How to connect to bitnami/cassandra using GoCQL in a Docker Compose multi-container application?

Question

I'm trying to build a simplified example of an app which uses username-and-password authentication to connect to Cassandra. To this end, I've adapted the docker-compose.yml from https://hub.docker.com/r/bitnami/cassandra/ as follows:

version: '2'

services:
  cassandra:
    image: 'docker.io/bitnami/cassandra:3-debian-10'
    ports:
      - '7000:7000'
      - '9042:9042'
    volumes:
      - 'cassandra_data:/bitnami'
    environment:
      - CASSANDRA_SEEDS=cassandra
      - CASSANDRA_PASSWORD_SEEDER=yes
      - CASSANDRA_PASSWORD=cassandra
  backend:
    build: .
    environment:
      - CASSANDRA_USERNAME=cassandra
      - CASSANDRA_PASSWORD=cassandra
volumes:
  cassandra_data:
    driver: local

where my directory structure is

.
├── Dockerfile
├── docker-compose.yml
├── go.mod
├── go.sum
└── main.go

the multi-stage Dockerfile is

FROM golang AS builder
WORKDIR /go/src/app/
COPY . .
RUN GO111MODULE=on CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

FROM alpine:latest  
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=builder /go/src/app/app .
CMD ["./app"]  

and main.go attempts to create a Cassandra session using gocql:

package main

import (
    "log"
    "os"

    "github.com/gocql/gocql"
)

func main() {
    cluster := gocql.NewCluster("cassandra")
    cluster.Authenticator = gocql.PasswordAuthenticator{
        Username: os.Getenv("CASSANDRA_USERNAME"),
        Password: os.Getenv("CASSANDRA_PASSWORD"),
    }

    if _, err := cluster.CreateSession(); err != nil {
        log.Fatalf("CreateSession: %v", err)
    }
}

The username and password have both been set to the default values of "cassandra" as in the "Connecting to other containers" example in https://hub.docker.com/r/bitnami/cassandra/.

The problem is that if I try to run this multi-container application using docker-compose build followed by docker-compose up, I get the following error:

backend_1    | 2020/11/04 15:10:34 CreateSession: gocql: unable to create session: unable to discover protocol version: dial tcp 172.18.0.3:9042: connect: connection refused

I don't understand what is going wrong here: the hostname, port number, username, and password all seem correct. Any ideas why the connection is refused?

Answer 1

Cassandra takes a few seconds to startup and begin accepting connections; backend is attempting to connect before it is ready. Your configuration works OK (for me) if you start up backend after a small delay (using docker-compose start backend). Modifying backend to retry the connection works reliably for me i.e.

for {
    _, err := cluster.CreateSession()
    if  err == nil {
        break
    }
    log.Printf("CreateSession: %v", err)
    time.Sleep(time.Second)
}
log.Printf("Connected OK")

Logs:

backend_1    | 2020/11/04 17:53:23 CreateSession: gocql: unable to create session: unable to discover protocol version: dial tcp 172.26.0.2:9042: connect: connection refused
backend_1    | 2020/11/04 17:53:24 CreateSession: gocql: unable to create session: unable to discover protocol version: dial tcp 172.26.0.2:9042: connect: connection refused
.......
backend_1    | 2020/11/04 17:53:36 CreateSession: gocql: unable to create session: unable to discover protocol version: dial tcp 172.26.0.2:9042: connect: connection refused
backend_1    | 2020/11/04 17:53:37 Connected OK