Rename cpp/design to cpp/design_docs to avoid conflict with DESIGN on windows.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1029686 13f79535-47bb-0310-9956-ffa450edef68

2 files changed, 908 insertions, 0 deletions

diff --git a/qpid/cpp/design_docs/new-cluster-design.txt b/qpid/cpp/design_docs/new-cluster-design.txt
new file mode 100644
index 0000000000..7adb46fee3
--- /dev/null
+++ b/qpid/cpp/design_docs/new-cluster-design.txt
@@ -0,0 +1,435 @@
+-*-org-*-
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+* A new design for Qpid clustering.
+
+** Issues with current design.
+
+The cluster is based on virtual synchrony: each broker multicasts
+events and the events from all brokers are serialized and delivered in
+the same order to each broker.
+
+In the current design raw byte buffers from client connections are
+multicast, serialized and delivered in the same order to each broker.
+
+Each broker has a replica of all queues, exchanges, bindings and also
+all connections & sessions from every broker. Cluster code treats the
+broker as a "black box", it "plays" the client data into the
+connection objects and assumes that by giving the same input, each
+broker will reach the same state.
+
+A new broker joining the cluster receives a snapshot of the current
+cluster state, and then follows the multicast conversation.
+
+*** Maintenance issues.
+
+The entire state of each broker is replicated to every member:
+connections, sessions, queues, messages, exchanges, management objects
+etc. Any discrepancy in the state that affects how messages are
+allocated to consumers can cause an inconsistency.
+
+- Entire broker state must be faithfully updated to new members.
+- Management model also has to be replicated.
+- All queues are replicated, can't have unreplicated queues (e.g. for management)
+
+Events that are not deterministically predictable from the client
+input data stream can cause inconsistencies. In particular use of
+timers/timestamps require cluster workarounds to synchronize.
+
+A member that encounters an error which is not encounted by all other
+members is considered inconsistent and will shut itself down. Such
+errors can come from any area of the broker code, e.g. different
+ACL files can cause inconsistent errors.
+
+The following areas required workarounds to work in a cluster:
+
+- Timers/timestamps in broker code: management, heartbeats, TTL
+- Security: cluster must replicate *after* decryption by security layer.
+- Management: not initially included in the replicated model, source of many inconsistencies.
+
+It is very easy for someone adding a feature or fixing a bug in the
+standalone broker to break the cluster by:
+- adding new state that needs to be replicated in cluster updates.
+- doing something in a timer or other non-connection thread.
+
+It's very hard to test for such breaks. We need a looser coupling
+and a more explicitly defined interface between cluster and standalone
+broker code.
+
+*** Performance issues.
+
+Virtual synchrony delivers all data from all clients in a single
+stream to each broker.  The cluster must play this data thru the full
+broker code stack: connections, sessions etc. in a single thread
+context in order to get identical behavior on each broker. The cluster
+has a pipelined design to get some concurrency but this is a severe
+limitation on scalability in multi-core hosts compared to the
+standalone broker which processes each connection in a separate thread
+context.
+
+** A new cluster design.
+
+Clearly defined interface between broker code and cluster plug-in.
+
+Replicate queue events rather than client data.
+- Broker behavior only needs to match per-queue.
+- Smaller amount of code (queue implementation) that must behave predictably.
+- Events only need be serialized per-queue, allows concurrency between queues
+
+Use a moving queue ownership protocol to agree order of dequeues.
+No longer relies on identical state and lock-step behavior to cause
+identical dequeues on each broker.
+
+Each queue has an associated thread-context. Events for a queue are executed
+in that queues context, in parallel with events for other queues.
+
+*** Requirements
+
+The cluster must provide these delivery guarantees:
+
+- client sends transfer: message must be replicated and not lost even if the local broker crashes.
+- client acquires a message: message must not be delivered on another broker while acquired.
+- client accepts message: message is forgotten, will never be delivered or re-queued by any broker.
+- client releases message: message must be re-queued on cluster and not lost.
+- client rejects message: message must be dead-lettered or discarded and forgotten.
+- client disconnects/broker crashes: acquired but not accepted messages must be re-queued on cluster.
+
+Each guarantee takes effect when the client receives a *completion*
+for the associated command (transfer, acquire, reject, accept)
+
+*** Broker receiving messages
+
+On recieving a message transfer, in the connection thread we:
+- multicast a message-received event.
+- enqueue and complete the transfer when it is self-delivered.
+
+Other brokers enqueue the message when they recieve the message-received event.
+
+Enqueues are queued up with other queue operations to be executed in the
+thread context associated with the queue.
+
+*** Broker sending messages: moving queue ownership
+
+Each queue is *owned* by at most one cluster broker at a time. Only
+that broker may acquire or dequeue messages. The owner multicasts
+notification of messages it acquires/dequeues to the cluster.
+Periodically the owner hands over ownership to another interested
+broker, providing time-shared access to the queue among all interested
+brokers.
+
+We assume the same IO-driven dequeuing algorithm as the standalone
+broker with one modification: queues can be "locked". A locked queue
+is not available for dequeuing messages and will be skipped by the
+output algorithm.
+
+At any given time only those queues owned by the local broker will be
+unlocked.
+
+As messages are acquired/dequeued from unlocked queues by the IO threads
+the broker multicasts acquire/dequeue events to the cluster.
+
+When an unlocked queue has no more consumers with credit, or when a
+time limit expires, the broker relinquishes ownership by multicasting
+a release-queue event, allowing another interested broker to take
+ownership.
+
+*** Asynchronous completion of accept
+### HERE
+In acknowledged mode a message is not forgotten until it is accepted,
+to allow for requeue on rejection or crash. The accept should not be
+completed till the message has been forgotten.
+
+On receiving an accept the broker:
+- dequeues the message from the local queue
+- multicasts an "accept" event
+- completes the accept asynchronously when the dequeue event is self delivered.
+
+NOTE: The message store does not currently implement asynchronous
+completions of accept, this is a bug.
+
+** Inconsistent errors.
+
+The new design eliminates most sources of inconsistent errors
+(connections, sessions, security, management etc.) The only points
+where inconsistent errors can occur are at enqueue and dequeue (most
+likely store-related errors.)
+
+The new design can use the exisiting error-handling protocol with one
+major improvement: since brokers are no longer required to maintain
+identical state they do not have to stall processing while an error is
+being resolved.
+
+#TODO: The only source of dequeue errors is probably an unrecoverable journal failure.
+
+** Updating new members
+
+When a new member (the updatee) joins a cluster it needs to be brought
+up to date with the rest of the cluster.  An existing member (the
+updater) sends an "update".
+
+In the old cluster design the update is a snapshot of the entire
+broker state.  To ensure consistency of the snapshot both the updatee
+and the updater "stall" at the start of the update, i.e. they stop
+processing multicast events and queue them up for processing when the
+update is complete. This creates a back-log of work to get through,
+which leaves them lagging behind the rest of the cluster till they
+catch up (which is not guaranteed to happen in a bounded time.)
+
+With the new cluster design only exchanges, queues, bindings and
+messages need to be replicated.
+
+Update of wiring (exchanges, queues, bindings) is the same as current
+design.
+
+Update of messages is different:
+- per-queue rather than per-broker, separate queues can be updated in parallel.
+- updates queues in reverse order to eliminate unbounded catch-up
+- does not require updater & updatee to stall during update.
+
+Replication events, multicast to cluster:
+- enqueue(q,m): message m pushed on back of queue q .
+- acquire(q,m): mark m acquired
+- dequeue(q,m): forget m.
+Messages sent on update connection:
+- update_front(q,m): during update, receiver pushes m to *front* of q
+- update_done(q): during update, update of q is complete.
+
+Updater:
+- when updatee joins set iterator i = q.end()
+- while i != q.begin(): --i; send update_front(q,*i) to updatee
+- send update_done(q) to updatee
+
+Updatee:
+- q initially in locked state, can't dequeue locally.
+- start processing replication events for q immediately (enqueue, dequeue, acquire etc.)
+- receive update_front(q,m): q.push_front(m)
+- receive update_done(q): q can be unlocked for local dequeing.
+
+Benefits:
+- Stall only for wiring update: updater & updatee can process multicast messages while messages are updated.
+- No unbounded catch-up: update consists of at most N update_front() messages where N=q.size() at start of update.
+- During update consumers actually help by removing messages before they need to be updated.
+- Needs no separate "work to do" queue, only the broker queues themselves.
+
+# TODO how can we recover from updater crashing before update complete?
+# Clear queues that are not updated & send request for udpates on those queues?
+
+# TODO updatee may receive a dequeue for a message it has not yet seen, needs
+# to hold on to that so it can drop the message when it is seen.
+# Similar problem exists for wiring?
+
+** Cluster API
+
+The new cluster API is similar to the MessageStore interface.
+(Initially I thought it would be an extension of the MessageStore interface,
+but as the design develops it seems better to make it a separate interface.)
+
+The cluster interface captures these events:
+- wiring changes: queue/exchange declare/bind
+- message enqueued/acquired/released/rejected/dequeued.
+
+The cluster will require some extensions to the Queue:
+- Queues can be "locked", locked queues are ignored by IO-driven output.
+- Cluster must be able to apply queue events from the cluster to a queue.
+  These appear to fit into existing queue operations.
+
+** Maintainability
+
+This design gives us more robust code with a clear and explicit interfaces.
+
+The cluster depends on specific events clearly defined by an explicit
+interface. Provided the semantics of this interface are not violated,
+the cluster will not be broken by changes to broker code.
+
+The cluster no longer requires identical processing of the entire
+broker stack on each broker. It is not affected by the details of how
+the broker allocates messages. It is independent of the
+protocol-specific state of connections and sessions and so is
+protected from future protocol changes (e.g. AMQP 1.0)
+
+A number of specific ways the code will be simplified:
+- drop code to replicate management model.
+- drop timer workarounds for TTL, management, heartbeats.
+- drop "cluster-safe assertions" in broker code.
+- drop connections, sessions, management from cluster update.
+- drop security workarounds: cluster code now operates after message decoding.
+- drop connection tracking in cluster code.
+- simper inconsistent-error handling code, no need to stall.
+
+** Performance
+
+The only way to verify the relative performance of the new design is
+to prototype & profile. The following points suggest the new design
+may scale/perform better:
+
+Some work moved from virtual synchrony thread to connection threads:
+- All connection/session logic moves to connection thread.
+- Exchange routing logic moves to connection thread.
+- On local broker dequeueing is done in connection thread
+- Local broker dequeue is IO driven as for a standalone broker.
+
+For queues with all consumers on a single node dequeue is all
+IO-driven in connection thread. Pay for time-sharing only if queue has
+consumers on multiple brokers.
+
+Doing work for different queues in parallel scales on multi-core boxes when
+there are multiple queues.
+
+One difference works against performance, thre is an extra
+encode/decode. The old design multicasts raw client data and decodes
+it in the virtual synchrony thread. The new design would decode
+messages in the connection thread, re-encode them for multicast, and
+decode (on non-local brokers) in the virtual synchrony thread. There
+is extra work here, but only in the *connection* thread: on a
+multi-core machine this happens in parallel for every connection, so
+it probably is not a bottleneck. There may be scope to optimize
+decode/re-encode by re-using some of the original encoded data, this
+could also benefit the stand-alone broker.
+
+** Asynchronous queue replication
+
+The existing "asynchronous queue replication" feature maintains a
+passive backup passive backup of queues on a remote broker over a TCP
+connection.
+
+The new cluster replication protocol could be re-used to implement
+asynchronous queue replication: its just a special case where the
+active broker is always the queue owner and the enqueue/dequeue
+messages are sent over a TCP connection rather than multicast.
+
+The new update update mechanism could also work with 'asynchronous
+queue replication', allowing such replication (over a TCP connection
+on a WAN say) to be initiated after the queue had already been created
+and been in use (one of the key missing features).
+
+** Increasing Concurrency and load sharing
+
+The current cluster is bottlenecked by processing everything in the
+CPG deliver thread. By removing the need for identical operation on
+each broker, we open up the possiblility of greater concurrency.
+
+Handling multicast enqueue, acquire, accpet, release etc: concurrency
+per queue.  Operatons on different queues can be done in different
+threads.
+
+The new design does not force each broker to do all the work in the
+CPG thread so spreading load across cluster members should give some
+scale-up.
+
+** Misc outstanding issues & notes
+
+Replicating wiring
+- Need async completion of wiring commands?
+- qpid.sequence_counter: need extra work to support in new design, do we care?
+
+Cluster+persistence:
+- finish async completion: dequeue completion for store & cluster
+- cluster restart from store: clean stores *not* identical, pick 1, all others update.
+- need to generate cluster ids for messages recovered from store.
+
+Live updates: we don't need to stall brokers during an update!
+- update on queue-by-queue basis.
+- updatee locks queues during update, no dequeue.
+- update in reverse: don't update messages dequeued during update.
+- updatee adds update messages at front (as normal), replicated messages at back.
+- updater starts from back, sends "update done" when it hits front of queue.
+
+Flow control: need to throttle multicasting
+1. bound the number of outstanding multicasts.
+2. ensure the entire cluster keeps up, no unbounded "lag"
+The existing design uses read-credit to solve 1., and does not solve 2.
+New design should stop reading on all connections while flow control
+condition exists?
+
+Can federation also be unified, at least in configuration?
+
+Consider queues (and exchanges?) as having "reliability" attributes:
+- persistent: is the message stored on disk.
+- backed-up (to another broker): active/passive async replication.
+- replicated (to a cluster): active/active multicast replication to cluster.
+- federated: federation link to a queue/exchange on another broker.
+
+"Reliability" seems right for the first 3 but not for federation, is
+there a better term?
+
+Clustering and scalability: new design may give us the flexibility to
+address scalability as part of cluster design. Think about
+relationship to federation and "fragmented queues" idea.
+
+* Design debates/descisions
+
+** Active/active vs. active passive
+
+An active-active cluster can be used in an active-passive mode. In
+this mode we would like the cluster to be as efficient as a strictly
+active-passive implementation.
+
+An active/passive implementation allows some simplifications over active/active:
+- drop Queue ownership and locking
+- don't need to replicate message acquisition.
+- can do immediate local enqueue and still guarantee order.
+
+Active/passive introduces a few extra requirements:
+- Exactly one broker hast to take over if primary fails.
+- Passive members must refuse client connections.
+- On failover, clients must re-try all known addresses till they find the active member.
+
+Active/active benefits:
+- A broker failure only affects the subset of clients connected to that broker.
+- Clients can switch to any other broker on failover
+- Backup brokers are immediately available on failover.
+- Some load sharing: reading from client + multicast only done on direct node.
+
+Active/active drawbacks:
+- Co-ordinating message acquisition may impact performance (not tested)
+- Code may be more complex that active/passive.
+
+Active/passive benefits:
+- Don't need message allocation strategy, can feed consumers at top speed.
+- Code may be simpler than active/active.
+
+Active/passive drawbacks:
+- All clients on one node so a failure affects every client in the system.
+- After a failure there is a "reconnect storm" as every client reconnects to the new active node.
+- After a failure there is a period where no broker is active, until the other brokers realize the primary is gone and agree on the new primary.
+- Clients must find the single active node, may involve multiple connect attempts.
+
+** Total ordering.
+
+Initial thinking: allow message ordering to differ between brokers.
+New thinking: use CPG total ordering, get identical ordering on all brokers.
+- Allowing variation in order introduces too much chance of unexpected behavior.
+- Usign total order allows other optimizations, see Message Identifiers below.
+
+** Message identifiers.
+
+Initial thinking: message ID = CPG node id + 64 bit sequence number.
+This involves a lot of mapping between cluster IDs and broker messsages.
+
+New thinking: message ID = queue name + queue position.
+- Removes most of the mapping and memory management for cluster code.
+- Requires total ordering of messages (see above)
+
+** Message rejection
+
+Initial thinking: add special reject/rejected points to cluster interface so
+rejected messages could be re-queued without multicast.
+
+New thinking: treat re-queueing after reject as entirely new message.
+- Simplifies cluster interface & implementation
+- Not on the critical path.
diff --git a/qpid/cpp/design_docs/new-cluster-plan.txt b/qpid/cpp/design_docs/new-cluster-plan.txt
new file mode 100644
index 0000000000..b5cd353fc0
--- /dev/null
+++ b/qpid/cpp/design_docs/new-cluster-plan.txt
@@ -0,0 +1,473 @@
+-*-org-*-
+
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+
+Notes on new cluster implementation. See also: new-cluster-design.txt
+
+* Implementation plan.
+
+Co-existence with old cluster code and tests:
+- Separate plugin cluster2, options --cluster2-*. Eventually renamed to replace cluster.
+- Double up tests with old version/new version as the new code develops.
+
+Minimal POC for message delivery & perf test.
+- no wiring replication, no updates, no failover, no persistence, no async completion.
+- just implement publish and acquire/dequeue locking protocol.
+- optimize the special case where all consumers are on the same node.
+- measure performance: compare active-passive and active-active modes of use.
+
+Full implementation of transient cluster
+- Update (based on existing update), async completion etc.
+- Passing all existing transient cluster tests.
+
+Persistent cluster
+- Make sure async completion works correctly.
+- InitialStatus protoocl etc. to support persistent start-up (existing code)
+- cluster restart from store: stores not identical. Load one, update the rest.
+ - assign cluster ID's to messages recovered from store, don't replicate.
+
+Improved update protocol
+- per-queue, less stalling, bounded catch-up.
+
+* Task list
+
+** TODO [#A] Minimal POC: publish/acquire/dequeue protocol.
+
+NOTE: as implementation questions arise, take the easiest option and make
+a note for later optimization/improvement.
+
+*** Tests
+- python test: 4 senders, numbered messages, 4 receivers, verify message set.
+- acquire then release messages: verify can be dequeued on any member
+- acquire then kill broker: verify can be dequeued other members.
+- acquire then reject: verify goes on alt-exchange once only.
+
+*** DONE broker::Cluster interface and call points.
+
+Initial interface commited.
+
+*** Main classes
+
+BrokerHandler:
+- implements broker::Cluster intercept points.
+- sends mcast events to inform cluster of local actions.
+- thread safe, called in connection threads.
+
+LocalMessageMap:
+- Holds local messages while they are being enqueued.
+- thread safe: called by both BrokerHandler and MessageHandler
+
+MessageHandler:
+- handles delivered mcast messages related to messages.
+- initiates local actions in response to mcast events.
+- thread unsafe, only called in deliver thread.
+- maintains view of cluster state regarding messages.
+
+QueueOwnerHandler:
+- handles delivered mcast messages related to queue consumer ownership.
+- thread safe, called in deliver, connection and timer threads.
+- maintains view of cluster state regarding queue ownership.
+
+cluster::Core: class to hold new cluster together (replaces cluster::Cluster)
+- thread safe: manage state used by both MessageHandler and BrokerHandler
+
+The following code sketch illustrates only the "happy path" error handling
+is omitted.
+
+*** BrokerHandler
+Types:
+- struct QueuedMessage { Message msg; QueueName q; SequenceNumber position; }
+- struct
+
+NOTE:
+- Messages on queues are identified by a queue name + a position.
+- Messages being routed are identified by a sequence number.
+
+Members:
+- thread_local bool noReplicate // suppress replication.
+- thread_local bool isRouting // suppress operations while routing
+- Message localMessage[SequenceNumber] // local messages being routed.
+- thread_local SequenceNumber routingSequence
+
+NOTE: localMessage is also modified by MessageHandler.
+
+broker::Cluster intercept functions:
+
+routing(msg)
+  if noReplicate: return
+  # Supress everything except enqueues while we are routing.
+  # We don't want to replicate acquires & dequeues caused by an enqueu,
+  # e.g. removal of messages from ring/LV queues.
+  isRouting = true
+
+enqueue(qmsg):
+  if noReplicate: return
+  if routingSequence == 0 # thread local
+    routingSequence = nextRoutingSequence()
+    mcast create(encode(qmsg.msg),routingSeq)
+  mcast enqueue(qmsg.q,routingSeq)
+
+routed(msg):
+  if noReplicate: return
+  isRouting = false
+
+acquire(qmsg):
+  if noReplicate: return
+  if isRouting: return # Ignore while we are routing a message.
+  if msg.id: mcast acquire(qmsg)
+
+release(QueuedMessage)
+  if noReplicate: return
+  if isRouting: return # Ignore while we are routing a message.
+  mcast release(qmsg)
+
+accept(QueuedMessage):
+  if noReplicate: return
+  if isRouting: return # Ignore while we are routing a message.
+  mcast accept(qmsg)
+
+reject(QueuedMessage):
+  isRejecting = true
+  mcast reject(qmsg)
+
+# FIXME no longer needed?
+drop(QueuedMessage)
+  cleanup(qmsg)
+
+*** MessageHandler and mcast messages
+Types:
+- struct QueueEntry { QueuedMessage qmsg; NodeId acquired; }
+- struct QueueKey { MessageId id; QueueName q; }
+- typedef map<QueueKey, QueueEntry> Queue
+- struct Node { Message routing[SequenceNumber]; list<QueueKey> acquired; }
+
+Members:
+- QueueEntry enqueued[QueueKey]
+- Node node[NodeId]
+
+Mcast messages in Message class:
+
+create(msg,seq)
+  if sender != self: node[sender].routing[seq] = decode(msg)
+
+enqueue(q,seq):
+  id = (sender,seq)
+  if sender == self:
+    enqueued[id,q] = (localMessage[seq], acquired=None)
+  else:
+    msg = sender.routing[seq]
+    enqueued[id,q] = (qmsg, acquired=None)
+    with noReplicate=true: qmsg = broker.getQueue(q).push(msg)
+
+routed(seq):
+  if sender == self: localMessage.erase(msg.id.seq)
+  else: sender.routing.erase(seq)
+
+acquire(id,q):
+  enqueued[id,q].acquired = sender
+  node[sender].acquired.push_back((id,q))
+  if sender != self:
+    with noReplicate=true: broker.getQueue(q).acquire(enqueued[id,q])
+
+release(id,q)
+  enqueued[id,q].acquired = None
+  node[sender].acquired.erase((id,q))
+  if sender != self
+    with noReplicate=true: broker.getQueue(q).requeue(enqueued[id,q])
+
+reject(id,q):
+  sender.routing[id] = enqueued[id,q] # prepare for re-queueing
+
+rejected(id,q)
+  sender.routing.erase[id]
+
+dequeue(id,q)
+  entry = enqueued[id,q]
+  enqueued.erase[id,q]
+  node[entry.acquired].acquired.erase(id,q)
+  if sender != self:
+    with noReplicate=true: broker.getQueue(q).dequeue(entry.qmsg)
+
+member m leaves cluster:
+  for key in node[m].acquired:
+   release(key.id, key.q)
+  node.erase(m)
+
+*** Queue consumer locking
+
+When a queue is locked it does not deliver messages to its consumers.
+
+New broker::Queue functions:
+- stopConsumers(): set consumersStopped flag, wait for currently busy consumers to exit.
+- startConsumers(): reset consumersStopped flag
+
+Implementation sketch, locking omitted:
+
+void Queue::stopConsumers() {
+  consumersStopped = true;
+  while (consumersBusy) consumersBusyMonitor.wait();
+}
+
+void Queue::startConsumers() {
+  consumersStopped = false;
+  listeners.notify();
+}
+
+bool Queue::dispatch(consumer) {
+   if (consumersStopped) return false;
+   ++consumersBusy;
+   do_regular_dispatch_body()
+   if (--consumersBusy == 0) consumersBusyMonitor.notify();
+}
+
+*** QueueOwnerHandler
+
+Invariants:
+- Each queue is owned by at most one node at any time.
+- Each node is interested in a set of queues at any given time.
+- A queue is un-owned if no node is interested.
+
+The queue owner releases the queue when
+- it loses interest i.e. queue has no consumers with credit.
+- a configured time delay expires and there are other interested nodes.
+
+The owner mcasts release(q). On delivery the new queue owner is the
+next node in node-id order (treating nodes as a circular list)
+starting from the old owner that is interested in the queue.
+
+Queue consumers initially are stopped, only started when we get
+ownership from the cluster.
+
+Thread safety: called by deliver, connection and timer threads, needs locking.
+
+Thread safe object per queue holding queue ownership status.
+Called by deliver, connection and timer threads.
+
+class QueueOwnership {
+  bool owned;
+  Timer timer;
+  BrokerQueue q;
+
+  drop(): # locked
+    if owned:
+      owned = false
+      q.stopConsumers()
+      mcast release(q.name, false)
+      timer.stop()
+
+  take(): # locked
+    if not owned:
+      owned = true
+      q.startConsumers()
+      timer.start(timeout)
+
+  timer.fire(): drop()
+}
+
+Data Members, only modified/examined in deliver thread:
+- typedef set<NodeId> ConsumerSet
+- map<QueueName, ConsumerSet> consumers
+- map<QueueName, NodeId> owner
+
+Thread safe data members, accessed in connection threads (via BrokerHandler):
+- map<QueueName, QueueOwnership> ownership
+
+Multicast messages in QueueOwner class:
+
+consume(q):
+  if sender==self and consumers[q].empty(): ownership[q].take()
+  consumers[q].insert(sender)
+
+release(q):
+  asssert(owner[q] == sender and owner[q] in consumers[q])
+  owner[q] = circular search from sender in consumers[q]
+  if owner==self: ownership[q].take()
+
+cancel(q):
+  assert(queue[q].owner != sender) # sender must release() before cancel()
+  consumers[q].erase(sender)
+
+member-leaves:
+  for q in queue: if owner[q] = left: left.release(q)
+
+Need 2 more intercept points in broker::Cluster:
+
+consume(q,consumer,consumerCount) - Queue::consume()
+  if consumerCount == 1: mcast consume(q)
+
+cancel(q,consumer,consumerCount) - Queue::cancel()
+  if consumerCount == 0:
+    ownership[q].drop()
+  mcast cancel(q)
+
+#TODO: lifecycle, updating cluster data structures when queues are destroyed
+
+*** Increasing concurrency
+The major performance limitation of the old cluster is that it does
+everything in the single CPG deliver thread context.
+
+We can get additional concurrency by creating a thread context _per queue_
+for queue operations: enqueue, acquire, accept etc.
+
+We associate a PollableQueue of queue operations with each AMQP queue.
+The CPG deliver thread would
+- build messages and associate with cluster IDs.
+- push queue ops to the appropriate PollableQueue to be dispatched the queues thread.
+
+Serializing operations on the same queue avoids contention, but takes advantage
+of the independence of operations on separate queues.
+
+*** Re-use of existing cluster code
+- re-use Event
+- re-use Multicaster
+- re-use same PollableQueueSetup (may experiment later)
+- new Core class to replace Cluster.
+- keep design modular, keep threading rules clear.
+
+** TODO [#B] Large message replication.
+Multicast should encode messages in fixed size buffers (64k)?
+Can't assume we can send message in one chunk.
+For 0-10 can use channel numbers & send whole frames packed into larger buffer.
+** TODO [#B] Batch CPG multicast messages
+The new cluster design involves a lot of small multicast messages,
+they need to be batched into larger CPG messages for efficiency.
+** TODO [#B] Genuine async completion
+Replace current synchronous waiting implementation with genuine async completion.
+
+Test: enhance test_store.cpp to defer enqueueComplete till special message received.
+
+Async callback uses *requestIOProcessing* to queue action on IO thread.
+
+** TODO [#B] Async completion of accept when dequeue completes.
+Interface is already there on broker::Message, just need to ensure
+that store and cluster implementations call it appropriately.
+
+** TODO [#B] Replicate wiring.
+From messageStore create/destroy/bind, replicate encoded declare/destroy/bind command.
+
+** TODO [#B] New members joining - first pass
+
+Re-use update code from old cluster but don't replicate sessions &
+connections.
+
+Need to extend it to send cluster IDs with messages.
+
+Need to replicate the queue ownership data as part of the update.
+
+** TODO [#B] Persistence support.
+InitialStatus protoocl etc. to support persistent start-up (existing code)
+
+Only one broker recovers from store, update to others.
+
+Assign cluster IDs to messages recovered from store, don't replicate. See Queue::recover.
+
+** TODO [#B] Handle other ways that messages can leave a queue.
+
+Other ways (other than via a consumer) that messages are take off a queue.
+
+NOTE: Not controlled by queue lock, how to make them consistent?
+
+Target broker may not have all messages on other brokers for purge/destroy.
+- Queue::move() - need to wait for lock? Replicate?
+- Queue::get() - ???
+- Queue::purge() - replicate purge? or just delete what's on broker ?
+- Queue::destroy() - messages to alternate exchange on all brokers.?
+
+Need to add callpoints & mcast messages to replicate these?
+
+** TODO [#B] Flow control for internal queues.
+
+Need to bound the size of internal queues: delivery and multicast.
+- stop polling for read on client connections when we reach a bound.
+- restart polling when we get back under it.
+
+That will stop local multicasting, we still have to deal with remote
+multicasting (note existing cluster does not do this.) Something like:
+- when over bounds multicast a flow-control event.
+- on delivery of flow-control all members stop polling to read client connections
+- when back under bounds send flow-control-end, all members resume
+- if flow-controling member dies others resume
+
+** TODO [#B] Integration with transactions.
+Do we want to replicate during transaction & replicate commit/rollback
+or replicate only on commit?
+No integration with DTX transactions.
+** TODO [#B] Make new cluster work with replication exchange.
+Possibly re-use some common logic. Replication exchange is like clustering
+except over TCP.
+** TODO [#B] Better concurrency, scalabiility on multi-cores.
+Introduce PollableQueue of operations per broker queue. Queue up mcast
+operations (enqueue, acquire, accept etc.) to be handled concurrently
+on different queue. Performance testing to verify improved scalability.
+** TODO [#C] Async completion for declare, bind, destroy queues and exchanges.
+Cluster needs to complete these asynchronously to guarantee resources
+exist across the cluster when the command completes.
+
+** TODO [#C] Allow non-replicated exchanges, queues.
+
+Set qpid.replicated=false in declare arguments, set flag on Exchange, Queue objects.
+- save replicated status to store.
+- support in management tools.
+Replicated exchange: replicate binds to replicated queues.
+Replicated queue: replicate all messages.
+
+** TODO [#C] New members joining - improved.
+
+Replicate wiring like old cluster, stall for wiring but not for
+messages.  Update messages on a per-queue basis from back to front.
+
+Updater:
+- stall & push wiring: declare exchanges, queues, bindings.
+- start update iterator thread on each queue.
+- unstall and process normally while iterator threads run.
+
+Update iterator thread:
+- starts at back of updater queue, message m.
+- send update_front(q,m) to updatee and advance towards front
+- at front: send update_done(q)
+
+Updatee:
+- stall, receive wiring, lock all queues, mark queues "updating", unstall
+- update_front(q,m): push m to *front* of q
+- update_done(q): mark queue "ready"
+
+Updatee cannot take the queue consume lock for a queue that is  updating.
+Updatee *can* push messages onto a queue that is updating.
+
+TODO: Is there any way to eliminate the stall for wiring?
+
+** TODO [#C] Refactoring of common concerns.
+
+There are a bunch of things that act as "Queue observers" with intercept
+points in similar places.
+- QueuePolicy
+- QueuedEvents (async replication)
+- MessageStore
+- Cluster
+
+Look for ways to capitalize on the similarity & simplify the code.
+
+In particular QueuedEvents (async replication) strongly resembles
+cluster replication, but over TCP rather than multicast.
+** TODO [#C] Concurrency for enqueue events.
+All enqueue events are being processed in the CPG deliver thread context which
+serializes all the work. We only need ordering on a per queue basis, can we
+enqueue in parallel on different queues and will that improve performance?
+** TODO [#C] Handling immediate messages in a cluster
+Include remote consumers in descision to deliver an immediate message?