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From Ken Giusti <kgiu...@redhat.com>
Subject Re: [QMF] Proposed API for QMFv2 - RFC (updated)
Date Tue, 01 Dec 2009 22:15:23 GMT

I've updated the proposed api:

*) Took an initial swag at the Agent class objects
*) Updated the schema classes based on some prototyping, added definitions for the map representation
of schema classes.

enjoy, and apologies if it does not render well - I'm still doing "confluence-by-hand"...



h1. Proposal for "Next Generation" QMF API

h2. Goals

 * Simplify QMF API

 * Use new QPID Messaging API

 * Implement QMF protocol via QPID Map messages

 * Improve thread safety by simplifying the callback notification mechanism.

 * Reorganize QMF code into common API, Console API, and Agent API modules.

h2. Common Object Classes

Object classes that are used by Agent and Console components.

h3. Schema

Schemas are used by QMF to describe the structure of management data.

h4. Schema Types

There are two classes (or types) of Schema - those that describe
QmfData objects, and those that describe QmfEvent objects.


These types may be represented by the strings "data" and "event",

h4. Schema Identifier

Schema are identified by a combination of their package name and class
name.  A hash value over the body of the schema provides a revision
identifier.  The class SchemaClassId represents this Schema

class SchemaClassId:
      <constructor>( package=<package-name-str>, 
                     hash=<hash-str, format="%08x-%08x-%08x-%08x">, 
      .getPackageName(): returns <package-name-str>
      .getClassName(): returns <class-name-str>
      .getHashString(): returns <hash-str, "%08x-%08x-%08x-%08x">
      .getType(): returns SchemaTypeObject or SchemaTypeEvent
      .mapEncode(): returns a map encoding of the instance.

If the hash is not supplied, then the default value
"00000000-00000000-00000000-00000000" will be supplied.  This may be
the case when a SchemaClass is being dynamically constructed, and a
proper hash is not yet available.

The map encoding of a SchemaClassId:

|"package_name"|N|string|The name of the associated package.|
|"class_name"|N|string|The name of the class within the package.|
|"type"|N|string|The type of schema, either "data" or "event".|
|"hash_str"|Y|string|The MD5 hash of the schema, in the format "%08x-%08x-%08x-%08x"|

h4. Schema for describing data Properties

The SchemaProperty class represents the description of a property
datum.  Once instantiated, the SchemaProperty is immutable.

class SchemaProperty:
      <constructor>( name=<name-value>, 
      .getName(): name of property (string)
      .getType(): typecode for property data
      .getAccess(): "RC"=read/create, "RW"=read/write, "RO"=read only (default)
      .isIndex(): True if this property is an index.
      .isOptional(): True if this property is optional
      .getUnit(): string describing units (optional)
      .getMin(): minimum value (optional)
      .getMax(): maximum value (optional)
      .getMaxLen(): maximum length for string values (optional)
      .getDesc(): optional string description of this Property
      .getReference(): if type==objId, name (str) of referenced class
      .isParentRef(): True if this property references an object in
                       which this object is in a child-parent
      .mapEncode(): returns a map encoding of the instance.

The map encoding of a SchemaProperty:

|"name"|N|string|The name of the property.|
|"type"|N|integer|The QMF type code indicating property's data type.|
|"access"|Y|string|The access allowed to this property, default "RO"|
|"index"|Y|boolean|True if this property is an index value, default False|
|"optional"|Y|boolean|True if this property is optional, default False|
|"unit"|Y|string|Description of the units used to express this property.|
|"min"|Y|integer|The minimum allowed value for this property|
|"max"|Y|integer|The maximun allowed value for this property|
|"maxlen"|Y|integer|For string types, this is the maximum length in 
bytes required to represent the longest permittable instance of this string.|
|"desc"|Y|string|Human-readable description of this property.|
|"parent_ref"|Y|boolean|True if this property references an object 
in which this object is in a child-parent relationship. Default False|

h4. Schema for describing a Statistic

The SchemaStatistic class represents the description of a statistic
datum.  Once instantiated, the SchemaStatistic is immutable.

class SchemaStatistic:
      <constructor>( name=<name-value>, 
                     ... )
      .getName(): name of the statistic (string)
      .getType(): typecode for statistic
      .getUnit(): string describing units (optional)
      .getDesc(): string description of this statistic (optional)
      .mapEncode(): returns a map encoding of the instance.

The map encoding of a SchemaStatistic:

|"name"|N|string|The name of the statistic.|
|"type"|N|integer|The QMF type code indicating statistic's data type.|
|"unit"|Y|string|Description of the units used to express this statistic.|
|"desc"|Y|string|Human-readable description of this statistic.|

h4. Schema for describing Method Calls

The Schema for describing a method call involves two classes.  The
SchemaMethod class describes the method's structure, and contains a
SchemaArgument class for each argument declared by the method.
An Agent may construct a SchemaMethod's argument list.

class SchemaMethod:
      <constructor>( name=<name-value>,
                     [args=<list of SchemaArguments>]
      .getName(): name of method
      .getArgCount(): count of arguments
      .getArguments(): returns a list of SchemaArgument instances, one
                       for each argument.
      .getArg(index): return the SchemaArgument at index 
      .addArgument(SchemaArgument): adds a SchemaArgument 
      .mapEncode(): returns a map encoding of the instance.

class SchemaArgument:
      <constructor>( name=<name-value>,
      .getName(): name of argument
      .getType(): type of argument
      .getDirection(): "I"=input, "O"=output, or "IO"=input/output
      .getUnit(): string describing units (optional)
      .getMin(): minimum value (optional)
      .getMax(): maximum value (optional)
      .getMaxLen(): maximum length for string values (optional)
      .getDesc(): string description of this argument (optional)
      .getDefault(): default value.
      .getRefPackage(): return name of the package referenced by this argument
      .getRefClass(): return name of the class referenced by this argument
      .mapEncode(): returns a map encoding of the instance.

The map encoding of a SchemaArgument:

|"name"|N|string|The name of the argument.|
|"type"|N|integer|The QMF type code indicating the argument's data type.|
|"dir"|N|string|Direction for an argument when passed to a Method call: "I", "O", "IO", default
value: "I"|
|"default"|Y|based on "type"|Default value to use if none provided.|
|"unit"|Y|string|Description of the units used to express this argument.|
|"min"|Y|integer|The minimum allowed value for this argument|
|"max"|Y|integer|The maximun allowed value for this argument|
|"maxlen"|Y|integer|For string types, this is the maximum length in bytes required to represent
the longest permittable instance of this string.|
|"desc"|Y|string|Human-readable description of this argument.|
|"ref_package"|Y|string|Name of package referenced by this argument.|
|"ref_class"|Y|string|Name of class referenced by this argument.|

The map encoding of a SchemaMethod:

|"name"|N|string|The name of the method.|
|"arguments"|Y|list|Ordered list of maps, one for each argument to the method.|
|"desc"|Y|string|Human-readable description of this method.|

h4. Schema for Data Objects and Events

A top-level data object in QMF is a collection of statistics,
properties, and methods. Data objects are described by the class
SchemaObjectClass.  Data objects may be managed (owned) by an Agent. 

Events describe a change within the underlying management system.  Unlike
data objects, events do not correspond to a managable entity.  They
are composed of a list of arguments that apply to the event. Event
objects are described by the class SchemaEventClass. 

Both of these classes derive from the virtual base SchemaClass.

Agent applications may dynamically construct instances of these
objects by adding properties/statistics/methods/etc. at run time.
However, once the Schema is published, it must be considered
immutable, as the hash value must be constant once the Schema is
in use.

class SchemaClass:
      <constructor>( package=<package-name-str>, 
                     hash=<hash-str, format="%08x-%08x-%08x-%08x">)
      .getClassId(): return the SchemaClassId that identifies this
                      Schema instance.
      .generateHash(): generate a hash over the body of the schema,
                       and return a string representation of the hash
                       in format  "%08x-%08x-%08x-%08x"
      .mapEncode(): returns a map encoding of the instance.

class SchemaObjectClass(SchemaClass):
      <constructor>( package=<package-name-str>, 
                     hash=<hash-str, format="%08x-%08x-%08x-%08x">,
                     _properties=[list of SchemaProperty instances],
                     _stats=[list of SchemaStatistics instances],
                     _methods=[list of SchemaMethod instances])
      .getPropertyCount(): return the number of properties.
      .getStatisticCount(): return the number of statistics.
      .getMethodCount(): return the number of methods.
      .getProperty(index): return the index'th SchemaProperty.
      .getStatistic(index): return the index'th SchemaStatistic.
      .getMethod(index): return the index'th SchemaMethod

      .addProperty(SchemaProperty): append a new property
      .addStatistic(SchemaStatistic): append a new statistic
      .addMethod(SchemaMethod): append a new method.

class SchemaEventClass(SchemaClass):
      <constructor>( package=<package-name-str>, 
                     hash=<hash-str, format="%08x-%08x-%08x-%08x">,
                     _arguments=[list of SchemaArgument instances])
      .getArgCount(): return the count of arguments.
      .getArgument(index): return the index'th SchemaArgument.

      .addArgument(SchemaArgument): append a new argument.

The map encoding of a SchemaObjectClass:

|"class_id"|N|map|Map containing the SchemaClassId for this object.|
|"properties"|Y|list|Ordered list of SchemaProperty maps.  One map for each property in this
|"statistics"|Y|list|Ordered list of SchemaStatistics maps.  One map for each statistic in
this object.|
|"methods"|Y|list|Ordered list of SchemaMethod maps.  One map for each method in this object.|

The map encoding of a SchemaEventClass:

|"class_id"|N|map|Map containing the SchemaClassId for this object.|
|"arguments"|Y|list|Order list of SchemaArgument maps, one map for each argument in this event.|

h3. Data Model

The QMF defines three _layers_ of data representation:

 # arbitrarily structured data 
 # structured data
 # managed structured data

Arbitrarily structured data is represented by a map of named data
items.  There is no schema associated with this data, as its structure
is arbitrary.  This class of data is represented by the QmfData class.

Data that has a formally defined structure is represented by the
QmfDescribed class.  This class extends the QmfData class by
associating the data with a formal schema.

Managed structured data extends the concept of structured data by
providing an owner for the data.  In QMF, this owner is an Agent.
This class of data extends the QmfDescribed class by adding an
identifier of the owning Agent.

h4. QmfData Class

class QmfData:
      <constructor>( map of "name=value" pairs )

h4. QmfDescribed Class

class QmfDescribed(QmfData):
      <constructor>( class QmfData, class SchemaClassId )
      .getSchemaClassId(): returns the identifier of the Schema that
                            describes the structure of the data.
      .getIndex(): return a string composed from "index" property
      .getProperties(): return a list of (SchemaProperty, value) pairs
                        for each of the object's properties.
      .getStatistics(): return a list of (SchemaStatistics, value)
                        pairs for each of the object's statistics.

h4. QmfManaged Class

The QmfManaged class represents a data object that is owned by a
particular Agent.  All QmfManaged objects contain a unique object

The QmfManaged class extends the QmfDescribed class:

class QmfManaged(QmfDescribed):
      .getObjectId(): return ObjectId for this object.

h5. Object Identification

An instance of managed object must be uniquely identified within the
management system.  Each managed object is given a name that is unique
within the domain of the object's managing Agent.  Note that these
names are not unique across Agents.  Therefore, a globally unique name
for an instance of a managed object is the concatenation of the
object's name and the managing Agent's name.  This unique object
identifier is represented by the ObjectId class.

ObjectIds are considered _hashable_.  That is, it is a requirement
that ObjectIds are deterministically sortable.

class ObjectId:
      <constructor>(agent="agent name", 
                    name="object name")
      .getAgentName(): returns <agent name> string.
      .getObjectName(): returns <object name> string.
      .operators(==, !=, <, >) supported.

h4. QmfEvent Class

A QMF Event is a type of QmfDescribed data that is not managed.
Events are notifications that are sent by Agents. 
Unlike QmfManaged objects, events do not correspond to managed data on the
Agent.  Instead, an event notifies a Console of a change in some
aspect of the system under managment.  For example, an event may
indicate that a threshold was exceeded, or a resource was returned.
The structure of an event is described by the SchemaEventClass.  An
instance of an event is represented by the QmfEvent class.

class QmfEvent(QmfDescribed):
      <constructor>( timestamp, argument_list )
      .getTimestamp(): return a timestamp indicating when the Event
      .getArguments(): return a list of (SchemaArgument, value) pairs
      .getArgument(<name>): return the value of argument <name>
      ?tbd? do we need to identify the originating Agent?

h3. Data Management

The role of a QMF component determines how it will interact with
managment data. Therefore the QmfManaged class is subclassed
to provide an Agent specific implementation and a Console specific

The Console application represents a managed data object by the
QmfConsoleData class.  The Console has "read only" access to the
statistics and properties in the data object via this class.  The
Console can also invoke the methods defined by the object via this
class.  The actual data stored in this object is cached from the
Agent.  In order to update the cached values, the Console invokes the
refresh() method.

Note that the refresh() and invokeMethod() methods require
communication with the remote Agent.  As such, they may block.  For
these two methods, the Console has the option of blocking in the call
until the call completes.  Optionally, the Console can receive a
notification asynchronously when the operation is complete.  See below
for more detail regarding synchronous and asynchronous API calls.

class QmfConsoleData(QmfManaged):
      .getTimestamps(): returns a list of timestamps describing the
                        lifecycle of the object:
                        [0] = time of last update from Agent,
                        [1] = creation timestamp 
                        [2] = deletion timestamp, or zero if not deleted.
      .isDeleted(): True if deletion timestamp not zero.
      .refresh([reply-handle | timeout]): request that the Agent
                    update the value of this object's contents.
      .invokeMethod(name, inArgs[], [[reply-handle] | [timeout]]): 
                          invoke the named method.
      .getStatistic(name): return the value of the named statistic
      .getProperty(name): return the value of the named property.

The Agent that manages the data represents it by the
QmfAgentData class.  The Agent is responsible for managing the
values of the properties and statistics within the object, as well as
servicing the object's method calls.  Unlike the Console, the Agent
has full control of the state of the object.

class QmfAgentObject(QmfObject):
      <constructor>( ObjectId, ?tbd?)
      .destroy(): mark the object as deleted by setting the deletion
                  timestamp to the current time.
      .setObjectId(ObjectId): set the value of the ObjectId.
      .setProperty(name, value): update the value of the property.
      .setStatistic(name, value): set the value of a statistic.
      .incStatistic(name, delta): add the delta to the statistic.
      .decStatistic(name, delta): subtract the delta from the statistic.

Note that some languages will allow properties, statistics, and
methods to act as attributes of the QmfObject itself.  As an example,
assume an QmfConsoleObject exists with a property named "state". In
python, it will be possible to access the value of "state" as if it
where an attribute of the QmfConsoleObject.  The following two
statements are equivalent:

           print("My state is '%s'" %

           print("My state is '%s'" %

Be aware - a QmfConsoleObject provides read-only access, so the
following statement would be illegal:

           myQmfConsoleObject.state = "up"

h5. Invoking Methods

A managed object's methods provide a mechanisms for a Console application to
perform a "remote procedure call" against the object.  The method
actually executes on the Agent, and the result is passed back to the
Console on completion.

The value(s) returned to the Console when the method call completes
are represented by the MethodResult class.  The MethodResult class
indicates whether the method call succeeded or not, and, on success,
provides access to all data returned by the method call.

Should a method call result in a failure, this failure is indicated by
the presence of a QmfData object in the MethodResult.  This object is
described as an "exception", and contains a description of the reason
for the failure. There are no returned arguments when a method call fails.

A successful method invokation is indicated by the absence of an
exception in the MethodResult.  In addition, a map of returned
argument values may be present.  The structure of the returned
arguments is described by the SchemaArgument list associated with the
MethodSchema that defines the method.  The map contains only those
arguments that the SchemaArgument marks with an "output" direction.

class MethodResult:
      <constructor>( QmfData <exception> | <argument-map> )
      .getException(): returns exception data if method fails.
      .getArgumentCount(): returns number of arguments.
      .getArguments(): returns a list of (SchemaArgument, value) pairs
                       for each returned argument.
      .getArgument(<name>): returns value of argument named "name".

h4. Queries

A Query is a mechanism for interrogating the management database.  A
Query represents a selector which is sent to an Agent.  The Agent
applies the Query against the management database, and 
returns those objects which meet the constraints contained in the query.  

class Query:
      <constructor>(<SchemaClassId> | 
                    <ObjectId> | 
                    <package-name>[, <class-name>])

The target for a Query is determined by the constructor.  When a Query
is created using a SchemaClassId, it will match against the
SchemaClass identified by that key.  When a Query is created using an
ObjectId, it will match against the QmfObject or Event identified by
that ObjectId.  When a Query is created with only a package name, it
will return all class names that belong to that package.  If both the
package and class names are provided, the Query will match against the
SchemaClassId identified by the combination of those names.

h4. Subscriptions

A subscription is a mechanism for monitoring management data for
changes in its state. A Console creates a subscription with an Agent
based on a Query.  The Query specifies the set of management data that
is to be monitored. When the Agent detects changes to the selected
set, a notification is sent to the subscribing Console(s).  A
Subscription is represented by the SubscriptionId class.  A Console
must cancel the subscription when the console no longer needs to
monitor the data.

class SubscriptionId:

h4. Agent Identifiers

An Agent is uniquely identified by an AgentId.  An AgentId is a
three-tuple containing:

 * the name of the vendor that produced the agent
 * the name of the product using the agent
 * the name of the agent component within the product.

This naming convention allows for a single product to host multiple
distinct Agents.  The Agent identifier is represented by the AgentId
class. An AgentId is considered _hashable_.  That is, it is a
requirement that AgentIds are deterministically sortable.

class AgentId:
      <constructor>( vendor, product, name )
      .getVendor(): return the vendor name string
      .getProduct(): return the product name string
      .getName(): return agent component name string

By convention, AgentIds are specified as a string containing the
vendor, product, and name strings separated by colons ":", in that
order.  For example: "company.com:OurProduct:AnAgent"

h2. Console application model

A QMF console component is represented by a Console class.  This class
is the topmost object of the console application's object model.

A Console is composed of the following objects:

 * a connection to the AMQP bus
 * a queue of inbound work items
 * a collection of all known schemas
 * a list of all known remote Agents
 * a cache of QmfConsoleObject proxies

The connection to the AMQP bus is used to communicate with remote
Agents.  The queue is used as a source for notifications coming from
remote Agents.

h3. Asychronous event model.

The original QMF API defined a set of callback methods that a Console
application needed to provide in order to process asynchronous QMF
events.  Each asynchonous event defined its own callback method.

The new API replaces this callback model with a work-queue approach.
All asynchronous events are represented by a WorkItem object.  When
a QMF event occurs it is translated into a WorkItem object and placed
in a FIFO queue.  It is left to the console application to drain
this queue as needed.

This new API does require the console application to provide a single
callback.  The callback is used to notify the console application that
WorkItem object(s) are pending on the work queue.  This callback is
invoked by QMF when the work queue transitions from the empty to the
non-empty state.  To avoid any potential threading issues, the console
application is _not_ allowed to call any QMF API from within the
callback context.  The purpose of the callback is to allow the console
application to schedule itself to drain the work queue at the next
available opportunity.  

For example, a console application may be designed using a {{select()}}
loop.  The application waits in the {{select()}} for any of a number
of different descriptors to become ready.  In this case, the callback
could be written to simply make one of the descriptors ready, and then
return.  This would cause the application to exit the wait state, and
start processing pending events.

The callback is represented by the Notifier virtual base class.  This
base class contains a single method.  A console application derives a
custom handler from this class, and makes it available to the Console

class Notifier:
    .indication():  Called when the internal work queue becomes
    non-empty due to the arrival of one or more WorkItems. This method
    will be called by the internal QMF management thread - it is
    illegal to invoke any QMF APIs from within this callback.  The
    purpose of this callback is to indicate that the application
    should schedule itself to process the work items.  

The WorkItem class represents a single notification event that is read
from the work queue:

class WorkItem:
    # Enumeration of the types of WorkItems produced by the Console
    NEW_CLASS = 4

    .getType(): Identifies the type of work item by returning one of
    the above type codes. 

    .getHandle(): return the handle for an asynchronous operation, if present.

    .getParams(): Returns the data payload of the work item.  The type
    of this object is determined by the type of the workitem (?TBD?). 

h3. Local representation of a remote Agent.

The console application maintains a list of all known remote Agents.
Each Agent is represented by the Agent class:

class Agent:
      <constructor>( AgentId )
      .getName(): returns the AgentId

h3. The Console Object.

The Console class is the top-level object used by a console
application.  All QMF console functionality is made available by this
object.  A console application must instatiate one of these objects.

As noted below, some Console methods require contacting a remote
Agent.  For these methods, the caller has the option to either block
for a (non-infinite) timeout waiting for a reply, or to allow the
method to complete asynchonously.  When the asynchronous approach is
used, the caller must provide a unique handle that identifies the
request.  When the method eventually completes, a WorkItem will be
placed on the work queue.  The WorkItem will contain the handle that
was provided to the corresponding method call.

All blocking calls are considered thread safe - it is possible to have
a multi-threaded implementation have multiple blocking calls in flight

If a name is supplied, it must be unique across all Consoles attached
to the AMQP bus.  If no name is supplied, a unique name will be
synthesized in the format: "qmfc-<hostname>.<pid>"

class Console:
                    notifier=<class Notifier>,
                    timeout=<default for all blocking calls>,
                    subscription_duration=<default lifetime of a subscription>)

      .destroy(timeout=None): Must be called to release Console's resources.

      .addConnection(QPID Connection): Connect the console to the AMQP cloud.

      .removeConnection(conn): Remove the AMQP connection from the
          console.  Un-does the addConnection() operation, and
          releases any agents associated with the connection.  All
          blocking methods are unblocked and given a failure status.
          All outstanding asynchronous operations are cancelled
          without producing WorkItems.

          Get the AMQP address this Console is listening to (type str).

      .findAgent( class AgentId, [timeout | handle] ): Query for the
      presence of a specific agent. Returns a class Agent if the agent
      is present.  May be called blocking (with default timeout
      override), or may allow asynchronous completion (producing a
      WorkItem with the given handle).

      .enableAgentDiscovery(): Called to enable the asynchronous
          Agent Discovery process. Once enabled, AGENT_ADD work items
          can arrive on the WorkQueue.

      .disableAgentDiscovery(): Called to disable the async Agent
      Discovery process enabled by calling enableAgentDiscovery().  

      .getWorkItemCount(): Returns the count of pending WorkItems that
      can be retrieved. 

      .getNextWorkItem([timeout=0]): Obtains the next pending work
      item, or None if none available. 

      .releaseWorkItem(wi): Releases a WorkItem instance obtained by
      getNextWorkItem(). Called when the application has finished
      processing the WorkItem. 

      .getAgents(): Returns a list of available agents (class Agent)

      .getAgent( class AgentId ): Return the class Agent for the
          named agent, if known. 

      .getPackages( [class Agent] ): Returns a list of the names of
          all known packages.  If an optional Agent is provided, then
          only those packages available from that Agent are returned.

      .getClasses( [class Agent] ):  Returns a list of SchemaClassIds
          for all available Schema.  If an optional Agent is provided,
          then the returned SchemaClassIds are limited to those
          Schema known to the given Agent.

      .getSchema( class SchemaClassId [, class Agent]): Return a list
          of all available class SchemaClass across all known agents.
          If an optional Agent is provided, restrict the returned
          schema to those supported by that Agent.

      .makeObject( SchemaClassId, **kwargs ): returns an uninitialized
      instance of a QmfDescribed data object.

      .getObjects( _SchemaClassId= | _package=, _class= |
                   [list-of-class-Agent] ): perform a blocking query
           for QmfConsoleObjects.  Returns a list (possibly empty) of matching
           objects. The selector for the query may be either:
           * class SchemaClassId - all objects whose schema match the
                    schema identified by _SchemaClassId parameter.
           * package/class name - all objects whose schema are
                    contained by the named package and class.
           * the object identified by _objectId
           This method will block until all known agents reply, or the
                    timeout expires. Once the timeout expires, all
                    data retrieved to date is returned.  
           If a list of agents is supplied, then the query is sent to
                    only those agents.  

      .createSubscription( class Query [, duration=<secs> [, list of agents] ): creates
            subscription using the given Query.  If a list of agents
            is provided, the Query will apply only to those agents.
            Otherwise it will apply to all active agents, including
            those discovered during the lifetime of the subscription.
            The duration argument can be used to override the
            console's default subscription lifetime for this
            subscription.  Returns a class SubscriptionId.

      .refreshSubscription( SubscriptionId [, duration=<secs>] ):
      (re)activates a subscription.  Uses the console default duration
      unless the duration is explicitly specified.

      .cancelSubscription( SubscriptionId ): terminates the

h2. Example Console Application

The following pseudo-code performs a blocking query for a particular agent.

logging.info( "Starting Connection" )
conn = Connection("localhost")

logging.info( "Starting Console" )
myConsole = Console()
myConsole.addConnection( conn )

logging.info( "Finding Agent" )
myAgent = myConsole.findAgent( AgentId( "aCompany.com", "Examples", "anAgent" ), _timeout=5

if myAgent:
   logging.info( "Agent Found: %s" % myAgent )
   logging.info( "No Agent Found!")

logging.info( "Removing connection" )
myConsole.removeConnection( conn )

logging.info( "Destroying console:" )
myConsole.destroy( _timeout=10 )

The following pseudo-code performs a non-blocking query for all
agents.  It completes when at least one agent is found.

class MyNotifier(Notifier):
    def __init__(self, context):
        self._myContext = context
        self.WorkAvailable = False

    def indication(self):
        print("Indication received! context=%d" % self._myContext)
        self.WorkAvailable = True

noteMe = MyNotifier( 668 )

logging.info( "Starting Connection" )
conn = Connection("localhost")

myConsole = Console(notifier=noteMe)
myConsole.addConnection( conn )


while not noteMe.WorkAvailable:
    print("No work yet...sleeping!")

print("Work available = %d items!" % myConsole.getWorkItemCount())
wi = myConsole.getNextWorkitem(timeout=0)
while wi:
    print("work item %d:%s" % (wi.getType(), str(wi.getParams())))
    wi = myConsole.getNextWorkitem(timeout=0)

logging.info( "Removing connection" )
myConsole.remove_connection( conn )

logging.info( "Destroying console:" )
myConsole.destroy( 10 )

h2. Agent Application Model

A QMF agent component is represented by a instance of the Agent
class.  This class is the topmost object of the agent application's
object model.  Associated with a particular agent are

* the set of objects managed by that agent
* a collection of consoles that are interfacing with the agent

The Agent class communicates with the application using the same
notification-based model as the console.  The agent maintains a
work-queue of pending requests.  Each pending request is
associated with a handle.  When the application is done servicing the
work request, it passes the response to the agent along with the
handle associated with the originating request.

An agent can support one of two different models for managing its
object database: internal or external store.

h3. Internal Object Store

An agent that implements internal object store takes full
responsibility for managing its associated objects.  In this model,
the application passes a reference for each managed objects to the
agent.  The agent manages the set of objects internally, directly
accessing the contents of the object in order to service console

With this model, the application's complexity is reduced.  The
application needs to instantiate the object and register it with the
agent.  The application also maintains a reference to the object, as
the application is responsible for updating the object's properties
and the statistics as necessary.

The application must also service method calls invoked on the object.
The agent notifies the application when a method call has been
requested by a console.  The application services the method call,
passing the result of the method back to the agent. The agent then
relays the response to the originating console.

The application may decide to delete an object instance.  The
application does this by invoking the destroy() method on the object.
This notifies the agent, which will mark the object as deleted in its
database. Once the application invokes the destroy() method on an
object, it must no longer access the object.  The agent will clean up
the object at a later point in time.

Internal object store is the default model for agent object

h4. Data Consistency

The internal object store requires sharing of the managed data
between the agent and the application.  The application is responsible
for keeping the data up to date, while the agent is responsible for
providing the data to client consoles.  It is likely that these
components may be implemented in separate execution contexts. This
raises the possibility that a data item could be in the process of
being written to by the application at the same moment the agent
attempts to read it.  This would result in invalid data being read.  

To prevent this from occuring, the QmfAgentObject class provides
accessors for all property and statistic data in the object. These
accessors provide atomic access to the underlying data.  Therefore,
both the agent and the application code *must* use these accessors
to manipulate a shared object's data.

h3. External Object Store

An alternate agent implementation allows the application to take
full responsibility for managing the objects.  With this model, all
instances of managed objects exist external to the agent. When a
console requests an action against an object, that action is
transferred from the agent to the application.  The application then
must process the request, and send the result to the agent.  The agent
then sends a reply to the requesting console.

The model gives full control of the managed objects to the
application, but usually requires more application development work. 

h3. Agent Class

The base class for the agent object is the Agent class.  This base
class represents a single agent implementing internal store.

class Agent:
      <constructor>( vendor=<vendor-string>,
                     class Notifier ): the vendor/product/name
            combination must uniquely identify this agent instance
            within the QMF domain.  The Notifier is used to alert the
            application to incoming method requests.
      .setConnection( QPID Connection ): connect the agent to the AMQP cloud.
      .registerObjectClass( class SchemaObjectClass ): Register a
            schema for an object class with the agent.  The agent must
            have a registered schema for an object class before it can
            handle objects of that class.
      .registerEventClass( class SchemaEventClass ) : Register a
            schema for an event class with the agent.  The agent must
            have a registered schema for an event class before it can
            handle events of that class.
      .raiseEvent( class QmfEvent ): Cause the agent to raise the
            given event.
      .addObject( class QmfManaged, object_name=<object name str> ):
            passes a reference to an instance of a managed QMF object
            to the agent. The object name must uniquely identify this
            object among all objects known to this agent.  Returns a
            class ObjectId containing the object identifier. 
      .getWorkItemCount(): Returns the count of pending WorkItems that
            can be retrieved.
      .getNextWorkItem([timeout=0]): Obtains the next pending work
            item, or None if none available. 
      .releaseWorkItem(wi): Releases a WorkItem instance obtained by
            getNextWorkItem(). Called when the application has finished
            processing the WorkItem. 
      .methodResponse( handle=<handle from WorkItem>,
                       [output argument list],
                       result=<status code>,
                       exception=<QmfData> ): Indicate to the agent
            that the application has completed processing a method
            request. A result code of zero indicates success.  If the
            result code is non-zero, exception may optionally be set to a
            QmfData object that describes the failure.  On success, zero or
            more output arguments may be supplied as defined by the method's


h3. AgentExternal Class

The AgentExternal class must be used by those applications that
implement the external store model. The AgentExternal class extends
the Agent class by adding interfaces that notify the application when
it needs to service a request for management operations from the

class AgentExternal(Agent):
      <constructor>(vendor, product, name, class Notifier)
      .allocObjectId( name="object name"): create a class ObjectId
            using the object name provided.  The object name must be
            unique across all objects known to this agent.  Returns a
            class ObjectId.  Once this method returns, the agent will
            service requests from consoles referencing this ObjectId.
      .freeObjectId( class ObjectId ): releases the ObjectId
            previously allocated by allocObjectId() method.  Once
            this call is complete, the agent will reject all further
            requests from consoles referencing this ObjectId.
      .queryResponse( handle=<handle from WorkItem>,
                      class QmfAgentObject,
                      prop=<True | False>,
                      stat=<True | False>): send a managed object in
            reply to a received query.  If (prop), send object's
            properties. If (stat), send object's statistics.  Note
            that ownership of the object instance is returned to the
            caller on return from this call.
      .queryComplete( handle=<handle from WorkItem>, 
                      result=<status code> ):  Indicate to the agent
            that the application has completed processing a query request.
            Zero or more calls to the queryResponse() method should be
            invoked before calling queryComplete().  If the query should
            fail - for example, due to authentication error - the result
            should be set to a non-zero error code ?TBD?.
      .subscriptionResponse( handle=<handle from WorkItem>,
                             result=<status code>,
                             subscription_handle=<application context>):
            Indicate the status of a subscription request.  If result
            is zero, the subscription is accepted by the application,
            and an subscription handle is provided.  This handle must
            be passed to the application when the agent unsubscribes.

h3. Asychronous event model.

The Agent uses the same notification driven work queue model as the
Console.  In the Agent case, the WorkItem supports the following set
of work types: 


In the case of an internal store agent implementation, only the
METHOD_CALL work item is generated.  An external store agent must support
all work item types.

  METHOD_CALL parameters: ( name=<method name>, 
                            [argument list],
                            class ObjectId,
                            user_id=<authenticated id of the user> )

The METHOD_CALL WorkItem describes a method call that must be serviced by the
application on behalf of this agent.  On completion of the
method call, the application must invoke the agent's
methodResponse() method.

  QUERY parameters: ( class Query, 
                      user_id=<authenticated id of the user> )


The QUERY WorkItem describes a query that the application must
service. The application should call the queryResponse() method for
each object that satisfies the query.  When complete, the application
must call the queryComplete() method.  If a failure occurs, the
application should indicate the error to the agent by calling the
queryComplete() method with a description of the error.

  SUBSCRIBE parameters: ( class Query, 
                          user_id=<authenticated id of the user> )

The SUBSCRIBE WorkItem provides a query that the application should
monitor until the subscription is cancelled.  On receipt of this
WorkItem, the application should call the subscriptionResponse() agent
method to acknowledge the response.  Whenever the matching objects are
updated, the application should call queryResponse() for each updated
object, followed by a call to queryComplete() when done.  The
subscription remains in effect until an UNSUBSCRIBE WorkItem for the
subscription is received. 

  UNSUBSCRIBE parameters: ( <handle associated with subscription> )

Alerts the application that the corresponding subscription has been
cancelled.  The application should no longer supply query updates
against the subscribed query.

h2. Revision History

11/20/2009 - First Revision
11/24/2009 - Added agent classes
12/01/2009 - Cleaned up Schema api and added map definitions.

Apache Qpid - AMQP Messaging Implementation
Project:      http://qpid.apache.org
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