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From Jeff Jirsa <jeff.ji...@crowdstrike.com>
Subject Re: Three questions about cassandra
Date Thu, 26 Nov 2015 17:16:12 GMT
1) It comes online in its former state. The operator is responsible for consistency beyond
that point. Common solutions would be `nodetool repair` (and if you get really smart, you
can start the daemon with the thrift/native listeners disabled, run repair, and then enable
listeners, so that when it DOES serve requests, they’re not out of date)

2) Consistency level tells cassandra how many replicas it will wait to acknowledge the write
- it doesn’t necessarily tell us how many replicas will/won’t get the write (even writing
at QUORUM, it’s likely that replicas will get the write). Those that do not may get the
writes later via read repair, or explicit repair (`nodetool repair`).

3) Yes, joining nodes acquire a part of the token range, and data will be streamed to the
joining node





On 11/26/15, 7:10 AM, "Hadmut Danisch" <hadmut@danisch.de> wrote:

>Hi, 
>
>I'm currently reading through heaps of docs and web pages to learn
>cassandra, but there's still three questions I could not find answers
>for, maybe someone could help:
>
>
>1. What happens, if a node is down for some time (hours, days,
>   weeks,...) for whatever reason (hardware, power, or network
>   failure, maintenance...) and gets back online?
>
>   Does the node remain in its former state and thus become
>   inconsistent, have outdated data, or does it update the changes
>   that occured during its downtime from other nodes?
>
>   Can nodes be easily offline for some time, then return and proceed,
>   or do they have to be added as a fresh node replacement (of their
>   own) to start from scratch?
>
>
>
>2. cassandra allows to choose from several data consistency levels,
>   especially allowing write access that does not update all nodes
>   (i.e. QUORUM, ONE, TWO, THREE). 
>
>   What happens with those nodes who did not get an update? Will they
>   synchronize with the updated nodes automatically, or will they
>   remain in their old state (forever or until next explicit write
>   access)?
>
>
>
>
>
>3. What exactly happens, when a new node is added to a cluster? Will
>   all records now belonging to the new node be automatically shifted
>   from others?
>
>   Web page
>   http://docs.datastax.com/en/cassandra/2.0/cassandra/operations/ops_add_node_to_cluster_t.html
>   describes a "streaming process", which sounds as if a new node was
>   busy to collect it's belongings from others, but it also says to
>   perform a
>   
>   nodetool cleanup
>
>   on all the old nodes, which would "remove the keys no longer
>   belonging to those nodes", which rather sounds like a simple drop,
>   i.e. having those records lost. 
>
>   So does cassandra safely fill new nodes, or do they start as empty
>   ones and their data is lost?
>
>
>
>Thank you!
>
>regards
>Hadmut
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