diff options
Diffstat (limited to 'cpp/design_docs/new-cluster-design.txt')
-rw-r--r-- | cpp/design_docs/new-cluster-design.txt | 285 |
1 files changed, 99 insertions, 186 deletions
diff --git a/cpp/design_docs/new-cluster-design.txt b/cpp/design_docs/new-cluster-design.txt index 7adb46fee3..a162ea68ec 100644 --- a/cpp/design_docs/new-cluster-design.txt +++ b/cpp/design_docs/new-cluster-design.txt @@ -17,7 +17,6 @@ # under the License. * A new design for Qpid clustering. - ** Issues with current design. The cluster is based on virtual synchrony: each broker multicasts @@ -95,8 +94,9 @@ 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. +Use multiple CPG groups to process different queues in parallel. Use a +fixed set of groups and hash queue names to choose the group for each +queue. *** Requirements @@ -149,7 +149,7 @@ 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. @@ -162,19 +162,32 @@ On receiving an accept the broker: NOTE: The message store does not currently implement asynchronous completions of accept, this is a bug. +*** Multiple CPG groups. + +The old cluster was bottlenecked by processing everything in a single +CPG deliver thread. + +The new cluster uses a set of CPG groups, one per core. Queue names +are hashed to give group indexes, so statistically queues are likely +to be spread over the set of groups. + +Operations on a given queue always use the same group, so we have +order within each queue, but operations on different queues can use +different groups giving greater throughput sending to CPG and multiple +handler threads to process CPG messages. + ** 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.) +An inconsistent error means that after multicasting an enqueue, accept +or dequeue, some brokers succeed in processing it and others fail. -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. +The new design eliminates most sources of inconsistent errors in the +old broker: connections, sessions, security, management etc. Only +store journal errors remain. -#TODO: The only source of dequeue errors is probably an unrecoverable journal failure. +The new inconsistent error protocol is similar to the old one with one +major improvement: brokers do not have to stall processing while an +error is being resolved. ** Updating new members @@ -193,60 +206,44 @@ 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. +We update individual objects (queues and exchanges) independently. +- create queues first, then update all queues and exchanges in parallel. +- multiple updater threads, per queue/exchange. -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. +Queue updater: +- marks the queue position at the sync point +- sends messages starting from the sync point working towards the head of the queue. +- send "done" message. -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 +Queue updatee: +- enqueues received from CPG: add to back of queue as normal. +- dequeues received from CPG: apply if found, else save to check at end of update. +- messages from updater: add to the *front* of the queue. +- update complete: apply any saved dequeues. -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. +Exchange updater: +- updater: send snapshot of exchange as it was at the sync point. -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. +Exchange updatee: +- queue exchange operations after the sync point. +- when snapshot is received: apply saved operations. -# TODO how can we recover from updater crashing before update complete? -# Clear queues that are not updated & send request for udpates on those queues? +Note: +- Updater is active throughout, no stalling. +- Consuming clients actually reduce the size of the update. +- Updatee stalls clients until the update completes. + (Note: May be possible to avoid updatee stall as well, needs thought) -# 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? +** Internal cluster interface -** 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 new cluster interface is similar to the MessageStore interface, but +provides more detail (message positions) and some additional call +points (e.g. acquire) 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. +- transactional events. ** Maintainability @@ -273,106 +270,48 @@ A number of specific ways the code will be simplified: ** 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 +The standalone broker processes _connections_ concurrently, so CPU +usage increases as you add more connections. + +The new cluster processes _queues_ concurrently, so CPU usage increases as you +add more queues. + +In both cases, CPU usage peaks when the number of "units of + concurrency" (connections or queues) goes above the number of cores. + +When all consumers on a queue are connected to the same broker the new +cluster uses the same messagea allocation threading/logic as a +standalone broker, with a little extra asynchronous book-keeping. + +If a queue has multiple consumers connected to multiple brokers, the +new cluster time-shares the queue which is less efficient than having +all consumers on a queue connected to the same broker. +** Flow control +New design does not queue up CPG delivered messages, they are +processed immediately in the CPG deliver thread. This means that CPG's +flow control is sufficient for qpid. + +** Live upgrades + +Live upgrades refers to the ability to upgrade a cluster while it is +running, with no downtime. Each brokers in the cluster is shut down, +and then re-started with a new version of the broker code. + +To achieve this +- Cluster protocl XML file has a new element <version number=N> attached + to each method. This is the version at which the method was added. +- New versions can only add methods, existing methods cannot be changed. +- The cluster handshake for new members includes the protocol version + at each member. +- The cluster's version is the lowest version among its members. +- A newer broker can join and older cluster. When it does, it must restrict + itself to speaking the older version protocol. +- When the cluster version increases (because the lowest version member has left) + the remaining members may move up to the new version. + + +* Design debates ** Active/active vs. active passive An active-active cluster can be used in an active-passive mode. In @@ -385,7 +324,7 @@ An active/passive implementation allows some simplifications over active/active: - 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. +- Exactly one broker has 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. @@ -393,43 +332,17 @@ 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. +- As long as a client can connect to any broker in the cluster, it can be served. Active/passive benefits: -- Don't need message allocation strategy, can feed consumers at top speed. -- Code may be simpler than active/active. +- Don't need to replicate message allocation, can feed consumers at top speed. 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. +- No service if a partition separates a client from the active broker, + even if the client can see other brokers. -** 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. |