Attached is the PROTOCOL that can help us implement
Clustering without a DB. The PROTOCOL uses the concept
of dynamic partitioning (inspired from Jules Gosnell's
original idea). Also though it is not mentioned explicitly
in the document, many of the functions will operate using
JMX (thanks to Vivek Biswas :-) for enlightening me on this).
Summary :
The protocol makes the system "dynamically change itself"
from Jetty Like topology where "ALL SERVERS BACKING UP
EACH OTHER" to a simple "PRIMARY - SECONDARY" topology
that contains only one backup per server. Thus as LOAD
increases partitions of ever diminishing sizes are created.
Since back-ups happen only within a partition the system
can scale well (also administrators can specify MAX_SERVERS_IN_CLUSTER
and MIN_SERVERS_IN_CLUSTER).
The document is divided into
1. Concept
2. Case of Failover
3. Case of Node Added
4. Case of excessive load on a Server
while the attachment has topics 1 and 2 , i am still writing the
others. I though that topic-2 was particularly complex so i though
of putting it up on the mailing-list for all to review and
digest it till the relatively simple (i guess) topics 3and4 are ready
thanks
-hb
-----Original Message-----
From: Jules Gosnell [mailto:[EMAIL PROTECTED]
Sent: Thursday, October 23, 2003 11:55 PM
To: Bhagwat, Hrishikesh; Geronimo Developers List
Cc: Biswas, Vivek
Subject: Re: [Re] Web Clustering : Stick Sessions with Shared Store -
curr ent state of play.
I agree on needing to be able to run without a db.
If you think about it, a db is just a highly specialised node in the
cluster. It is much simpler, and therefore maintainable, if all nodes in
a Geronimo cluster are homogeneous. We can't lose the db your business
data lives in, but if we can avoid adding another for session
replication it might be of advantage.
To this end, my design should work without a db. You just tune it so
that passivation never occurs - i.e. unlimited active sessions. You
trade of in-vm space against db space.
Likewise, if you were an advocate of the 'shared store' approach, you
should be able to constrain the web container to keep '0' active
sessions in memory, but passivate everything immediately to the db.
So, yes, I'm sure we are on the same page.
Jules
Bhagwat, Hrishikesh wrote:
>hi Jules,
>
>I am happy that we are converging, in that, the following approach does very
>well cover some of the main goal that i was trying to achieve when i wrote my
>first proposal. I have marked those points below as [hb-X] with brief comments.
>
>Thus keeping the "Goal of the Architecture" the same I would like to propose a
>new
>scheme for "replication". While initially, I was keen on having NO DATA
>EXCHANGE BETWEEN
>SERVERS but rather on having them all just use the shared store to persist and
>retrieve session
>data, you were interested in the contrary. Your approach (mentioned in section
>2 of http://wiki.codehaus.org/geronimo/Architecture/Clustering) is about
>Session object exchange
>between servers which are clustered and about clusters that are partitioned
>(stat/dynamically).
>I think the discussion there stops with some issues that you think that may
>arise with that
>kind of a scheme.
>
>After Vivek Biswas first pointed out to me, I have been feeling increasing
>uncomfortable with the idea of
>Geronimo being DEPENDENT on an EXTERNAL system like a Database for
>implementing its web clustering.
>Some ppl on the mailing list have spoken about performanced issues with the DB
>approach but i
>think with techniques like asny-write-to-DB etc such problems can be
>circumvented. Thus though I
>still believe that its a solution that can solve the problem, I am not
>comfortable with the usage of
>an external system to aid in clustering. I dont know of any GOOD (hi-av) DBs
>that comes for FREE. This
>mean that even if Geronimo is available for FREE it cant be used without
>purchasing a DB. The solution
>as a whole, is then, not truely FREE-of-cost.
>
>I have been since then looking at many alternatives ... like the Jetty
>implemented solution of
>having all m/c in one cluster to a 1Primary:1Secondary solution.
>
>Presenly I have something cooking. As I come close to working on details I
>find it quite similar to
>your original solution of dynamic patitioning. This is what again convinces me
>that we seem to converge.
>my porposal in to exchange note and to jointly come up with a detailed design.
>
>Do let me know your thoughts on that .... I am tring to complete a document on
>this new schema ASAP and will
>mail you the same.
>
>thanks
>- hb
>
>
>
>-----Original Message-----
>From: Jules Gosnell [mailto:[EMAIL PROTECTED]
>Sent: Thursday, October 23, 2003 3:44 AM
>To: [EMAIL PROTECTED]
>Subject: Re: [Re] Web Clustering : Stick Sessions with Shared Store -
>current state of play.
>
>
>Guys,
>
>since this topic has come up again, I thought this would a useful point
>to braindump my current ideas for comment and as a common point of
>reference...
>
>Here goes :
>
>
>
>Each session has one 'primary' node and n 'replicant' nodes associated
>with it.
>
>Sticky load-balancing is a hard requirement.
>
>Changes to the session may only occur on the primary node.
>
>[hb-1] i always intended only the OWNER NODE (remember ... i wasn't keen on
>having
>Primary/Sec. concept so i am using this term)to make changes to the SESSION.
>Also
>only ON CHANGE the session would be sent for persistance.
>
>
>Such changes are then replicated (possibly asynchronously, depending
>on data integrity requirements) to the replicant nodes.
>
>[hb-2] My approach was about NOT USING REPLICATION AT ALL but using
>a shared data store. Though I never mentioned it explicitly (bocz I
>regarded this as a finer detail) ... i always intended on having an
>async sys do the persistence.
>
>If, for any reason, a session is required to 'migrate' to another node
>(fail-over or clusterwide-state-balancing), this 'target' node makes a
>request to the cluster for this session, the current 'source' node
>handshakes and the migration ensues, after which the target node is
>promoted to primary status.
>
>[hb-3] I am not sure how a target server can initiate a migration process
>I would imagine that on a fail-over or on a systematic-removal of a node
>or any other such action that requires a cluster wide state-balancing, it is
>the lb/adminS that would sense this first and initiate actions.
>
>Any inbound request landing on a node that is not primary for the
>required session results in a forward/redirect of the request to it's
>current primary, or a migration of the session to the receiving node
>and it's promotion to primary.
>
>[hb-4] Not sure when this scenario would occur when a "secondary" would
>receieve a HTTP request even when the primary is functionning well.
>
>A shared store is used to passivate sessions that have been inactive
>for a given period, or are surplus to constraints on a node's session
>cache size.
>
>[hb-5] yes this is very much a point that I have been saying since my
>first proposal. Just to quote from that doc. "With a little intelligence
>built in an MS can store away, less busy sessions to DB and retrieve them
>when needed thus offering something that is near to "virtually unlimited
>amount of sessions (section 1.1.1.1)"
>
>Once in the shared store, a session is disassociated from it's primary
>and replicant nodes. Any node in the cluster, receiving a relevant
>request, may load the session, become it's primary and choose
>replicant nodes for it.
>[hb-6] this is a good optimization to sit on the scheme mentioned in [hb-5]
>
>Correct tuning of this feature, in a situation where frequent
>migration is taking place, might cut this dramatically.
>
>
>The reason for the hard node-level session affinity requirement is to
>ensure maximum cache hits in e.g. the business tier. If a web session
>is interacting with cached resources that are not explicitly tied to
>it (and so could be associated with the same replicant nodes), the
>only way to ensure that subsequent uses of this session hit resources
>in these caches is to ensure that these occur on the same node as the
>cache - i.e. the session's primary node.
>
>By only having one node that can write to a session, we remove the
>possibility of concurrent writes occurring on different nodes and the
>subsequent complexity of deciding how to merge them.
>
>[hb-7] I complete agree
>
>The above strategy will work for a 'implicit-affinity' lb (e.g. BigIP),
>which remembers the last node that a session was successfully accessed
>on and rolls this value forward as and when it has to fail-over to a
>new node. We should be able to migrate sessions forward to the next
>node picked by the lb, underneath it, keeping the two in sync.
>
>With an 'explicit-affinity' lb (e.g. mod_jk), where routing info is
>actually encoded into the jsessionid/JSESSIONID value (or maybe an
>auxiliary path param or cookie), it should be possible, in the case of
>fail-over, to choose a (probably) replicant node to promote to primary
>and to stick
>requests falling elsewhere to this new primary by resetting this
>routing info on their jsessionid/JSESSIONID and redirecting/forwarding
>them to it.
>
>If, in the future, we write/enhance an lb to be Geronimo-aware, we can
>be even smarter in the case of fail-over and just ask the cluster to
>choose a (probably) replicant node to promote to primary and then direct
>requests
>directly to this node.
>
>The cluster should dynamically inform the lb about joining/leaving
>nodes, and sessions should likewise maintain their primary/replicant
>lists accordingly.
>
>[hb-8] I complete agree
>
>LBs also need to be kept up to date with the locations and access
>points of the various webapps deployed around the cluster, relevant
>node and webapp stats (on which to base balancing decisions), etc...
>
>All of this information should be available to any member of the
>cluster and a Geronimo-aware lb should be a full cluster member.
>
>On shutting down every node in the cluster all session state should
>end up in the shared store.
>
>
>These are fairly broad brushstrokes, but they have been placed after
>some thought and outline the sort of picture that I would like to see.
>
>Your thoughts ?
>
>
>Jules
>
>
>
>
>
>
>Bhagwat, Hrishikesh wrote:
>
>
>
>>>I am also not convinced it reduces the amount of net traffic. After each
>>>request the MS must write to the shared store, which is the same traffic as
>>>a unicast write to another node or a multicast write to the partition
>>>(discounting the processing power needed to receive the message).
>>>
>>>
>>>
>>>
>>I agree. However, this is based on the assumption that only one unicast
>>write is required. In other words, this is a primary/secondary topology. I
>>think that hd did not intended such a topology and hence his statement.
>>
>>[hb] Yes i was not assuming a Pri/Sec design but a layout where any active
>>server
>> can be request to pick up a client request which is destined to server
>> that has just failed
>>
>>-----Original Message-----
>>From: gianny DAMOUR [mailto:[EMAIL PROTECTED]
>>Sent: Sunday, October 19, 2003 7:35 AM
>>To: [EMAIL PROTECTED]
>>Subject: [Re] Web Clustering : Stick Sessions with Shared Store
>>
>>
>>Jeremy Boynes wrote:
>>
>>
>>
>>
>>
>>>However, as Andy says, the cost of storing a serialized object in a BLOB is
>>>significant. Other forms of shared store are available though which may
>>>offer better performance (e.g. a hi-av NFS server).
>>>
>>>
>>>
>>>
>>Do we need a shared repository or a replicated repository?
>>
>>
>>
>>
>>
>>
>>>The issue I have with hb's approach is the reliance on an Admin Server, of
>>>which there would need to be at least two and they would need to co-operate
>>>between themselves and with any load-balancers. I think this can be handled
>>>by the regular servers themselves just as efficiently.
>>>
>>>
>>>
>>>
>>I agree. It seems that in such a design an Admin Server is only used to
>>route incoming requests to the relevant node.
>>
>>However, I do not believe that regular servers can do this job. I assume
>>that they will implement a standard peer-to-peer cluster topology to provide
>>redundancies, however I do not see how they can handle the dispatch of
>>incoming requests.
>>
>>This feature seems to be either a client or a proxy one: I mean it should be
>>done prior to reach the nodes.
>>
>>For instance, this feature is treated on the client-side via a stub aware of
>>the available nodes in WebLogic. It seems that JBoss (correct me if I am
>>wrong) has also followed this design.
>>
>>
>>
>>
>>
>>>I am also not convinced it reduces the amount of net traffic. After each
>>>request the MS must write to the shared store, which is the same traffic as
>>>a unicast write to another node or a multicast write to the partition
>>>(discounting the processing power needed to receive the message).
>>>
>>>
>>>
>>>
>>I agree. However, this is based on the assumption that only one unicast
>>write is required. In other words, this is a primary/secondary topology. I
>>think that hd did not intended such a topology and hence his statement.
>>
>>Gianny
>>
>>_________________________________________________________________
>>MSN Search, le moteur de recherche qui pense comme vous !
>>http://search.msn.fr/
>>
>>
>>
>>
>
>
>
>
--
/*************************************
* Jules Gosnell
* Partner
* Core Developers Network (Europe)
* http://www.coredevelopers.net
*************************************/
Imagine a Cluster having N managed servers providing complete redundancy to
each other. Imagine that they are all arranged in a cirlce.
They continue to be one big cluster till the Internal Network traffic (INT) is
less than a certain thereshold. At this stage any server in the cluster is just
as good as any other server in the cluster to provide fail-over support. This
scheme cannot however scale well largely due to 2 factors
1. As client requests increase the number of server-to-server exchanges for
Session replication would
increase, stressing the network.
2. There would be an added overhead for each server to process ALL of these
Sessions objects.
A ----------------- B ------------------- C ----------- D
| ----> |
| |
| |
J E
| |
| |
| |
I ----------------- H -------------------- G ----------- F
Here is a solution :
-------------------
After the INT crosses a certain threshold value, "A" stops sending
Session-updates to "the server before itself" (J). It also sends a notification
to J to that effect. J can now forget all the session of A that it had stored
up untill now and thus get a chunk of free memory. Each server does the same
thus "B cuts off A" and "C cuts off B" and so on. The number of exchanges now
reduces from (n * n) to (n * (n-1)). In case of failure "A" still has a solid
backup (All server from B to I).
After somemore increase in the traffic the servers "PERGES" yet another server.
Thus this time "A" will stop sending Session Objects to I (and ofcourse J).
Follwing that it will send a notification (YOU_ARE_PURGED)to I . On the other
hand it "A" would have received a similar notification, this time, from C.
However A still has a strong enough backup (all servers from B to H) to fall
on. Every machine holds with itself a list of ALL peers (A-B-C- .. in the
correct order) and the recent "cutoff-count (no. of servers purged - this value
is obviously the same for all the servers)".
Note that, in a way partitions are being created in the single large cluster.
"A" OWNS a particition that originally contained B-J. It also is a member of 9
other particition owned respectively by B,C...J. As traffic and cliet-load
increased A starts pushing out members of its partition thus making it smaller.
In effect it gets removed from some other server's (say B .. for cutoff_count =
1 and from C when cutover_count = 2) partition.
As soon as a server is PURGED (goes out of a partition):
1. It need not any more receieve session-updates from the Partition OWNER -
saving network bandwidth
2. It need not remember any Session objects that were earlier given to it by
THAT partition owner - thus allowing it to have more free memory - to be
utilized for serving the increased client load.
Under this scheme, I think, the Cluster will have a fairly uniform turn around
time over a large range of "client load".
Even as all of this happens the LB will continue allocate NEW CLIENTS in a
simple round robin fashion while forwarding requests comming from all other
clients, who already have established session with a server, to their
respective servers.
Now we shall discuss 4 special cases
1. Failover
Now imagine that "A" (actully any randomly selected server) fails. When LB
finds out that it is not able to reach "A" it simple forwards the client
request (destined for A) to the next available server in the list, "B". B tries
to serve the request but doesnt find the SessionID in its list of "NATIVE"
sessions. THIS IS A SIGNAL TO "B" that "A"HAS failed. Now it goes on to do the
following CRUTIAL ACTIVITY :
It goes to its local Bucket that was dedicated to hold A's Sessions. It will
scope out a collection of these for EACH SERVER IN THE CLUSTER (not just for
the ones in the partition) ... So even if "A" owned a partition having A-B-C,
yet the Session objects will be divided into 8 (B-J) sets. The SETS are now
sent out to their respective owners. A special protocol (may be a different
Queue/Topic) may be used to specify to the receipients that they are supposed
to HEREAFTER treat these Sessions as being native to them (as if they have now
being promoted as the Primary Node for the Sessions they just received). While
B is doing this .. it may continue to get requests from A's clients (since LB
simply forwards them). TILL SUCH TIME THAT B HAS NOT SENT OUT THE "SETS" IT
WILL CONTINUE TO SERVICE SUCH REQUESTS. ALSO SUCH SESSIONS WOULD BE REMOVED
FROM THE SETS AND WOULD THEREAFTER BE TREATED AS NATIVE TO B. After B sends out
the SETS, it waits for acknowledgements from the servers. Before getting the
ACK if it receives any more requests from A's clients, "B" will service that
request and send the updated Session to the owner(say F). If after a stipulated
amount of time "B" still doesnt receieve any ACKs it will distribute F's SET
amongst the rest. For distribution it will again follow the same process. It
will send F a signal to IGNORE its MAKE_NATIVE call (through which it had
earlier handed over the Sesssions).
When a server acknowledges, B sends an UPDATE_LIST (sessions-newOWNER)to the
LB. LB is hereon expected to send the requests to the new OWNERS. "B" however
continues to maintain this (UPDATE_LIST) list with it. It will keep this list
with it till the time LB does not ACK having received the UPDATE_LIST. The
reason to do so is in the folliwng special case : "B" has sent the SETs and the
UPDATE-REPORT(LB) ... but LB has not yet received it. Just then a request
appears from A's client. LB still doesnt know about the new ownership of this
SESSION by another server ("D") and so continues with its policy to forwards it
to B. On receival B, finds out that it has received a call for A. It check for
the correct owner of the session and forwards the call to it.
By now all the Machines have shared the load of A and can continue serving all
clients, it is a good time now for the system to re-organize it self. what this
means is that; Partitions IJA and JAB are not complete (bocz "A" has failed)
also partition ABC no more exists. The seazure of ABC doesnt really matter but
for the first two partitions (IJA and JAB ) it means that their owners,
respectively I and J, do not have enough back up servers. Instead of two back
up servers as every one else has, they have just one available. Thus the SYSTEM
needs to reorganize.
In the current example we have said that the cutover_count is 3. The system can
take the new LIST OF SERVERS B-C-D-E-F-G-I-J and apply this cutover_count. BUT
since one server has gone down they would each have to service more load thus
the cutover_count is cut by ONE.
IMP NOTE: a system administrator can specify the MINIMUM and the MAXIMUM value
for cutover_count. LOWER THE CUTOVER_COUNT higher the performance (but lesser
the number of servers backing up one (each) server). On the other hand you may
have many servers backing up ONE (each) server .. thus assuring higher failover
capacity but at lower performance.
After getting ACK from all servers and all correspondign UPDATE-LIST-ACK from
the LB, B will send out a RE_ORGANIZE signal with LIST as B-C-D-E-F-G-I-J and
cutover_count = (cutover_count > MIN_ALLOWED_CUTOVER) ? cutover_count
-1 : MIN_CUTOVER
to all the SERVERS
NOTE : In this case, all through out B is acting as an ADMIN SERVER. But its
selection is DYNAMIC. If server C would have gone down "D" would have been the
ADMIN. This design thus does not have a STATICALLY CONFIGURED ADMIN on whose
failure the system cannot run.
ANOTHER NOTE : if "C" receives a request that was ment for "A" (and NOT B) then
that means that both A and B failed (almost simultaneously). C must then
execute the algo explained above for BOTH the servers.
How the Cluster re-organizes (actually re-partitions), we shall see in the next
section. However one case needs to be explained before that. All the discussion
above pertains to the case where failover of A was follwed immidiately by a
client request for A. The following is a discussion on how the system shall
behave for requests comming in for other servers
1.> B,C,D,E,F,G,H (Servers which do NOT have A as a part of partitions they
OWN) : Processing happens as normal. Capability to back them up is not
compromized by A going down.
2.> I,J (Servers that DO CONTAIN A as a part of the partition they OWN) : After
processing the client request they shall find out, while trying to send the
updated SESSION to back-up servers, that one of their backup servers(A) has
failed. Since each server has a list of all peers (and in the correct order)
they can ask the server (B) following the one that failed (A) to take
corrective action. B will then execute the same process that is mentioned above
ending eventually with a call to RE_ORG. It is only after RE_ORG that I and J
will have a second back up to save their SESSIONS. Meanwhile they will continue
to be backup on ONE backup server that is still available.
Reorganization
--------------
Reorganization happens at all Servers simulataneously. It is a simple process.
A server (say G) receives a RE_ORG signal from B. Along with this signal it
receives the new SERVER_LIST (B-C-D-E-F-G-H-I-J) and then new cutover_count
(say 3-1 = 2). G will then undertake this two step process
1. Here after send all updates to the session (including the new ones it has
just received from B; A's sessions that B distributed to it) to ONLY its new
member(s). In this case "H".
2. It will send a YOU_ARE_PERGED signal to all other signals (meaning that they
are no a part of G's partition).
This process happens at each server. Thus J will have I as backup and I will
have B.
Note: If in case the MIN_ALLOWED_CUTOVER_COUNT is 3. B cannot send the new
cutover_count of (2). It will continue with the then current value of 3. thus
new partitions would be like GHI, HIJ, IJB, JBC and so on.
