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Appendix #2: Tips and tricks

Making this tutorial took quite a lot of time for us. We want to collect some useful tips and tricks here for those who decide to play with the final example. Feel free to contribute if you find something which could help others.

Point to Centrifugo running on host (outside Docker)​

We did this ourselves while experimenting and measuring latency numbers in different scenarios. If you want to run the example, but need to point backend or Nginx to look at Centrifugo on your machine outside Docker, then you can use:

  • On Linux run ifconfig and find docker0 interface – use its ip address to point to Centrifugo. In our case it was 172.17.0.1, so we pointed Nginx to 172.17.0.1:8000 upstream (Centrifugo runs on port 8000 by default), and in Django code used http://172.17.0.1:8000/api/broadcast endpoint. Also, make sure you are using "address": "0.0.0.0" in Centrifugo configuration
  • On Macos – use host.docker.internal special name, i.e. host.docker.internal:8000 in Nginx and http://host.docker.internal:8000/api/broadcast as API endpoint for Django code.

Connect to PosgreSQL​

Run from within example repo root:

docker compose exec db psql postgresql://grandchat:grandchat@localhost:5432/grandchat

Inspect dockerized Kafka state​

List current Kafka topics (run from within example repo root):

docker compose exec kafka kafka-topics --bootstrap-server kafka:9092 --list

Describe the state of Kafka topic:

docker compose exec kafka kafka-topics --bootstrap-server kafka:9092 --describe --topic postgres.public.chat_cdc

Show state of consumer group – partitions, lag, offsets (centrifugo is a name of consumer group in our case):

docker compose exec kafka kafka-consumer-groups --bootstrap-server kafka:9092 --describe --group centrifugo

Tail new messages in topic:

docker compose exec kafka kafka-console-consumer --bootstrap-server kafka:9092 --topic postgres.public.chat_cdc

Pause Kafka Connect​

Or any other service in Docker compose when you need to test failure scenarios (use name of service from docker-compose.yml):

docker compose pause connect

To run again:

docker compose unpause connect