Initial TODO list import. Current release target version is: first alpha...

Initial TODO list import. Current release target version is: first alpha (numbering scheme to be decided).


git-svn-id: https://svn.erp5.org/repos/neo/branches/prototype3@922 71dcc9de-d417-0410-9af5-da40c76e7ee4

TODO

+RC = Release Critical (for next release)
+
+  General
+RC  - Don't provide a default cluster name in configuration
+      To avoid lazy admin naming 2 clusters on the same net with the same name
+
+  Documentation
+    - Clarify node state signification, and consider renaming them in the code.
+      Ideas:
+        TEMPORARILY_DOWN becomes UNAVAILABLE
+        DOWN becomes UNKNOWN
+        BROKEN is removed ?
+RC  - Clarify cell state signification
+    - Add docstrings (think of doctests)
+RC  - Update README (TODOs should be dropped/moved here)
+
+  Tests
+    - rewrite tests
+RC  - write ZODB-API-level tests
+
+  Code
+
+    Code changes often impact more than just one node. They are categorised by node where the most important changes are needed.
+
+    General
+RC  - Review XXX in the code (CODE)
+RC  - Review TODO in the code (CODE)
+RC  - Review FIXME in the code (CODE)
+RC  - Review output of pylint (CODE)
+    - Connections should be integrated to Node class instances (CODE)
+      Currently, connections are managed separately from nodes, and the code very often needs to find one from the other. As all connections are to a node, and as all nods can be reperesented as Node class instances, such instance should directly contain associated connection for code simplicity.
+    - Rework indexes in NodeManager class (CODE)
+      NodeManager should provide indexes to quickly find nodes by type, UUID, and (ip, port).
+    - Keep-alive (HIGH AVAILABILITY)
+      Consider the need to implement a keep-alive system (packets sent automatically when there is no activity on the connection for a period of time).
+RC  - Nodes must not stop running when receiving an eroneous/unexpected packet. (HIGH AVAILABILITY)
+    - Factorise packet data when sending partition table cells (BANDWITH)
+      Currently, each cell in a partition table update contains UUIDs of all involved nodes.
+      It must be changed to a correspondance table using shorter keys (sent in the packet) to avoid repeating the same UUIDs many times.
+    - Make IdleEvent know what message they are expecting (DEBUGABILITY)
+      If a PING packet is sent, there is currently no way to know which request created associated IdleEvent, nor which response is expected (knowing either should be enough).
+    - Consider using multicast for cluster-wide notifications. (BANDWITH)
+      Currently, multi-receivers notifications are sent in unicast to each receiver. Multicast should be used.
+    - Remove sleeps (LATENCY, CPU WASTE)
+      Code still contains many delays (explicit sleeps or polling timeouts). They must be removed to be either infinite (sleep until some condition becomes true, without waking up needlessly in the meantime) or null (don't wait at all).
+      There is such delay somewhere in master node startup (near the end of the election phase).
+    - Connections must support 2 simultaneous handlers (CODE)
+      Connections currently define only one handler, which is enough for monothreaded code. But when using multithreaded code, there are 2 possible handlers involved in a packet reception:
+      - The first one handles notifications only (nothing special to do regarding multithreading)
+      - The second one handles expected messages (such message must be directed to the right thread)
+      The second handler must be possible to set on the connection when that connection is thread-safe (MT version of connection classes).
+      Also, the code to detect wether a response is expected or not must be genericised and moved out of handlers.
+    - Pack (FEATURE)
+RC  - Migration scripts (FEATURE)
+    - Control that client processed all invalidations before starting a transaction (CONSISTENCY)
+      If a client strats a transaction before it received an invalidation message caused by a transaction commited, it will use outdated data. This is a bug known in Zeo.
+    - Factorise node initialisation for admin, client and storage (CODE)
+      The same code to ask/receive node list and partition table exists in too many places.
+
+    Storage
+    - Implement incremental storage verification (BANDWITH)
+      When a partition cell is in out-of-date state, the entire transition history is checked.
+      This is because there might be gaps in cell tid history, as an out-of-date node is writable (although non-readable).
+      It should use an incremental mechanism to only check transaction past a certain TID known to have no gap.
+    - Use embeded MySQL database instead of a stand-alone MySQL server. (LATENCY)
+      (to be discussed)
+    - Make replication work even in non-operational cluster state (HIGH AVAILABILITY)
+      When a master decided a partition change triggering replication, replication should happen independently of cluster state. (Maybe we still need a primary master, to void replicating from an outdated partition table setup.)
+    - Flush objects from partition cells not served (DISK SPACE)
+      Currently, when a node stops serving a partition cell, the objects from that cell are kept in MySQL. They should be removed (possibly asynchronously, to avoid performance impact).
+    - Close connections to other storage nodes (SYSTEM RESOURCE USAGE)
+      When a replication finishes, the connection is not closed currecntly. It should be closed (possibly asynchronously, and possibly by detecting that connection is idle - similar to keep-alive principle)
+    - Notify master when storage becomes available for clients (LATENCY)
+      Currently, storage presence is broadcasted to client nodes too early, as the storage node would refuse them until it has only up-to-date data (not only up-to-date cells, but also a partition table and node states).
+    - Improve replication process (BANDWITH)
+      Current implementation do this way to replicate objects (for a given TID) :
+        S1 > S2 : Ask for a range of OIDs
+        S1 < S2 : Answer the range fo OIDs
+        For each OID :
+          S1 > S2 : Ask a range of the object history
+          S1 < S2 : Answer the object history
+          For each missing version of the object :
+            S1 > S2 : Ask object data
+            S1 < S2 : Answer object data
+      Proposal (just to keep the basics in mind):
+        S1 > S2 : Send its object state list, with last serial for each oid
+        S1 < S2 : Answer object data for latter state of each object
+        Or something like that, the idea is to say what we have instead or check
+        what we don't have.
+
+    Master
+    - Master node data redundancy (HIGH AVAILABILITY)
+      Secondary master nodes should replicate primary master data (ie, primary master should inform them of such changes).
+      This data takes too long to extract from storage nodes, and loosing it increases the risk of starting from underestimated values.
+      This risk is (currently) unavoidable when all nodes stop running, but this case must be avoided.
+    - Don't reject peers during startup phases (STARTUP LATENCY)
+      When (for example) a client sends a RequestNodeIdentification to the primary master node while the cluster is not yet operational, the primary master should postpone the node acceptance until the cluster is operational, instead of closing the connection immediately. This would avoid the need to poll the master to know when it is ready.
+    - Differential partition table updates (BANDWITH)
+      When a storage asks for current partition table (when it connects to a cluster in service state), it must update its knowledge of the partition table. Currently it's done by fetching the entire table. If the master keeps a history of a few last changes to partition table, it would be able to only send a differential update (via the incremental update mechanism)
+    - During recovery phase, store multiple partition tables (ADMINISTATION)
+      When storage nodes know different version of the partition table, the master should be abdle to present them to admin to allow him to choose one when moving on to next phase.
+
+    Client
+    - Client should prefer storage nodes it's already connected to when retrieving objects (LOAD LATENCY)
+    - Implement C version of mq.py (LOAD LATENCY)
+    - Move object data replication task to storage nodes (COMMIT LATENCY)
+      Currently the client node must send a single object data to all storage nodes in charge of the partition cell containing that object. This increases the time the client has to wait for storage reponse, and increases client-to-storage bandwith usage. It must be possible to send object data to only one stroage and that storage should automatically replicate on other storages. Locks on objects would then be released by storage nodes.
+RC  - Use Zope-logging-facility-friendly logging code (CODE)
+    - Use generic bootstrap module (CODE)
+    - Extend waitMessage to expect more than one response, on multiple connections (LATENCY)
+      To be able to pipeline requests, waitMessage must be extended to allow responses to arrive out of order.
+      The extreme case is when we must ask multiple nodes for object history (used to support undo) because different msg_ids are expected on different connections.
+
+    Admin
+RC  - Fix admin node behaviour when not connected to a primary master node (ADMINISTATION)
+      Make admin node refuse modification commands, but accept to display read-only commands.
+
+    neoctl
+RC  - rewrite to separate cleanly in a lib + frontends
+
+RC Known bugs
+    General
+    - Message id logging format is too narrow (16 bits, but value is 32 bits).
+    Admin
+    - Fix primary master node reconnection
+      It happens that, once disconnected from primary master node, admin node becomes unable to re-establish the connection.
+      Make admin not re-ask partition table on reconnection to primary master.
+    Client
+    - Fix inconsistencies between client oid pool and the last oid returned by storage nodes.
+      Currently, the max oid returned by a storage is the max of the oid column, which ignores oids generated by master node which are not used yet in any object.
+      The oid pool at client side can then contain oids greater than loid known by primary master.
+
+  Later
+    - Consider auto-generating cluster name upon initial startup (it might actualy be a partition property).
+    - Consider ways to centralise the configuration file, or make the configuration updatable automaticaly on all nodes.
+    - Consider storing some metadata on master nodes (partition table [version], ...). This data should be treated non-authoritatively, as a way to lower the probability to use an outdated partition table.
+    - Decentralize primary master tasks as much as possible (consider distributed lock mechanisms, ...)
+    - Make admin node able to monitor multiple clusters simultaneously