【Docker】Docker-Compose内置DNS负载均衡失效问题
Docker Compose实现负载均衡
还是对前面的例子docker-compose.yml稍微修改：
version: "3.8"
services:
flask-demo:
build:
context: .
dockerfile: Dockerfile
image: flask-demo:latest
environment:
- REDIS_HOST=redis-server
- REDIS_PASS=${REDIS_PASS}
healthcheck:
test: ["CMD", "curl", "-f", "http://localhost:5000"]
interval: 30s
timeout: 3s
retries: 3
start_period: 40s
depends_on:
- redis-server
deploy:
replicas: 3
networks:
- backend
- frontend
redis-server:
image: redis:latest
command: redis-server --requirepass ${REDIS_PASS}
networks:
- backend
nginx:
image: nginx:stable-alpine
ports:
- 8000:80
depends_on:
flask-demo:
condition: service_healthy
volumes:
- ./nginx.conf:/etc/nginx/conf.d/default.conf:ro
- ./log/nginx:/var/log/nginx
networks:
- frontend
networks:
backend:
frontend:
主要是修改flask-demo启动3个容器。
nginx.conf文件的内容如下：
server {
listen 80 default_server;
location / {
proxy_pass http://flask-demo:5000;
}
}
启动服务：
$ docker-compose up -d
Creating network "app8_backend" with the default driver
Creating network "app8_frontend" with the default driver
Creating app8_redis-server_1 ... done
Creating app8_flask-demo_1 ... done
Creating app8_flask-demo_2 ... done
Creating app8_flask-demo_3 ... done
Creating app8_nginx_1 ... done
$ docker-compose ps
Name Command State Ports
----------------------------------------------------------------------------------------------------------
app8_flask-demo_1 flask run -h 0.0.0.0 Up (healthy) 5000/tcp
app8_flask-demo_2 flask run -h 0.0.0.0 Up (healthy) 5000/tcp
app8_flask-demo_3 flask run -h 0.0.0.0 Up (healthy) 5000/tcp
app8_nginx_1 /docker-entrypoint.sh ngin ... Up 0.0.0.0:8000->80/tcp,:::8000->80/tcp
app8_redis-server_1 docker-entrypoint.sh redis ... Up 6379/tcp
访问服务：
$ curl localhost:8000
Hello Container World! I have been seen 4 times and my hostname is 448b5d70d3d8.
$ curl localhost:8000
Hello Container World! I have been seen 5 times and my hostname is 77b2a2314533.
$ curl localhost:8000
Hello Container World! I have been seen 9 times and my hostname is 4eee9c8d54f1.
从运行结果可以发现我们可以根据service的名称访问容器，Docker会使用内置的DNS服务器将service的名称解析成IP，如果service对应的容器有多个，nginx会进行负载均衡。
Docker带有一个内置的DNS服务器。默认情况下，可以通过127.0.0.11:53访问服务器。这个DNS的IP可以通过进入容器内部查看/etc/resolv.conf获得：
$ docker-compose exec nginx cat /etc/resolv.conf
nameserver 127.0.0.11
options ndots:0
负载均衡的实现
在上面的例子中我们将flask-demo容器的个数提高到6个：
$ docker-compose up -d --scale flask-demo=6
app8_redis-server_1 is up-to-date
Creating app8_flask-demo_4 ... done
Creating app8_flask-demo_5 ... done
Creating app8_flask-demo_6 ... done
app8_nginx_1 is up-to-date
$ docker-compose ps
Name Command State Ports
----------------------------------------------------------------------------------------------------------
app8_flask-demo_1 flask run -h 0.0.0.0 Up (healthy) 5000/tcp
app8_flask-demo_2 flask run -h 0.0.0.0 Up (healthy) 5000/tcp
app8_flask-demo_3 flask run -h 0.0.0.0 Up (healthy) 5000/tcp
app8_flask-demo_4 flask run -h 0.0.0.0 Up (healthy) 5000/tcp
app8_flask-demo_5 flask run -h 0.0.0.0 Up (healthy) 5000/tcp
app8_flask-demo_6 flask run -h 0.0.0.0 Up (healthy) 5000/tcp
app8_nginx_1 /docker-entrypoint.sh ngin ... Up 0.0.0.0:8000->80/tcp,:::8000->80/tcp
app8_redis-server_1 docker-entrypoint.sh redis ... Up 6379/tcp
$ docker container ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS
NAMES
6e1494379165 flask-demo:latest "flask run -h 0.0.0.0" 53 seconds ago Up 51 seconds (healthy) 5000/tcp
app8_flask-demo_6
62733bdccdb8 flask-demo:latest "flask run -h 0.0.0.0" 53 seconds ago Up 51 seconds (healthy) 5000/tcp
app8_flask-demo_5
77e74622fa4e flask-demo:latest "flask run -h 0.0.0.0" 53 seconds ago Up 51 seconds (healthy) 5000/tcp
app8_flask-demo_4
b67132189d90 nginx:stable-alpine "/docker-entrypoint.…" 2 minutes ago Up 2 minutes 0.0.0.0:8000->80/tcp, :::8000->80/tcp app8_nginx_1
448b5d70d3d8 flask-demo:latest "flask run -h 0.0.0.0" 2 minutes ago Up 2 minutes (healthy) 5000/tcp
app8_flask-demo_1
4eee9c8d54f1 flask-demo:latest "flask run -h 0.0.0.0" 2 minutes ago Up 2 minutes (healthy) 5000/tcp
app8_flask-demo_2
77b2a2314533 flask-demo:latest "flask run -h 0.0.0.0" 2 minutes ago Up 2 minutes (healthy) 5000/tcp
app8_flask-demo_3
215beaad114a redis:latest "docker-entrypoint.s…" 2 minutes ago Up 2 minutes 6379/tcp
app8_redis-server_1
再次访问我们的服务：
$ curl localhost:8000
Hello Container World! I have been seen 28 times and my hostname is 448b5d70d3d8.
$ curl localhost:8000
Hello Container World! I have been seen 29 times and my hostname is 77b2a2314533.
$ curl localhost:8000
Hello Container World! I have been seen 30 times and my hostname is 4eee9c8d54f1.
$ curl localhost:8000
Hello Container World! I have been seen 31 times and my hostname is 448b5d70d3d8.
$ curl localhost:8000
Hello Container World! I have been seen 32 times and my hostname is 77b2a2314533.
$ curl localhost:8000
Hello Container World! I have been seen 33 times and my hostname is 4eee9c8d54f1.
连续访问6次之后发现请求并没有转发到我们新启动的3个容器中，这是为什么呢？
负载均衡失效原因分析
初步怀疑是DNS解析失效，我们进入nginx容器查看DNS在解析flask-demo时解析出哪些IP：
$ docker-compose exec nginx sh
/ # apk update
fetch https://dl-cdn.alpinelinux.org/alpine/v3.14/main/x86_64/APKINDEX.tar.gz
fetch https://dl-cdn.alpinelinux.org/alpine/v3.14/community/x86_64/APKINDEX.tar.gz
v3.14.10-47-g7553b19fe26 [https://dl-cdn.alpinelinux.org/alpine/v3.14/main]
v3.14.10-42-gd8ce7b89082 [https://dl-cdn.alpinelinux.org/alpine/v3.14/community]
OK: 14983 distinct packages available
/ # apk add bind-tools
(1/11) Installing fstrm (0.6.1-r0)
(2/11) Installing krb5-conf (1.0-r2)
(3/11) Installing libcom_err (1.46.2-r1)
(4/11) Installing keyutils-libs (1.6.3-r0)
(5/11) Installing libverto (0.3.2-r0)
(6/11) Installing krb5-libs (1.18.5-r0)
(7/11) Installing json-c (0.15-r1)
(8/11) Installing protobuf-c (1.3.3-r6)
(9/11) Installing libuv (1.41.0-r0)
(10/11) Installing bind-libs (9.16.39-r0)
(11/11) Installing bind-tools (9.16.39-r0)
Executing busybox-1.33.1-r6.trigger
OK: 31 MiB in 53 packages
/ # dig flask-demo
; <<>> DiG 9.16.39 <<>> flask-demo
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 55582
;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;flask-demo. IN A
;; ANSWER SECTION:
flask-demo. 600 IN A 192.168.96.7
flask-demo. 600 IN A 192.168.96.2
flask-demo. 600 IN A 192.168.96.3
flask-demo. 600 IN A 192.168.96.4
flask-demo. 600 IN A 192.168.96.8
flask-demo. 600 IN A 192.168.96.6
;; Query time: 0 msec
;; SERVER: 127.0.0.11#53(127.0.0.11)
;; WHEN: Tue Oct 10 08:45:53 UTC 2023
;; MSG SIZE rcvd: 184
发现DNS解析出了6个IP，然后我们在nginx容器中访问flask-demo：
/ # curl flask-demo:5000
Hello Container World! I have been seen 281 times and my hostname is 77e74622fa4e.
/ # curl flask-demo:5000
Hello Container World! I have been seen 282 times and my hostname is 4eee9c8d54f1.
/ # curl flask-demo:5000
Hello Container World! I have been seen 283 times and my hostname is 62733bdccdb8.
/ # curl flask-demo:5000
Hello Container World! I have been seen 284 times and my hostname is 448b5d70d3d8.
/ # curl flask-demo:5000
Hello Container World! I have been seen 285 times and my hostname is 6e1494379165.
/ # curl flask-demo:5000
Hello Container World! I have been seen 289 times and my hostname is 77b2a2314533.
在nginx容器内访问就能访问到6个节点，那就说明nginx容器中DNS的解析没有问题，问题出在Nginx服务上。
Nginx在启动的时候会将代理服务器域名解析的ip地址缓存起来，后续不会再更新这个缓存，除非重启。
我们可以给nginx的缓存设置一个失效事件来解决这个问题，nginx.conf配置修改如下：
server {
listen 80 default_server;
location / {
resolver 127.0.0.11 valid=1s; #设置dns服务器，缓存时间改为1s
set $backend http://web:5000
proxy_pass $backend;
}
}
然后再让nginx重新加载配置，再访问服务，发现已经生效了：
$ docker-compose exec nginx nginx -s reload
$ curl localhost:8000
Hello Container World! I have been seen 414 times and my hostname is 77b2a2314533.
$ curl localhost:8000
Hello Container World! I have been seen 415 times and my hostname is 448b5d70d3d8.
$ curl localhost:8000
Hello Container World! I have been seen 417 times and my hostname is 4eee9c8d54f1.
$ curl localhost:8000
Hello Container World! I have been seen 420 times and my hostname is 77e74622fa4e.
$ curl localhost:8000
Hello Container World! I have been seen 421 times and my hostname is 6e1494379165.
$ curl localhost:8000
Hello Container World! I have been seen 422 times and my hostname is 62733bdccdb8.
docker compose参考例子
- 投票app练习：https://github.com/dockersamples/example-voting-app
- docker-compose example：https://github.com/docker/awesome-compose