Hi Lars,
Thanks for answering. Something where I might not have been sufficiently
specific is that we only store changes that happen at a given point in
time. That is, a row is created with some columns at some initial time
and in between changes to existing columns, or new columns get stored at
later points in time. Changes do not necessarily occur at increasing
times, historic changes are possible.
That means, in order to get the latest at some point in time, we rely on
HBase doing all the version skipping. That could indeed become
expensive. Most records will have a short time trail though.
The alternative would indeed be to store a complete copy for every time
there is a change and in that case it would be easiest to add the time
dimension to the row key (simple scanning in ordered time). We would
then pay in space rather than in time.
Seems I need to go back and check for the most typical access paths (at
time, or as history trail)...
Thanks,
Henning
On 08/11/2013 07:21 AM, lars hofhansl wrote:
If you want deletes to work correctly you should enable KEEP_DELETED_CELLS for
your column families (I still think that should be the default anyway).
Otherwise time-range queries will not be correct w.r.t. deleted data
(specifically you cannot get back at deleted data even if you specify a time
range before the delete and even if you column family as unlimited versions).
Depending on what your typical queries are, you might run into performance
issues. HBase sorts all versions of a KeyValue adjacent to each other.
If you now want to query only along the latest data (the last version), HBase
will have to skip a lot of other versions. In the worst case the latest version
of all KeyVales are on separate (HFile) blocks.
The question of whether to use the builtin timestamps or model the time as part
of the row keys (or even a time-column), is an interesting one.
Generally the row-key identifies your row. If you want a new row for each TS in
your logical model you should manage the time dimension yourself.
Otherwise if you identities (i.e. row) with many versions, the builtin TS might
be better.
-- Lars
________________________________
From: Henning Blohm <[email protected]>
To: user <[email protected]>
Sent: Saturday, August 10, 2013 6:26 AM
Subject: Using HBase timestamps as natural versioning
Hi,
we are managing some naturally time versioned data in HBase. That is,
there are change events that have a specific time set and when such
event is handled, data in HBase, pertaining to the exact same point in
time, is updated.
So far we are using HBase time stamps to model the time dimension. All
columns have unlimited number of versions. That worked ok so far, and
HBase's way of providing access to data at a given time or time range
seemed a natural fit.
We are aware of some tricky issues around timestamp handling (e.g. in
particular in conjunction with deletes). As we need to migrate HBase
stored data (for other reasons) shortly we are wondering, if our
approach has some long-term drawbacks that we should pay attention to
now and possibly re-design our timestamp handling as well.
So my question is:
* Is there problematic experience with using HBase timestamps as time
dimension of your data (assuming it has some natural time-based versioning)?
* Is it generally better to model time-based versioning of data within
the data structure itself (e.g. in the row key) and why?
* In case you used HBase timestamps similar to the way we use them,
feedback on how that worked is welcome as well!
Thanks,
Henning
--
Henning Blohm
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