supermq/consumers/writers/README.md

Writers

Writers consume messages from the message broker, normalize them (SenML or JSON), and persist them to a storage backend. Magistrala provides two writer services:

• Postgres writer: Stores data in PostgreSQL.
• Timescale writer: Stores data in TimescaleDB and uses hypertables for time-series workloads.

Writers are optional services and are treated as plugins. Core services and the message broker must be running first. For platform dependencies, see Docker Compose.

Configuration

Values shown are from docker/.env and the add-on compose files in docker/addons/*-writer/docker-compose.yaml.

Postgres writer

Postgres Service endpoints

Variable	Description	Default
MG_POSTGRES_WRITER_LOG_LEVEL	Service log level	debug
MG_POSTGRES_WRITER_CONFIG_PATH	Config file path (subjects/transformer)	/config.toml
MG_POSTGRES_WRITER_HTTP_HOST	HTTP host	postgres-writer
MG_POSTGRES_WRITER_HTTP_PORT	HTTP port	9007
MG_POSTGRES_WRITER_HTTP_SERVER_CERT	HTTPS server certificate path	""
MG_POSTGRES_WRITER_HTTP_SERVER_KEY	HTTPS server key path	""
MG_POSTGRES_WRITER_INSTANCE_ID	Instance ID	""

Postgres Database

Variable	Description	Default
MG_POSTGRES_HOST	PostgreSQL host	postgres
MG_POSTGRES_PORT	PostgreSQL port	5432
MG_POSTGRES_USER	PostgreSQL user	supermq
MG_POSTGRES_PASS	PostgreSQL password	supermq
MG_POSTGRES_NAME	PostgreSQL database name	messages
MG_POSTGRES_SSL_MODE	PostgreSQL SSL mode	disable
MG_POSTGRES_SSL_CERT	PostgreSQL SSL client cert	""
MG_POSTGRES_SSL_KEY	PostgreSQL SSL client key	""
MG_POSTGRES_SSL_ROOT_CERT	PostgreSQL SSL root cert	""

Postgres Message broker and observability

Variable	Description	Default
SMQ_MESSAGE_BROKER_URL	Message broker URL	nats://nats:4222
SMQ_JAEGER_URL	Jaeger collector endpoint	http://jaeger:4318/v1/traces
SMQ_JAEGER_TRACE_RATIO	Trace sampling ratio	1.0
SMQ_SEND_TELEMETRY	Send telemetry to Magistrala call-home server	true

Timescale writer

Timescale Service endpoints

Variable	Description	Default
MG_TIMESCALE_WRITER_LOG_LEVEL	Service log level	debug
MG_TIMESCALE_WRITER_CONFIG_PATH	Config file path (subjects/transformer)	/config.toml
MG_TIMESCALE_WRITER_HTTP_HOST	HTTP host	timescale-writer
MG_TIMESCALE_WRITER_HTTP_PORT	HTTP port	9012
MG_TIMESCALE_WRITER_HTTP_SERVER_CERT	HTTPS server certificate path	""
MG_TIMESCALE_WRITER_HTTP_SERVER_KEY	HTTPS server key path	""
MG_TIMESCALE_WRITER_INSTANCE_ID	Instance ID	""

Timescale Database

Variable	Description	Default
MG_TIMESCALE_HOST	TimescaleDB host	timescale
MG_TIMESCALE_PORT	TimescaleDB port	5432
MG_TIMESCALE_USER	TimescaleDB user	supermq
MG_TIMESCALE_PASS	TimescaleDB password	supermq
MG_TIMESCALE_NAME	TimescaleDB database name	supermq
MG_TIMESCALE_SSL_MODE	TimescaleDB SSL mode	disable
MG_TIMESCALE_SSL_CERT	TimescaleDB SSL client cert	""
MG_TIMESCALE_SSL_KEY	TimescaleDB SSL client key	""
MG_TIMESCALE_SSL_ROOT_CERT	TimescaleDB SSL root cert	""

Timescale Message broker and observability

Timescale writer uses the same broker and telemetry variables listed for Postgres writer.

Writer config file

Both writers read a config file defined by *_WRITER_CONFIG_PATH. The default add-on config files are:

• docker/addons/postgres-writer/config.toml
• docker/addons/timescale-writer/config.toml

The config file controls subscription subjects and, for Postgres, optional transformer settings:

["subscriber"]
subjects = ["writers.>"]

[transformer]
format = "senml"
content_type = "application/senml+json"
time_fields = [
  { field_name = "seconds_key", field_format = "unix",    location = "UTC" },
  { field_name = "millis_key",  field_format = "unix_ms", location = "UTC" },
  { field_name = "micros_key",  field_format = "unix_us", location = "UTC" },
  { field_name = "nanos_key",   field_format = "unix_ns", location = "UTC" }
]

NATS uses subject writers.> and RabbitMQ uses routing key writers.# (both are handled by consumers/writers/brokers).

Features

• Message persistence: Stores incoming SenML messages into PostgreSQL or TimescaleDB.
• JSON payload support: Saves JSON payloads into dynamically created tables.
• Broker-backed ingestion: Consumes from NATS JetStream or RabbitMQ topics.
• Configurable subscription: Limits ingestion to specific writers.* subjects.
• Observability: Exposes /health and /metrics endpoints, with Jaeger tracing.

Architecture

Runtime flow

1. The message broker publishes messages under writers.*.
2. The writer loads config.toml to select subjects and transformer settings.
3. The consumer converts messages to SenML or JSON payloads.
4. The repository writes records to the target database.

Components

• Message broker adapter: consumers/writers/brokers (NATS JetStream or RabbitMQ).
• Writer services: consumers/writers/postgres and consumers/writers/timescale.
• HTTP API: consumers/writers/api exposes /health and /metrics.
• Migrations: consumers/writers/*/init.go defines the schema and indexes.

PostgreSQL schema (SenML messages)

Defined in consumers/writers/postgres/init.go:

Column	Type	Description
id	UUID	Message ID
channel	UUID	Channel ID
subtopic	VARCHAR(254)	Subtopic
publisher	UUID	Publisher (client) ID
protocol	TEXT	Protocol name
name	TEXT	SenML name
unit	TEXT	SenML unit
value	FLOAT	Numeric value
string_value	TEXT	String value
bool_value	BOOL	Boolean value
data_value	BYTEA	Data value
sum	FLOAT	Sum value
time	FLOAT	Measurement time
update_time	FLOAT	Update time

Primary key: (time, publisher, subtopic, name)

TimescaleDB schema (SenML messages)

Defined in consumers/writers/timescale/init.go:

Column	Type	Description
time	BIGINT	Measurement time
channel	UUID	Channel ID
subtopic	VARCHAR(254)	Subtopic
publisher	VARCHAR(254)	Publisher (client) ID
protocol	TEXT	Protocol name
name	VARCHAR(254)	SenML name
unit	TEXT	SenML unit
value	FLOAT	Numeric value
string_value	TEXT	String value
bool_value	BOOL	Boolean value
data_value	BYTEA	Data value
sum	FLOAT	Sum value
update_time	FLOAT	Update time

Primary key: (time, channel, subtopic, protocol, publisher, name)

Timescale writer creates a hypertable on messages and adds time-series indexes for common query paths.

JSON payload tables (dynamic)

If the transformer emits JSON payloads, the writers create a table named after the payload format:

Postgres JSON table: id UUID, created BIGINT, channel VARCHAR(254), subtopic VARCHAR(254), publisher VARCHAR(254), protocol TEXT, payload JSONB (PK: id)

Timescale JSON table: created BIGINT, channel VARCHAR(254), subtopic VARCHAR(254), publisher VARCHAR(254), protocol TEXT, payload JSONB (PK: created, publisher, subtopic)

Deployment

Build and run locally

Postgres writer:

make postgres-writer

MG_POSTGRES_WRITER_LOG_LEVEL=debug \
MG_POSTGRES_WRITER_CONFIG_PATH=./docker/addons/postgres-writer/config.toml \
MG_POSTGRES_WRITER_HTTP_PORT=9007 \
MG_POSTGRES_HOST=localhost \
MG_POSTGRES_PORT=5432 \
MG_POSTGRES_USER=supermq \
MG_POSTGRES_PASS=supermq \
MG_POSTGRES_NAME=messages \
SMQ_MESSAGE_BROKER_URL=nats://localhost:4222 \
SMQ_JAEGER_URL=http://localhost:4318/v1/traces \
./build/postgres-writer

Timescale writer:

make timescale-writer

MG_TIMESCALE_WRITER_LOG_LEVEL=debug \
MG_TIMESCALE_WRITER_CONFIG_PATH=./docker/addons/timescale-writer/config.toml \
MG_TIMESCALE_WRITER_HTTP_PORT=9012 \
MG_TIMESCALE_HOST=localhost \
MG_TIMESCALE_PORT=5432 \
MG_TIMESCALE_USER=supermq \
MG_TIMESCALE_PASS=supermq \
MG_TIMESCALE_NAME=supermq \
SMQ_MESSAGE_BROKER_URL=nats://localhost:4222 \
SMQ_JAEGER_URL=http://localhost:4318/v1/traces \
./build/timescale-writer

Docker Compose

Postgres writer add-on:

docker compose -f docker/docker-compose.yaml -f docker/addons/postgres-writer/docker-compose.yaml up

Timescale writer add-on:

docker compose -f docker/docker-compose.yaml -f docker/addons/timescale-writer/docker-compose.yaml up

Health check

curl -X GET http://localhost:9007/health \
  -H "accept: application/health+json"

Testing

go test ./consumers/writers/...

Usage

Writers do not expose a message ingestion API. Messages are written via the message broker. The HTTP API provides only health and metrics endpoints.

Endpoint	Description
GET /health	Service health check
GET /metrics	Prometheus metrics

For an in-depth explanation of Writers, see the official documentation.