pgwalstreams

Event sourcing from Postgresql database to Nats Jetstreams in kubernetes with wal-listener:

Overview

A service that helps implement the Event-driven architecture.

To maintain the consistency of data in the system, we will use transactional messaging - publishing events in a single transaction with a domain model change.

The service allows you to subscribe to changes in the PostgreSQL database using its logical decoding capability and publish them to the NATS Jetstreams server.

Logic of work

To receive events about data changes in our PostgreSQL DB   we use the standard logic decoding module (pgoutput) This module converts changes read from the WAL into a logical replication protocol.   And we already consume all this information on our side. Then we filter out only the events we need and publish them in the queue

Event publishing

NATS Jetstreams is used as a message broker. Service publishes the following structure. The name of the topic for subscription to receive messages is formed from:

• the topic prefix
• the name of the database
• the name of the table
• the action on the record

Message structure: prefix.schema_table.table.action

Example: db.public.users.insert

{
	ID        uuid.UUID   # unique ID           
	Schema    string                 
	Table     string                 
	Action    string                 
	Data      map[string]interface{} 
	EventTime time.Time   # commit time          
}

Filter configuration example

database:
  filter:
    tables:
      users:
        - insert
        - update

This filter means that we only process events occurring with the users table, and in particular insert and update data.

Filter tables from postgresql events is updated when database.filter config is updated via k8s configmap / docker-compose volume.

DB setting

You must make the following settings in the db configuration (postgresql.conf)

• wal_level >= “logical”
• max_replication_slots >= 1

The publication & slot created automatically when the service starts (for all tables and all actions). You can delete the default publication and create your own (name: wal-listener) with the necessary filtering conditions, and then the filtering will occur at the database level and not at the application level.

https://www.postgresql.org/docs/current/sql-createpublication.html

If you change the publication, do not forget to change the slot name or delete the current one.

Tutorial

Requirements

• kubectl
• k3d / kind
• helm

Deploy

• Start local k8s:

k3d cluster create -c deploy/k8s/k3d.yaml
export KUBECONFIG=$(k3d kubeconfig write poc)
kubectl cluster-info

• Helm-charts repositories:

helm repo add bitnami https://charts.bitnami.com/bitnami
helm repo add nats https://nats-io.github.io/k8s/helm/charts/
helm repo update

• Create postgresql resouces:

kubectl create configmap postgresql-conf --from-file=deploy/postgresql/postgresql.conf
helm upgrade --install postgres -f deploy/k8s/postgres-values.yaml bitnami/postgresql

• Create nats resouces:

kubectl apply -f https://raw.githubusercontent.com/nats-io/nack/v0.6.0/deploy/crds.yml
helm upgrade --install nats -f deploy/k8s/nats-values.yaml nats/nats
helm upgrade --install nack -f deploy/k8s/nack-values.yaml nats/nack

• Create pgwalstreams resouces:

kubectl apply -f deploy/k8s/pgwalstreams.yaml

Develop

• Start dev container with hot-reload using okteto:

okteto up

Test

cat <<EOF | kubectl apply -f -
apiVersion: jetstream.nats.io/v1beta2
kind: Stream
metadata:
  name: db
spec:
  name: db
  subjects: ["db.>"]
  storage: memory
  maxAge: 1h
  replicas: 1
EOF

cat <<EOF | kubectl apply -f -
apiVersion: jetstream.nats.io/v1beta2
kind: Consumer
metadata:
  name: public-users
spec:
  streamName: db
  durableName: public-users
  deliverPolicy: all
  filterSubject: db.public.users.>
  maxDeliver: 20
  ackPolicy: explicit
EOF

• Adds new records to the database tables

kubectl exec -ti pod/postgresql-0 -- bash -c 'PGPASSWORD=$POSTGRES_POSTGRES_PASSWORD psql -U postgres -p 5432 -d app' < deploy/postgresql/init.sql

• Check pgwalstreams logs

kubectl logs deployment.apps/pgwalstreams

• Check nats streams queue

kubectl exec -it deploy/nats-box -- /bin/sh -l
nats-box:~# nats consumer next db public-users

• Monitoring:

  https://docs.nats.io/reference/faq#how-can-i-monitor-my-nats-cluster

kubectl exec -it deploy/nats-box -- /bin/sh -l
nats-box:~# nats-top

cat <<EOF | kubectl apply -f -
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: nats
spec:
  rules:
    - host: nats.k8s.local
      http:
        paths:
          - path: /
            pathType: ImplementationSpecific
            backend:
              service:
                name: nats
                port:
                  number: 8222
EOF

echo "127.0.0.1 nats.k8s.local" | sudo tee -a /etc/hosts

firefox http://nats.k8s.local:8080/

License

This project is licensed under the terms of the APACHE license.

Original work comes from ihippik/wal-listener