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Arroyo supports Postgres as a source and sink, via Debezium. A native Postgres connector is planned for the future. Debezium is a CDC (change data capture) tool that monitors the Postgres transaction log and publishes changes to Kafka. Arroyo can read these changes from Kafka and present them as streaming tables which can be queried like any other table. Currently Debezium tables must be created via CREATE TABLE calls in SQL and must be configured with format = 'debezium_json'. Otherwise, the options are the same as for other Kafka sources.

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Postgres Sources

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Setting up Debezium

To read from Postgres via Debezium, you will first need to set up Debezium to read from your Postgres changelogs and publish them to Kafka. See the Debezium Postgres docs for more details on how to set this up. An example Debezium config coud look like this:
connector-config.json
{
    "name": "arroyo-connector",
    "config": {
        "connector.class": "io.debezium.connector.postgresql.PostgresConnector",
        "database.hostname": "localhost",
        "database.port": "5432",
        "database.user": "user",
        "database.password": "password",
        "database.dbname": "my_database",
        "tombstones.on.delete": "false",
        "table.include.list": "public.table_name",
        "topic.prefix": "debezium",
        "plugin.name": "pgoutput",
        "value.converter": "org.apache.kafka.connect.json.JsonConverter",
        "value.converter.schemas.enable": "false"
    }
}
Arroyo requires access to the full Postgres transaction log, so you must set the following option on your table:
ALTER TABLE <table_name> REPLICA IDENTITY FULL;
This will create a Kafka topic called debezium.public.table_name which will contain the changes to the public.table_name table in Postgres. To verify that this is working, you can use the kafka-console-consumer tool in your Kafka installation to read from the topic: 
kafka-console-consumer --bootstrap-server localhost:9092 --topic debezium.public.table_name
You should see a stream of JSON objects representing the changes to the table that look like this: 
{"before":null,"after":{"id":1,"name":"foo","age":42},"source":{"version":"2.3.2.Final","connector":"postgresql","name":"debezium","ts_ms":1691786140166,"snapshot":"false","db":"timescale","sequence":"[\"10642191632\",\"10642191688\"]","schema":"public","table":"table_name","txId":418604,"lsn":10642191688,"xmin":null},"op":"c","ts_ms":1691786140421,"transaction":null}
{"before":null,"after":{"id":1,"name":"bar","age":42},"source":{"version":"2.3.2.Final","connector":"postgresql","name":"debezium","ts_ms":1691786140167,"snapshot":"false","db":"timescale","sequence":"[\"10642191968\",\"10642191968\"]","schema":"public","table":"table_name","txId":418605,"lsn":10642191968,"xmin":null},"op":"u","ts_ms":1691786140424,"transaction":null}
{"before":{"id":1,"name":null,"age":null},"after":null,"source":{"version":"2.3.2.Final","connector":"postgresql","name":"debezium","ts_ms":1691786149069,"snapshot":"false","db":"timescale","sequence":"[\"10642192096\",\"10642192272\"]","schema":"public","table":"table_name","txId":418606,"lsn":10642192272,"xmin":null},"op":"d","ts_ms":1691786149093,"transaction":null}

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Creating a Debezium table in Arroyo

Once you have a Debezium topic set up, you can create a table in Arroyo that will read from that topic. This is a normal Kafka source table, but you must set the format to debezium_json and the topic to the Debezium topic you created above. 
CREATE TABLE debezium_table (
    id int,
    name string,
    age int
) WITH (
    format = 'debezium_json',
    connector = 'kafka',
    type = 'source',
    bootstrap_servers = 'localhost:9092',
    topic = 'debezium.public.table_name'
)
This will create a table called debezium_table that will read from the topic debezium.public.table_name and will have the schema id int, name string, age int. You can then query this table like any other table in Arroyo: 
SELECT * FROM debezium_table;
which should produce output like this:

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Postgres Sinks

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Setting up a Postgres sink

Arroyo can write query results to Postgres via the Debezium connector for JDBC. There are several steps to setting up a Postgres sink.

Create topic

Create a Kafka topic to write to. This can be done via the kafka-topics tool in your Kafka installation:
./kafka-topics.sh --bootstrap-server localhost:9092 --create \
    --topic my_sink

Create table

Create the destination table in Postgres:
psql
CREATE TABLE my_sink (
    time TIMESTAMP,
    product TEXT,
    price FLOAT
);

Configure Debezium

Configure the Debezium sink connector to write to the Postgres table:
connector.json
{
    "name": "jdbc-connector",
    "config": {
        "connector.class": "io.debezium.connector.jdbc.JdbcSinkConnector",
        "consumer.override.auto.offset.reset": "latest",
        "tasks.max": "1",
        "connection.url": "jdbc:postgresql://localhost/database",
        "connection.username": "user",
        "connection.password": "password",
        "database.time_zone": "UTC",
        "topics": "my_sink",
        "key.converter": "org.apache.kafka.connect.storage.StringConverter",
        "value.converter.schemas.enable": true,
        "value.converter": "org.apache.kafka.connect.json.JsonConverter"
    }
}

curl -XPOST localhost:8083/connectors -d@connector.json \
  -H "Content-Type: application/json"

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Writing to a Postgres sink

Now that we’ve set up Debezium, we can write to the sink table from Arroyo.
The Debezium connector for JDBC requires that your data has a schema associated with it. There are several mechanisms of providing a schema (including via Avro and the Confluent Schema Registry), but as of now Arroyo only supports the embedded-JSON schema format, which is why the value.converter is set to org.apache.kafka.connect.json.JsonConverter. and value.converter.schemas.enable is set to true.On the Arroyo side, you must set the format to debezium_json to indicate that the data is in this format and set 'json.include_schema' = true to include the schema in the JSON data.
A Postgres Debezium sink table is a normal Kafka sink table, with some additional options set. 
CREATE TABLE prices (
  time TIMESTAMP,
  product_id TEXT,
  price FLOAT
) WITH (
  connector = 'kafka',
  bootstrap_servers = 'localhost:9092',
  topic = 'my_sink',
  type = 'sink',
  format = 'debezium_json',
  'json.include_schema' = 'true'
);
This can be inserted into via a query like 
INSERT INTO prices
SELECT window.end_time, price, product_id FROM (
SELECT
  avg(CAST(price as FLOAT)) as price,
  hop(interval '5' second, interval '1 minute') as window,
  product_id
from tickers
GROUP BY window, product_id);
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