4:30 a.m.
On this specific case, it's related to HAWKULAR-881 (Join org workflow
without email): I'd need new "tables" and a new "column" in an
existing
table, possibly with a default value.
The details on this specific case aren't that important, as it's more of
a general question. As far as I know, we don't have an uniform approach
to this problem across the components.
- Juca.
On 05.01.2016 19:15, John Doyle wrote:
Can we back up and describe what we're migrating from and to, and
the
event thats triggered the migration? Maybe that's implicit in the
question for everyone else, not for me.
thx
~jd
On Mon, Jan 4, 2016 at 10:35 AM, John Sanda <jsanda(a)redhat.com
<mailto:jsanda@redhat.com>> wrote:
On Jan 4, 2016, at 5:43 AM, Juraci Paixão Kröhling
<jpkroehling(a)redhat.com <mailto:jpkroehling@redhat.com>> wrote:
>
> Team,
>
> What's the recommended approach for handling data migrations? Is there a
> library similar to liquibase?
>
> - Juca.
>
Liquibase is designed specifically for the RDBMS. When RHQ started
moving to Cassandra, I started working on a patch for Liquibase to
add support for Cassandra. After some discussion on the liquibase
dev list, I eventually decided to abandon the effort because of the
amount of changes involved and because it became clearer that
liquibase was not a good fit because of it being very RDBMS-centric.
We decided to implement our own solution in RHQ to address our
immediate needs. It has been a while since I have looked to see what
other solutions might be out there. I have come across something for
Rails applications, and I think someone may have tried to add
support in Flyway.
There are some things that need to be taken into consideration. I
will briefly discuss some of those now.
* Should the migrations be done at installation/deployment time or
at runtime?
This is probably the most important consideration because everything
else in large part stems from it. Some changes like adding/removing
a column or adding/removing a table are fast and efficient in
Cassandra. I therefore think it is acceptable to do these types of
changes at deployment time. Other changes like adding data or
moving/transforming data that could be long running operations.
While it increases application code complexity, these changes should
be done at runtime generally speaking.
* How should migrations be implemented?
With the RDBMS, we can easily manipulate, transform, and move data
with SQL. That is not the case with CQL. We have to resort to
writing code on top of the driver to make the changes. In some
situations a better approach might be to generate new SSTables and
stream those into Cassandra. For larger data migrations is likely to
be a faster as you completely bypass the whole CQL layer.
Ultimately, I think both approaches need to be an option.
* Where should migration meta data be stored?
We need to keep track of migrations that have been applied. There
might be migrations that are specific to a particular environment,
e.g., dev vs prod. Since we are trying to avoid additional data
stores, I think it makes sense to store migration meta data in
Cassandra. Maybe we a migrations keyspace that tracks the migrations
for each of the hawkular keyspaces.
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