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From Charlie Mason <charlie....@gmail.com>
Subject Re: Consolidating records and TTL
Date Thu, 05 Jun 2014 19:38:18 GMT
Hi All,

Thanks for the replies. These detailed explanations are extremely useful.
The are much appreciated!

One other question that came to mind with this. Is there any way I can use
a TTLs to keep the old account entries around for a fixed length of time
rather than deleting them immediately? That way if I had an issue I could
replay the process form one of these earlier points. For the regular reads
of the account table I can use a limit of 1, with the reverse comparator
set I should always get the newest record without hitting an older record
first.

I can't do the initial account insert with a TTL as I can't guarantee when
a new value would come along and so replace this account record. However
when I insert the new account record, instead of deleting the old one could
I reinsert it with a TTL of say 1 month.

How would compaction handle this. Would the original record get compacted
away after 1 month + the GC Grace period or would it hang around still?

Thanks,

Charlie M


On Thu, Jun 5, 2014 at 5:32 PM, James Campbell <james@breachintelligence.com
> wrote:

>  Thanks for creating and opening the discussion on this use case.  I have
> been evaluating Cassandra for a very similar problem, but with the
> small twist that I'd like to roll up the ledger entries into the
> aggregated 'account' information on a regular basis to ensure that even
> rarely-read (but often written) data will be periodically rolled up and
> ready for quick reads.  There is also a larger twist that for me 'account'
> is a compound key and the typical read pattern will include reading from multiple
> rows within the same partition (each of which would require separate
> merging from the ledger).
>
>
>  This response prompts three questions for me about that:
>
>
>  1. Just to clarify the requirement that only one client thread does this
> at a time, I assume you mean only one thread should handle any given
> 'account' at a time, right?  There wouldn't be problems having multiple
> clients do this data maintenance on different accounts at the same time, I
> hope.
>
>
>  2. For my use case, hadoop integration seems the natural fit, because
> I'd like to batch these updates to run at night during query down-time, but
> the current hadoop tools don't really support reading from or writing to
> multiple tables, as is required for this implementation.  I've seen a few
> posts from people who have written homegrown input/output formats for
> hadoop, but I haven't tried to use them or evaluate their stability.  Is
> there another mechanism I should be thinking about for that sort of batch
> updating?​
>
>
>  James Campbell
>  ------------------------------
> *From:* Aaron Morton <aaron@thelastpickle.com>
> *Sent:* Thursday, June 5, 2014 5:26 AM
> *To:* Cassandra User
> *Cc:* charlie.mas@gmail.com
> *Subject:* Re: Consolidating records and TTL
>
>  As Tyler says, with atomic batches which are enabled by default the
> cluster will keep trying to replay the insert / deletes.
>
>  Nodes check their local batch log for failed batches, ones where the
> coordinator did not acknowledge it had successfully completed, every 60
> seconds. So there is a window where it’s possible for not all mutations in
> the batch to be completed. This could happen when a write timeout occurs
> when processing a batch of 2 rows; the request CL will not have been
> achieved on one or more of the rows. The coordinator will leave it up to
> the batch log to replay the request, and the client driver will (by default
> config) not retry.
>
>  You can use a model like this.
>
>  create table ledger (
> account int,
> tx_id  timeuuid,
> sub_total  int,
> primary key (account, tx_id)
> );
>
>  create table account (
> account  int,
> total int,
> last_tx_id timeuuid,
> primary key (account)
> );
>
>  To get the total:
>
>  select * from account where account = X;
>
>  Then get the ledger entries you need
>
>  select * from ledger where account = X and tx_id > last_tx_id;
>
>  This query will degrade when the partition size in the ledger table gets
> bigger, as it will need to read the column index
> (see column_index_size_in_kb in yaml). It will use that to find the first
> page that contains the rows we are interested in and then read forwards to
> the end of the row. It’s not the most efficient type of read but if you are
> going to delete ledger entries this *should* be able to skip over the
> tombstones without reading them.
>
>  When you want to update the total in the account write to the account
> table and update both the total and the last_tx_id. You can then delete
> ledger entries if needed. Don’t forget to ensure that only one client
> thread is doing this at a time.
>
>  Hope that helps.
> Aaron
>
>
>       -----------------
> Aaron Morton
> New Zealand
> @aaronmorton
>
>  Co-Founder & Principal Consultant
> Apache Cassandra Consulting
> http://www.thelastpickle.com
>
>  On 5/06/2014, at 10:37 am, Tyler Hobbs <tyler@datastax.com> wrote:
>
>  Just use an atomic batch that holds both the insert and deletes:
> http://www.datastax.com/dev/blog/atomic-batches-in-cassandra-1-2
>
>
> On Tue, Jun 3, 2014 at 2:13 PM, Charlie Mason <charlie.mas@gmail.com>
> wrote:
>
>> Hi All.
>>
>>  I have a system thats going to make possibly several concurrent changes
>> to a running total. I know I could use a counter for this. However I have
>> extra meta data I can store with the changes which would allow me to reply
>> the changes. If I use a counter and it looses some writes I can't recover
>> it as I will only have its current total not the extra meta data to know
>> where to replay from.
>>
>>  What I was planning to do was write each change of the value to a CQL
>> table with a Time UUID as a row level primary key as well as a partition
>> key. Then when I need to read the running total back I will do a query for
>> all the changes and add them up to get the total.
>>
>>  As there could be tens of thousands of these I want to have a period
>> after which these are consolidated. Most won't be any where near that but a
>> few will which I need to be able to support. So I was also going to have a
>> consolidated total table which holds the UUID of the values consolidated up
>> to. Since I can bound the query for the recent updates by the UUID I should
>> be able to avoid all the tombstones. So if the read encounters any changes
>> that can be consolidated it inserts a new consolidated value and deletes
>> the newly consolidated changes.
>>
>>  What I am slightly worried about is what happens if the consolidated
>> value insert fails but the deletes to the change records succeed. I would
>> be left with an inconsistent total indefinitely. I have come up with a
>> couple of ideas:
>>
>>
>>  1, I could make it require all nodes to acknowledge it before deleting
>> the difference records.
>>
>>  2, May be I could have another period after its consolidated but before
>> its deleted?
>>
>>  3, Is there anyway I could use the TTL to allow to it to be deleted
>> after a period of time? Chances are another read would come in and fix the
>> value.
>>
>>
>>  Anyone got any other suggestions on how I could implement this?
>>
>>
>>  Thanks,
>>
>>  Charlie M
>>
>
>
>
> --
> Tyler Hobbs
> DataStax <http://datastax.com/>
>
>
>

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