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Subject [GitHub] [qpid-broker-j] alex-rufous commented on a change in pull request #36: QPID-8361: [Broker-J] Create a developer guide for Qpid Broker-J
Date Tue, 17 Sep 2019 13:21:48 GMT
alex-rufous 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_r325162398

 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
+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.
+![Broker Model](images/model.png)
+## 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
+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
+ * 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)
+`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
+`ConfigurationChangeListener` are used to trigger the update of the storage each time a `ConfiguredObject`
is changed.
+## AMQP Transport Layer
+At the high level, the transport layer
+ * accepts bytes from the wire and passes them to the protocol engines.
+ * pulls bytes from the protocol engines and pushes them down the wire.
+There are two AMQP Transport Layers in Broker-J.
+ * Traditional TCP/IP connections
+ * Websocket
+We'll consider the two layers separately below.
+The transport is responsible for TLS.  The TLS configuration is defined on the `Port`, `Keystore`
and `Truststore`
+model objects.  If so configured, it is the transport's responsibility to manage the TLS
+### TCP/IP
+This layer is implemented from first principles using Java NIO.
+It is non-blocking in nature.
+It uses a `Selector` to monitor all connected sockets (and the accepting socket) for work.
+Once work is detected (i.e. the `selector` returns) the connection work is serviced by threads
drawn from
+an IO thread pool.  An [eat-what-you-kill](https://webtide.com/eat-what-you-kill/) pattern
is used to reduce dispatch latency.
+This works in the following way. The worker thread that performed the select, after adding
all the ready connections
+to the work queue, adds the selector task to the work queue and then starts to process the
work queue itself
+(this is the eat what you kill bit).  This approach potentially avoids the dispatch latency
between the thread
+ that performed select and another thread from the IO thread pool. The `Selector` is the
responsibility of the
+`SelectorThread` class.
+A connections to a client is represented by a `NonBlockingConnection` instance.  The `SelectorThread`
causes the
+`NonBlockingConnections` that require IO work to be executed (`NonBlockingConnection#doWork`)
on a thread from
+an IO thread pool (owned by `NetworkConnectionScheduler`).  On each work cycle, the `NonBlockingConnection`
first goes
+through a write phase where pending work is pulled from the protocol engine producing bytes
for the wire in the process.
+If all the pending work is sent completely (i.e. the outbound network buffer is not exhausted),
+the next phase is a read phase. The bytes are consumed from the channel and fed into the
protocol engine.
+Finally, there is a further write phase to send any new bytes resulting from the input we
have just read.
+The write/read/write sequence is organised so in order that the `Broker` first evacuates
as much state from memory
+ as possible (thus freeing memory) before reading new bytes from the wire.
+In addition to the `NonBlockingConnection` being scheduled when singled by the `Selector`,
the `Broker` may need
+to awaken them at other times.  For instance, if a message arrives on a queue that is suitable
for a consumer,
+the `NonBlockingConnection` associated with that consumer must awoken. The mechanism that
does this is
+`NetworkConnectionScheduler#schedule` method which adds it to the work queue. This is wired
to the protocol engine via
+a listener.
+### Threading
+The only threads that execute `NonBlockingConnnections` are those of the `NetworkConnectionScheduler`.
+Furthermore, it is imperative that no `NonBlockingConnnection` is executed by more than one
thread at once.
+It is the job of `ConnectorProcessor` to organise this exclusivity. Updates made by `NonBlockingConnnection`
+be published safely so they can be read consistently by the other threads in the pool.
+There is a `NetworkConnectionScheduler` associated with each AMQP Port and each `VirtiualHost`.
+When a connection is made to the `Broker`, the initial exchanges between peer and broker
+(protocol headers, authentication etc) take place on the thread pool of the `NetworkConnectionScheduler`
of the `Port`.
+Once the connection has indicated which `VirtualHost` it wishes to connect to, responsibility
for the
+`NonBlockingConnection` shifts to the `NetworkConnectionScheduler` of the `VirtualHost`.
+### TLS
+The TCP/IP transport layer responds to the TLS configuration provided by the `Port`, `Keystore`
and `Truststore model` objects.
+It does this using the `NonBlockingConnectionDelegates`.
+ * The `NonBlockingConnectionUndecidedDelegate` is used to allow Plain/TLS port unification
+    (that is support for plain and TLS from the same port).  It sniffs the initial incoming
bytes to determine
+    if the peer is trying to negotiate a TLS connection or not.  Once the determination is
made one of the following
+    delegates is substituted in its place.
+ * NonBlockingConnectionTLSDelegate is responsible for TLS connections.  It feeds the bytes
through an SSLEngine.
+ * NonBlockingConnectionPlainDelegate is used for non-TLS connections.
+### Idle timeout
+All versions of the AMQP protocol support the idea of the peers regularly passing null data
to keep a wire that would
+otherwise by silent (during quiet times) busy. This is called idle timeout or heartbeating.
It is configured during
+connection establishment.  If a peer detects that a other has stopped sending this data,
it can infer
+that the network connection has failed or the peer has otherwise become inoperable and close
the connection.
+Sending of the null data is the responsibility of the `ServerIdleWriteTimeoutTicker`.  Responsibility
of detecting
+the absence of data from the peer is `ServerIdleReadTimeoutTicker`.  When the `Selector`
blocks awaiting activity
+the timeout is the minimum timeout value of all Tickers.
+### Websocket
+AMQP 1.0 specification defines AMQP 1.0 over web sockets.  The earlier version of the protocols
didn't do this
+but the implementation within the `Broker` actually supports Websocket transport.
+The websocket transport layer (`WebSocketProvider`)  uses Jetty's websocket module. The methods
of class
+`AmqpWebSocket` is annotated with the Jetty websocket annotations `OnWebSocketConnect`, `OnWebSocketMethod`,
+and `OnWebSocketClose`. The method implementation cause `ProtocolEngine` instances to the
create, bytes passed
+to the engine, or closed respectively.   When the protocol engine signals the need to work,
+a Jetty thread is used to pull the pending bytes bytes from the protocol engine
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