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From "Jon Haddad (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-10546) Custom MV support
Date Tue, 10 Nov 2015 18:25:11 GMT


Jon Haddad commented on CASSANDRA-10546:

Would CASSANDRA-8844 cover what you need here?  It seems like all you're really looking to
do is be notified of underlying mutations.

> Custom MV support
> -----------------
>                 Key: CASSANDRA-10546
>                 URL:
>             Project: Cassandra
>          Issue Type: New Feature
>            Reporter: Matthias Broecheler
> The MV implementation should be generalized to allow for custom materialized view implementations.
Like with MV, the logic would be triggered by a mutation to some base table on which the custom
MV is registered. A custom MV would allow for custom logic to determine the "derived" mutations
that need to be applied as a result of the base table mutation. It would then ensure that
those derived mutations are applied (to other tables) as the current MV implementation does.
> Note, that a custom MV implementation is responsible for ensuring that any tables that
derived mutations are written into exist. As such, a custom MV implementation has an initialization
logic which can create those tables upon registration if needed. There should be no limit
on what table a custom MV can write derived records to (even existing ones).
> Example:
> (Note, that this example is somewhat construed for simplicity)
> We have a table in which we track user visits to certain properties with timestamp:
> {code}
> CREATE TABLE visits (
>   userId bigint,
>   visitAt timestamp,
>   property varchar,
>   PRIMARY KEY (userId, visitAt)
> );
> {code}
> Every time a user visits a property, a record gets added to this table. Records frequently
come in out-of-order.
> At the same time, we would like to know who is currently visiting a particular property
(with their last entry time).
> For that, we create a custom MV registered against the {{visits}} table which upon registration
creates the following table:
> {code}
> CREATE TABLE currentlyVisiting (
>   property varchar,
>   userId bigint,
>   enteredOn timestamp,
>   PRIMARY KEY (property, userId)
> );
> {code}
> Now, when a record (u,v,p) gets inserted into the {{visits}} table the custom MV logic
gets invoked:
> # It reads the most recent visit record for user u: (u,v',p').
> # If no such record exists, it emits (p,u,v) targeting table {{currentlyVisiting}} as
a derived record to be persisted.
> # If such a record exists and v'>=v then it emits nothing. But if v'<v it emits
records (p',u,v') to be deleted and (p,u,v) to be added to table {{currentlyVisiting}}.
> The MV framework ensures that the emitted records get persisted correctly.

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