tet123/documentation/modules/ROOT/pages/configuration/signalling.adoc

// Category: debezium-using
// Type: assembly
[id="sending-signals-to-a-debezium-connector"]
= Sending signals to a {prodname} connector
ifdef::community[]
:toc:
:toc-placement: macro
:linkattrs:
:icons: font
:source-highlighter: highlight.js

toc::[]

== Overview
endif::community[]

The {prodname} signaling mechanism provides a way to modify the behavior of a connector, or to trigger a one-time action, such as initiating an xref:debezium-signaling-ad-hoc-snapshots[ad hoc snapshot] of a table.
To use signals to trigger a connector to perform a specified action, you can configure the connector to use one or more of the following channels:

SourceSignalChannel:: You can issue a SQL command to add a signal message to a specialized signaling {data-collection}.
The signaling {data-collection}, which you create on the source database, is designated exclusively for communicating with {prodname}.
KafkaSignalChannel:: You submit signal messages to a configurable Kafka topic.
Custom:: You submit signals to a xref:debezium-custom-signaling-channel[custom channel] that you implement.

When {prodname} detects that a new xref:debezium-signaling-example-of-a-logging-record[logging record] or xref:debezium-signaling-example-of-an-ad-hoc-signal-record[ad hoc snapshot record] is added to the channel, it reads the signal, and initiates the requested operation.

Signaling is available for use with the following {prodname} connectors:

* Db2
* MongoDB (Technology Preview)
* MySQL
* Oracle
* PostgreSQL
* SQL Server

You can specify which channel is enabled by setting the `signal.enabled.channels` configuration property. The property lists the names of the channels that are enabled. By default, {prodname} provides the following channels: `source`  and `kafka`.
The `source` channel is enabled by default, because it is required for incremental snapshot signals.


// Type: procedure
// Title: Enabling {prodname} source signaling channel
[id="debezium-signaling-enabling-source-signaling-channel"]
== Enabling source signaling channel

By default, the {prodname} source signaling channel is enabled.

You must explicitly configure signaling for each connector that you want to use it with.

.Procedure

. On the source database, create a signaling data collection table for sending signals to the connector.
  For information about the required structure of the signaling data collection, see xref:debezium-signaling-data-collection-structure[Structure of a signaling data collection].

. For source databases such as Db2 or SQL Server that implement a native change data capture (CDC) mechanism, enable CDC for the signaling table.

. Add the name of the signaling data collection to the {prodname} connector configuration. +
  In the connector configuration, add the property `signal.data.collection`, and set its value to the fully-qualified name of the signaling data collection that you created in Step 1. +
 +
For example, `signal.data.collection = inventory.debezium_signals`. +
 +
The format for the fully-qualified name of the signaling collection depends on the connector. +
The following example shows the naming formats to use for each connector:

Db2:: `_<schemaName>_._<tableName>_`
MongoDB:: `_<databaseName>_._<collectionName>_`
MySQL:: `_<databaseName>_._<tableName>_`
Oracle:: `_<databaseName>_._<schemaName>_._<tableName>_`
PostgreSQL:: `_<schemaName>_._<tableName>_`
SQL Server:: `_<databaseName>_._<schemaName>_._<tableName>_` +
 +
For more information about setting the `signal.data.collection` property, see the table of configuration properties for your connector.

// Type: reference
// ModuleID: debezium-signaling-required-structure-of-a-signaling-data-collection
// Title: Required structure of a {prodname} signaling data collection
[id="debezium-signaling-data-collection-structure"]
=== Structure of a signaling data collection

A signaling data collection, or signaling table, stores signals that you send to a connector to trigger a specified operation.
The structure of the signaling table must conform to the following standard format.

* Contains three fields (columns).
* Fields are arranged in a specific order, as shown in xref:debezium-signaling-description-of-required-structure-of-a-signaling-data-collection[Table 1].

.Structure of a signaling data collection
[id="debezium-signaling-description-of-required-structure-of-a-signaling-data-collection"]
.Required structure of a signaling data collection
[cols="1,1,9",options="header"]
|===
|Field | Type | Description

|`id` +
(required)
|`string`

|An arbitrary unique string that identifies a signal instance. +
You assign an `id` to each signal that you submit to the signaling table. +
Typically the ID is a UUID string. +
You can use signal instances for logging, debugging, or de-duplication. +
When a signal triggers {prodname} to perform an incremental snapshot, it generates a signal message with an arbitrary `id` string.
The `id` string that the generated message contains is unrelated to the `id` string in the submitted signal.

|`type` +
(required)
|`string`

|Specifies the type of signal to send. +
You can use some signal types with any connector for which signaling is available, while other signal types are available for specific connectors only.

|`data` +
(optional)
|`string`

|Specifies JSON-formatted parameters to pass to a signal action. +
Each signal type requires a specific set of data.

|===

NOTE: The field names in a data collection are arbitrary.
The preceding table provides suggested names.
If you use a different naming convention, ensure that the values in each field are consistent with the expected content.

// Type: procedure
// Title: Creating a {prodname} signaling data collection
[id="debezium-signaling-creating-a-signal-data-collection"]
=== Creating a signaling data collection

You create a signaling table by submitting a standard SQL DDL query to the source database.

.Prerequisites

* You have sufficient access privileges to create a table on the source database.

.Procedure

* Submit a SQL query to the source database to create a table that is consistent with the xref:debezium-signaling-description-of-required-structure-of-a-signaling-data-collection[required structure], as shown in the following example: +
 +
`CREATE TABLE _<tableName>_ (id VARCHAR(_<varcharValue>_) PRIMARY KEY, type VARCHAR(__<varcharValue>__) NOT NULL, data VARCHAR(_<varcharValue>_) NULL);` +

[NOTE]
====
The amount of space that you allocate to the `VARCHAR` parameter of the `id` variable must be sufficient to accommodate the size of the ID strings of signals sent to the signaling table. +
If the size of an ID exceeds the available space, the connector cannot process the signal.
====

The following example shows a `CREATE TABLE` command that creates a three-column `debezium_signal` table:

[source,sql]
----
CREATE TABLE debezium_signal (id VARCHAR(42) PRIMARY KEY, type VARCHAR(32) NOT NULL, data VARCHAR(2048) NULL);
----

// Type: procedure
// Title: Enabling the {prodname} Kafka signaling channel
[id="debezium-signaling-enabling-kafka-signaling-channel"]
== Enabling Kafka signaling channel

You can enable the Kafka signaling channel by adding it to the `signal.enabled.channels` configuration property, and then adding the name of the topic that receives signals to the `signal.kafka.topic` property.

=== Message format

The key of the Kafka message must match the value of the `topic.prefix` connector configuration option.

The value is a JSON object with `type` and `data` fields.

When the signal type is set to `execute-snapshot`, the `data` field must include the fields that are listed in the following table:

.Execute snapshot data fields
[cols="2,2,6",options="header"]
|===
|Field | Default | Value

|`type`
|`incremental`
| The type of the snapshot to run. 
Currently {prodname} supports only the `incremental` type.

|`data-collections`
|_N/A_
| An array of comma-separated regular expressions that match the fully-qualified names of the tables to include in the snapshot. +
Specify the names by using the same format as is required for the xref:{context}-property-signal-data-collection[signal.data.collection] configuration option.

|`additional-condition`
|_N/A_
| An optional string that specifies a condition that the connector evaluates to designate a subset of records to include in a snapshot.
|===

The following example shows a typical `execute-snapshot` Kafka message:

----
Key = `test_connector`

Value = `{"type":"execute-snapshot","data": {"data-collections": ["schema1.table1", "schema1.table2"], "type": "INCREMENTAL"}}`
----

// Type: concept
[id="debezium-custom-signaling-channel"]
== Custom signaling channel
The signaling mechanism is designed to be extensible. 
You can implement channels as needed to send signals to {prodname} in a manner that works best in your environment.

Adding a signaling channel involves several steps:

1. xref:debezium-signaling-enabling-custom-signaling-channel[Create a Java project for the channel] to implement the channel, and xref:debezium-signaling-core-module-dependencies[add `{prodname} Core` as a dependency].
2. xref:deploying-a-debezium-custom-signaling-channel[Deploy the custom signaling channel].
3. xref:configuring-connectors-to-use-a-custom-signaling-channel[Enable connectors to use the custom signaling channel by modifying the connector configuration].

// Type: procedure
// Title: Providing a custom {prodname} signaling channel
// ModuleID: debezium-signaling-providing-a custom-signaling-channel
[id="debezium-signaling-enabling-custom-signaling-channel"]
=== Provide custom signaling channel

Custom signaling channels are Java classes that implement the `io.debezium.pipeline.signal.channels.SignalChannelReader` service provider interface (SPI).
For example:

[source,java,indent=0]
----
public interface SignalChannelReader {

    String name(); // <1>

    void init(CommonConnectorConfig connectorConfig); // <2>

    List<SignalRecord> read(); // <3>

    void close(); // <4>
}
----
<1> The name of the reader. 
To enable {prodname} to use the channel, specify this name in the connector's `signal.enabled.channels` property.
<2> Initializes specific configuration, variables, or connections that the channel requires.
<3> Reads signal from the channel. 
The `SignalProcessor` class calls this method to retrieve the signal to process.
<4> Closes all allocated resources. 
{prodname} calls this methods when the connector is stopped.

// Type: concept
[id="debezium-signaling-core-module-dependencies"]
=== {prodname} core module dependencies

A custom signaling channel Java project has compile dependencies on the {prodname} core module.
You must include these compile dependencies in your project's `pom.xml` file, as shown in the following example:

[source,xml]
----
<dependency>
    <groupId>io.debezium</groupId>
    <artifactId>debezium-core</artifactId>
    <version>${version.debezium}</version> // <1>
</dependency>
----
<1> `${version.debezium}` represents the version of the {prodname} connector.

Declare your implementation in the `META-INF/services/io.debezium.pipeline.signal.channels.SignalChannelReader` file.

// Type: procedure
[id="deploying-a-debezium-custom-signaling-channel"]
=== Deploying a custom signaling channel

.Prerequisites
* You have a custom signaling channel Java program.

.Procedure
* To use a custom signaling channel with a {prodname} connector, export the Java project to a JAR file, and copy the file to the directory that contains the JAR file for each {prodname} connector that you want to use it with. +
 +
For example, in a typical deployment, the {prodname} connector files are stored in subdirectories of a Kafka Connect directory (`/kafka/connect`), with each connector JAR in its own subdirectory (`/kafka/connect/debezium-connector-db2`, `/kafka/connect/debezium-connector-mysql`, and so forth).

NOTE: To use a custom signaling channel with multiple connectors, you must place a copy of the custom signaling channel JAR file in the subdirectory for each connector.

// Type: procedure
[id="configuring-connectors-to-use-a-custom-signaling-channel"]
=== Configuring connectors to use a custom signaling channel

Add the name of the custom signaling channel to the `signal.enabled.channels` configuration property.

// Type: concept
// ModuleID: debezium-signaling-types-of-signal-actions
// Title: Types of {prodname} signal actions
== Signal actions

You can use signaling to initiate the following actions:

* xref:debezium-signaling-logging[Add messages to the log].
* xref:debezium-signaling-ad-hoc-snapshots[Trigger ad hoc snapshots].
* xref:debezium-signaling-stop-ad-hoc-snapshots[Stop execution of an ad hoc snapshot].
* xref:debezium-signaling-pause-incremental-snapshots[Pause incremental snapshots].
* xref:debezium-signaling-resume-incremental-snapshots[Resume incremenal snapshots].

Some signals are not compatible with all connectors.

// Type: concept
[id="debezium-signaling-logging"]
=== Logging signals

You can request a connector to add an entry to the log by creating a signaling table entry with the `log` signal type.
After processing the signal, the connector prints the specified message to the log.
Optionally, you can configure the signal so that the resulting message includes the streaming coordinates.

[id="debezium-signaling-example-of-a-logging-record"]
.Example of a signaling record for adding a log message
[cols="1,9,9",options="header"]
|===
|Column | Value | Description

|id
|`924e3ff8-2245-43ca-ba77-2af9af02fa07`
|

|type
|`log`
|The action type of the signal.

|data
|`{"message": "Signal message at offset {}"}`
| The `message` parameter specifies the string to print to the log. +
If you add a placeholder (`{}`) to the message, it is replaced with streaming coordinates.
|===

// Type: concept
[id="debezium-signaling-ad-hoc-snapshots"]
=== Ad hoc snapshot signals

You can request a connector to initiate an ad hoc snapshot by creating a signal with the `execute-snapshot` signal type.
After processing the signal, the connector runs the requested snapshot operation.

Unlike the initial snapshot that a connector runs after it first starts, an ad hoc snapshot occurs during runtime, after the connector has already begun to stream change events from a database.
You can initiate ad hoc snapshots at any time.

Ad hoc snapshots are available for the following {prodname} connectors:

* Db2
ifdef::community[]
* MongoDB
endif::community[]
* MySQL
* Oracle
* PostgreSQL
* SQL Server

[id="debezium-signaling-example-of-an-ad-hoc-signal-record"]
.Example of an ad hoc snapshot signal record
[cols="1,9",options="header"]
|===
|Column | Value

|id
|`d139b9b7-7777-4547-917d-e1775ea61d41`

|type
|`execute-snapshot`

|data
|`{"data-collections": ["public.MyFirstTable", "public.MySecondTable"]}`

|===

[id="debezium-signaling-example-of-an-ad-hoc-signal-message"]
.Example of an ad hoc snapshot signal message
[cols="1,9",options="header"]
|===
|Key | Value

|test_connector
|`{"type":"execute-snapshot","data": {"data-collections": ["public.MyFirstTable"], "type": "INCREMENTAL", "additional-condition":"color=blue AND brand=MyBrand"}}`

|===


For more information about ad hoc snapshots, see the _Snapshots_ topic in the documentation for your connector.


.Additional resources

* {link-prefix}:{link-db2-connector}#db2-ad-hoc-snapshots[Db2 connector ad hoc snapshots]
ifdef::community[]
* {link-prefix}:{link-mongodb-connector}#mongodb-ad-hoc-snapshots[MongoDB connector ad hoc snapshots]
endif::community[]
* {link-prefix}:{link-mysql-connector}#mysql-ad-hoc-snapshots[MySQL connector ad hoc snapshots]
* {link-prefix}:{link-oracle-connector}#oracle-ad-hoc-snapshots[Oracle connector ad hoc snapshots]
* {link-prefix}:{link-postgresql-connector}#postgresql-ad-hoc-snapshots[PostgreSQL connector ad hoc snapshots]
* {link-prefix}:{link-sqlserver-connector}#sqlserver-ad-hoc-snapshots[SQL Server connector ad hoc snapshots]

[id="debezium-signaling-stop-ad-hoc-snapshots"]
=== Ad hoc snapshot stop signals

You can request a connector to stop an in-progress ad hoc snapshot by creating a signal table entry with the `stop-snapshot` signal type.
After processing the signal, the connector will stop the current in-progress snapshot operation.

You can stop ad hoc snapshots for the following {prodname} connectors:

* Db2
ifdef::community[]
* MongoDB
endif::community[]
* MySQL
* Oracle
* PostgreSQL
* SQL Server

[id="debezium-signaling-example-of-a-stop-ad-hoc-signal-record"]
.Example of a stop ad hoc snapshot signal record
[cols="1,9",options="header"]
|===
|Column | Value

|id
|`d139b9b7-7777-4547-917d-e1775ea61d41`

|type
|`stop-snapshot`

|data
|`{"type":"INCREMENTAL", "data-collections": ["public.MyFirstTable"]}`

|===

You must specify the `type` of the signal.
The `data-collections` field is optional.
Leave the `data-collections` field blank to request the connector to stop all activity in the current snapshot.
If you want the incremental snapshot to proceed, but you want to exclude specific collections from the snapshot, provide a comma-separated list of the names of the collections or regular expressions to exclude.
After the connector processes the signal, the incremental snapshot proceeds, but it excludes data from the collections that you specify.

// Type: concept
[id="debezium-signaling-incremental-snapshots"]
=== Incremental snapshots

Incremental snapshots are a specific type of ad hoc snapshot.
In an incremental snapshot, the connector captures the baseline state of the tables that you specify, similar to an initial snapshot.
However, unlike an initial snapshot, an incremental snapshot captures tables in chunks, rather than all at once.
The connector uses a watermarking method to track the progress of the snapshot.

By capturing the initial state of the specified tables in chunks rather than in a single monolithic operation, incremental snapshots provide the following advantages over the initial snapshot process:

* While the connector captures the baseline state of the specified tables, streaming of near real-time events from the transaction log continues uninterrupted.
* If the incremental snapshot process is interrupted, it can be resumed from the point at which it stopped.
* You can initiate an incremental snapshot at any time.

[id="debezium-signaling-pause-incremental-snapshots"]
==== Incremental snapshot pause signals

You can request a connector to pause an in-progress incremental snapshot by creating a signal table entry with the `pause-snapshot` signal type.
After processing the signal, the connector will stop pause current in-progress snapshot operation.
Therefor it's not possible to specify the data collection as the snapshot processing will be paused in position where it is in time of processing of the signal.

You can pause incremental snapshots for the following {prodname} connectors:

* Db2
ifdef::community[]
* MongoDB
endif::community[]
* MySQL
* Oracle
* PostgreSQL
* SQL Server

[id="debezium-signaling-example-of-a-pause-incremental-signal-record"]
.Example of a pause incremental snapshot signal record
[cols="1,9",options="header"]
|===
|Column | Value

|id
|`d139b9b7-7777-4547-917d-e1775ea61d41`

|type
|`pause-snapshot`

|===

You must specify the `type` of the signal.
The `data` field is ignored.

[id="debezium-signaling-resume-incremental-snapshots"]
==== Incremental snapshot resume signals

You can request a connector to resume a paused incremental snapshot by creating a signal table entry with the `resume-snapshot` signal type.
After processing the signal, the connector will resume previously paused snapshot operation.

You can resume incremental snapshots for the following {prodname} connectors:

* Db2
ifdef::community[]
* MongoDB
endif::community[]
* MySQL
* Oracle
* PostgreSQL
* SQL Server

[id="debezium-signaling-example-of-a-resume-incremental-signal-record"]
.Example of a resume incremental snapshot signal record
[cols="1,9",options="header"]
|===
|Column | Value

|id
|`d139b9b7-7777-4547-917d-e1775ea61d41`

|type
|`resume-snapshot`

|===

You must specify the `type` of the signal.
The `data` field is ignored.

For more information about incremental snapshots, see the _Snapshots_ topic in the documentation for your connector.

.Additional resources

* {link-prefix}:{link-db2-connector}#db2-incremental-snapshots[Db2 connector incremental snapshots]
ifdef::community[]
* {link-prefix}:{link-mongodb-connector}#mongodb-incremental-snapshots[MongoDB connector incremental snapshots]
endif::community[]
* {link-prefix}:{link-mysql-connector}#mysql-incremental-snapshots[MySQL connector incremental snapshots]
* {link-prefix}:{link-oracle-connector}#oracle-incremental-snapshots[Oracle connector incremental snapshots]
* {link-prefix}:{link-postgresql-connector}#postgresql-incremental-snapshots[PostgreSQL connector incremental snapshots]
* {link-prefix}:{link-sqlserver-connector}#sqlserver-incremental-snapshots[SQL Server connector incremental snapshots]