MongoDB Outbox Event Router

Table of Contents

Example outbox message
Basic outbox collection
Basic configuration
Options for applying the transformation selectively
Using Avro as the payload format
Emitting messages with additional fields
Expanding escaped JSON string as JSON
Configuration options
Distributed tracing

This SMT is for use with the Debezium MongoDB connector only. For information about using the outbox event router SMT for relational databases, see 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 collection
Apply the Debezium MongoDB outbox event router single message transformation (SMT)

A Debezium connector that is configured to apply the MongoDB outbox SMT should capture changes that occur in an outbox collection only. For more information, see Options for applying the transformation selectively.

A connector can capture changes in more than one outbox collection only if each outbox collection has the same structure.

To use this SMT, operations on the actual business collection(s) and the insert into the outbox collection must be done as part of a multi-document transaction, which have been being supported since MongoDB 4.0, to prevent potential data inconsistencies between business collection(s) and outbox collection. For future update, to enable updating existing data and inserting outbox event in an ACID transaction without multi-document transactions, we have planned to support additional configurations for storing outbox events in a form of a sub-document of the existing collection, rather than an independent outbox collection.

For more information about the outbox pattern, see Reliable Microservices Data Exchange With the Outbox Pattern.

Example outbox message

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

# Kafka Topic: outbox.event.order
# Kafka Message key: "b2730779e1f596e275826f08"
# Kafka Message Headers: "id=596e275826f08b2730779e1f"
# Kafka Message Timestamp: 1556890294484
{
  "{\"id\": {\"$oid\": \"da8d6de63b7745ff8f4457db\"}, \"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 MongoDB outbox event router SMT generates the preceding message by transforming a raw Debezium change event message as in the following example:

# Kafka Message key: { "id": "{\"$oid\": \"596e275826f08b2730779e1f\"}" }
# Kafka Message Headers: ""
# Kafka Message Timestamp: 1556890294484
{
  "patch": null,
  "after": "{\"_id\": {\"$oid\": \"596e275826f08b2730779e1f\"}, \"aggregateid\": {\"$oid\": \"b2730779e1f596e275826f08\"}, \"aggregatetype\": \"Order\", \"type\": \"OrderCreated\", \"payload\": {\"_id\": {\"$oid\": \"da8d6de63b7745ff8f4457db\"}, \"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}}",
  "source": {
    "version": "2.1.4.Final",
    "connector": "mongodb",
    "name": "fulfillment",
    "ts_ms": 1558965508000,
    "snapshot": false,
    "db": "inventory",
    "rs": "rs0",
    "collection": "customers",
    "ord": 31,
    "h": 1546547425148721999
  },
  "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 collection structure and event routing based on aggregates. To customize behavior, the outbox event router SMT provides numerous configuration options.

Basic outbox collection

To apply the default MongoDB outbox event router SMT configuration, your outbox collection is assumed to have the following fields:

{
  "_id": "objectId",
  "aggregatetype": "string",
  "aggregateid": "objectId",
  "type": "string",
  "payload": "object"
}

Table 1. Descriptions of expected outbox collection fields
Field	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 collection field, set the `collection.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 documents to the outbox collection. In the first document, the value in the `aggregatetype` field is `customers`. In the second document, the value in the `aggregatetype` field is `orders`. The connector emits the first document to the `outbox.event.customers` topic. The connector emits the second document to the `outbox.event.orders` topic. To obtain this value from a different outbox collection field, 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 collection field, set the `collection.field.event.key` SMT option in the connector configuration.
`payload`	A representation of the outbox change event. The default structure is JSON. By default, the Kafka message value is solely comprised of the `payload` value. However, if the outbox event is configured to include additional fields, the Kafka message value contains an envelope encapsulating both payload and the additional fields, and each field is represented separately. For more information, see Emitting messages with additional fields. To obtain the event payload from a different outbox collection field, set the `collection.field.event.payload` SMT option in the connector configuration.
Additional custom fields	Any additional fields from the outbox collection can be added to outbox events either within the payload section or as a message header. One example could be a field `eventType` which conveys a user-defined value that helps to categorize or organize events.

Basic configuration

To configure a Debezium MongoDB connector to support the outbox pattern, configure the outbox.MongoEventRouter SMT. To obtain the default behavior of the SMT, add it to the connector configuration without specifying any options, as in the following example:

transforms=outbox,...
transforms.outbox.type=io.debezium.connector.mongodb.transforms.outbox.MongoEventRouter

Customizing the configuration

The connector might emit many types of event messages (for example, heartbeat messages, tombstone messages, or metadata messages about transactions). To apply the transformation only to events that originate in the outbox collection, define an SMT predicate statement that selectively applies the transformation to those events only.

Options for applying the transformation selectively

In addition to the change event messages that a Debezium connector emits when a database change occurs, the connector also emits other types of messages, including heartbeat messages, and metadata messages about schema changes and transactions. Because the structure of these other messages differs from the structure of the change event messages that the SMT is designed to process, it’s best to configure the connector to selectively apply the SMT, so that it processes only the intended data change messages. You can use one of the following methods to configure the connector to apply the SMT selectively:

Configure an SMT predicate for the transformation.
Use the route.topic.regex configuration option for the SMT.

Using Avro as the payload format

The MongoDB outbox event router SMT supports arbitrary payload formats. The payload field value in an outbox collection 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.connector.mongodb.transforms.outbox.MongoEventRouter
value.converter=io.debezium.converters.ByteArrayConverter

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

Note that this differs from the BinaryDataConverter suggested for other SMTs. This is due to the different approach MongoDB takes to storing byte arrays internally.

The Debezium connectors may be configured to emit heartbeat, transaction metadata, or schema change events (support varies by connector). These events cannot be serialized by the ByteArrayConverter so additional configuration must be provided so the converter knows how to serialize these events. As an example, the following configuration illustrates using the Apache Kafka JsonConverter with no schemas:

transforms=outbox,...
transforms.outbox.type=io.debezium.connector.mongodb.transforms.outbox.MongoEventRouter
value.converter=io.debezium.converters.ByteArrayConverter
value.converter.delegate.converter.type=org.apache.kafka.connect.json.JsonConverter
value.converter.delegate.converter.type.schemas.enable=false

The delegate Converter implementation is specified by the delegate.converter.type option. If any extra configuration options are needed by the converter, they can also be specified, such as the disablement of schemas shown above using schemas.enable=false.

Emitting messages with additional fields

Your outbox collection might contain fields whose values you want to add to the emitted outbox messages. For example, consider an outbox collection that has a value of purchase-order in the aggregatetype field and another field, eventType, whose possible values are order-created and order-shipped. Additional fields can be added with the syntax field:placement:alias.

The allowed values for placement are: - header - envelope - partition

To emit the eventType field value in the outbox message header, configure the SMT like this:

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

The result will be a header on the Kafka message with type as its key, and the value of the eventType field as its value.

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

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

To control which partition the outbox message is produced on, configure the SMT like this:

transforms=outbox,...
transforms.outbox.type=io.debezium.transforms.outbox.EventRouter
transforms.outbox.collection.fields.additional.placement=partitionField:partition

Note that for the partition placement, adding an alias will have no effect.

Expanding escaped JSON string as JSON

By default, the payload of the Debezium outbox message is represented as a string. When the original source of the string is in JSON format, the resulting Kafka message uses escape sequences to represent the string, as shown in the following example:

# Kafka Topic: outbox.event.order
# Kafka Message key: "1"
# Kafka Message Headers: "id=596e275826f08b2730779e1f"
# Kafka Message Timestamp: 1556890294484
{
  "{\"id\": {\"$oid\": \"da8d6de63b7745ff8f4457db\"}, \"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}"
}

You can configure the outbox event router to expand the message content, converting the escaped JSON back to its original, unescaped JSON format. In the converted string, the companion schema is deduced from the original JSON document. The following examples shows the expanded JSON in the resulting Kafka message:

# Kafka Topic: outbox.event.order
# Kafka Message key: "1"
# Kafka Message Headers: "id=596e275826f08b2730779e1f"
# Kafka Message Timestamp: 1556890294484
{
  "id": "da8d6de63b7745ff8f4457db", "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
}

To enable string conversion in the transformation, set the value of collection.expand.json.payload to true and use the StringConverter as shown in the following example:

transforms=outbox,...
transforms.outbox.type=io.debezium.connector.mongodb.transforms.outbox.MongoEventRouter
transforms.outbox.collection.expand.json.payload=true
value.converter=org.apache.kafka.connect.storage.StringConverter

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
`collection.op.invalid.behavior`	`warn`	Collection	Determines the behavior of the SMT when there is an update operation on the outbox collection. Possible settings are: `warn` - The SMT logs a warning and continues to the next outbox collection document. `error` - The SMT logs an error and continues to the next outbox collection document. `fatal` - The SMT logs an error and the connector stops processing. All changes in an outbox collection are expected to be an insert or delete operation. That is, an outbox collection functions as a queue; updates to documents in an outbox collection are not allowed. The SMT automatically filters out delete operations (for removing proceeded outbox events) on an outbox collection.
`collection.field.event.id`	`_id`	Collection	Specifies the outbox collection field that contains the unique event ID. This ID will be stored in the emitted event’s headers under the `id` key.
`collection.field.event.key`	`aggregateid`	Collection	Specifies the outbox collection field that contains the event key. When this field 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.
`collection.field.event.timestamp`		Collection	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 collection field that contains the timestamp that you want to be in emitted outbox messages.
`collection.field.event.payload`	`payload`	Collection	Specifies the outbox collection field that contains the event payload.
`collection.expand.json.payload`	`false`	Collection	Specifies whether the JSON expansion of a String payload should be done. If no content found or in case of parsing error, the content is kept "as is". Fore more details, please see the expanding escaped json section.
`collection.fields.additional.placement`		Collection, Envelope	Specifies one or more outbox collection fields 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 field 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 field, 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`. Configuration examples are in emitting additional fields in Debezium outbox messages.
`collection.field.event.schema.version`		Collection, Schema	When set, this value is used as the schema version as described in the Kafka Connect Schema Javadoc.
`route.by.field`	`aggregatetype`	Router	Specifies the name of a field in the outbox collection. By default, the value specified in this field becomes a part of the name of the topic to which the connector emits the outbox messages. For an example, see the description of the expected outbox collection.
`route.topic.regex`	`(?<routedByValue>.*)`	Router	Specifies a regular expression that the outbox SMT applies in the RegexRouter to outbox collection documents. 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 `${routedByValue}` variable in the setting of the `route.topic.replacement` SMT option with the setting of the `route.by.field` outbox SMT option.
`route.topic.replacement`	`outbox.event.${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` field value in the outbox collection document. 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 field. Set the `route.topic.regex` option to a different regular expression.
`route.tombstone.on.empty.payload`	`false`	Router	Indicates whether an empty or `null` payload causes the connector to emit a tombstone event.
`tracing.span.context.field`	`tracingspancontext`	Tracing	The name of the field containing tracing span context.
`tracing.operation.name`	`debezium-read`	Tracing	The operation name representing the Debezium processing span.
`tracing.with.context.field.only`	`false`	Tracing	When `true` only events that have serialized context field should be traced.

Distributed tracing

The outbox event routing SMT has support for distributed tracing. See tracing documentation for more details.