Docker-compose network : how to connect to other container?

Question

Im running zabbix with docker compose with multiple containers.

I have an issue with connecting two containers to each other (see : docker containerized zabbix server monitoring same host running the zabbix server : connection refused ).

So im wondering how connection between containers works in docker-compose : do i need to use links in the docker-compose.yml ? Do I need to specify an ip adress in network in docker-compose.yml and then use this ip adress in my apps?

In particular, if i want to connect to container A ip named containerA in docker-compose.yml to container B named containerB in docker-compose.yml, can I use container name as it appears in docker ps -a ? (the container name is often not the same as the container name in docker-compose.yml) Or should I use the service name as it appears in docker-compose.yml? Or should I use links service:alias so i can use the alias in my app?

I have tried to use links but I had a circular link problem as i was linking to container to each other.

This is the yml file (notice the network alias is the same as the first service name...):

version: '3.5'
services:
 zabbix-server:
  container_name: zabbixserver
  image: zabbix/zabbix-server-pgsql:centos-6.0-latest
  ports:
   - "10051:10051"
  volumes:
   - /etc/localtime:/etc/localtime:ro
   - /etc/timezone:/etc/timezone:ro
   - ./zbx_env/usr/lib/zabbix/alertscripts:/usr/lib/zabbix/alertscripts:ro
   - ./zbx_env/usr/lib/zabbix/externalscripts:/usr/lib/zabbix/externalscripts:ro
   - ./zbx_env/var/lib/zabbix/export:/var/lib/zabbix/export:rw
   - ./zbx_env/var/lib/zabbix/modules:/var/lib/zabbix/modules:rw
   - ./zbx_env/var/lib/zabbix/enc:/var/lib/zabbix/enc:ro
   - ./zbx_env/var/lib/zabbix/ssh_keys:/var/lib/zabbix/ssh_keys:ro
   - ./zbx_env/var/lib/zabbix/mibs:/var/lib/zabbix/mibs:ro
   - snmptraps:/var/lib/zabbix/snmptraps:rw
#   - ./env_vars/.ZBX_DB_CA_FILE:/run/secrets/root-ca.pem:ro
#   - ./env_vars/.ZBX_DB_CERT_FILE:/run/secrets/client-cert.pem:ro
#   - ./env_vars/.ZBX_DB_KEY_FILE:/run/secrets/client-key.pem:ro
  ulimits:
   nproc: 65535
   nofile:
    soft: 20000
    hard: 40000
  deploy:
   resources:
    limits:
     cpus: '0.70'
     memory: 1G
    reservations:
     cpus: '0.5'
     memory: 512M
  env_file:
   - ./env_vars/.env_db_pgsql
   - ./env_vars/.env_srv
  secrets:
   - POSTGRES_USER
   - POSTGRES_PASSWORD
  depends_on:
   - postgres-server
  networks:
   zbx_net_backend:
    aliases:
     - zabbix-server
     - zabbix-server-pgsql
     - zabbix-server-centos-pgsql
     - zabbix-server-pgsql-centos
   zbx_net_frontend:
#  devices:
#   - "/dev/ttyUSB0:/dev/ttyUSB0"
  stop_grace_period: 30s
  sysctls:
   - net.ipv4.ip_local_port_range=1024 65000
   - net.ipv4.conf.all.accept_redirects=0
   - net.ipv4.conf.all.secure_redirects=0
   - net.ipv4.conf.all.send_redirects=0
  labels:
   com.zabbix.description: "Zabbix server with PostgreSQL database support"
   com.zabbix.company: "Zabbix LLC"
   com.zabbix.component: "zabbix-server"
   com.zabbix.dbtype: "pgsql"
   com.zabbix.os: "centos"
zabbix-agent:
  image: zabbix/zabbix-agent:centos-6.0-latest
  ports:
   - "10050:10050"
  volumes:
   - /etc/localtime:/etc/localtime:ro
   - /etc/timezone:/etc/timezone:ro
   - ./zbx_env/etc/zabbix/zabbix_agentd.d:/etc/zabbix/zabbix_agentd.d:ro
   - ./zbx_env/var/lib/zabbix/modules:/var/lib/zabbix/modules:ro
   - ./zbx_env/var/lib/zabbix/enc:/var/lib/zabbix/enc:ro
   - ./zbx_env/var/lib/zabbix/ssh_keys:/var/lib/zabbix/ssh_keys:ro
  deploy:
   resources:
    limits:
     cpus: '0.2'
     memory: 128M
    reservations:
     cpus: '0.1'
     memory: 64M
   mode: global
  links:
   - zabbix-server:zabbix-server
  env_file:
   - ./env_vars/.env_agent
  privileged: true
  pid: "host"
  networks:
   zbx_net_backend:
    aliases:
     - zabbix-agent
     - zabbix-agent-passive
     - zabbix-agent-centos
  stop_grace_period: 5s
  labels:
   com.zabbix.description: "Zabbix agent"
   com.zabbix.company: "Zabbix LLC"
   com.zabbix.component: "zabbix-agentd"
   com.zabbix.os: "centos"

Answer 1

Use the other container's Compose service name and the port the process inside that container is listening on. In your example, assuming the second ports: numbers are both correct, both containers should be able to access zabbix-server:10051 and zabbix-agent:10050. Also see Networking in Compose in the Docker documentation.

Do I need to use links in docker-compose.yml?

The links: option is obsolete and you should delete it if present. expose: is similarly only used by the obsolete first-generation Docker networking, and there are no consequences to deleting it from your Compose file.

Do I need to specify an IP address in networks in docker-compose.yml?

No, Docker can assign container-private IP addresses on its own. These are an internal implementation detail of Docker. It's useful to know they exist (in particular, since each container has a private IP address, multiple containers can each use the same port internally) but you never need to directly specify them or look them up.

You rarely if ever need to specify networks: { aliases: } or to override a container's generated container_name:. The docker ps names won't match what's in the Compose file but that's not a practical problem. If you need to directly manage an individual container you can e.g. docker-compose stop zabbix-server, and as previously described you can use the Compose service names for container-to-container communication.

In fact, for most practical cases, you can delete all of the networks: blocks entirely. Compose provides a network named default for you, and you don't usually need to configure anything.

So, in the file you originally show, I'd suggest deleting all of the networks:, links:, and container_name: options. The ports: are required only if you want to call into these containers from outside of Docker. Even deleting these you can use the host names and ports shown initially.