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Re: Difference between heartbeat and generation on a Gossip packet


Hi Joe,

Thanks for the details it is more clear for me now !

Kind regards.
Abdelkarim.

Le jeu. 28 juin 2018 08:29, Joseph Lynch <joe.e.lynch@xxxxxxxxx> a écrit :

> Hi Abdelkarim,
>
> Other people on this list are much more knowledgeable than me and can
> correct me if I'm wrong, but my understanding is that the combination of
> generation and version (aka heartbeat) form a logical clock tuple
> consisting of (generation, version) and that combination is the
> HeartBeatState.
>
> The generation is the really important part and roughly corresponds to the
> last start time of that particular Cassandra process in seconds since epoch
> plus any forced increments due to e.g. the gossiper stopping or starting
> (nodetool disable/enable gossip). The generation is further stored on disk
> in the system.local table so that during a crash or restart, even if the
> system's clock moves backwards, the Cassandra node's generation should
> never move backwards. Whenever a node's generation number changes it's
> considered a major gossip state update by other nodes because they have to
> do things like ensure they are speaking the right protocol version, compare
> schema, etc ... in addition to all the versioned state changes seen below.
>
> The version is a counter used to show the passage of time within a
> generation and is used to signal versioned gossip state changes. It starts
> at zero on process start and increases by one roughly every second. There
> are various pieces of metadata like a node's status, schema, rack, dc, host
> id, tokens, etc... which are all versioned using this version counter when
> they change (whatever shows up in nodetool gossipinfo is a good example of
> these states). When the gossiper is enabled, every second, each node
> increments
> their local version by one, picks another peer to gossip with, and sends
> out their map of versioned items to that peer; other nodes know to pick up
> any new data if the version has increased. Since nodes are all gossiping
> with each other, any update to one node's versioned data get's propagated
> out quickly even if that node may not have directly gossiped with everyone.
> Naturally, the version number only increases for a given generation, but if
> the generation changes the version moves backwards (resets to zero).
>
> So yea, think of (generation, version) as forming a logical clock which
> roughly corresponds to (~last process start in seconds since the epoch,
> ~seconds since last process start) for each node. This logical clock is
> used to create ordering in gossip state changes.
>
> Hope that was helpful,
> -Joey Lynch
>
> On Tue, Jun 26, 2018 at 3:09 PM Abdelkrim Fitouri <abdou.abf@xxxxxxxxx>
> wrote:
>
> > Hello,
> >
> > I  am studying the gossip part on casssandra and wondering about the
> > difference between the heartbeat and generation data exchanged for the
> > autodiscovery.
> >
> > many thanks for any help.
> >
> > --
> >
> > Best Regards.
> >
> > Abdelkarim.
> >
>