Proof of concept

Author: MvRens

.gitignore

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+node_modules
+
+config/*
+!config/*.example.js

README.md

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-# ConsulTemplate-NodeJS
-Like Consul-template, but different. And written in Node.js.
+# ConsulWatcher
+
+This is a much simplified version of HashiCorp's Consul-Template, because what it accomplishes is awesome but I really dislike the Go template syntax. All configuration in this version are plain JavaScript.
+
+It's main purpose is creating output files based on a Consul catalog, much like Consul-Template, but since the update handlers are just JavaScript functions you are free to do whatever you want, like calling a webservice or using a template library.

config/included.example.js

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+module.exports = async (catalog, logger) =>
+{
+  // Use catalog parameter to generate output
+};

config/index.example.js

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+const config = {
+  onUpdate: [],
+  afterUpdate: null
+};
+
+
+/*
+
+  Consul agent configuration
+    Determines the agent or server which will be queried and
+    monitored for the service catalog.
+
+    Recommended to be a local agent connected to the cluster.
+
+    Passed to the initialization of the Node Consul client.
+    For all options, see: https://github.com/silas/node-consul#init
+
+*/
+config.consul = {
+  host: 'localhost'
+}
+
+
+
+/*
+
+  Logging
+    See: https://github.com/winstonjs/winston#transports
+
+*/
+const winston = require('winston');
+
+config.logging = {
+  transports: [
+    new winston.transports.Console({
+      level: 'debug',
+      timestamp: true,
+      format: winston.format.combine(
+        winston.format.colorize(),
+        winston.format.simple()
+      )
+    })
+  ]
+};
+
+
+/*
+
+  onUpdate handlers
+    When a change occurs in the Consul catalog, each handler is called in order.
+    Callbacks may return a Promise. Note that when using multiple handlers,
+    they are not awaited immediately but at the end using Promise.all(), which
+    means the handlers effectively run in parallel.
+
+    The catalog parameter has a services property to enumerate the registered
+    services. See the Readme for the documentation of the ConsulCatalog class.
+
+    For more information about a service, including it's address and health,
+    an additional call to Consul is required. Calling these methods will result
+    in the service being watched for changes as well.
+
+    The second parameter is a reference to the Winston logger instance.
+
+*/
+const fs = require('fs').promises;
+
+
+config.onUpdate.push((catalog, logger) =>
+{
+  // Use catalog parameter to generate output
+  let output = '';
+
+  for (const service of catalog.services)
+  {
+    output +=
+`Service: ${service.name}
+  Tags:    ${JSON.stringify(service.tags)}
+  Address: ${await service.getAddress()}
+  Port:    ${await service.getPort()}
+
+`;
+  };
+
+  await fs.writeFile('example-output.txt', output);
+});
+
+
+/*
+  afterUpdate handler
+    This is a single handler which is called after all the onUpdate handlers
+    have finished, including any Promises returned.
+
+    It can be used for example to reload a proxy server after the configuration
+    changes have been written in onUpdate.
+*/
+
+config.afterUpdate = (catalog, logger) =>
+{
+  // Call a reload script
+
+}
+
+
+
+module.exports = config;

consulwatcher.service

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+[Unit]
+Description=ConsulWatcher
+Requires=network-online.target
+After=network-online.target
+
+[Service]
+Restart=on-failure
+ExecStart=/usr/local/bin/node /srv/consulwatcher/index.js
+KillSignal=SIGINT
+
+[Install]
+WantedBy=multi-user.target

index.js

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+const ConsulCatalog = require('./lib/consulcatalog');
+const logger = require('./lib/logger');
+const config = require('./config');
+
+const catalog = new ConsulCatalog(logger, config);
+
+// TODO detect if the connection is down for too long, allow a custom notification to be sent
+
+// TODO provide a way to easily switch between configs, for multiple environments