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From "Kenneth Brotman (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-14271) The Hints web page in the web site is empty
Date Thu, 01 Mar 2018 12:55:00 GMT


Kenneth Brotman commented on CASSANDRA-14271:

>From the Dynamo paper:


4.6 Handling Failures: Hinted Handoff If Dynamo used a traditional quorum approach it would
be unavailable during server failures and network partitions, and would have reduced durability
even under the simplest of failure conditions. To remedy this it does not enforce strict quorum
membership and instead it uses a “sloppy quorum”; all read and write operations are performed
on the first N healthy nodes from the preference list, which may not always be the first N
nodes encountered while walking the consistent hashing ring.  Consider the example of Dynamo
configuration given in Figure 2 with N=3. In this example, if node A is temporarily down or
unreachable during a write operation then a replica that would normally have lived on A will
now be sent to node D. This is done to maintain the desired availability and durability guarantees.
The replica sent to D will have a hint in its metadata that suggests which node was the intended
recipient of the replica (in this case A).  Nodes that receive hinted replicas will keep
them in a separate local database that is scanned periodically. Upon detecting that A has
recovered, D will attempt to deliver the replica to A.  Once the transfer succeeds, D may
delete the object from its local store without decreasing the total number of replicas in
the system.  Using hinted handoff, Dynamo ensures that the read and write operations are
not failed due to temporary node or network failures. Applications that need the highest level
of availability can set W to 1, which ensures that a write is accepted as long as a single
node in the system has durably written the key it to its local store. Thus, the write request
is only rejected if all nodes in the system are unavailable. However, in practice, most Amazon
services in production set a higher W to meet the desired level of durability. A more detailed
discussion of configuring N, R and W follows in section 6.  It is imperative that a highly
available storage system be capable of handling the failure of an entire data center(s). Data
center failures happen due to power outages, cooling failures, network failures, and natural
disasters. Dynamo is configured such that each object is replicated across multiple data centers.
In essence, the preference list of a key is constructed such that the storage nodes are spread
across multiple data centers. These datacenters are connected through high speed network links.
This scheme of replicating across multiple datacenters allows us to handle entire data center
failures without a data outage.  4

> The Hints web page in the web site is empty
> -------------------------------------------
>                 Key: CASSANDRA-14271
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Documentation and Website
>            Reporter: Kenneth Brotman
>            Priority: Major
> []
> is empty.  Please contribute content.  Myself or someone else will take it from there.

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