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From Stephen Henderson <>
Subject A few questions on row caching and read consistency ONE
Date Thu, 18 Aug 2011 09:01:05 GMT

We're currently in the planning stage of a new project which needs a low latency, persistent
key/value store with a roughly 60:40 read/write split. We're trying to establish if Cassandra
is a good fit for this and in particular what the hardware requirements would be to have the
majority of rows cached in memory (other nosql platforms like Couchbase/Membase seem like
a more natural fit but we're already reasonably familiar with cassandra and would rather stick
with what we know if it can work). 

If anyone could help answer/clarify the following questions it would be a great help (all
assume that row-caching is enabled for the column family).

Q. If we're using read consistency ONE does the read request get sent to all nodes in the
replica set and the first to reply is returned (i.e. all replica nodes will then have that
row in their cache), OR does the request only get sent to a single node in the replica set?
If it's the latter would the same node generally be used for all requests to the same key
or would it always be a random node in the replica set? (i.e. if we have multiple reads for
one key in quick succession would this entail potentially multiple disk lookups until all
nodes in the set have been hit?). 

Q. Related to the above, if only one node recieves the request would the client (hector in
this case) know which node to send the request to directly or would there be potentially one
extra network hop involved (client -> random node -> node with key).

Q. Is it possible to do a warm cache load of the most recently accessed keys on node startup
or would we have to do this with a client app?

Q. With write consistency ANY is it correct that following a write request all nodes in the
replica set will end up with that row in their cache, as well as on disk, once they receive
the write? i.e. total cache size is (cache_memory_per_node * num_nodes) / num_replicas.

Q. If the cluster only has a single column family, random partitioning and no secondary indexes,
is there a good metric for estimating how much heap space we would need to leave aside for
everything that isn't the row-cache? Would it be proportional to the row-cache size or fairly


Stephen Henderson - Lead Developer (Onsite), Cognitive Match |

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