NO-JIRA: Remove out of date qpid/cpp/src/qpid/ha/README.md

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-<!--
-    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.
--->
-
-Overview of HA replication
-==========================
-
-Message Identifiers
--------------------
-
-Replication IDs are sequence numbers assigned to messages *before* a message is
-enqueued.  Originally the queue position number was used, but that was
-insufficient for two reasons:
--  We sometimes need to identify messages that are not yet enqueued, for example messages in an open transaction.
-- We don't want to require maintaining identical message sequences on every broker e.g. so transactions can be committed independently by each broker.
-
-We use the IDs to:
-- identify messages to dequeue on a backup.
-- remove extra messages from backup on failover.
-- avoid downloading messages already on backup on failover.
-
-
-On the primary
---------------
-
-The main classes on the primary are as follows:
-
-`RemoteBackup`: Represents a remote backup broker. Container for per-queue
-information about the broker.
-
-Each (queue,backup) pair has an instance of either or both of the following
-classes:
-
-`QueueGuard`: A queue observer that delays completion of messages as they are
-enqueued and completes messages when they are acknowledged or dequeued.
-
-`RepicatingSubscription`: A queue browser that sends messages to the backup and
-receives acknowledgments. Forwards acknowledgments to the `QueueGuard`
-
-`ReplicatingSubscription` and `QueueGuard` are separate because the guard
-can be created before the subscription.
-
-Events intercepted by HA code:
-
-- enqueue: Message published to queue, completion delayed (QueueGuard)
-- deliver: Message delivered to ReplicatingSubscription and sent to backup.
-- acknowledge: Message acknowledged by backup (ReplicatingSubscription)
-- dequeue: Message removed from queue by a consumer (QueueGuard)
-
-Message states:
-- new: initial state.
-- sent: ReplicatingSubscription has sent message to backup.
-- delayed: QueueGuard has delayed completion.
-- delayed-sent: Both sent and delayed.
-- safe: Replication code is done with the message: it is acknowledged or dequeued.
-
-Events:
-- enqueue: message enqueue on queue
-- deliver: message delivered to ReplicatingSubscription
-- acknowledged: message is acknowledged by backup
-- dequeued: message is dequeued by consumer.
-
-State transition diagram:
-
-    (new)--deliver-->(sent)--acknowledged/dequeued---------------->(safe)
-      |                 L---dequeued- -------------------------------^
-      L-enqueue->(delayed)--dequeued---------------------------------|
-                  |                                                  |
-                  L--deliver->(delayed-sent)--acknowled/dequeued-----|
-
-
-A QueueGuard is set on the queue when a backup subscribes or when a backup is
-promoted. Messages before the _first guarded position_ cannot be delayed
-because they may have already been acknowledged to clients.
-
-A backup sends a set of pre-acknowledged messages when subscribing, messages
-that are already on the backup and therefore safe.
-
-A `ReplicatingSubscription` is _ready_ when all messages are safe or delayed.  We
-know this is the case when all the following conditions hold:
-
-- The `ReplicatingSubscription` has reached the position preceeding the first guarded position AND
-- All messages prior to the first guarded position are safe.