vavrtom commented on a change in pull request #36: QPID-8361: [Broker-J] Create
a developer guide for Qpid Broker-J
URL: https://github.com/apache/qpid-broker-j/pull/36#discussion_r324197112
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File path: doc/developer-guide/src/main/markdown/architecture.md
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+# High Level Architecture
+
+This article provides a high level description of the architecture of Qpid
Broker-J.
+
+Broker-J is messaging broker that implements the AMQP protocols (version 0-8,
0-9, 0-91, 0-10 and 1.0).
+Any AMQP compliant messaging library can be used with the Broker. The Broker
supports on the fly message translation
+from one AMQP protocol to another, meaning it is possible to use the Broker to
allow clients that use different
+AMQP protocol version to exchange messages. It can be managed over a built in
HTTP interface
+(that presents a REST API and a Web Management Console), or by AMQP Management
(early draft implementation).
+
+The Broker has a highly pluggable architecture that allows alternative
implementations to be substituted for any concern.
+For instance, you can simply build a module delegating to your own storage or
own authentication provider linking
+to your enterprise authentication backend.
+
+Broker-J is 100% pure Java. It can be run standalone or embedded within
another Java applications.
+
+## Model
+
+A tree of manageable categories, all of which extend of the interface
`ConfiguredObject`, underpin the `Broker`.
+A `ConfiguredObject` has zero or more attributes, zero or more children and
zero or more context variable name/value pairs.
+A `ConfiguredObject` may be persisted to a configuration store so its state
can be restored when the Broker is restarted.
+The manageable categories are arranged into a tree structure. `SystemConfig`
is at the root and has a single descendent
+`Broker`. The `Broker` itself has children: `Port`, `AuthenticationProvider`,
`VirtualHostNode` amongst others.
+`VirtualHostNode` has a child `VirtualHost`. The children of the
`VirtualHost` are categories that directly involved
+in messaging such as `Queue`. The diagram below illustrates the category
hierarchy but many categories are elided for brevity.
+The model tree structure is codified in the `BrokerModel` class.
+
+
+
+## Category Specializations
+
+Some categories have specialisations. An example is the category `Queue`. It
has specialisations corresponding to
+the queue types supported by the `Broker` e.g. `StandardQueue`,
`PrirorityQueue` etc.
+
+### Attributes
+
+Each `ConfiguredObject` instance has zero or more attributes. Attributes
have a name and a value which can be
+a Java primitive value or an instance of any class for which an
`AttributeValueConverter` exist. This mechanism allows
+ attribute values to be `Lists`, `Sets`, `Maps`, or arbitrary structured types
`ManagedAttributeValues`.
+
+Attributes are marked up in the code with method annotations
`@ManagedAttribute` which defines things
+whether the attribute is mandatory or mutable. Attributes can also be marked
a secure which indicates restrictions
+ no how the attribute is used (used for attributes that that store passwords
or private-keys).
+
+Attributes can have default values. The default value applies if the user
omits to supply a value when the object
+is created. Defaults themselves can be defined in terms of `context variable`
references.
+
+### Context Variables
+
+Each `ConfiguredObject` instance has zero or more context variable
assignments. These are simply name/value pairs
+where both name and value are strings.
+
+When resolving an attribute's value, if the attribute's value (or attribute's
default) contains a context variable
+reference (e.g. `${foo}`), the variable is first resolved using the
`ConfiguredObject`'s own context variables.
+If the `ConfiguedObject` has no definition for the context variable, the
entity's parent is tried,
+then its grandparent and so forth, all the way until the `SystemContext` is
reached.
+If the `SystemContext` provides no value, the JVM's system properties are
consulted.
+
+A context variable's value can be defined in terms of other context variables.
+
+Context variables are useful for extracting environment specific information
from configuration for instance path stems
+or port numbers.
+
+## Lifecycle
+
+`ConfiguredObjects` have a lifecycle.
+
+A `ConfiguredObject` is created exactly once by a call its parent's
`#createChild()` method.
+This brings the object into existence. It goes through a number of phases
during creation (`ConfiguredObject#create`)
+
+ * resolution (where the attribute values are resolved and assigned to the
object)
+ * creation validation (ensuring business rules are adhered to)
+ * registration with parents
+ * implementation specific creation (`#onCreate`)
+ * implementation specific opening (`#onOpen`)
+
+When the `Broker` is restarted objects that exist in the configuration store
are said to be recovered.
+During recovery, they follow the opening (`ConfiguredObject#open`)
+
+ * resolution (where the attribute values are resolved and assigned to the
object)
+ * validation (ensuring business rules are adhered to)
+ * implementation specific opening (#onOpen)
+
+Some `ConfiguredObjects` support starting (`ConfiguredObject#start()`) and
stopping (`ConfiguredObject#stop()`)
+ but this have not yet been extended to all objects.
+
+`ConfiguredObject#delete()` caused the object to be deleted.
+
+## AbstractConfiguredObject
+
+Most configured object implementations extend `AbstractConfiguredObject`
(ACO). ACO provides the mechanics
+behind the configured implementations: attributes, context variables, state
and lifecycle,
+and a listener mechanism: `ConfigurationChangeListener`.
+
+## Threading
+
+The threading model used by the model must be understood before changes can be
made safely.
+
+The `Broker` and `VirtualHost` `ConfiguredObject` instances have a task
executor backed by single configuration thread.
+Whenever the a configuration object needs to be changed, that change MUST be
made by the nearest ancestor's
+configuration thread. This approach ensures avoids the need to employ
locking. Any thread is allowed to observe
+ the state of a `ConfiguredObject` at any time. For this reasons, changes
must be published safely, so they can be
+read consistently by the observing threads.
+
+The implementations of the mutating methods (`#setAttributes()`, `#start()`,
#`stop()`, etc) within
+`AbstractConfiguredObject` are already implemented to adhere to these rules.
+
+## Configuration Persistence
+
+`ConfiguredObject` categories such as `SystemConfig` and `VirtualhostNode`
take responsibility for managing the storage
+of their children. This is marked up in the model with the `@ManagedObject`
annotation (`#managesChildren`).
+These objects utilise a `DurableConfigurationStore` to persist their durable
children to storage.
+`ConfigurationChangeListener` are used to trigger the update of the storage
each time a `ConfiguredObject` is changed.
Review comment:
'Are' should be most likely 'is'
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