ingest and persist data. Connecting to external
data input (**sources**) and external data storage (**sinks**) is achieved with
interfaces called **connectors**.
+
+Since connectors are such important components, Flink ships with [connectors
for some popular
systems](https://ci.apache.org/projects/flink/flink-docs-release-1.13/docs/connectors/table/overview/).
But sometimes you may need to read in an uncommon data format and what Flink
provides is not enough. This is why Flink also provides [APIs](#) for building
custom connectors if you want to connect to a system that is not supported by
an existing connector.
+
+Once you have a source and a sink defined for Flink, you can use its
declarative APIs (in the form of the [Table API and
SQL](https://ci.apache.org/projects/flink/flink-docs-release-1.13/docs/dev/table/overview/))
to execute queries for data analysis without modification to the underlying
data.
+
+The **Table API** has the same operations as **SQL** but it extends and
improves SQL's functionality. It is named Table API because of its relational
functions on tables: how to obtain a table, how to output a table, and how to
perform query operation on the table.
+
+In this two-part tutorial, you will explore some of these APIs and concepts by
implementing your own custom source connector for reading in data from a
mailbox. You will use Flink to process an email inbox through the IMAP protocol
and sort them out by subject to a sink.
+
+Part one will focus on building a custom source connector and [part two](#)
will focus on integrating it.
+
+# Goals
+
+Part one of this tutorial will teach you how to build and run a custom source
connector to be used with Table API and SQL, two high-level abstractions in
Flink.
+
+You are encouraged to follow along with the code in this
[repository](github.com/Airblader/blog-imap). It provides a boilerplate project
that also comes with a bundled
[docker-compose](https://docs.docker.com/compose/) setup that lets you easily
run the connector. You can then try it out with Flink’s SQL client.
+
+
+# Prerequisites
+
+This tutorial assumes that you have some familiarity with Java and
objected-oriented programming.
+
+It would also be useful to have
[docker-compose](https://docs.docker.com/compose/install/) installed on your
system in order to use the script included in the repository that builds and
runs the connector.
+
+
+# Understand the infrastructure required for a connector
+
+In order to create a connector which works with Flink, you need:
+
+1. A _factory class_ (a blueprint for creating other objects) that tells Flink
with which identifier (in this case, “imap”) our connector can be addressed,
which configuration options it exposes, and how the connector can be
instantiated. Since Flink uses the Java Service Provider Interface (SPI) to
discover factories located in different modules, you will also need to add some
configuration details.
+
+2. The _table source_ object as a specific instance of the connector during
the planning stage. It tells Flink some information about this instance and how
it can create the connector runtime implementation. There are also more
advanced features, such as
[abilities](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/connector/source/abilities/package-summary.html),
that can be implemented to improve connector performance.
+
+3. A _runtime implementation_ from the connector obtained during the planning
stage. The runtime logic is implemented in Flink's core connector interfaces
and does the actual work of producing rows of dynamic table data. The runtime
instances are shipped to the Flink cluster.
+
+Let us look at this sequence (factory class → table source → runtime
implementation) in reverse order.
+
+# Establish the runtime implementation of the connector
+
+You first need to have a source connector which can be used in Flink's runtime
system, defining how data goes in and how it can be executed in the cluster.
There are a few different interfaces available for implementing the actual
source of the data and have it be discoverable in Flink.
+
+For complex connectors, you may want to implement the [Source
interface](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/api/connector/source/Source.html)
which gives you a lot of control. For simpler use cases, you can use the
[SourceFunction
interface](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/streaming/api/functions/source/SourceFunction.html),
which is the base interface for all stream data sources in Flink. There are
already a few different implementations of SourceFunction interfaces for common
use cases such as the
[FromElementsFunction](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/streaming/api/functions/source/FromElementsFunction.html)
class and the
[RichSourceFunction](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/streaming/api/functions/source/RichSourceFunction.html)
class. You will use the latter.
+
+`RichSourceFunction` is a base class for implementing a parallel data source
that has access to context information and some lifecycle methods. There is a
`run()` method inherited from the `SourceFunction` interface that you need to
implement. It is invoked once and can be used to produce the data either once
for a bounded result or within a loop for an unbounded stream.
+
+For example, to create a bounded data source, you could implement this method
so that it reads all existing emails and then closes. To create an unbounded
source, you could only look at new emails coming in while the source is active.
You can also combine these behaviors and expose them through configuration
options.
+
+When you first create the class and implement the interface, it should look
something like this:
+
+```java
+import org.apache.flink.streaming.api.functions.source.RichSourceFunction;
+import org.apache.flink.table.data.RowData;
+
+public class ImapSourceFunction extends RichSourceFunction<RowData> {
+ @Override
+ public void run(SourceContext<RowData> ctx) throws Exception {}
+
+ @Override
+ public void cancel() {}
+}
+```
+
+In the `run()` method, you get access to a
[context](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/streaming/api/functions/source/SourceFunction.SourceContext.html)
object inherited from the SourceFunction interface, which is a bridge to Flink
and allows you to output data. Since the source does not produce any data yet,
the next step is to make it produce some static data in order to test that the
data flows correctly:
+
+```java
+import org.apache.flink.streaming.api.functions.source.RichSourceFunction;
+import org.apache.flink.table.data.GenericRowData;
+import org.apache.flink.table.data.RowData;
+import org.apache.flink.table.data.StringData
+
+public class ImapSourceFunction extends RichSourceFunction<RowData> {
+ @Override
+ public void run(SourceContext<RowData> ctx) throws Exception {
+ ctx.collect(GenericRowData.of(
+ StringData.fromString("Subject 1"),
+ StringData.fromString("Hello, World!")
+ ));
+ }
+
+ @Override
+ public void cancel() {}
+}
+```
+
+// explain the collect function?
+
+You do not need to implement the `cancel()` method yet because the source
finishes instantly.
+
+
+# Create and configure a dynamic table source for the data stream
+
+[Dynamic
tables](https://ci.apache.org/projects/flink/flink-docs-release-1.13/docs/dev/table/concepts/dynamic_tables/)
are the core concept of Flink’s Table API and SQL support for streaming data
and, like its name suggests, change over time. You can imagine a data stream
being logically converted into a table that is constantly changing. For this
tutorial, the emails that will be read in will be interpreted as a (source)
table that is queryable. It can be viewed as a specific instance of a connector
class.
+
+You will now implement a
[DynamicTableSource](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/connector/source/DynamicTableSource.html)
interface. There are two types of dynamic table sources:
[ScanTableSource](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/connector/source/ScanTableSource.html)
and
[LookupTableSource](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/connector/source/LookupTableSource.html).
Scan sources read the entire table on the external system while lookup sources
look for specific rows based on keys. The former will fit the use case of this
tutorial.
+
+This is what a scan table source implementation would look like:
+
+```java
+import org.apache.flink.table.connector.ChangelogMode;
+import org.apache.flink.table.connector.source.DynamicTableSource;
+import org.apache.flink.table.connector.source.ScanTableSource;
+import org.apache.flink.table.connector.source.SourceFunctionProvider;
+
+public class ImapTableSource implements ScanTableSource {
+ @Override
+ public ChangelogMode getChangelogMode() {
+ return ChangelogMode.insertOnly();
+ }
+
+ @Override
+ public ScanRuntimeProvider getScanRuntimeProvider(ScanContext ctx) {
+ boolean bounded = true;
+ final ImapSourceFunction sourceFunction = new ImapSourceFunction();
+ return SourceFunctionProvider.of(sourceFunction, bounded);
+ }
+
+ @Override
+ public DynamicTableSource copy() {
+ return new ImapTableSource();
+ }
+
+ @Override
+ public String asSummaryString() {
+ return "IMAP Table Source";
+ }
+}
+```
+
+[ChangelogMode](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/connector/ChangelogMode.html)
informs Flink of expected changes that the planner can expect during runtime.
For example, whether the source produces only new rows, also updates to
existing ones, or whether it can remove previously produced rows. Our source
will only produce (`insertOnly()`) new rows.
+
+[ScanRuntimeProvider](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/connector/source/ScanTableSource.ScanRuntimeProvider.html)
allows Flink to create the actual runtime implementation you established
previously (for reading the data). Flink even provides utilities like
[SourceFunctionProvider](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/connector/source/SourceFunctionProvider.html)
to wrap it into an instance of
[SourceFunction](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/streaming/api/functions/source/SourceFunction.html),
which is the base unit of all stream data sources in Flink.
+
+You will also need to indicate whether the source is bounded or not.
Currently, this is the case but you will have to change this later.
+
+# Create a factory class for the connector so it can be discovered by Flink
+
+You now have a working source connector, but in order to use it in Table API
or SQL, it needs to be discoverable by Flink. You also need to define how the
connector is addressable from a SQL statement when creating a source table.
+
+You need to implement a
[Factory](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/factories/Factory.html),
which is a base interface that create object instances from a list of
key-value pairs in Flink's Table API and SQL. A factory is uniquely identified
by its class name and `factoryIdentifier()`. For this tutorial, you will
implement the more specific
[DynamicTableSourceFactory](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/factories/DynamicTableSourceFactory.html),
which allows you to configure a dynamic table connector as well as create
`DynamicTableSource` instances.
+
+```java
+import java.util.HashSet;
+import org.apache.flink.configuration.ConfigOption;
+import org.apache.flink.table.connector.source.DynamicTableSource;
+import org.apache.flink.table.factories.DynamicTableSourceFactory;
+import org.apache.flink.table.factories.FactoryUtil;
+
+public class ImapTableSourceFactory implements DynamicTableSourceFactory {
+ @Override
+ public String factoryIdentifier() {
+ return "imap";
+ }
+
+ @Override
+ public Set<ConfigOption<?>> requiredOptions() {
+ return new HashSet<>();
+ }
+
+ @Override
+ public Set<ConfigOption<?>> optionalOptions() {
+ return new HashSet<>();
+ }
+
+ @Override
+ public DynamicTableSource createDynamicTableSource(Context ctx) {
+ final FactoryUtil.TableFactoryHelper factoryHelper =
FactoryUtil.createTableFactoryHelper(this, ctx);
+ factoryHelper.validate();
+
+ return new ImapTableSource();
+ }
+}
+```
+
+There are currently no configuration options but they can be added and also
validated within the `createDynamicTableSource()` function. There is a small
helper utility,
[TableFactoryHelper](https://ci.apache.org/projects/flink/flink-docs-master/api/java/org/apache/flink/table/factories/FactoryUtil.TableFactoryHelper.html),
that Flink offers which ensures that required options are set and that no
unknown options are provided.
+
+Finally, you need to register your factory for Java's Service Provider
Interfaces (SPI). Classes that implement this interface can be discovered and
should be added to this file
`src/main/resources/META-INF/services/org.apache.flink.table.factories.Factory`
with the fully classified class name of your factory:
+
+```java
+// if you created your class in the package org.example.acme, it should be
named the following:
+org.example.acme.ImapTableSourceFactory
+```
+
+# Test the custom connector
+
+You should now have a working source connector. If you are following along
with the provided repository, you can test it by running:
+
+```sh
+$ cd testing/
+$ ./build_and_run.sh
+```
+
+This builds the connector, starts a Flink cluster, a [test email
server](https://greenmail-mail-test.github.io/greenmail/) which you will need
later), and the [SQL
client](https://ci.apache.org/projects/flink/flink-docs-release-1.13/docs/dev/table/sqlclient/)
for you. You may need to run this script as a super user. If successful, you
should see the SQL CLI:
+
+
Review comment:
Haha you're right... I was trying to do **DRY** but wasn't careful here.
:see_no_evil:
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