Outbox Event Router

The outbox pattern is a way to safely and reliably exchange data between multiple (micro) services. An outbox pattern implementation avoids inconsistencies between a service’s internal state (as typically persisted in its database) and state in events consumed by services that need the same data.

To implement the outbox pattern in a Debezium application, configure a Debezium connector to:

  • Capture changes in an outbox table

  • Apply the Debezium outbox event router single message transformation (SMT)

A Debezium connector that is configured to apply the outbox SMT should capture changes in only an outbox table. A connector can capture changes in more than one outbox table only if each outbox table has the same structure.

The outbox event router SMT is under active development. The structure of the emitted message or other details might change as development progresses.

See Reliable Microservices Data Exchange With the Outbox Pattern to learn about why the outbox pattern is useful and how it works.

For an example that you can run, see the outbox pattern demo, which is in the Debezium examples repository. It includes an example of how to configure a Debezium connector to run the outbox event router SMT.

The outbox event router SMT does not support the MongoDB connector.

Example outbox message

To learn about how to configure the Debezium outbox event router SMT, consider the following example of a Debezium outbox message:

# Kafka Topic: outbox.event.order
# Kafka Message key: "1"
# Kafka Message Headers: "id=4d47e190-0402-4048-bc2c-89dd54343cdc"
# Kafka Message Timestamp: 1556890294484
{
  "{\"id\": 1, \"lineItems\": [{\"id\": 1, \"item\": \"Debezium in Action\", \"status\": \"ENTERED\", \"quantity\": 2, \"totalPrice\": 39.98}, {\"id\": 2, \"item\": \"Debezium for Dummies\", \"status\": \"ENTERED\", \"quantity\": 1, \"totalPrice\": 29.99}], \"orderDate\": \"2019-01-31T12:13:01\", \"customerId\": 123}"
}

A Debezium connector that is configured to apply the outbox event router SMT generates the above message by transforming a Debezium raw message like this:

# Kafka Message key: "406c07f3-26f0-4eea-a50c-109940064b8f"
# Kafka Message Headers: ""
# Kafka Message Timestamp: 1556890294484
{
  "before": null,
  "after": {
    "id": "406c07f3-26f0-4eea-a50c-109940064b8f",
    "aggregateid": "1",
    "aggregatetype": "Order",
    "payload": "{\"id\": 1, \"lineItems\": [{\"id\": 1, \"item\": \"Debezium in Action\", \"status\": \"ENTERED\", \"quantity\": 2, \"totalPrice\": 39.98}, {\"id\": 2, \"item\": \"Debezium for Dummies\", \"status\": \"ENTERED\", \"quantity\": 1, \"totalPrice\": 29.99}], \"orderDate\": \"2019-01-31T12:13:01\", \"customerId\": 123}",
    "timestamp": 1556890294344,
    "type": "OrderCreated"
  },
  "source": {
    "version": "0.9.3.Final",
    "connector": "postgresql",
    "name": "dbserver1-bare",
    "db": "orderdb",
    "ts_usec": 1556890294448870,
    "txId": 584,
    "lsn": 24064704,
    "schema": "inventory",
    "table": "outboxevent",
    "snapshot": false,
    "last_snapshot_record": null,
    "xmin": null
  },
  "op": "c",
  "ts_ms": 1556890294484
}

This example of a Debezium outbox message is based on the default outbox event router configuration, which assumes an outbox table structure and event routing based on aggregates. To customize behavior, the outbox event router SMT provides numerous configuration options.

Basic outbox table

To apply the default outbox event router SMT configuration, your outbox table is assumed to have the following columns:

Column        |          Type          | Modifiers
--------------+------------------------+-----------
id            | uuid                   | not null
aggregatetype | character varying(255) | not null
aggregateid   | character varying(255) | not null
type          | character varying(255) | not null
payload       | jsonb                  |
Table 1. Descriptions of expected outbox table columns

Column

Effect

id

Contains the unique ID of the event. In an outbox message, this value is a header. You can use this ID, for example, to remove duplicate messages.

To obtain the unique ID of the event from a different outbox table column, set the table.field.event.id SMT option in the connector configuration.

aggregatetype

Contains a value that the SMT appends to the name of the topic to which the connector emits an outbox message. The default behavior is that this value replaces the default ${routedByValue} variable in the route.topic.replacement SMT option.

For example, in a default configuration, the route.by.field SMT option is set to aggregatetype and the route.topic.replacement SMT option is set to outbox.event.${routedByValue}. Suppose that your application adds two records to the outbox table. In the first record, the value in the aggregatetype column is customers. In the second record, the value in the aggregatetype column is orders. The connector emits the first record to the outbox.event.customers topic. The connector emits the second record to the outbox.event.orders topic.

To obtain this value from a different outbox table column, set the route.by.field SMT option in the connector configuration.

aggregateid

Contains the event key, which provides an ID for the payload. The SMT uses this value as the key in the emitted outbox message. This is important for maintaining correct order in Kafka partitions.

To obtain the event key from a different outbox table column, set the table.field.event.key SMT option in the connector configuration.

type

A user-defined value that helps categorize or organize events.

payload

The representation of the event itself. The default structure is JSON. The content in this field becomes one of these:

  • Part of the outbox message payload.

  • If other metadata, including eventType is delivered as headers, the payload becomes the message itself without encapsulation in an envelope.

To obtain the event payload from a different outbox table column, set the table.field.event.payload SMT option in the connector configuration.

Basic configuration

To configure a Debezium connector to support the outbox pattern, configure the outbox.EventRouter SMT. For example, the basic configuration in a .properties file looks like this:

transforms=outbox,...
transforms.outbox.type=io.debezium.transforms.outbox.EventRouter

Using Avro as the payload format

The outbox event router SMT supports arbitrary payload formats. The payload column value in an outbox table is passed on transparently. An alternative to working with JSON is to use Avro. This can be beneficial for message format governance and for ensuring that outbox event schemas evolve in a backwards-compatible way.

How a source application produces Avro formatted content for outbox message payloads is out of the scope of this documentation. One possibility is to leverage the KafkaAvroSerializer class to serialize GenericRecord instances. To ensure that the Kafka message value is the exact Avro binary data, apply the following configuration to the connector:

transforms=outbox,...
transforms.outbox.type=io.debezium.transforms.outbox.EventRouter
value.converter=io.debezium.converters.ByteBufferConverter

By default, the payload column value (the Avro data) is the only message value. Configuration of ByteBufferConverter as the value converter propagates the payload column value as-is into the Kafka message value.

Emitting messages with additional fields

Your outbox table might contain columns whose values you want to add to the emitted outbox messages. For example, consider an outbox table that has a value of purchase-order in the aggregatetype column and another column, eventType, whose possible values are order-created and order-shipped. To emit the eventType column value in the outbox message header, configure the SMT like this:

transforms=outbox,...
transforms.outbox.type=io.debezium.transforms.outbox.EventRouter
transforms.outbox.table.fields.additional.placement=type:header:eventType

To emit the eventType column value in the outbox message envelope, configure the SMT like this:

transforms=outbox,...
transforms.outbox.type=io.debezium.transforms.outbox.EventRouter
transforms.outbox.table.fields.additional.placement=type:envelope:eventType

Configuration options

The following table describes the options that you can specify for the outbox event router SMT. In the table, the Group column indicates a configuration option classification for Kafka.

Table 2. Descriptions of outbox event router SMT configuration options

Option

Default

Group

Description

id

Table

Specifies the outbox table column that contains the unique event ID.

aggregateid

Table

Specifies the outbox table column that contains the event key. When this column contains a value, the SMT uses that value as the key in the emitted outbox message. This is important for maintaining correct order in Kafka partitions.

Table

By default, the timestamp in the emitted outbox message is the Debezium event timestamp. To use a different timestamp in outbox messages, set this option to an outbox table column that contains the timestamp that you want to be in emitted outbox messages.

payload

Table

Specifies the outbox table column that contains the event payload.

aggregateid

Table

Specifies the outbox table column that contains the payload ID.

Table, Envelope

Specifies one or more outbox table columns that you want to add to outbox message headers or envelopes. Specify a comma-separated list of pairs. In each pair, specify the name of a column and whether you want the value to be in the header or the envelope. Separate the values in the pair with a colon, for example:

id:header,my-field:envelope

To specify an alias for the column, specify a trio with the alias as the third value, for example:

id:header,my-field:envelope:my-alias

The second value is the placement and it must always be header or envelope.

Table, Schema

When set, this value is used as the schema version as described in the Kafka Connect Schema Javadoc.

aggregatetype

Router

Specifies the name of a column in the outbox table. The default behavior is that the value in this column becomes a part of the name of the topic to which the connector emits the outbox messages. An example is in the description of the expected outbox table.

(?<routedByValue>.*)

Router

Specifies a regular expression that the outbox SMT applies in the RegexRouter to outbox table records. This regular expression is part of the setting of the route.topic.replacement SMT option.

The default behavior is that the SMT replaces the default ${routedByValue} variable in the setting of the route.topic.replacement SMT option with the setting of the route.by.field outbox SMT option.

outbox.event​.${routedByValue}

Router

Specifies the name of the topic to which the connector emits outbox messages. The default topic name is outbox.event. followed by the aggregatetype column value in the outbox table record. For example, if the aggregatetype value is customers, the topic name is outbox.event.customers.

To change the topic name, you can:

  • Set the route.by.field option to a different column.

  • Set the route.topic.regex option to a different regular expression.

false

Router

Indicates whether an empty or null payload causes the connector to emit a tombstone event.

warn

Debezium

Determines the behavior of the SMT when there is an UPDATE operation on the outbox table. Possible settings are:

  • warn - The SMT logs a warning and continues to the next outbox table record.

  • error - The SMT logs an error and continues to the next outbox table record.

  • fatal - The SMT logs an error and the connector stops processing.

All changes in an outbox table are expected to be INSERT operations. That is, an outbox table functions as a queue; updates to records in an outbox table are not allowed. The SMT automatically filters out DELETE operations on an outbox table.