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From Reid Pinchback <rpinchb...@tripadvisor.com>
Subject Re: Dynamo autoscaling: does it beat cassandra?
Date Tue, 10 Dec 2019 16:15:29 GMT
Note that DynamoDB I/O throughput scaling doesn’t work well with brief spikes.  Unless you
write your own machinery to manage the provisioning, by the time AWS scales the I/O bandwidth
your incident has long since passed.  It’s not a thing to rely on if you have a latency
SLA.  It really only works for situations like a sustained alteration in load, e.g. if you
have a sinusoidal daily traffic pattern, or periodic large batch operations that run for an
hour or two, and you need the I/O adjustment while that takes place.

Also note that DynamoDB routinely chokes on write contention, which C* would rarely do.  About
the only benefit DynamoDB has over C* is that more of its operations function as atomic mutations
of an existing row.

One thing to also factor into the comparison is developer effort.  The DynamoDB API isn’t
exactly tuned to making developers productive.  Most of the AWS APIs aren’t, really, once
you use them for non-toy projects. AWS scales in many dimensions, but total developer effort
is not one of them when you are talking about high-volume tier one production systems.

To respond to one of the other original points/questions, yes key and row caches don’t seem
to be a win, but that would vary with your specific usage pattern.  Caches need a good enough
hit rate to offset the GC impact.  Even when C* lets you move things off heap, you’ll see
a fair number of GC-able artifacts associated with data in caches.  Chunk cache somewhat wins
with being off-heap, because it isn’t just I/O avoidance with that cache, you’re also
benefitting from the decompression.  However I’ve started to wonder how often sstable compression
is worth the performance drag and internal C* complexity.  If you compare to where a more
traditional RDBMS would use compression, e.g. Postgres, use of compression is more selective;
you only bear the cost in the places already determined to win from the tradeoff.

From: Dor Laor <dor@scylladb.com>
Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Date: Monday, December 9, 2019 at 5:58 PM
To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Subject: Re: Dynamo autoscaling: does it beat cassandra?

Message from External Sender
The DynamoDB model has several key benefits over Cassandra's.
The most notable one is the tablet concept - data is partitioned into 10GB
chunks. So scaling happens where such a tablet reaches maximum capacity
and it is automatically divided to two. It can happen in parallel across the entire
data set, thus there is no concept of growing the amount of nodes or vnodes.
As the actual hardware is multi-tenant, the average server should have plenty
of capacity to receive these streams.

That said, when we benchmarked DynamoDB and just hit it with ingest workload,
even when it was reserved, we had to slow down the pace since we received many
'error 500' which means internal server errors. Their hot partitions do not behave great
as well.

So I believe a growth of 10% the capacity with good key distribution can be handled well
but a growth of 2x in a short time will fail. It's something you're expect from any database
but Dynamo has an advantage with tablets and multitenancy and issues with hot partitions
and accounting of hot keys which will get cached in Cassandra better.

Dynamo allows you to detach compute from the storage which is a key benefit in a serverless,
spiky deployment.

On Mon, Dec 9, 2019 at 1:02 PM Jeff Jirsa <jjirsa@gmail.com<mailto:jjirsa@gmail.com>>
wrote:
Expansion probably much faster in 4.0 with complete sstable streaming (skips ser/deser), though
that may have diminishing returns with vnodes unless you're using LCS.

Dynamo on demand / autoscaling isn't magic - they're overprovisioning to give you the burst,
then expanding on demand. That overprovisioning comes with a cost. Unless you're actively
and regularly scaling, you're probably going to pay more for it.

It'd be cool if someone focused on this - I think the faster streaming goes a long way. The
way vnodes work today make it difficult to add more than one at a time without violating consistency,
and thats unlikely to change, but if each individual node is much faster, that may mask it
a bit.



On Mon, Dec 9, 2019 at 12:35 PM Carl Mueller <carl.mueller@smartthings.com.invalid>
wrote:
Dynamo salespeople have been pushing autoscaling abilities that have been one of the key temptations
to our management to switch off of cassandra.

Has anyone done any numbers on how well dynamo will autoscale demand spikes, and how we could
architect cassandra to compete with such abilities?

We probably could overprovision and with the presumably higher cost of dynamo beat it, although
the sales engineers claim they are closing the cost factor too. We could vertically scale
to some degree, but node expansion seems close.

VNode expansion is still limited to one at a time?

We use VNodes so we can't do netflix's cluster doubling, correct? With cass 4.0's alleged
segregation of the data by token we could though and possibly also "prep" the node by having
the necessary sstables already present ahead of time?

There's always "caching" too, but there isn't a lot of data on general fronting of cassandra
with caches, and the row cache continues to be mostly useless?
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