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_r324587001
########## File path: doc/developer-guide/src/main/markdown/architecture.md ########## @@ -0,0 +1,399 @@ +# 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. + +![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 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. + +## 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 connection. + +### 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` must +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 feature + (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 +`WebSocketProvider.ConnectionWrapper#doWork`. The websocket transport tries to remain as close to the TCP/IP transport layer. + +The `Port`, `Keystore` and `Truststore` model objects are used to configure the websocket connection according +to the TLS requirements. + + +## AMQP Protocol Engines + +The `ProtocolEngine`: + + * accepts bytes from the transport (`ProtocolEngine#received`). + * exposes a public method (`ProtocolEngine#processPendingIterator`) which is used by the transport layer + to pull pending tasks that produce bytes for the wire from the engine. + +The engine never pushes bytes onto the transport. + +### Accepting bytes + +The transport references an instance of the `MultiVersionProtocolEngine`. Internally the `MultiVersionProtocolEngine` +delegates to other `ProtocolEngine` implementations. It switches from one implementation to another during +this connection's life. + +In this beginning, the `MultiVersionProtocolEngine` does not know which version of the AMQP protocol the peer wishes to use. +Internally it begins by delegating to a `SelfDelegateProtocolEngine` until sufficient header bytes have arrived from +the wire to make a determination (all AMQP protocols begin with the bytes AMQP followed by a version number). +Once a determination is made, a `ProtocolEngine` that supports the correct AMQP protocol is substituted in its place +(an implementation of `AMQPConnection`). The other alternative is that the desired protocol is not supported. +In this case a supported AMQP header is sent down the wire and the connection closed. + +There is an implementation of `AMQPConnection` for every AMQP protocol: + + * `AMQPConnection_0_8Impl` - for AMQP 0-8..0-91 + * `AMQPConnection_0_10Impl` - for AMQP 0-10 + * `AMQPConnection_1_0Impl` - for AMQP 1.0 + +The `AMQPConnection#received` method accepts the raw bytes. The connection implementation uses AMQP codecs +to turn this stream of bytes into a stream of object representing the AMQP frames. Review comment: There is missing plural in word 'object' ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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