Pencil is a simple service-registration tool for Docker and uses Consul as a backend for Service-Discovery. It syncronizes only the delta ∆ between the local state on every node (Running Docker Containers) and the remote state on Consul registry every (n) seconds. The default is set to 10s.

Pencil have been used in many different production systems and successfully synchronized hundreds of thouthands of contianers without problems.

docker run -it -v /var/run/docker.sock:/var/run/docker.sock alaa/pencil:2 <consul-registry-address>

Or build the docker image yourself:

Build the Docker image

docker build -t pencil .

Run it

docker run -d \
           -v /var/run/docker.sock:/tmp/docker.sock \
           pencil <consul-registry-address>

we need to mount the docker-engine socket into Pencil container in order to give it a privilige to observe the contaners state on the host.

You can pass array of strings to Consul Tags using container environment variables.

• All tags should start with SERVICE_

• All tags keys should be passed as upper case letters as SERVICE_TAG not SERVICE_tag

• Tag keys should match the following regex: /^SERVICE_[A-Z0-9-_]+[A-Z0-9]$/

• Tag keys should be greater than 5 chars and less than 40 chars

• Tag values should match the following regex: /^[a-z0-9-_]+[a-z0-9]$/

• Tag values should be greater than 3 chars and less than 200 chars

For example: docker run -P -e "SERVICE_CLUSTER=staging_01" nginx

By default Pencil registers every docker contianer that exposes a TCP Port. and uses the following convention for registering services on consul: <docker-image-name>-<exposed-container-port>.

For example: docker run -P nginx will let Pencil to register this container under the following name: nginx-80 as nginx image exposes TCP port 80

If you wish to register your service as an alternative name, you can pass SERVICE_NAME to the container and Pencil will use that string as the preferred name for consul registration.

For example: docker run -P -e 'SERVICE_NAME=testing-microservice' nginx

You can pass custom health check to Consul via docker container environment variables. Pencil fills out the host and port dynamically on the run time. For example:

docker run -P nginx -e "SERVICE_HEALTH_CHECK='curl -Ss http://%<host>s:%<port>s/health'"

docker run -P nginx -e "SERVICE_HEALTH_CHECK='nc -vz %<host>s %<port>s'"

Pencil replaces the host with the value of <consul-registry-address> passed to the agent and replaces the port dynamically by inspecting container.PortMapping

The following example uses SERVICE_ lables in a way to build nginx ingress rules: I will exaplain here the lables I choose to use for this particular example:

SERVICE_SCOPE: If I want to register only public services on this ingress or only the internal services.

SERVICE_TYPE : If I want to configure HTTP or TCP or WebSockets in a specific blocks.

SERVICE_NAME : The service name I choose to use as sub-domain on the ingress config: i.e: nginx virtual host

SERVICE_ACL : If I want to pass service Access List, especeially if I am exposing or registering this services on a public nginx ingress.

@upstream_domain and @server_name: These are just variables can be configured to set the domain name you wish to expose your ingress on. You can replace these variavles starting with @ to your desired static values.

{{ range services }}
{{ $tags := .Tags | join "," }}
  {{ if $tags | regexMatch "SERVICE_SCOPE=public" }}

    {{ if $tags | regexMatch "SERVICE_TYPE=(http)" }}
      {{ $services := service .Name }}
      {{ $len := len $services }}
        {{ if gt $len 0 }}
          upstream {{.Name}}.<%=@upstream_domain%> {
          {{ range service .Name }}
            server {{.Address}}:{{.Port}};
          {{end}}
          }
        {{end}}
    {{ end }}

  {{ end }}
{{ end }}

{{ range services }}
{{ $tags := .Tags | join "," }}
  {{ if $tags | regexMatch "SERVICE_SCOPE=public" }}

    {{ if $tags | regexMatch "SERVICE_TYPE=(http)" }}
      {{ $services := service .Name }}
      {{ $len := len $services }}
        {{ if gt $len 0 }}
        server {
          server_name <%=@server_names%>;

          # Fetch Service ACL
          {{range $tag := .Tags}}
            {{ if $tag | regexMatch "SERVICE_ACL" }}
              {{ $acls := (index ($tag | split "=") 1) | split "," }}
                {{ range $acls }} allow {{ . }}; {{ end }}
                  deny all;
            {{end}}
          {{end}}

          location / {
            proxy_pass http://{{.Name}}.<%=@upstream_domain%>;
          }
        }
        {{end}}
    {{ end }}
  {{ end }}
{{ end }}

• Refactoring
• Write tests