2.0.3 • Public • Published


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Module monitors consul key-value store and notifies about changes of a value or a set of values. It relies on blocking queries and detects changes as soon as consul agent receives them. This module uses consul module that must be installed too.

Please, check the full documentation below.

Table of Contents

Installation and Usage

Using npm:

$ npm install --save consul-kv-monitor consul

Using yarn:

$ yarn add consul-kv-monitor consul

After you've installed both consul and consul-kv-monitor, set it up and start the service in your code.

const consul = require('consul');
const consulClient = consul({
    host: '',
    port: 8500
const {ConsulKvMonitor, Errors} = require('consul-kv-monitor');
const monitorConfig = {
    keysPrefix: 'lcm',
    timeoutMsec: 1000,
    json: false
const monitor = new ConsulKvMonitor(monitorConfig, consulClient);
function printData(consulKvData) {
    const keys = consulKvData.getKeys();
    keys.forEach(key => {
        let value = consulKvData.getValue(key);
        if (value === Object(value)) {
            // we received json decoded data, so just stringifying to pretty output in this example
            value = JSON.stringify(value);
        const metadata = consulKvData.getMetadata(key);
        console.log(`${key} => ${value}, metadata: ${JSON.stringify(metadata)}`);
monitor.on('changed', (kvData) => {
    console.log('Some data has changed');
    console.log('Response headers:', JSON.stringify(monitor.getConsulHeaders()));
monitor.on('error', (error) => {
    if (error instanceof Errors.InvalidDataError) {
        console.log(`Oh, key "${error.extra.key}" value can not be decoded as JSON, actual value is "${error.extra.value}"`);
        console.log('And here is a raw error');
    } else {
        console.log(`Error occured, class: ${}`);
        console.log('And here is a raw error');
    .then(initialData => {
        console.log('Keys discovered on start:');
        console.log('Response headers:', JSON.stringify(monitor.getConsulHeaders()));
    .catch(err => {
        console.log(err instanceof Errors.WatchTimeoutError);

Lets add some data to kv:

$ consul kv put lcm/server-1
Success! Data written to: lcm/server-1

Then start this example. Right after initialization we may discover our key

$ node index.js
Keys discovered on start:
Response headers: {"x-consul-index":"337151414","x-consul-knownleader":"true","x-consul-lastcontact":"0"}
lcm/server-1 =>, metadata: {"LockIndex":0,"Key":"lcm/server-1","Flags":0,"Value":"","CreateIndex":337151414,"ModifyIndex":337151414}

Lets add one more key to subpath under monitoring while example is running.

$ consul kv put lcm/server-2
Success! Data written to: lcm/server-2

Monitor detects this change:

Some data has changed
Response headers: {"x-consul-index":"337156828","x-consul-knownleader":"true","x-consul-lastcontact":"0"}
lcm/server-1 =>, metadata: {"LockIndex":0,"Key":"lcm/server-1","Flags":0,"Value":"","CreateIndex":337151414,"ModifyIndex":337151414}
lcm/server-2 =>, metadata: {"LockIndex":0,"Key":"lcm/server-2","Flags":0,"Value":"","CreateIndex":337156828,"ModifyIndex":337156828}

Delete lcm/server-1 and update lcm/server-2:

$ consul kv delete lcm/server-1
Success! Deleted key: lcm/server-1
$ consul kv put lcm/server-2
Success! Data written to: lcm/server-2

Monitor detects our changes:

Some data has changed
Response headers: {"x-consul-index":"337158175","x-consul-knownleader":"true","x-consul-lastcontact":"0"}
lcm/server-2 =>, metadata: {"LockIndex":0,"Key":"lcm/server-2","Flags":0,"Value":"","CreateIndex":337156828,"ModifyIndex":337156828}
Some data has changed
Response headers: {"x-consul-index":"337158295","x-consul-knownleader":"true","x-consul-lastcontact":"0"}
lcm/server-2 =>, metadata: {"LockIndex":0,"Key":"lcm/server-2","Flags":0,"Value":"","CreateIndex":337156828,"ModifyIndex":337158295}

And finally delete all keys:

$ consul kv delete lcm/server-2
Success! Deleted key: lcm/server-2

Monitor emits changed event with an object without any keys.

Some data has changed
Response headers: {"x-consul-index":"337159004","x-consul-knownleader":"true","x-consul-lastcontact":"0"}

Monitor Config

There are few options available for the config object:

  • keysPrefix (String): a path to specific key or path to set of keys, monitor always use recurse option of the consul client
  • timeoutMsec (Number, optional, deafult: 5000): number of milliseconds to wait initial response from consul
  • json (Boolean, optional, default: false): if true tries to decode json object from stringified value from consul, make sense only if you store values as stringified objects


const monitorConfig = {
    keysPrefix: 'path/to/key',
    timeoutMsec: 1000,
    json: true
const monitor = new ConsulKvMonitor(monitorConfig, consulClient);

Constructor throws error TypeError if invalid values passes.

Start and stop


To start monitoring just call start method of the monitor.

Errors = require('consul-kv-monitor').Errors;
try {
    const initialData = await monitor.start();
} catch (err) {
    console.log(err instanceof Errors.WatchTimeoutError);

start method returns promise that may be resolved with values under monitoring or be rejected.

Promise rejects with one of the following errors (all of them are in Errors set):

  • AlreadyInitializedError if service is already started.
  • WatchTimeoutError if either initial data nor error received for timeoutMsec msec
  • WatchError on error from consul underlying method

Promise resolved only once. Rejection of promise means that watcher was stopped and no retries will be done. To receive updates you may add listeners.

After successful start monitor never gives up and tries to reconnect even after failures. Of course, it notifies about failures.


To stop monitor just call stop method. It returns monitor object itself.


This method returns false in the following scenarios:

  • before start of monitoring
  • after failure and till recovering from that failure
  • after stop of monitor

After successful start it returns true till one of the event described above occurs.

Listening of changes

Monitor emits changed event if values in consul or data's metadata changes. It's possible to catch changed event with a data without any modifications comparing to previous catch. It happens because consul's fields like ModifyIndex or LockIndex may change without modification of data.

So, it's a good idea to compare actual changes.

Anyway, monitor emits instance of ConsulKvData class that has the following methods:

  • ConsulKvData.hasKey(key): checks presence of the given key in a set of monitored keys and returns Boolean
  • ConsulKvData.getKeys(): returns an array of keys
  • ConsulKvData.getValue(key): returns decoded json data or raw string data, if key is absent returns undefined
  • ConsulKvData.getMetadata(key): returns all fields received from consul

Also, you may get ConsulKvData object by direct call to monitor at any moment after start:

const keys = monitor.getData().getKeys();

monitor.getData() returns ConsulKvData object even if there are no keys at all.


While monitor listens for changes it can lose connection with consul agent or consul agent can lose connection with a master server, so keys under monitoring may become inconsistent. Monitor will run normally and will try to recover as soon as possible using reconnection with backoff. But business logic may require to detect such situations and make some actions. At LCMApps we stop processing requests while kv connection is unhealthy.

So, monitor emits unhealthy event if it detects failure. You may still get last seen values from KV or consul headers but always remember that requested data may be stale.

If a unhealthy state is caused by consul error then monitor emits error event with WatchError instance right after unhealthy event.

After successful recovery to healthy state monitor emits healthy event and may emit changed event with updated keys.

At any time you may get health status running method explicitly:


Method returns true or false.

Also there is special case when monitor emits error events. If you pass json: true option in constructor monitor tries to decode string in value field as a JSON. But if value is not JSON monitor can't decode value and emits error event passing object of InvalidDataError class. For example, if consul KV stores 3 values not in stringified JSON format monitor emits 3 errors.

Let's use an example from the start of this readme, but with json: true option.

Set invalid JSON value

$ consul kv put lcm/server-2 '{"a":1,"b"}'
Success! Data written to: lcm/server-2

Monitor emits error

Some data has changed
Response headers: {"x-consul-index":"337175801","x-consul-knownleader":"true","x-consul-lastcontact":"0"}
Oh, key "lcm/server-2" value can not be decoded as JSON, actual value is "{"a":1,"b"}"
And here is a raw error
{ InvalidDataError: Invalid JSON of Value field of KV is received from consul, record will be skipped
    at validRecords.forEach.record (/home/vss-services-rel/mt/node_modules/consul-kv-monitor/src/Factory.js:31:31)
    at Array.forEach (<anonymous>)
    at Object.buildConsulKvData (/home/vss-services-rel/mt/node_modules/consul-kv-monitor/src/Factory.js:22:18)
    at ConsulKvMonitor._onWatcherChange (/home/vss-services-rel/mt/node_modules/consul-kv-monitor/src/ConsulKvMonitor.js:274:50)
    at emitTwo (events.js:126:13)
    at Watch.emit (events.js:214:7)
    at /home/vss-services-rel/mt/node_modules/consul/lib/watch.js:179:14
    at Consul.<anonymous> (/home/vss-services-rel/mt/node_modules/consul/lib/kv.js:71:5)
    at next (/home/vss-services-rel/mt/node_modules/papi/lib/client.js:313:25)
    at IncomingMessage.<anonymous> (/home/vss-services-rel/mt/node_modules/papi/lib/client.js:611:7)
  extra: { key: 'lcm/server-2', value: '{"a":1,"b"}' },
  name: 'InvalidDataError' }

Set correct JSON

$ consul kv put lcm/server-2 '{"a":1,"b":[]}'
Success! Data written to: lcm/server-2

Monitor sees changes

Some data has changed
Response headers: {"x-consul-index":"337176075","x-consul-knownleader":"true","x-consul-lastcontact":"0"}
lcm/server-2 => {"a":1,"b":[]}, metadata: {"LockIndex":0,"Key":"lcm/server-2","Flags":0,"Value":"{\"a\":1,\"b\":[]}","CreateIndex":337171339,"ModifyIndex":337176075}




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