lock

README

Lock service

How it works

1. Starts and listens for HTTP requests

Responds with 423 Locked until initialized. Binding host and port are configurable.

2. Checks TCP/HTTP endpoints if configured

Waits till all dependent endpoints are accessible. Then allows acquiring of the lock. Endpoints are checked constantly. Locking is allowed only if all are OK.

3. First request(s) acquires the lock

Client gets 200 OK. Lock is acquired for a specific time. Multiple locks can be configured for parallel acquiring. Custom lock duration may be specified in the request itself.

4. Subsequent requests are denied to acquire the lock

Client gets 423 Locked until lock timeout exceeds.

That's it. Steps 2 - 4 are constantly repeated

Request custom lock duration

You can configure default lock timeout. But each client may request custom duration with GET parameter, for example, http://localhost:8888?duration=60 To acquire a lock for 60 seconds.

Dependent Endpoints check

This is useful when you need to wait for certain service(s) start before allowing starting of applications in the cluster.

• You may specify http/https endpoint, like http://myelasticsearch.local:9200 Response with HTTP code 2XX is considered as a success.
• You may specify tcp endpoint, like tcp://mymongodb:27017 Established TCP connection is considered as a success.

Configuration

You may specify additional command line options to override defaults:

Option	Default	Description
--host	0.0.0.0	Address to bind
--port	8888	Port to bind
--locks	1	Number of locks allowed to be acquired at the same time
--timeout	10	Default time until the acquired lock is released, seconds
--check	none	List of endpoints to check before allow locking, see example below
--failHc	10	Pause between endpoint checks if the previous check failed, seconds
--passHc	60	Pause between endpoint checks if the previous check succeeded, seconds

How to run locally

Example with some command line options:

go run lock/main.go --port 9000 --locks 2 --check http://myelasticsearch:9200 --check tcp://mymongodb:27017

How to deploy to Kubernetes

The preferable way is to deploy as a DaemonSet. Sample deployment YAML (notice checked endpoint):

apiVersion: extensions/v1beta1
kind: DaemonSet
metadata:
  name: startup-lock
spec:
  template:
    metadata:
      labels:
        app: startup-lock
        version: '1.0'
    spec:
      hostNetwork: true
      nodeSelector:
        kubernetes.io/role: node
      containers:
        - name: startup-lock-container
          image: ssamoilenko/startup-lock
          args: ["--port", "8888", "--locks", "1", "--check", "http://$(HOST_IP):9999"]
          ports:
            - name: http
              containerPort: 8888
          env:
            - name: HOST_IP
              valueFrom:
                fieldRef:
                  fieldPath: status.hostIP