Specification: MicroProfile Health
Version: 4.0
Status: Final
Release: November 01, 2021

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MicroProfile Health

Rationale

The Eclipse MicroProfile Health specification defines a single container runtime mechanism for validating the availability and status of a MicroProfile implementation. This is primarily intended as a machine to machine (M2M) mechanism for use in containerized environments like cloud providers. Example of existing specifications from those environments include Cloud Foundry Health Checks and Kubernetes Liveness, Readiness and Startup Probes.

In this scenario health checks are used to determine if a computing node needs to be discarded (terminated, shutdown) and eventually replaced by another (healthy) instance.

The MicroProfile Health architecture consists of three /health/ready, /health/live and /health/started endpoints in a MicroProfile runtime that respectively represent the readiness, the liveness and the startup health of the entire runtime. These endpoints are linked to health check procedures defined with specifications API and annotated respectively with @Readiness, @Liveness and @Startup annotations.

A 4th endpoint /health is also available and can be used to provide a combination of the previous endpoints.

These endpoints are expected to be associated with a configurable context, such as a web application deployment, that can be configured with settings such as port, virtual-host, security, etc. Further, MicroProfile Health defines the notion of a procedure that represents the health of a particular subcomponent of an application.

In an application, there can be zero or more procedures bound to a given health endpoint. The overall application health for a given endpoint is the logical AND of all of the procedures bound to it.

The current version of the MicroProfile Health specification does not define how the defined endpoints may be partitioned in the event that the MicroProfile runtime supports deployment of multiple applications. If an implementation wishes to support multiple applications within a MicroProfile runtime, the semantics of individual endpoints are expected to be the logical AND of all the application in the runtime. The exact details of this are deferred to a future version of the MicroProfile Health specification.

Proposed solution

The proposed solution breaks down into two parts:

A Java API to implement health check procedures
A health checks protocol and wireformat

Contributors

John Ament
Heiko Braun
Clément Escoffier
Emily Jiang
Werner Keil
Jeff Mesnil
Andrew Pielage
Heiko Rupp
Antoine Sabot-Durand
Scott Stark
Martin Stefanko
Kevin Sutter

Java API Usage

This specification provides the following API to define health check procedures.

Common API check

The main API to provide health check procedures (readiness or liveness) on the application level is the HealthCheck interface:

@FunctionalInterface
public interface HealthCheck {

    HealthCheckResponse call();
}

Applications provide health check procedures (implementation of a HealthCheck), which will be used by the runtime hosting the application to verify the healthiness of the computing node.

Different kinds of Health Checks

This specification provides different kinds of health check procedures. Difference between them is only semantic. The nature of the procedure is defined by annotating the HealthCheck procedure with a specific annotation.

Readiness checks defined with @Readiness annotation
Liveness checks defined with @Liveness annotation
Startup checks defined with @Startup annotation

A HealthCheck procedure with none of the above annotations is not an active procedure and should be ignored.

Readiness check

A Health Check for readiness allows third party services to know if the application is ready to process requests or not.

The @Readiness annotation must be applied on a HealthCheck implementation to define a readiness check procedure, otherwise, this annotation is ignored.

Liveness check

A Health Check for liveness allows third party services to determine if the application is running. This means that if this procedure fails the application can be discarded (terminated, shutdown).

The @Liveness annotation must be applied on a HealthCheck implementation to define a Liveness check procedure, otherwise, this annotation is ignored.

Startup check

A Health check for startup allows applications to define startup probes that are used for initial verification of the application before the liveness probe takes over. This is useful for applications which require additional startup time on their first initialization.

The @Startup annotation must be applied on a HealthCheck implementation to define a startup check procedure, otherwise, this annotation is ignored.

Multiple HealthChecks procedures for a given kind

There can be one or several HealthCheck exposed for a given kind, they will all be invoked when an inbound protocol request is received (i.e. HTTP).

If more than one HealthCheck are invoked, they will be called in an unpredictable order.

The runtime will call() each HealthCheck which in turn creates a HealthCheckResponse that signals the health status to a consuming end:

public class HealthCheckResponse {

    public enum Status { UP, DOWN }

    public abstract String getName();

    public abstract Status getStatus();

    public abstract Optional<Map<String, Object>> getData();

    [...]
}

The status of all HealthCheck 's determines the overall status for the given Health check kind.

Combining multiple kinds of checks

A HealthCheck implementation may be annotated with multiple kinds of checks. The procedure will be used to resolve every kind of health check for which it is annotated.

For instance this procedure will be used to resolve liveness and readiness health check.

@Liveness
@Readiness
public class MyCheck implements HealthCheck {

  public HealthCheckResponse call() {
    ...
  }
}

Constructing `HealthCheckResponse` 's

Application level code is expected to use one of static methods on HealthCheckResponse to retrieve a HealthCheckResponseBuilder used to construct a response, i.e. :

public class SuccessfulCheck implements HealthCheck {
    @Override
    public HealthCheckResponse call() {
        return HealthCheckResponse.up("successful-check");
    }
}

The name is used to tell the different checks apart when a human operator looks at the responses. It may be that one check of several fails and it’s useful to know which one. It’s required that a response defines a name.

HealthCheckResponse 's also support a free-form information holder, that can be used to supply arbitrary data to the consuming end:

public class CheckDiskspace implements HealthCheck {

    @Override
    public HealthCheckResponse call() {
        return HealthCheckResponse.named("diskspace")
                .withData("free", "780mb")
                .up()
                .build();
    }
}

HealthCheckResponse also provides a constructor to allow instantiation on the consuming end.

Integration with CDI

Any enabled bean with a bean of type org.eclipse.microprofile.health.HealthCheck and @Liveness, @Readiness, or @Startup qualifier can be used as health check procedure.

Contextual references of health check procedures are invoked by runtime when the outermost protocol entry point (i.e. http://HOST:PORT/health) receives an inbound request.

@ApplicationScoped
@Liveness
@Readiness
@Startup
public class MyCheck implements HealthCheck {

    public HealthCheckResponse call() {
        [...]
    }
}

Health check procedures are CDI beans, therefore, they can also be defined with CDI producers:

@ApplicationScoped
class MyChecks {

  @Produces
  @Liveness
  HealthCheck check1() {
    return () -> HealthCheckResponse.named("heap-memory").status(getMemUsage() < 0.9).build();
  }

  @Produces
  @Readiness
  HealthCheck check2() {
    return () -> HealthCheckResponse.named("cpu-usage").status(getCpuUsage() < 0.9).build();
  }

  @Produces
  @Startup
  HealthCheck check3() {
    return () -> HealthCheckResponse.named("startup-cpu-usage").status(getCpuUsage() < 0.95).build();
  }
}

Protocol and Wireformat

Abstract

This document defines the protocol to be used by components that need to ensure a compatible wireformat, agreed upon semantics and possible forms of interactions between system components that need to determine the “liveliness” or "readiness" of computing nodes in a bigger system.

Guidelines

Note that the force of these words is modified by the requirement level of the document in which they are used.

MUST This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification.
MUST NOT This phrase, or the phrase "SHALL NOT", mean that the definition is an absolute prohibition of the specification.
SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.
MAY – This word, or the adjective “OPTIONAL”, mean that an item is truly discretionary.

Goals

MUST be compatibility with well known cloud platforms (i.e. http://kubernetes.io/docs/user-guide/liveness/)
MUST be appropriate for machine-to-machine communication
SHOULD give enough information for a human administrator

Terms used

Term

Description

Producer

The service/application that is checked

Consumer

The probing end, usually a machine, that needs to verify the liveness or readiness of a Producer

Health Check Procedure

The code executed to determine the liveliness of a Producer

Producer status

The overall status, determined by considering all health check procedure results

Health check procedure result

The result of single check

Protocol Overview

Consumer invokes the health check of a Producer through any of the supported protocols
Producer enforces security constraints on the invocation (i.e. authentication)
Producer executes a set of Health check procedures (could be a set with one element)
Producer determines the overall status
The status is mapped to outermost protocol (i.e. HTTP status codes)
The payload is written to the response stream
The consumer reads the response
The consumer determines the overall status

Protocol Specifics

This section describes the specifics of the HTTP protocol usage.

Interacting with producers

How are the health checks accessed and invoked ? We don’t make any assumptions about this, except for the wire format and protocol.

Protocol Mappings

Health checks (innermost) can and should be mapped to the actual invocation protocol (outermost). This section described some of guidelines and rules for these mappings.

Producers MAY support a variety of protocols but the information items in the response payload MUST remain the same.
Producers SHOULD define a well known default context to perform checks
Each response SHOULD integrate with the outermost protocol whenever it makes sense (i.e. using HTTP status codes to signal the overall status)
Inner protocol information items MUST NOT be replaced by outer protocol information items, rather kept redundantly.
The inner protocol response MUST be self-contained, that is carrying all information needed to reason about the producer status

Mandatory and optional protocol types

REST/HTTP interaction

Producer MUST provide a HTTP endpoint that follows the REST interface specifications described in Appendix A.

Protocol Adaptor

Each provider MUST provide the REST/HTTP interaction, but MAY provide other protocols such as TCP or JMX. When possible, the output MUST be the JSON output returned by the equivalent HTTP calls (Appendix B). The request is protocol specific.

Healthcheck Response information

The primary information MUST be boolean, it needs to be consumed by other machines. Anything between available/unavailable doesn’t make sense or would increase the complexity on the side of the consumer processing that information.
The response information MAY contain an additional information holder
Consumers MAY process the additional information holder or simply decide to ignore it
The response information MUST contain the boolean status of each check
The response information MUST contain the name of each check

Wireformats

Producer MUST support JSON encoded payload with simple UP/DOWN statuses
Producers MAY support an additional information holder with key/value pairs to provide further context (i.e. disk.free.space=120mb).
The JSON response payload MUST be compatible with the one described in Appendix B
The JSON response MUST contain the name entry specifying the name of the check, to support protocols that support external identifier (i.e. URI)
The JSON response MUST contain the status entry specifying the status as String: “UP” or “DOWN”
The JSON MAY support an additional information holder to carry key value pairs that provide additional context

Health Check Procedures

A producer MUST support custom, application level health check procedures
A producer SHOULD support reasonable out-of-the-box procedures
A producer with no liveness procedures expected or installed MUST return positive overall status (i.e. HTTP 200)
A producer with no readiness procedures expected or installed MUST return positive overall status (i.e. HTTP 200)
A producer with no startup procedures expected or installed MUST return positive overall status (i.e. HTTP 200)
A producer with liveness procedures expected but not yet installed MUST return positive overall status (i.e. HTTP 200)
A producer with readiness procedures expected but not yet installed MUST return negative overall status (i.e. HTTP 503)
A producer with startup procedures expected but not yet installed MUST return negative overall status (i.e. HTTP 503)

A producer with no procedures expected or installed can be for instance an application server without any deployments (focusing on some different function).

A producer with procedures expected but not yet installed can be for instance a standalone runtime packaged with the user application, or an application server with already deployed user applications. User defined health check procedures thus can be expected but they do not have to be already processed when the first invocations of /health/* endpoints are received. See also the Empty default readiness and startup health check responses section.

Policies to determine the overall status

When multiple procedures are installed all procedures MUST be executed and the overall status needs to be determined.

Consumers MUST support a logical conjunction policy to determine the status
Consumers MUST use the logical conjunction policy by default to determine the status
Consumers MAY support custom policies to determine the status

Empty default readiness and startup health check responses

As readiness determines whether the container may consume requests, the container is required to return negative overall status until the user defined readiness checks can be executed. However, in some cases (e.g., the containers allowing multiple deployments) the container itself may be required to become ready before user defined readiness procedures can be deployed. This means that the container cannot know whether some procedures will be expected. For this reason, users are allowed to specify MicroProfile Config configuration value mp.health.default.readiness.empty.response to UP to give the container a hint that it can become ready.

The property mp.health.default.readiness.empty.response which defaults to DOWN (if not configured) should only be taken into account, when the user defined readiness check procedures are not processed or deployed yet. In other words, if the container is still starting up, the overall default readiness check status will reflect the mp.health.default.readiness.empty.response property status, with an empty payload response. Once the container has started up, the overall readiness check status and the payload response will reflect the processed readiness check procedures that are defined by the user. If there are not any readiness check procedures defined by the user, after the container has started, the overall default readiness check status MUST return a positive UP overall status (i.e. HTTP 200).

Similarly, for the startup health check, the users can additionally define mp.health.default.startup.empty.response to UP to achieve the same effect for the startup probes.

Executing procedures

When executing health check procedures a producer MUST handle any unchecked exceptions and synthesize a substitute response.

The synthesized response MUST contain a status entry with a value of "DOWN".
The synthesized response MUST contain a name entry with a value set to the runtime class name of the failing check.
The synthesized response MAY contain additional information about the failure (i.e. exception message or stack trace)

Disabling default vendor procedures

An implementation is allowed to supply a reasonable default (out-of-the-box) procedures as defined in the Health Check Procedures section. To disable all default vendor procedures users can specify a MicroProfile Config configuration property mp.health.disable-default-procedures to true. This allows the application to process and display only the user-defined health check procedures.

Security

Aspects regarding the secure access of health check information.

A producer MAY support security on all health check invocations (i.e. authentication)
A producer MUST NOT enforce security by default, it SHOULD be an opt-in feature (i.e. configuration change)

Appendix A: REST interfaces specifications

Context

Verb

Status Code

Kind of procedure called

Response

/health/live

GET

200, 500, 503

Liveness

See Appendix B

/health/ready

GET

200, 500, 503

Readiness

See Appendix B

/health/started

GET

200, 500, 503

Startup

See Appendix B

/health

GET

200, 500, 503

Liveness + Readiness + Startup

See Appendix B

Status Codes:

200 for a health check with a positive status (UP)
503 in case the overall status is negative (DOWN)
500 in case the producer wasn’t able to process the health check request (i.e. error in procedure)

Appendix B: JSON payload specification

Response Codes and status mappings

The following table gives valid health check responses for all kinds of health checks:

Request

HTTP Status

JSON Payload

Status

Comment