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Dataset server capabilities
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Summary
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Dataset servers can be run on various hardware configurations, and
PyCTOH cluster can be highly heterogeneous.

Optimizing DAP server is a matter of carefully choosing which dataset
server will be requested.

A capability negociation system permits DAP servers to select dataset
servers that will best answer the request.


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Rationale
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Capability that can be advertised by dataset servers:
 - processing power: cores, bogomips, memory, ...
 - location relative to data: rack-id, network bandwidth, swift-proxy
   address, ...
 - CUDA informations: cores, memory, ...
 - supported checksums for byzantine processes checks (cf
   :doc:`Byzantine Dataset servers <byzantine_processes>`)

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Design
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Implementation
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