Health Check Response Format for HTTP APIs
draft-inadarei-api-health-check-00

Abstract

This document proposes a service health check response format for HTTP APIs.

Note to Readers

RFC EDITOR: please remove this section before publication

The issues list for this draft can be found at https://github.com/inadarei/rfc-healthcheck/issues.

The most recent draft is at https://inadarei.github.io/rfc-healthcheck/.

Recent changes are listed at https://github.com/inadarei/rfc-healthcheck/commits/master.

See also the draft’s current status in the IETF datatracker, at https://datatracker.ietf.org/doc/draft-inadarei-api-health-check/.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on July 21, 2018.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. Notational Conventions
• 3. API Health Response
• 4. Details Object
• 5. Security Considerations
• 6. IANA Considerations
• 6.1. Media Type Registration
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Acknowledgements
• Appendix B. Creating and Serving Health Responses
• Appendix C. Consuming Health Check Responses
• Author's Address

1. Introduction

The vast majority of modern APIs driving data to web and mobile applications use HTTP [RFC7230] as their protocol. The health and uptime of these APIs determine availability of the applications themselves. In distributed systems built with a number of APIs, understanding the health status of the APIs and making corresponding decisions, for failover or circuit-breaking, are essential for providing highly available solutions.

There exists a wide variety of operational software that relies on the ability to read health check response of APIs. There is currently no standard for the health check output response, however, so most applications either rely on the basic level of information included in HTTP status codes [RFC7231] or use task-specific formats.

Usage of task-specific or application-specific rformats creates significant challenges, disallowing any meaningful interoprerability across different implementations and between different tooling.

Standardizing a format for health checks can provide any of a number of benefits, including:

• Flexible deployment - since operational tooling and API clients can rely on rich, uniform format, they can be safely combined and substituted as needed.
• Evolvability - new APIs, conforming to the standard, can safely be introduced in any environment and ecosystem that also conforms to the same standard, without costly coordination and testing requirements.

This document defines a “health check” format using the JSON format [RFC8259] for APIs to use as a standard point for the health information they offer. Having a well-defined format for this purpose promotes good practice and tooling.

2. Notational Conventions

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

3. API Health Response

An API Health Response Format (or, interchangeably, “health check response”) uses the format described in JSON [RFC8259] and has the media type “application/vnd.health+json”.

Its content consists of a single mandatory root field (“status”) and several optional fields:

• status: (required) indicates whether the service status is acceptable or not. API publishers SHOULD use following values for the field:
  • “pass”: healthy,
  • “fail”: unhealthy, and
  • “warn”: healthy, with some concerns.

  For “pass” and “warn” statuses HTTP response code in the 2xx - 3xx range MUST be used. for “fail” status HTTP response code in the 4xx - 5xx range MUST be used. In case of the “warn” status, additional information SHOULD be provided, utilizing optional fields of the response.

• version: (optional) public version of the service.
• release_id: (optional) in well-designed APIs, backwards-compatible changes in the service should not update a version number. APIs usually change their version number as infrequently as possible, to preserve stable interface. However implementation of an API may change much more frequently, which leads to the importance of having separate “release number” or “release_id” that is different from the public version of the API.
• uptime: (optional) current uptime in seconds since the last restart
• connections: (optional) current number of active connections
• notes: (optional) array of notes relevant to current state of health
• output: (optional) raw error output, in case of “fail” or “warn” states. This field SHOULD be omitted for “pass” state.
• details: (optional) an array of objects optionally providing additional information regarding the various sub-components of the service.
• links: (optional) an array of objects containing link relations and URIs [RFC3986] for external links that MAY contain more information about the health of the endpoint. Per web-linking standards [RFC5988] a link relationship SHOULD either be a common/registered one or be indicated as a URI, to avoid name clashes.
• serviceID: (optional) unique identifier of the service, in the application scope.
• description: (optional) human-friendly description of the service.

For example:

  GET /health HTTP/1.1
  Host: example.org
  Accept: application/vnd.health+json

  HTTP/1.1 200 OK
  Content-Type: application/vnd.health+json
  Cache-Control: max-age=3600
  Connection: close

  {
    "status": "pass",
    "version" : "1",
    "release_id" : "1.2.2",
    "uptime": "1209600.245",
    "connections" : 25,
    "notes": [""],
    "output": "",
    "details": [
      {
        "componentId": "dfd6cf2b-1b6e-4412-a0b8-f6f7797a60d2",
        "componentName": "Cassandra",
        "componentType" : "datastore",
        "metricName" : "responseTime",
        "metricValue": 250,
        "metricUnit" : "milliseconds",
        "status": "pass",
        "time" : "2018-01-17T03:36:48Z",
        "output": ""
      },
      {
        "componentId": "dfd6cf2b-1b6e-4412-a0b8-f6f7797a60d2",
        "componentName": "Cassandra",
        "type" : "datastore",
        "metricName" : "connections",
        "metricValue": 75,
        "status": "warn",
        "time" : "2018-01-17T03:36:48Z",
        "output": ""
      },
      {
        "componentId": "6fd416e0-8920-410f-9c7b-c479000f7227",
        "componentName": "cpu",
        "type" : "system",
        "metricName" : "utilization",
        "metricValue": 85,
        "metricUnit" : "percent",
        "status": "warn",
        "time" : "2018-01-17T03:36:48Z",
        "output": ""
      },
      {
        "componentId": "6fd416e0-8920-410f-9c7b-c479000f7227",
        "componentName": "cpu",
        "type" : "system",
        "metricName" : "utilization",
        "metricValue": 85,
        "metricUnit" : "percent",
        "status": "warn",
        "time" : "2018-01-17T03:36:48Z",
        "output": ""
      },
      {
        "componentId": "6fd416e0-8920-410f-9c7b-c479000f7227",
        "componentName": "memory",
        "type" : "system",
        "node" : 1,
        "metricName" : "utilization",
        "metricValue": 8.5,
        "metricUnit" : "gb",
        "status": "warn",
        "time" : "2018-01-17T03:36:48Z",
        "output": ""
      },
      {
        "componentId": "6fd416e0-8920-410f-9c7b-c479000f7227",
        "componentName": "memory",
        "node" : 2,
        "type" : "system",
        "metricName" : "utilization",
        "metricValue": 5500,
        "metricUnit" : "mb",
        "status": "pass",
        "time" : "2018-01-17T03:36:48Z",
        "output": ""
      }
    ],
    "links": [
      {"rel": "about", "uri": "http://api.example.com/about/authz"},
      {
        "rel": "http://api.example.com/rel/thresholds",
        "uri": "http://api.example.com/about/authz/thresholds"
      }
    ],
    "serviceID": "f03e522f-1f44-4062-9b55-9587f91c9c41",
    "description": "health of authz service"
  }

4. Details Object

Following fields MAY appear and rules SHOULD be used for the details objects of the reponse.

• componentId: (required) unique identifier of an instance of a specific sub-component/dependency of a service. Multiple objects with the same componentId MAY appear in the details, if they are from different nodes.
• status: (required) “pass”, “fail” or “warn”. Same semantic meaning as at the top level.
• componentName: (optional) human-readable name for the component.
• componentType: (optional) SHOULD be present if componentName is present. Type of the component. Could be one of:
  • Pre-defined value from this spec. Pre-defined values include:
    • component
    • datastore
    • system
  • A common and standard term from a well-known source such as schema.org, IANA or microformats.
  • A URI that indicates extra semantics and processing rules that MAY be provided by a resource at the other end of the URI. URIs do not have to be dereferenceable, however. They are just a namespace, and the meaning of a namespace CAN be provided by any convenient means (e.g. publishing an RFC, Swagger document or a nicely printed book).
• metricName: (optional) Could be one of:
  • Pre-defined value from this spec. Pre-defined values include:
    • utilization
    • responseTime
    • connections
  • A common and standard term from a well-known source such as schema.org, IANA or microformats.
  • A URI that indicates extra semantics and processing rules that MAY be provided by a resource at the other end of the URI. URIs do not have to be dereferenceable, however. They are just a namespace, and the meaning of a namespace CAN be provided by any convenient means (e.g. publishing an RFC, Swagger document or a nicely printed book).
• metricValue: (optional) could be any valid JSON value, such as: string, number, object, array or literal.
• metricUnit: (optional) SHOULD be present if metricValue is present. Could be one of:
  • Pre-defined value from this spec. Pre-defined values include:
    • data size abbreviations: kb, mb, gb, tb, or pb that respectively stand for: kilobyte, megabyte, gigabyte, terabyte and petabyte.
    • time abbreviations: ns, ms, s, hr, min, d, yr that respectively stand for: nanosecond, millisecond, second, hour, day, and year.
  • A common and standard term from a well-known source such as schema.org, IANA or microformats.
  • A URI that indicates extra semantics and processing rules that MAY be provided by a resource at the other end of the URI. URIs do not have to be dereferenceable, however. They are just a namespace, and the meaning of a namespace CAN be provided by any convenient means (e.g. publishing an RFC, Swagger document or a nicely printed book).
• output: (optional) raw error output, in case of “fail” or “warn” states. This field SHOULD be omitted for “pass” state.

5. Security Considerations

Clients need to exercise care when reporting health information. Malicious actors could use this information for orchestrating attacks. In some cases the health check endpoints may need to be authenticated and institute role-based access control.

6. IANA Considerations

6.1. Media Type Registration

TODO: application/vnd.health+json is being submitted for registration per [RFC6838]

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Acknowledgements

Thanks to Mike Amundsen, Erik Wilde, Justin Bachorik and Randall Randall for their suggestions and feedback. And to Mark Nottingham for blueprint for authoring RFCs easily.

Appendix B. Creating and Serving Health Responses

When making an health check endpoint available, there are a few things to keep in mind:

• A health response endpoint is best located at a memorable and commonly-used URI, such as “health” because it will help self-discoverability by clients.
• Health check responses can be personalized. For example, you could advertise different URIs, and/or different kinds of link relations, to afford different clients access to additional health check information.
• Health check responses must be assigned a freshness lifetime (e.g., “Cache-Control: max-age=3600”) so that clients can determine how long they could cache them, to avoid overly frequent fetching and unintended DDOS-ing of the service.
• Custom link relation types, as well as the URIs for variables, should lead to documentation for those constructs.

Appendix C. Consuming Health Check Responses

Clients might use health check responses in a variety of ways.

Note that the health check response is a “living” document; links from the health check response MUST NOT be assumed to be valid beyond the freshness lifetime of the health check response, as per HTTP’s caching model [RFC7234].

As a result, clients ought to cache the health check response (as per [RFC7234]), to avoid fetching it before every interaction (which would otherwise be required).

Likewise, a client encountering a 404 (Not Found) on a link is encouraged to obtain a fresh copy of the health check response, to assure that it is up-to-date.

Author's Address