上一节部署了只有一个副本的Service，不过对于web服务，我们通常会运行多个实例，这样可以做负载均衡，同时也能提高可用性。
swarm 要实现这个目标非常简单，增加Service 的副本数就可以了。在 swarm-manager 上执行如下命令
root@host03:~# docker service scale web_server=5
web_server scaled to 5
overall progress: 5 out of 5 tasks
1/5: running [==================================================>]
2/5: running [==================================================>]
3/5: running [==================================================>]
4/5: running [==================================================>]
5/5: running [==================================================>]
verify: Service converged
root@host03:~# docker service ps web_server # 副本数增加过程中
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
jykct1jmfrte web_server.1 httpd:latest host03 Running Running 20 minutes ago
n4afxlx16tny web_server.2 httpd:latest host03 Running Running 13 seconds ago
kmtovvs8i1rk web_server.3 httpd:latest host01 Running Preparing 14 seconds ago
mlsb1ey4n65r web_server.4 httpd:latest host02 Running Preparing 14 seconds ago
a08aabmfiprh web_server.5 httpd:latest host02 Running Preparing 14 seconds ago
root@host03:~# docker service ps web_server # 副本数增加完毕
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
jykct1jmfrte web_server.1 httpd:latest host03 Running Running 21 minutes ago
n4afxlx16tny web_server.2 httpd:latest host03 Running Running 52 seconds ago
kmtovvs8i1rk web_server.3 httpd:latest host01 Running Running 29 seconds ago
mlsb1ey4n65r web_server.4 httpd:latest host02 Running Running 31 seconds ago
a08aabmfiprh web_server.5 httpd:latest host02 Running Running 31 seconds ago
root@host03:~# docker service ls # 要求副本数和实际副本数均已达到 5
ID NAME MODE REPLICAS IMAGE PORTS
9aa10i3l6xjw web_server replicated 5/5 httpd:latest
设置 swarm-manager 为仅manager模式
root@host03:~# docker node update --availability drain host03 # 将 swarm-manager（host03）设置为仅 manager
host03
root@host03:~# docker node ls # host03 AVAILABILITY 已经变成 Drain
ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS ENGINE VERSION
hvt2ez9e7zvqm2hz8nix1eke7 host01 Ready Active 18.09.6
asn5ufnogzkyqigk4mizatoer host02 Ready Active 18.09.6
h6rzavsz2vjxstwj3pytiebjb * host03 Ready Drain Leader 18.09.6
root@host03:~# docker service ps web_server # 可以看到 host03 上的容器都已 shutdown，另外在host01 上新起了两个容器
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
tznw0lrn8wh8 web_server.1 httpd:latest host01 Running Running 14 seconds ago
jykct1jmfrte \_ web_server.1 httpd:latest host03 Shutdown Shutdown 15 seconds ago
4x32c9x1hizg web_server.2 httpd:latest host01 Running Running 14 seconds ago
n4afxlx16tny \_ web_server.2 httpd:latest host03 Shutdown Shutdown 15 seconds ago
kmtovvs8i1rk web_server.3 httpd:latest host01 Running Running about an hour ago
mlsb1ey4n65r web_server.4 httpd:latest host02 Running Running about an hour ago
a08aabmfiprh web_server.5 httpd:latest host02 Running Running about an hour ago
弹性收缩，将副本数由 5 降到 3
root@host03:~# docker service scale web_server=3 # 副本数设置为 3
web_server scaled to 3
overall progress: 3 out of 3 tasks
1/3: running [==================================================>]
2/3: running [==================================================>]
3/3: running [==================================================>]
verify: Service converged
root@host03:~# docker service ps web_server # 可以查看编号为 3 和 5 的容器被删除了
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
tznw0lrn8wh8 web_server.1 httpd:latest host01 Running Running 2 minutes ago
jykct1jmfrte \_ web_server.1 httpd:latest host03 Shutdown Shutdown 2 minutes ago
4x32c9x1hizg web_server.2 httpd:latest host01 Running Running 2 minutes ago
n4afxlx16tny \_ web_server.2 httpd:latest host03 Shutdown Shutdown 2 minutes ago
mlsb1ey4n65r web_server.4 httpd:latest host02 Running Running about an hour ago