Docker Stack Swarm - Service Replicas are not spread for Mutli Service Stack - docker

Question

I have deployed a stack with a of 4 services on two hosts (docker compose version 3).
The services are Elasticsearch, Kibana. Redis, Visualiser and finally my Web App. I have't set any resource restrictions yet.
I spun two virtual host via docker-machine , one with 2GB and one with 1GB.
Then I increased the replicas of my web app to 2 replicas, which resolved to the following distribution:
Host1 (Master):
Kibana, Redis, Web App, Visualiser, WebApp
Host2 (Worker):
Elasticsearch
Why is the Swarm Manager distributing both Web App Containers to the same host. Wouldn't it be smarter if Web App is distributed to both hosts?
Besides node tagging I couldn't find any other way in the docs to influence the distribution.
Am I missing something?
Thanks
Bjorn
docker-compose.yml
version: "3"
services:
visualizer:
image: dockersamples/visualizer:stable
ports:
- "8080:8080"
volumes:
- "/var/run/docker.sock:/var/run/docker.sock"
deploy:
placement:
constraints: [node.role == manager]
networks:
- webnet
elasticsearch:
image: docker.elastic.co/elasticsearch/elasticsearch:5.4.3
environment:
ES_JAVA_OPTS: -Xms1g -Xmx1g
ulimits:
memlock: -1
nofile:
hard: 65536
soft: 65536
nproc: 65538
deploy:
resources:
limits:
cpus: "0.5"
memory: 1g
volumes:
- esdata:/usr/share/elasticsearch/data
ports:
- 9200:9200
- 9300:9300
networks:
- webnet
web:
# replace username/repo:tag with your name and image details
image: bjng/workinseason:swarm
deploy:
replicas: 2
restart_policy:
condition: on-failure
ports:
- "80:6000"
networks:
- webnet
kibana:
image: docker.elastic.co/kibana/kibana:5.4.3
deploy:
placement:
constraints: [node.role == manager]
ports:
- "5601:5601"
networks:
- webnet
redis:
image: "redis:alpine"
networks:
- webnet
volumes:
esdata:
driver: local
networks:
webnet:

Answer 1

Docker schedules tasks (containers) based on available resources; if two nodes have enough resources, the container can be scheduled on either one.
Recent versions of Docker use "HA" scheduling by default, which means that tasks for the same service are spread over multiple nodes, if possible (see this pull request) https://github.com/docker/swarmkit/pull/1446