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Shard awareness extension for Scylla patchset 2 #1211

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Shard awareness extension for Scylla patchset 2 #1211

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Commits on Mar 26, 2019

  1. conn: introduced supported field containing payload of SUPPORTED frame 

    Co-authored-by: Henrik Johansson <henrik@scylladb.com>
Co-authored-by: Michał Matczuk <michal@scylladb.com>
    @mmatczuk
    mmatczuk and Henrik Johansson committed Mar 26, 2019
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  2. SelectedHost: added Token function 

    This allows tokenAwareHostPolicy return token from ExecutableQuery.

Co-authored-by: Henrik Johansson <henrik@scylladb.com>
Co-authored-by: Michał Matczuk <michal@scylladb.com>
    @mmatczuk
    mmatczuk and Henrik Johansson committed Mar 26, 2019
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  3. hostConnPool: introduced ConnPicker interface to abstract pool storage 

    The hostConnPool logic around conns slice is moved to defaultConnPicker.

Co-authored-by: Henrik Johansson <henrik@scylladb.com>
Co-authored-by: Michał Matczuk <michal@scylladb.com>
    @mmatczuk
    mmatczuk and Henrik Johansson committed Mar 26, 2019
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  4. scylla: dedicated token aware ConnPicker 

    This is an extension to tokenAwareHostPolicy supported by the Scylla 2.3
and onwards. It allows driver to select a connection to shard on a host
based on the token. The protocol extension spec is available at [1].

[1] https://github.com/scylladb/scylla/blob/master/docs/protocol-extensions.md

Co-authored-by: Henrik Johansson <henrik@scylladb.com>
Co-authored-by: Michał Matczuk <michal@scylladb.com>
    @mmatczuk
    mmatczuk and Henrik Johansson committed Mar 26, 2019
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  5. scylla: lazily close excess connections. 

    A list of excess connections is maintained to allow for lazy removal of.
excess connections. Keeping excess connections open helps reaching equilibrium
faster since the likelihood of hitting the same shard decreases with the number
of connections to the shard. The excess connections list is currently
capped to 10 times the number of shards. This magic number has no
backing statistics but sounds reasonable.
    Henrik Johansson committed Mar 26, 2019
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  6. scylla: give out random connections before pool equilibrium 

    When the pool is not yet fully materialized we give the caller
a random connection to allow the session to start working.
    Henrik Johansson committed Mar 26, 2019
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  7. scylla: ensure we use the correct token to determine shard 

    The driver silently replaced the current token with the closest one
from the token ring that it could find. This worked well enough
to route the request to the correct host but to pick the right shard
we need the actual token for the qiven query.
    Henrik Johansson committed Mar 26, 2019
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