machbase-neo
MQTT API
MQTT Write API

MQTT Write API

The examples below shows how to efficiently write data with mqtt client (mosquitto_pub). The destination topic should be db/append/+table_name.

sequenceDiagram
    CLIENT->> SERVER: CONNECT
    activate SERVER
    SERVER -->> CLIENT: CONNACK
    deactivate SERVER
    loop
        CLIENT ->> SERVER: PUBLISH 'db/append/{table_name}'
        activate SERVER
        SERVER -->> CLIENT: PUBACK
        deactivate SERVER
    end
    CLIENT->> SERVER: DISCONNECT

Topic

The topic to write data is named with table name of the destination.

The full syntax of the topic is

db/{method}/{table}:{format}:{compress}

method

There are two methods for writing data - append and write. The append is recommened for the general situation of MQTT environment.

  • append writing data in append mode
  • write writing data in INSERT sql statement.

format

Current version of machbase-neo supports json and csv. The default format is json

  • json
  • csv

compress

Currently gzip is supported.

examples

  • db/append/EXAMPLE means writing data to the table EXAMPLE in append method and the payload is JSON.

  • db/append/EXAMPLE:json is equivalent the example above. The last :json part can be omitted, because json is the default format.

  • db/append/EXAMPLE:json:gzip means writing data to the table EXAMPLE in append method and the payload is gzip compressed JSON.

  • db/append/EXAMPLE:csv means writing data to the table EXAMPLE in append method and the payload is CSV.

  • db/write/EXAMPLE:csv means writing data to the table EXAMPLE with INSERT INTO... SQL statement and the payload is CSV.

  • db/write/EXAMPLE:csv:gzip means writing data to the table EXAMPLE with INSERT INTO... SQL statement and the payload is gzip compressed CSV.

Payload formats

Compose the MQTT topic with table name, payload format and compression separated by colon(:) to use the payload other than plain JSON.

📌
In this example, we use mosquitto_pub just for demonstration. Since it makes a connection to MQTT server and close when it finishes to publish a single message. You will barely see performance gains of append feature against HTTP write api.
Use this MQTT method only when a client can keep a connection relatively long time and send multiple messages.

JSON

PUBLISH multiple records

mosquitto_pub -h 127.0.0.1 -p 5653 \
    -t db/append/EXAMPLE \
    -f ./mqtt-data.json
  • mqtt-data.json
[
    [ "my-car", 1670380342000000000, 32.1 ],
    [ "my-car", 1670380343000000000, 65.4 ],
    [ "my-car", 1670380344000000000, 76.5 ]
]

Payload form of above example is array of tuple (array of array in JSON), it appends the table with multiple records received through a mqtt message. It is also possible to publish single tuple like below. Machbase Neo accepts both types of payload via mqtt.

PUBLISH single record

mosquitto_pub -h 127.0.0.1 -p 5653 \
    -t db/append/EXAMPLE \
    -m '[ "my-car", 1670380345000000000, 87.6 ]'

Since MQTT is connection oriented protocol, a client program can continuously send data while it keeps the same mqtt session. It is the real benefit for using MQTT instead of HTTP for writing data.

JSON gzip

Topic = Table + ‘:’ + “json:gzip”

mosquitto_pub -h 127.0.0.1 -p 5653 \
    -t db/append/EXAMPLE:json:gzip \
    -f mqtt-data.json.gz

CSV

Topic = Table + ‘:’ + “csv”

Since there is no way to tell machbase-neo whether the first line is header or data. The payload should not contain header.

mosquitto_pub -h 127.0.0.1 -p 5653 \
    -t db/append/EXAMPLE:csv \
    -f mqtt-data.csv
my-car,1670380346000000000,87.7
my-car,1670380347000000000,98.6
my-car,1670380348000000000,99.9

CSV gzip

Topic = Table + “:csv:gzip”

mosquitto_pub -h 127.0.0.1 -p 5653 \
    -t db/append/EXAMPLE:csv:gzip \
    -f mqtt-data.csv.gz

PUBLISH for INSERT INTO…

Topic db/write/{table} is for INSERT.

📌
It is strongly recommended using db/append/{table} for the better performance through MQTT. Refer this example of db/write/{table} only for the inevitable situation.
mosquitto_pub -h 127.0.0.1 -p 5653 \
    -t db/write/EXAMPLE \
    -f data-write.json

Since db/write works in INSERT INTO... SQL statement, it is required the columns in json payload. The example of data-write.json is below.

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{
  "data": {
    "columns": ["name", "time", "value"],
    "rows": [
      [ "wave.pi", 1687481466000000000, 1.2345],
      [ "wave.pi", 1687481467000000000, 3.1415]
    ]
  }
}

PUBLISH to tql

Topic db/tql/{file.tql} is for transforming.

When the data transforming is required for writing into the database, prepare the proper tql script and publish the data to the topic named db/tql/+{tql_file.tql}.

Please refer to the As Writing API for the writing data via MQTT and tql.

Max message size

The maximum size of payload in a PUBLISH message is 256MB by MQTT specification. If a malicious or malfunctioning client sends large messages continuously it can consume all of network bandwidth and computing resource of server side then it may lead server to out of service status. It is good practice to set max message limit as just little more than what client applications demand. The default mqtt max message size is 1MB (1048576), it can be adjusted by command line flag like below or MaxMessageSizeLimit in the configuration file.

machbase-neo serve --mqtt-max-message 1048576
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