[infinispan-dev] Infinispan and change data capture

Fri Dec 9 12:25:56 EST 2016

Randall and I had a chat on $subject. Here is a proposal worth
exploring as it is very lightweight on Infinispan's code.

Does an operation has a unique id sahred by the master and replicas?
If not could we add that?

The proposal itself:

The total order would not be global but per key.
Each node has a Debezium connector instance embedded that listens to the
operations happening (primary and replicas alike).
All of this process is happening async compared to the operation.
Per key, a log of operations is kept in memory (it contains the key, the
operation, the operation unique id and a ack status.
If on the key owner, the operation is written by the Debezium connector
to Kafka when it has been acked (whatever that means is where I'm less
knowledgable - too many bi-cache, tri-cache and quadri latency mixed in
my brain).
On a replica, the kafka partition is read regularly to clear the
in-memory log from operations stored in Kafka
If the replica becomes the owner, it reads the kafka partition to see
what operations are already in and writes the missing ones.

There are a few cool things:
- few to no change in what Infinispan does
- no global ordering simplifies things and frankly is fine for most
  Debezium cases. In the end a global order could be defined after the
  fact (by not partitioning for example). But that's a pure downstream
  concern.
- everything is async compared to the Infinispan ops
- the in-memory log can remain in memory as it is protected by replicas
- the in-memory log is self cleaning thanks to the state in Kafka

Everyone wins. But it does require some sort of globally unique id per
operation to dedup.

Emmanuel

On Fri 16-12-09 10:08, Randall Hauch wrote:
>
>> On Dec 9, 2016, at 3:13 AM, Radim Vansa <rvansa at redhat.com> wrote:
>>
>> On 12/08/2016 10:13 AM, Gustavo Fernandes wrote:
>>>
>>> I recently updated a proposal [1] based on several discussions we had
>>> in the past that is essentially about introducing an event storage
>>> mechanism (write ahead log) in order to improve reliability, failover
>>> and "replayability" for the remote listeners, any feedback greatly
>>> appreciated.
>>
>> Hi Gustavo,
>>
>> while I really like the pull-style architecture and reliable events, I
>> see some problematic parts here:
>>
>> 1) 'cache that would persist the events with a monotonically increasing id'
>>
>> I assume that you mean globally (for all entries) monotonous. How will
>> you obtain such ID? Currently, commands have unique IDs that are
>> <Address, Long> where the number part is monotonous per node. That's
>> easy to achieve. But introducing globally monotonous counter means that
>> there will be a single contention point. (you can introduce another
>> contention points by adding backups, but this is probably unnecessary as
>> you can find out the last id from the indexed cache data). Per-segment
>> monotonous would be probably more scalabe, though that increases complexity.
>
>It is complicated, but one way to do this is to have one “primary” node maintain the log and to have other replicate from it. The cluster does need to use consensus to agree which is the primary, and to know which secondary becomes the primary if the primary is failing. Consensus is not trivial, but JGroups Raft (http://belaban.github.io/jgroups-raft/ <http://belaban.github.io/jgroups-raft/>) may be an option. However, this approach ensures that the replica logs are identical to the primary since they are simply recording the primary’s log as-is. Of course, another challenge is what happens during a failure of the primary log node, and can any transactions be performed/completed while the primary is unavailable.
>
>Another option is to have each node maintain their own log, and to have an aggregator log that merges/combines the various logs into one. Not sure how feasible it is to merge logs by getting rid of duplicates and determining a total order, but if it is then it may have better fault tolerance characteristics.
>
>Of course, it is possible to have node-specific monotonic IDs. For example, MySQL Global Transaction IDs (GTIDs) use a unique UUID for each node, and then GTIDs consists of the node’s UUID plus a monotonically-increasing value (e.g., “31fc48cd-ecd4-46ad-b0a9-f515fc9497c4:1001”). The transaction log contains a mix of GTIDs, and MySQL replication uses a “GTID set” to describe the ranges of transactions known by a server (e.g., “u1:1-100,u2:1-10000,u3:3-5” where “u1”, “u2”, and “u3” are actually UUIDs). So, when a MySQL replica connects, it says “I know about this GTID set", and this tells the master where that client wants to start reading.
>
>>
>> 2) 'The write to the event log would be async in order to not affect
>> normal data writes'
>>
>> Who should write to the cache?
>> a) originator - what if originator crashes (despite the change has been
>> added)? Besides, originator would have to do (async) RPC to primary
>> owner (which will be the primary owner of the event, too).
>> b) primary owner - with triangle, primary does not really know if the
>> change has been written on backup. Piggybacking that info won't be
>> trivial - we don't want to send another message explicitly. But even if
>> we get the confirmation, since the write to event cache is async, if the
>> primary owner crashes before replicating the event to backup, we lost
>> the event
>> c) all owners, but locally - that will require more complex
>> reconciliation if the event did really happen on all surviving nodes or
>> not. And backups could have some trouble to resolve order, too.
>>
>> IIUC clustered listeners are called from primary owner before the change
>> is really confirmed on backups (@Pedro correct me if I am wrong,
>> please), but for this reliable event cache you need higher level of
>> consistency.
>
>This could be handled by writing a confirmation or “commit” event to the log when the write is confirmed or the transaction is committed. Then, only those confirmed events/transactions would be exposed to client listeners. This requires some buffering, but this could be done in each HotRod client.
>
>>
>> 3) The log will also have to filter out retried operations (based on
>> command ID - though this can be indexed, too). Though, I would prefer to
>> see per-event command-id log to deal with retries properly.
>
>IIUC, a “commit” event would work here, too.
>
>>
>> 4) Client should pull data, but I would keep push notifications that
>> 'something happened' (throttled on server). There could be use case for
>> rarely updated caches, and polling the servers would be excessive there.
>
>IMO the clients should poll, but if the server has nothing to return it blocks until there is something or until a timeout occurs. This makes it easy for clients and actually reduces network traffic compared to constantly polling.
>
>BTW, a lot of this is replicating the functionality of Kafka, which is already quite mature and feature rich. It’s actually possible to *embed* Kafka to simplify operations, but I don’t think that’s recommended. And, it introduces a very complex codebase that would need to be supported.
>
>>
>> Radim
>>
>>>
>>>
>>> [1]
>>> https://github.com/infinispan/infinispan/wiki/Remote-Listeners-improvement-proposal
>>>
>>> Thanks,
>>> Gustavo
>>>
>>>
>>>
>>> _______________________________________________
>>> infinispan-dev mailing list
>>> infinispan-dev at lists.jboss.org
>>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
>>
>>
>> --
>> Radim Vansa <rvansa at redhat.com>
>> JBoss Performance Team
>>
>> _______________________________________________
>> infinispan-dev mailing list
>> infinispan-dev at lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
>

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