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Open Clustering Framework Resource Agent API

Editor: Lars Marowsky-Brée lmb@suse.de

URL: http://www.opencf.org/standards/resource-agent-api.html

License

Copyright (c) 2002 Lars Marowsky-Brée.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
copy of the license can be found at http://www.gnu.org/licenses/fdl.txt.

Abstract

Resource Agents (RA) are the middle layer between the Resource Manager (RM) and the actual resources being managed. They aim to integrate the resource type with the RM without any modifications to the actual resource provider itself, by encapsulating it carefully and providing generic methods (actions) to operate on them.

The RAs are obviously very specific to the resource type they operate on, however there is no reason why they should be specific to a particular RM.

The API described in this document should be general enough that a compliant Resource Agent can be used by all existing resource managers / switch-over systems who chose to implement this API either exclusively or in addition to their existing one.

Scope

This document describes a common API for the RM to call the RAs so the pool of available RAs can be shared by the different clustering solutions.

It does NOT define any libraries or helper functions which RAs might share with regard to common functionality like external command execution, cluster logging et cetera, as these are NOT specific to RA and are defined in the respective standards.

API version described

This document currently describes version 1.0 of the API.

Terms used in this document

"Resource"

A single physical or logical entity that provides a service to clients or other resources. For example, a resource can be a single disk volume, a particular network address, or an application such as a web server. A resource is generally available for use over time on two or more nodes in a cluster, although it usually can be allocated to only one node at any given time.

Resources are identified by a name that must be unique to the particular resource type. This is any name chosen by the administrator to identify the resource instance and passed to the RA as a special environment variable.

A resource may also have instance parameters which provide additional information required for Resource Agent to control the resource.

"Resource types"

A resource type represents a set of resources which share a common set of instance parameters and a common set of actions which can be performed on resource of the given type.

The resource type name is chosen by the provider of the RA.

"Resource agent"

A RA provides the actions ("member functions") for a given type of resources; by providing the RA with the instance parameters, it is used to control a specific resource.

They are usually implemented as shell scripts, but the API described here does not require this.

Although this is somewhat similar to LSB init scripts, there are some differences explained below.

"Instance parameters"

Instance parameters are the attributes which describe a given resource instance. It is recommended that the implementor minimize the set of instance parameters.

The meta data allows the RA to flag one or more instance parameters as unique. This is a hint to the RM or higher level configuration tools that the combination of these parameters must be unique to the given resource type.

An instance parameter has a given name and value. They are both case sensitive and must satisfy the requirements of POSIX environment name/value combinations.

API

API Version Numbers

The version number is of the form x.y, where x and y are positive numbers greater or equal to zero. x is referred to as the "major" number, and y as the "minor" number.

The major number must be increased if a backwards incompatible change is made to the API. A major number mismatch between the RA and the RM must be reported as an error by both sides.

The minor number must be increased if any change at all is made to the API. If the major is increased, the minor number should be reset to zero. The minor number can be used by both sides to see whether a certain additional feature is supported by the other party.

Paths

The Resource Agents are located in subdirectories under /usr/ocf/resource.d.

The subdirectories allow the installation of multiple RAs for the same type, but from different vendors or package versions.

The filename within the directories equals the resource type name provided by the RA and may be a link to the real location.

Example directory structure:

FailSafe -> FailSafe-1.1.0/
FailSafe-1.0.4/
FailSafe-1.1.0/
heartbeat -> heartbeat-1.1.2/
heartbeat-1.1.2/
heartbeat-1.1.2/IPAddr
heartbeat-1.1.2/IP -> IPAddr

How the RM choses an agent for a specific resource type name from the available set is implementation specific.

Execution syntax

After the RM has identified the executable to call, the RA will be called with the requested action as its sole argument.

To allow for further extensions, the RA shall ignore all other arguments.

Resource Agent actions

A RA must be able to perform the following actions on a given resource instance on request by the RM; additional actions may be supported by the script for example for LSB compliance.

The actions are all required to be idempotent. Invoking any operation twice - in particular, the start and stop actions - shall succeed and leave the resource instance in the requested state.

In general, a RA should not assume it is the only RA of its type running at any given time because the RM might start several RA instances for multiple independent resource instances in parallel.

Mandatory actions must be supported; optional operations must be advertised in the meta data if supported. If the RM tries to call a unsupported action the RA shall return an error as defined below.

  • start

    Mandatory.

    This brings the resource instance online and makes it available for use. It should NOT terminate before the resource instance has either been fully started or an error has been encountered.

    It may try to implement recovery actions for certain cases of startup failures.

    start must succeed if the resource instance is already running.

    start must return an error if the resource instance is not fully started.

  • stop

    Mandatory.

    This stops the resource instance. After the stop command has completed, no component of the resource shall remain active and it must be possible to start it on the same node or another node or an error must be returned.

    The stop request by the RM includes the authorization to bring down the resource even by force as long data integrity is maintained; breaking currently active transactions should be avoided, but the request to offline the resource has higher priority than this. If this is not possible, the RA shall return an error to allow higher level recovery.

    The stop action should also perform clean-ups of artifacts like leftover shared memory segments, semaphores, IPC message queues, lock files etc.

    stop must succeed if the resource is already stopped.

    stop must return an error if the resource is not fully stopped.

  • monitor

    Mandatory.

    Checks and returns the current status of the resource instance. The thoroughness of the check is further influenced by the weight of the check, which is further explained under Action specific extensions..

    It is accepted practice to have additional instance parameters which are not strictly required to identify the resource instance but are needed to monitor it or customize how intrusive this check is allowed to be.

    Note that monitor shall also return a well defined error code (see below) for stopped instances, ie before start has ever been invoked.

  • recover

    Optional.

    A special case of the start action, this should try to recover a resource locally.

    It is recommended that this action is not advertised unless it is advantageous to use when compared to a stop/start operation.

    If this is not supported, it may be mapped to a stop/start action by the RM.

    An example includes "recovering" an IP address by moving it to another interface; this is much less costly than initiating a full resource group fail over to another node.

  • reload

    Optional.

    Notifies the resource instance of a configuration change external to the instance parameters; it should reload the configuration of the resource instance without disrupting the service.

    It is recommended that this action is not advertised unless it is advantageous to use when compared to a stop/start operation.

    If this is not supported, it may be mapped to a stop/start action by the RM.

  • meta-data

    Mandatory.

    Returns the resource agent meta data via stdout.

  • validate-all

    Optional.

    Validate the instance parameters provided.

    Perform a syntax check and if possible, a semantic check on the instance parameters.

Parameter passing

The instance parameters and some additional attributes are passed in via the environment; this has been chosen because it does not reveal the parameters to an unprivileged user on the same system and environment variables can be easily accessed by all programming languages and shell scripts.

The entire environment variable name space starting with OCF_ is considered to be reserved for OCF use.

Syntax for instance parameters

They are directly converted to environment variables; the name is prefixed with OCF_RESKEY_.

The instance parameter force with the value yes thus becomes OCF_RESKEY_force=yes in the environment.

See the terms section on instance parameters for a more formal explanation.

Global OCF attributes

Currently, the following additional environment variables are defined:

  • OCF_RA_VERSION_MAJOR
  • OCF_RA_VERSION_MINOR

    Version number of the OCF Resource Agent API. If the script does not support this revision, it should report an error.

    See API Version Numbers for an explanation of the versioning scheme used. The version number is split into two numbers for ease of use in shell scripts.

    These two may be used by the RA to determine whether it is run under an OCF compliant RM.

    Example:

    OCF_RA_VERSION_MAJOR=1
    OCF_RA_VERSION_MINOR=0
    
  • OCF_ROOT

    Referring to the root of the OCF directory hierarchy.

    Example: OCF_ROOT=/usr/ocf

  • OCF_RESOURCE_INSTANCE

    The name of the resource instance.

  • OCF_RESOURCE_TYPE

    The name of the resource type being operated on.

Action specific extensions

These environment variables are not required for all actions, but only supported by some.

Parameters specific to the 'monitor' action

  • OCF_CHECK_LEVEL

    • 0

      The most lightweight check possible, which should not have an impact on the QoS.

      Example: Check for the existence of the process.

    • 10

      A medium weight check, expected to be called multiple times per minute, which should not have a noticeable impact on the QoS.

      Example: Send a request for a static page to a webserver.

    • 20

      A heavy weight check, called infrequently, which may impact system or service performance.

      Example: An internal consistency check to verify service integrity.

Service must remain available during all of these operation. All other number are reserved.

It is recommended that if a requested level is not implemented, the RA should perform the next lower level supported.

Exit status codes

These exit status codes are the ones documented in the LSB 1.1.0 specification, with additional explanations of how they shall be used by RAs. In general, all non-zero status codes shall indicate failure in accordance to the best current practices.

All operations

  • 0

    No error, action succeeded completely

  • 1

    Generic or unspecified error (current practice) The "monitor" operation shall return this for a crashed, hung or otherwise non-functional resource.

  • 2

    Invalid or excess argument(s) Likely error code for validate-all, if the instance parameters do not validate. Any other action is free to also return this exit status code for this case.

  • 3

    Unimplemented feature (for example, "reload")

  • 4

    User had insufficient privilege

  • 5

    Program is not installed

  • 6

    Program is not configured

  • 7

    Program is not running

    Note: This is not the error code to be returned by a successful "stop" operation. A successful "stop" operation shall return 0. The "monitor" action shall return this value only for a cleanly stopped resource. If in doubt, it should return 1.

  • 8-99

    Reserved for future LSB use

  • 100-149

    Reserved for distribution use

  • 150-199

    Reserved for application use

  • 200-254

    Reserved

Relation to the LSB

It is required that the current LSB spec is fully supported by the system.

The API tries to make it possible to have RA function both as a normal LSB init script and a cluster-aware RA, but this is not required functionality. The RAs could however use the helper functions defined for LSB init scripts.

RA meta data

Format

The API has the following requirements which are not fulfilled by the LSB way of embedding meta data into the beginning of the init scripts:

  • Independent of the language the RA is actually written in,
  • Extensible,
  • Structured,
  • Easy to parse from a variety of languages.

This is why the API uses simple XML to describe the RA meta data. The DTD for this API can be found at this location.

Semantics

An example of a valid meta data output is provided in ra-metadata-example.xml.

To-do list

  • Move the terminology definitions out into a separate document common to all OCF work.
  • An interface where the RA asynchronously informs the RM of failures is planned but not defined yet.

Contributors