consul introduction

CP model.

consul deployment on k8s

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apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: consul
  name: consul
spec:
  selector:
    matchLabels:
      app: consul
  template:
    metadata:
      labels:
        app: consul
    spec:
      containers:
      - args:
        - agent
        - -enable-script-checks
        - -dev
        - -client
        - 0.0.0.0
        image: consul:1.8.4
        name: consul

Service registration

Configuration file method

Specify the configuration directory via -config-dir.

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consul agent -data-dir=/consul/data -config-dir=/consul/config -dev -client 0.0.0.0

Create a configuration file in the directory. As follows, create a service named web.

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{
  "service": {
    "name": "web",
    "tags": [
      "rails"
    ],
    "port": 80
  }
}

consul does not automatically monitor the directory and needs to send a SIGHUP signal to the consul process (kill -HUP {pid}) before it will reload.

The configuration can also be re-read via the command line consul reload.

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$ consul reload
Configuration reload triggered

Service discovery

DNS method

When consul is started, a dns server is started synchronously, listening on port 8600. registered services are added with the .service.consul suffix.

A record (IP address) or SRV record (IP+Port) can be requested.

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$ dig @10.244.6.25 -p 8600  web.service.consul
...
;; ANSWER SECTION:
web.service.consul. 0   IN  A   127.0.0.1
$ dig @10.244.6.25 -p 8600  web.service.consul SRV
...
;; ANSWER SECTION:
web.service.consul. 0   IN  SRV 1 1 80 consul-6c464f5d4b-gv52l.node.dc1.consul.

;; ADDITIONAL SECTION:
consul-6c464f5d4b-gv52l.node.dc1.consul. 0 IN A 127.0.0.1
consul-6c464f5d4b-gv52l.node.dc1.consul. 0 IN TXT "consul-network-segment="

API method

It can also be requested through the API method

Query all services

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$ curl http://127.0.0.1:8500/v1/catalog/services
{
    "consul": [],
    "web": [
        "rails"
    ]
}

Query the details of a service

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$ curl http://127.0.0.1:8500/v1/catalog/service/web
[
    {
        "ID": "0b617f2a-58d6-3779-268a-8583f1d56e1f",
        "Node": "consul-6c464f5d4b-gv52l",
        "Address": "127.0.0.1",
        "Datacenter": "dc1",
...

Query the health status of a service

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$ curl 127.0.0.1:8500/v1/health/service/web
[
    {
        "Node": {
            "ID": "291dec46-80c2-cfe4-e13e-10b4521e9ccc",
            "Node": "consul-655bc59456-nj5l8",
            "Address": "127.0.0.1",
            "Datacenter": "dc1",
...
        "Checks": [
            {
                "Node": "consul-655bc59456-nj5l8",
                "CheckID": "serfHealth",
                "Name": "Serf Health Status",
                "Status": "passing",
...
            }
        ]

Register service

student.json

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{
    "ID":"student-service-e5",
    "Name":"student-service",
    "Tags":[

    ],
    "Meta":{

    },
    "Port":9098,
    "Address":"127.0.0.1",
    "TaggedAddresses":{
        "lan_ipv4":{
            "Address":"127.0.0.1",
            "Port":9098
        },
        "wan_ipv4":{
            "Address":"127.0.0.1",
            "Port":9098
        }
    },
    "Weights":{
        "Passing":1,
        "Warning":1
    },
    "EnableTagOverride":false
}

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$ curl --request PUT --data @student.json "http://192.168.0.54:8500/v1/agent/service/register?replace-existing-checks=true"

Logout service

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$ curl --request PUT http://192.168.0.54:8500/v1/agent/service/deregister/student-service

Health Detection

consul can configure health monitoring for the service. Note that -enable-script-checks needs to be enabled.

As follows, the service is checked every 10 seconds to see if it is OK. If it is not normal, DNS resolution will not return an IP address.

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{
  "service": {
    "name": "web",
    "tags": [
      "rails"
    ],
    "port": 80,
    "check": {
      "args": [
        "curl",
        "localhost"
      ],
      "interval": "10s"
    }
  }
}

The status is already cirtical through API detection.

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$ curl 127.0.0.1:8500/v1/health/service/web
[
    {
        "Node": {
            "ID": "291dec46-80c2-cfe4-e13e-10b4521e9ccc",
            "Node": "consul-655bc59456-nj5l8",
            "Address": "127.0.0.1",
            "Datacenter": "dc1",
            {
                "Node": "consul-655bc59456-nj5l8",
                "CheckID": "service:web",
                "Name": "Service 'web' check",
                "Status": "critical",
...
$ curl http://10.244.6.25:8500/v1/health/service/web?passing
[]