[infinispan-dev] Infinispan and change data capture

Fri Jul 29 09:47:37 EDT 2016

--
Galder Zamarreño
Infinispan, Red Hat

> On 11 Jul 2016, at 16:41, Randall Hauch <rhauch at redhat.com> wrote:
> 
>> 
>> On Jul 11, 2016, at 3:42 AM, Adrian Nistor <anistor at redhat.com> wrote:
>> 
>> Hi Randall,
>> 
>> Infinispan supports both push and pull access models. The push model is supported by events (and listeners), which are cluster wide and are available in both library and remote mode (hotrod). The notification system is pretty advanced as there is a filtering mechanism available that can use a hand coded filter / converter or one specified in jpql (experimental atm). Getting a snapshot of the initial data is also possible. But infinispan does not produce a transaction log to be used for determining all changes that happened since a previous connection time, so you'll always have to get a new full snapshot when re-connecting. 
>> 
>> So if Infinispan is the data store I would base the Debezium connector implementation on Infinispan's event notification system. Not sure about the other use case though.
>> 
> 
> Thanks, Adrian, for the feedback. A couple of questions.
> 
> You mentioned Infinispan has a pull model — is this just using the normal API to read the entries?
> 
> With event listeners, a single connection will receive all of the events that occur in the cluster, correct? Is it possible (e.g., a very unfortunately timed crash) for a change to be made to the cache without an event being produced and sent to listeners?

^ Yeah, that can happen due to async nature of remote events. However, there's the possibility for clients, upon receiving a new topology, to receive the current state of the server as events, see [1] and [2]

[1] http://infinispan.org/docs/dev/user_guide/user_guide.html#client_event_listener_state_consumption
[2] http://infinispan.org/docs/dev/user_guide/user_guide.html#client_event_listener_failure_handling

> What happens if the network fails or partitions? How does cross site replication address this?

In terms of cross-site, depends what the client is connected to. Clients can now failover between sites, so they should be able to deal with events too in the same as explained above.

> 
> Has there been any thought about adding to Infinispan a write ahead log or transaction log to each node or, better yet, for the whole cluster?

Not that I'm aware of but we've recently added security audit log, so a transaction log might make sense too.

Cheers,

> 
> Thanks again!
> 
>> Adrian
>> 
>> On 07/09/2016 04:38 PM, Randall Hauch wrote:
>>> The Debezium project [1] is working on building change data capture connectors for a variety of databases. MySQL is available now, MongoDB will be soon, and PostgreSQL and Oracle are next on our roadmap. 
>>> 
>>> One way in which Debezium and Infinispan can be used together is when Infinispan is being used as a cache for data stored in a database. In this case, Debezium can capture the changes to the database and produce a stream of events; a separate process can consume these change and evict entries from an Infinispan cache.
>>> 
>>> If Infinispan is to be used as a data store, then it would be useful for Debezium to be able to capture those changes so other apps/services can consume the changes. First of all, does this make sense? Secondly, if it does, then Debezium would need an Infinispan connector, and it’s not clear to me how that connector might capture the changes from Infinispan.
>>> 
>>> Debezium typically monitors the log of transactions/changes that are committed to a database. Of course how this works varies for each type of database. For example, MySQL internally produces a transaction log that contains information about every committed row change, and MySQL ensures that every committed change is included and that non-committed changes are excluded. The MySQL mechanism is actually part of the replication mechanism, so slaves update their internal state by reading the master’s log. The Debezium MySQL connector [2] simply reads the same log.
>>> 
>>> Infinispan has several mechanisms that may be useful:
>>> 
>>> 	• Interceptors - See [3]. This seems pretty straightforward and IIUC provides access to all internal operations. However, it’s not clear to me whether a single interceptor will see all the changes in a cluster (perhaps in local and replicated modes) or only those changes that happen on that particular node (in distributed mode). It’s also not clear whether this interceptor is called within the context of the cache’s transaction, so if a failure happens just at the wrong time whether a change might be made to the cache but is not seen by the interceptor (or vice versa).
>>> 	• Cross-site replication - See [4][5]. A potential advantage of this mechanism appears to be that it is defined (more) globally, and it appears to function if the remote backup comes back online after being offline for a period of time.
>>> 	• State transfer - is it possible to participate as a non-active member of the cluster, and to effectively read all state transfer activities that occur within the cluster?
>>> 	• Cache store - tie into the cache store mechanism, perhaps by wrapping an existing cache store and sitting between the cache and the cache store
>>> 	• Monitor the cache store - don’t monitor Infinispan at all, and instead monitor the store in which Infinispan is storing entries. (This is probably the least attractive, since some stores can’t be monitored, or because the store is persisting an opaque binary value.)
>>> 
>>> Are there other mechanism that might be used?
>>> 
>>> There are a couple of important requirements for change data capture to be able to work correctly:
>>> 
>>> 	• Upon initial connection, the CDC connector must be able to obtain a snapshot of all existing data, followed by seeing all changes to data that may have occurred since the snapshot was started. If the connector is stopped/fails, upon restart it needs to be able to reconnect and either see all changes that occurred since it last was capturing changes, or perform a snapshot.               (Performing a snapshot upon restart is very inefficient and undesirable.) This works as follows: the CDC connector only records the “offset” in the source’s sequence of events; what this “offset” entails depends on the source. Upon restart, the connector can use this offset information to coordinate with the source where it wants to start reading. (In MySQL and PostgreSQL, every event includes the filename of the log and position in that file. MongoDB includes in each event the monotonically increasing timestamp of the transaction.
>>> 	• No change can be missed, even when things go wrong and components crash.
>>> 	• When a new entry is added, the “after” state of the entity will be included. When an entry is updated, the “after” state will be included in the event; if possible, the event should also include the “before” state. When an entry is removed, the “before” state should be included in the event.
>>> 
>>> Any thoughts or advice would be greatly appreciated.
>>> 
>>> Best regards,
>>> 
>>> Randall
>>> 
>>> 
>>> [1] http://debezium.io
>>> [2] http://debezium.io/docs/connectors/mysql/
>>> [3] http://infinispan.org/docs/stable/user_guide/user_guide.html#_custom_interceptors_chapter
>>> [4] http://infinispan.org/docs/stable/user_guide/user_guide.html#CrossSiteReplication
>>> [5] https://github.com/infinispan/infinispan/wiki/Design-For-Cross-Site-Replication
>>> 
>>> 
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>> 
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