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+                ===========================================
+                         MySQL Replication Servers
+                     Lars Thalmann and Johan Andersson
+                              2003 MySQL AB
+                ===========================================
+ 
+-------------------------------------------------------------------------------
+
+                PRIMARY SYSTEM               STANDBY SYSTEM 
+              REPLICATION SERVER           REPLICATION SERVER
+             (PS or SOURCE SYSTEM)     (SS or DESTINATION SYSTEM)
+             +------------------+     +-------------------------+
+             |     RepMain      |     |    RepMain [Requests]   |
+             |                  |     +-------------------------+
+             |                  |     |   Requestor [Executes]  |
+             +------------------+     +-------------------------+
+      PS --- | ExtNDB | TransPS | --- |   TransSS  |  AppNDB    | --- SS
+             +------------------+     +-------------------------+
+                (GCIContainer)               (GCIContainer)
+                                               (RepState)
+
+                      Figure 1: Replication Server Threads 
+
+Component List
+--------------
+RepMain
+  Main thread that runs command-line interpreter [Requests]
+
+Requestor
+  Thread that runs RepState::execute [Executes]
+
+ExtNDB
+  Extracts transaction epochs from NDB Cluster
+
+TransPS, TransSS
+  Transfers information (control and epoch buffers) between 
+  Replication Servers.
+
+AppNDB
+  Applies transaction epochs to NDB Cluster
+
+-------------------------------------------------------------------------------
+
+               RepState Control
+                    Object
+             +------------------+    
+             |     RepState     |    
+             |    [Requests]    |    
+             |    [Executes]    |    
+             +------------------+    
+             |  RepStateRequest | --- ExtSender
+             +------------------+ 
+
+                      Figure 2: RepState Object
+
+
+The RepState object is shared by all components.
+
+
+-------------------------------------------------------------------------------
+
+Dependent Directories
+---------------------
+rep/adapters    Appliers and Extractors
+  All code dependent on the database system
+
+rep/transfer
+  Depends on NDB transporters
+
+rep/state
+  Shared resources for all components
+
+Independent Directories
+-----------------------
+rep/storage     Storage of epochs 
+  Should not depend on any transporters/NDB specific
+
+rep/repstate
+  Should only have a reference to an ExtSender for the external PS REP node
+
+
+-------------------------------------------------------------------------------
+
+Replication Teminology
+----------------------
+GLOBAL CHECKPOINT <global checkpoint id - gci>
+A global checkpoint is a point in time when all database server
+are synchronized.
+
+NODE
+A database server with information.
+
+NODE GROUP <node group id>
+A set of database servers, all storing the same information.
+
+SUBSCRIPTION <subscription id>.
+A "subscription" is a collection of services that a source system 
+provides.  The main to services belonging to a subscription are 
+"log" and "scan".  Log provides the replication servers with 
+log entries (epochs) and scan provides the replication servers
+with scanned data (also stored in epochs).
+
+EPOCH <subscription id, gci>
+An "epoch" is a log of all database changes between two time points.
+(An epoch can have redundant log entries.)  An epoch is named by the
+number of the time slice between the two time points.
+
+EPOCH BUFFER <subscription id, gci, node group id>
+An "epoch buffer" is a part of the log belonging to an epoch.  An
+epoch buffer does not contain any redundancy.  
+
+Two epoch buffers with the same subscription id and gci can be
+"complements" or "duplicates" to each other.  If they are complements,
+they store different information, if they are duplicates then they
+store equivalent information (the information need not be identical,
+but it is equivalent for the purpose of restoring the original
+information).  If they are duplicates then they have the same name,
+i.e. same subscription id, gci, and node group id.
+
+CHANNEL <subscription id>
+A "channel" is a collection of epoch buffers belonging to
+a specific subscription.  (The channel can exist before it is 
+assigned to a subscription.)
+
+SUBSCRIPTION CONSISTENT
+A database is "subscription consistent" or "consistent with respect
+to a subscription" if ...
+
+Architectural Terminology
+-------------------------
+ADAPTER
+An "adapter" is either an applier or an extractor.
+
+APPLIER 
+An "applier" is a a collection of threads in the replication server
+that applies epochs to a destination database system.
+
+EXTRACTOR
+An "extractor" is a collection of theads in the replication server
+that receives epochs from a source database system.
+
+TRANSFER COMPONENT 
+A "transfer component" is a thread in the replication server that is
+responsible for the connection with another replication server.
+
+REQUESTOR 
+A thread in the replication server that controls replication.