Happy Feet

Application state management with triggers that span both urgent and non-urgent states to account for more graceful and staggered recoveries.

State Management

Sometimes healthy isn't so straight forward. Intermediate states can help fill the void.

const happy = require('happy-feet')();

// by default, happy.state === happy.STATE.HAPPY

// sometimes apps want to differentiate between unhealthy, and not-yet-healthy...
happy.state = happy.STATE.STARTING;

// app is now running
happy.state = happy.STATE.HAPPY;

// something went wrong, but we're still operational.
// eventual escalation to UNHAPPY
happy.state = happy.STATE.WARN;

// other times, maybe you just want your own custom state -- who are we to judge?
happy.state = 'BLACK_HOLE';

// DANGER WILL ROBINSON!
happy.state = happy.STATE.UNHAPPY;

// once state set to UNHAPPY it is irreversible, future states will be ignored

Usage

The core functionality of happy-feet lies in setting the state property of your happy-feet instance. This state may be set automatically by happy-feet as it monitors your process or set by your application. The value of this state can be automatically reflected at a web service URL via one of the provided middlewares. This can be used to signal to supervising processes that are monitoring this health indicator when they need to take corrective action.

You may also decide to manually use the happy-feet API to roll your own state management logic:

const happy = require('happy-feet')({ /* optional options */ });

// happy.state === happy.STATE.HAPPY by default

if (happy.state !== happy.STATE.HAPPY) {
  // do something about it
}

There are three built-in values that have a special meaning to happy-feet:

State	Meaning
happy.STATE.HAPPY	This indicates that your service is in a healthy state
happy.STATE.WARN	This means your service is having problems. The health check page will reflect an OK status, but eventually the state will automatically transition to UNHAPPY.
happy.STATE.UNHAPPY	This signals that your service instance should be considered ready to be terminated/restarted due to being in a bad state.

You can also use custom states if you're rolling your own health status implementation.

Automatic state change triggers include soft and hard limits. Soft limits trigger a warning state (a temporary successful state) indicating the eventual need to be restarted, while hard limits trigger a state of immediate urgency.

The automatic transition from the WARN state to UNHAPPY is done after a random amount of time, which is configurable with the escalationSoftLimitMin and escalationSoftLimitMax settings. The purpose of this randomness is to avoid each member of a service cluster being taken down at the same time in case of full cluster impacting faults.

Here's a full list of configuration options for happy-feet:

Option	Type	Default	Info
escalationSoftLimitMin	number	20	Minimum time (in seconds) before a WARN state may be escalated to an UNHAPPY state.
escalationSoftLimitMax	number	300	Maximum time (in seconds) before a WARN state may be escalated to an UNHAPPY state.
uncaughtExceptionSoftLimit	number	1	Number of uncaught exceptions before WARN state.
uncaughtExceptionHardLimit	number	undefined	Number of uncaught exceptions before UNHAPPY state. Disabled by default.
unhandledRejectionSoftLimit	number	undefined	Number of unhandled rejections before WARN state. Disabled by default.
unhandledRejectionHardLimit	number	undefined	Number of unhandled rejections before UNHAPPY state. Disabled by default.
rssSoftLimit	number	undefined	Memory Resident Set Size (in bytes) before WARN state. Disabled by default.
rssHardLimit	number	undefined	Memory Resident Set Size (in bytes) before UNHAPPY state. Disabled by default.
eventLoopSoftLimit	number	undefined	Event Loop delay (in ms) before WARN state. Disabled by default. Recommended value of 150 or higher.
eventLoopHardLimit	number	undefined	Event Loop delay (in ms) before UNHAPPY state. Disabled by default. Recommended value of 500 or higher.
timeLimitMin	number	undefined	Minimum time (in seconds) before UNHAPPY state. Disabled by default. Useful for periodic application restarts. Both timeLimitMin and timeLimitMax must be set to use this feature.
timeLimitMax	number	undefined	Maximum time (in seconds) before UNHAPPY state. Disabled by default. Useful for periodic application restarts. Both timeLimitMin and timeLimitMax must be set to use this feature.
logger	{ warn,error }	console	Logging interface to use when state changes occur. Defaults to use console.

A happy-feet instance has this interface:

Property	Type	Info
state	string	Get/set the current state of your process.
updateState(state, reason, code)	function	Calling this does the same thing as setting the state property, except it allows you to log a reason for the change. The code argument is there to enable identifying the reason for transitions programmatically (see events below).
destroy()	function	Use this to free the instance.

Additionally, each happy-feet instance is an EventEmitter. It emits a change event with the following parameters whenever its state changes:

Parameter	Type	Info
state	string	The new state
reason	string	The textual description of the change
code	string	Can be any value for custom implementations or manual if the state was changed through assignment. For automatic transitions can be one of the following: uncaughtExceptions, unhandledRejections, memory, eventLoop, or escalation

Connect Usage

If you've already got an (Connect) API, attach a handler like so:

const happyConnect = require('happy-feet/connect');
const handler = happyConnect({ /* options */ }, { /* optional happy options */ });

app.use(handler);

// Manually change the state
handler.happy.state = handler.happy.STATE.WARN; 

Option	Type	Default	Info
url	string	"/_health"	Route to return health status. Defaults to common Kubernetes healthcheck route.
method	string	"GET"	Method required to return health status.
errorStatus	number	500	Status code returned if in UNHAPPY state.
status	object	{}	A collection of custom overrides for status responses based on individual states.
status[STATE].statusCode	number	200|500	Status code returned for the given state. By default ${errorStatus} is returned for UNHAPPY state, otherwise 200.
status[STATE].body	string	${STATE}	Body message returned for the given state. By default ${STATE} will be returned verbatim.
status[STATE].contentType	string	text/plain	Content type to respond with.

Return Property	Type	Info
happy	Happy	Instance of Happy.

Express Usage

If you've already got an (Express) API, attach a handler like so:

const happyExpress = require('happy-feet/express');

const handler = happyExpress({ /* options */ }, { /* optional happy options */ });
app.get('/_health', handler);

// Manually change the state
handler.happy.state = handler.happy.STATE.WARN; 

Option	Type	Default	Info
errorStatus	number	500	Status code returned if in UNHAPPY state.
status	object	{}	A collection of custom overrides for status responses based on individual states.
status[STATE].statusCode	number	200|500	Status code returned for the given state. By default ${errorStatus} is returned for UNHAPPY state, otherwise 200.
status[STATE].body	string	${STATE}	Body message returned for the given state. By default ${STATE} will be returned verbatim.
status[STATE].contentType	string	text/plain	Content type to respond with.

Return Property	Type	Info
happy	Happy	Instance of Happy.

API Usage

Or if your service does not expose an API, you can use this helper to expose your healthcheck for you.

const happyApi = require('happy-feet/api');

const handler = happyApi({ /* options */ }, { /* optional happy options */ });

Option	Type	Default	Info
port	'number'	80	HTTP port to bind to.
url	string	"/_health"	Route to return health status. Defaults to common Kubernetes healthcheck route.
method	string	"GET"	Method required to return health status.
errorStatus	number	500	Status code returned if in UNHAPPY state.
status	object	{}	A collection of custom overrides for status responses based on individual states.
status[STATE].statusCode	number	200|500	Status code returned for the given state. By default ${errorStatus} is returned for UNHAPPY state, otherwise 200.
status[STATE].body	string	${STATE}	Body message returned for the given state. By default ${STATE} will be returned verbatim.
status[STATE].contentType	string	text/plan	Content type to responde with.

Return Property	Type	Info
happy	Happy	Instance of Happy.
server	http.Server	Instance of http.Server.

Advanced Healthchecks

The main advantage of Advanced Healthchecks over the default escalation threshold is that a centralized system can coordinate restarts in a more graceful and controlled manner without impact to availability.

If your system for monitoring healthchecks is capable of discerning between catastrophic (remove from LB immediately) and unhealthy (some undesirable event or threshold, operational but eventually should be restarted), happy.STATE.WARN (HTTP statusMessage of WARN) can be leveraged to gracefully handle rolling restarts in whatever fashion deemed acceptable. By default, WARN states will be automatically escalated to UNHAPPY after a period of time to avoid vulnerabilities resulting in mass concurrent crashes and ultimately impact to customers and availability of your services.