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From Vladimir Ozerov <voze...@gridgain.com>
Subject Re: Distributed MetaStorage discussion
Date Fri, 25 Jan 2019 13:39:01 GMT
Ivan,

The change you describe is extremely valuable thing as it allows to detect
changes into global configuration which is of great importance for SQL.
Will topology and affinity changes be reflected in metastore history as
well? From SQL perspective it is important for us to be able to understand
whether cluster topology, data distribution or SQL schema has changed
between two versions. Is it possible to have a kind of composite version
instead of hashed counter? E.g.

class ConfigurationVersion {
    long globalVer; // Global counter
    long topVer; // Increasing topology version
    long affVer; // Increasing affinity version which is incremented every
time data distribution is changed (node join/leave, baseline changes, late
affinity assignment)
    long sqlSchemaVer; // Incremented every time SQL schema changes
}

Vladimir.


On Fri, Jan 25, 2019 at 11:45 AM Ivan Bessonov <bessonov.ip@gmail.com>
wrote:

> Hello, Igniters!
>
> Here's more info "Distributed MetaStorage" feature [1]. It is a part of
> Phase II for
> IEP-4 (Baseline topology) [2] and was mentioned in recent "Baseline
> auto-adjust`s
> discuss" topic. I'll partially duplicate that message here.
>
> One of key requirements is the ability to store configuration data (or any
> other data)
> consistently and cluster-wide. There are also other tickets that require
> similar
> mechanisms, for example [3]. Ignite doesn't have any specific API for such
> configurations and we don't want to have many similar implementations of
> the
> same feature across the code.
>
> There are several API methods required for the feature:
>
>  - read(key) / iterate(keyPrefix) - access to the distributed data. Should
> be
>    consistent for all nodes in cluster when it's in active state.
>  - write / remove - modify data in distributed metastorage. Should
> guarantee that
>    every node in cluster will have this update after the method is
> finished.
>  - writeAsync / removeAsync (not yet implemented) - same as above, but
> async.
>    Might be useful if one needs to update several values one after another.
>  - compareAndWrite / compareAndRemove - helpful to reduce number of data
>    updates (more on that later).
>  - listen(keyPredicate) - a way of being notified when some data was
> changed.
>    Normally it is triggered on "write/remove" operation or node activation.
> Listener
>    itself will be notified with <key, oldValue, newValue>.
>
> Now some implementation details:
>
> First implementation is based on existing local metastorage API for
> persistent
> clusters (in-memory clusters will store data in memory). Write/remove
> operation
> use Discovery SPI to send updates to the cluster, it guarantees updates
> order
> and the fact that all existing (alive) nodes have handled the update
> message.
>
> As a way to find out which node has the latest data there is a "version"
> value of
> distributed metastorage, which is basically <number of all updates, hash of
> all
> updates>. Whole updates history until some point in the past is stored
> along with
> the data, so when an outdated node connects to the cluster it will receive
> all the
> missing data and apply it locally. Listeners will also be invoked after
> such updates.
> If there's not enough history stored or joining node is clear then it'll
> receive
> shapshot of distributed metastorage so there won't be inconsistencies.
> "compareAndWrite" / "compareAndRemove" API might help reducing the size of
> the history, especially for Boolean or other primitive values.
>
> There are, of course, many more details, feel free to ask about them. First
> implementation is in master, but there are already known improvements that
> can
> be done and I'm working on them right now.
>
> See package "org.apache.ignite.internal.processors.metastorage" for the new
> interfaces and comment your opinion or questions. Thank you!
>
> [1] https://issues.apache.org/jira/browse/IGNITE-10640
> [2]
>
> https://cwiki.apache.org/confluence/display/IGNITE/IEP-4+Baseline+topology+for+caches
> [3] https://issues.apache.org/jira/browse/IGNITE-8717
>
> --
> Sincerely yours,
> Ivan Bessonov
>

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