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From Apache Wiki <wikidi...@apache.org>
Subject [Cassandra Wiki] Update of "ArchitectureGossip" by JingguoYao
Date Thu, 24 Mar 2011 08:30:48 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Cassandra Wiki" for change notification.

The "ArchitectureGossip" page has been changed by JingguoYao.
The comment on this change is: Disable wiki links.
http://wiki.apache.org/cassandra/ArchitectureGossip?action=diff&rev1=5&rev2=6

--------------------------------------------------

  
  == Data structures ==
  ==== HeartBeatState ====
- Consists of generation and version number. Generation stays the same when server is running
and grows every time the node is started. Used for distinguishing state information before
and after a node restart. Version number is shared with application states and guarantees
ordering. Each node has one HeartBeatState associated with it.
+ Consists of generation and version number. Generation stays the same when server is running
and grows every time the node is started. Used for distinguishing state information before
and after a node restart. Version number is shared with application states and guarantees
ordering. Each node has one !HeartBeatState associated with it.
  
  ==== ApplicationState ====
- Consists of state and version number and represents a state of single "component" or "element"
within Cassandra. For instance application state for "load information" could be (5.2, 45),
which means that node load is 5.2 at version 45. Similarly a node that is bootstrapping would
have "bootstrapping" application state: (bxLpassF3XD8Kyks, 56) where first one is bootstrap
token, and the second is version. Version number is shared by application states and HeartBeatState
to guarantee ordering and can only grow.
+ Consists of state and version number and represents a state of single "component" or "element"
within Cassandra. For instance application state for "load information" could be (5.2, 45),
which means that node load is 5.2 at version 45. Similarly a node that is bootstrapping would
have "bootstrapping" application state: (bxLpassF3XD8Kyks, 56) where first one is bootstrap
token, and the second is version. Version number is shared by application states and !HeartBeatState
to guarantee ordering and can only grow.
  
  ==== EndPointState ====
- Includes all ApplicationStates and HeartBeatState for certain endpoint (node). EndPointState
can include only one of each type of ApplicationState, so if EndPointState already includes,
say, load information, new load information will overwrite the old one. ApplicationState version
number guarantees that old value will not overwrite new one.
+ Includes all !ApplicationStates and !HeartBeatState for certain endpoint (node). !EndPointState
can include only one of each type of !ApplicationState, so if !EndPointState already includes,
say, load information, new load information will overwrite the old one. !ApplicationState
version number guarantees that old value will not overwrite new one.
  
  ==== endPointStateMap ====
- Internal structure in Gossiper that has EndPointState for all nodes (including itself) that
it has heard about.
+ Internal structure in Gossiper that has !EndPointState for all nodes (including itself)
that it has heard about.
  
  == Gossip Exchange ==
  === GossipDigestSynMessage ===
- Node starting gossip exchange sends GossipDigestSynMessage, which includes a list of gossip
digests. A single gossip digest consists of endpoint address, generation number and maximum
version that has been seen for the endpoint. In this context, maximum version number is the
biggest version number in EndPointState for this endpoint. An example to illustrate this better:
+ Node starting gossip exchange sends !GossipDigestSynMessage, which includes a list of gossip
digests. A single gossip digest consists of endpoint address, generation number and maximum
version that has been seen for the endpoint. In this context, maximum version number is the
biggest version number in !EndPointState for this endpoint. An example to illustrate this
better:
  
  Suppose that node 10.0.0.1 has following information in its endPointStateMap (remember that
endPointStateMap includes also node itself):
  
@@ -59, +59 @@

    ApplicationState "load-information": 6.7, generation 1259912942, version 3
    ApplicationState "normal": bj05IVc0lvRXw2xH, generation 1259912942, version 7
  }}}
- In this case max version number for these endpoints are 325, 61, 5 and 18 respectively.
A gossip digest for endpoint 10.0.0.2 would be "10.0.0.2:1259911052:61" and essentially says
"AFAIK endpoint 10.0.0.2 is running generation 1259911052 and maximum version is 61". When
the node sends GossipDigestSynMessage, there will be exactly one gossip digest per known endpoint.
That is, in this case GossipDigestSynMessage contents would be: "10.0.0.1:1259909635:325 10.0.0.2:1259911052:61
10.0.0.3:1259912238:5 10.0.0.4:1259912942:18". HeartBeatState version number is not necessarily
always the biggest, but that is the most common situation by far.
+ In this case max version number for these endpoints are 325, 61, 5 and 18 respectively.
A gossip digest for endpoint 10.0.0.2 would be "10.0.0.2:1259911052:61" and essentially says
"AFAIK endpoint 10.0.0.2 is running generation 1259911052 and maximum version is 61". When
the node sends !GossipDigestSynMessage, there will be exactly one gossip digest per known
endpoint. That is, in this case !GossipDigestSynMessage contents would be: "10.0.0.1:1259909635:325
10.0.0.2:1259911052:61 10.0.0.3:1259912238:5 10.0.0.4:1259912942:18". !HeartBeatState version
number is not necessarily always the biggest, but that is the most common situation by far.
  
  === Main code pointers: ===
  {{{
@@ -96, +96 @@

  Sort gossip digest list according to the difference in max version number between sender's
digest and our own information in descending order. That is, handle those digests first that
differ mostly in version number. Number of endpoint information that fits in one gossip message
is limited. This step is to guarantee that we favor sending information about nodes where
information difference is biggest (sending node has very old information compared to us).
  
  ==== Examine gossip digest list ====
- At this stage we go through the arriving gossip digest list and construct the two parts
of GossipDigestAckMessage mentioned above (gossip digest list and endpoint state list). Let
us go through the example digest one by one:
+ At this stage we go through the arriving gossip digest list and construct the two parts
of !GossipDigestAckMessage mentioned above (gossip digest list and endpoint state list). Let
us go through the example digest one by one:
  
  '''10.0.0.1:1259909635:325''' In our own endPointStateMap the generation is the same, so
10.0.0.1 has not rebooted since we have last heard of it. Version number in the digest is
bigger than our max version number (325 > 324), so we have to ask the sender what has happened
since version 324. For this purpose we include a gossip digest 10.0.0.1:1259909635:324, which
says "I know about 10.0.0.1 only until generation 1259909635, version 324, please tell me
anything that is newer than this".
  
- '''10.0.0.2:1259911052:61''' When examining this, we notice that we know more than the sender
about 10.0.0.2 (generations match, but our version is bigger 63 > 61). Sender's max version
is 61, so we look for any states that are newer than this. As we can see from the endPointStateMap,
there are two: Application state "normal" (version 62) and HeartBeatState (version 63). We
send these ApplicationStates to the sender. Please note that in this case we are not sending
digests, as digest only tells the maximum version number. In this case we already know that
there is difference, so we will send full ApplicationStates.
+ '''10.0.0.2:1259911052:61''' When examining this, we notice that we know more than the sender
about 10.0.0.2 (generations match, but our version is bigger 63 > 61). Sender's max version
is 61, so we look for any states that are newer than this. As we can see from the endPointStateMap,
there are two: Application state "normal" (version 62) and !HeartBeatState (version 63). We
send these !ApplicationStates to the sender. Please note that in this case we are not sending
digests, as digest only tells the maximum version number. In this case we already know that
there is difference, so we will send full !ApplicationStates.
  
  '''10.0.0.3:1259912238:5''' In this case generations do not match. Our generation is smaller
than the arriving, so 10.0.0.3 must have rebooted. We will ask all data from the sender for
generation 1259912238 starting from smallest version number 0. That is, we insert gossip digest
10.0.0.3:1259912238:0 to the reply.
  
  '''10.0.0.4:1259912942:18''' We do not know anything about this endpoint, so we proceed
in the same manner as 10.0.0.3 and ask for all information. Insert digest 10.0.0.4:1259912942:0
to the reply.
  
- At this point we have constructed GossipDigestAckMessage, which includes following information:
+ At this point we have constructed !GossipDigestAckMessage, which includes following information:
  
  {{{
  10.0.0.1:1259909635:324
@@ -114, +114 @@

  10.0.0.4:1259912942:0
  10.0.0.2:[ApplicationState "normal": AujDMftpyUvebtnn, generation 1259911052, version 62],
[HeartBeatState, generation 1259911052, version 63]
  }}}
- We now send this GossipAckMessage to the sender of GossipSynMessage
+ We now send this !GossipAckMessage to the sender of !GossipSynMessage
  
  === Main Code Pointers: ===
  {{{

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