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From "Ariel Weisberg (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-13442) Support a means of strongly consistent highly available replication with tunable storage requirements
Date Mon, 09 Oct 2017 16:55:00 GMT


Ariel Weisberg commented on CASSANDRA-13442:

bq. Considering this seems to be mostly about reducing storage costs so write bound workloads
can run "dense" nodes, 
It's not either/or as these two things compose further reducing costs.

Dense nodes don't reduce costs 10-20x. With dense nodes you still need to pay for the additional
RAM, disk, and datacenter power, cooling do go up slightly as you stuff more power utilization
into each box. Dense nodes also still need to process each request so you also need to scale
up read and write throughput which is not always a dimension we are claiming to improve with
dense nodes.

Dense nodes won't let you increase replication on hardware where you can't fit an entire replica
of your data set in most cases. Such as racks or DCs in a region that have limited capacity
and by limited I mean many times less capacity. What do we expect from dense nodes? 2x? 4x?
Are all use cases going to behave well with various strategies we use to get to dense nodes?

bq. While this idea does seem interesting, it seems very complex and you are still trading
off replicas for additional storage. 
The target is 10x to 20x less storage. So additional storage yes, but not the same order of
magnitude. In other words we pay something (complexity) and we get something (some replicas
require 10-20x less hardware).

I also think 10-20x storage savings is a conservative estimate assuming the worse case utilization
during an outage where transient data must be stored at transient replicas. With vnodes data
would be spread out over several nodes so the additional utilization at each node could be
substantially less.

bq. Seems that the primary use case would be multiple datacenters with transient replicas,
which granted would be nice, 
Multiple data centers aren't required to benefit. Many people will be able to go from RF=3
in a DC today to RF=5 and lose two nodes with no availability or data loss instead of just
one node.

There are other permutations where being able to inexpensively add a transient replica can
increase availability like RF=3 with one replica at each DC. Write at CL.ALL, read at LOCAL_ONE,
fall back to reading from a remote DC if LOCAL_ONE fails. You get strong consistency, but
not write availability. Add a transient replicas at each DC and write at EACH_QUORUM and you
get write availability after a single node fails.

bq. you're probably able to just store less replicas in each datacenter anyway, at least if
we had more flexible consistency levels.
I'm not sure what you mean by flexibility.

Not without losing either availability or consistency under failure scenarios. If you run
RF=3 today with strong consistency you can't drop to RF=2 without losing availability if there
is a node failure.

> Support a means of strongly consistent highly available replication with tunable storage
> -----------------------------------------------------------------------------------------------------
>                 Key: CASSANDRA-13442
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Compaction, Coordination, Distributed Metadata, Local Write-Read
>            Reporter: Ariel Weisberg
> Replication factors like RF=2 can't provide strong consistency and availability because
if a single node is lost it's impossible to reach a quorum of replicas. Stepping up to RF=3
will allow you to lose a node and still achieve quorum for reads and writes, but requires
committing additional storage.
> The requirement of a quorum for writes/reads doesn't seem to be something that can be
relaxed without additional constraints on queries, but it seems like it should be possible
to relax the requirement that 3 full copies of the entire data set are kept. What is actually
required is a covering data set for the range and we should be able to achieve a covering
data set and high availability without having three full copies. 
> After a repair we know that some subset of the data set is fully replicated. At that
point we don't have to read from a quorum of nodes for the repaired data. It is sufficient
to read from a single node for the repaired data and a quorum of nodes for the unrepaired
> One way to exploit this would be to have N replicas, say the last N replicas (where N
varies with RF) in the preference list, delete all repaired data after a repair completes.
Subsequent quorum reads will be able to retrieve the repaired data from any of the two full
replicas and the unrepaired data from a quorum read of any replica including the "transient"
> Configuration for something like this in NTS might be something similar to { DC1="3-1",
DC2="3-2" } where the first value is the replication factor used for consistency and the second
values is the number of transient replicas. If you specify { DC1=3, DC2=3 } then the number
of transient replicas defaults to 0 and you get the same behavior you have today.

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