Stefan Podkowinski commented on CASSANDRA-11748:

I'm not sure introducing a hard cap on pending outgoing pull requests and 
simply dopping anything from there is the way to go here. The good thing about 
the approach is that it's pretty much stateless, except from the atomic 
counter. But we should at least take the schema Ids and/or endpoints into 
account as well. It just doesn't make sense to queue 50 requests for the same 
schema Id and potentially drop requests for a different schema afterwards. Also 
as already noted, issuing pulls in parallel is probably not what we want, as 
this could lead to the described OOM issue, when too many responses get queued 
and applied at the same time. So I think we don't get around managing some more 
state, such as schema Ids, endpoints, last request time, delay, .., that we can 
use to schedule pulls in a more efficient way, by doing one request after 

But we should also not forget to look at the receiver side for incoming pull 
requests. Joining the cluster with a schema mismatch should not cause a node to 
answer each of those in parallel. If we keep track of pending incoming schema 
requests, we could introduce a delay before responding and create the schema 
mutations just once as payload to be used for all of them. We might have to 
bump up the MIGRATION_REQUEST timeout a in that case, but otherwise just 
delaying a few seconds should make a notable difference for nodes joining the 
cluster and having to answer to many migration requests in a short time frame.

> Schema version mismatch may leads to Casandra OOM at bootstrap during a 
> rolling upgrade process
> -----------------------------------------------------------------------------------------------
>                 Key: CASSANDRA-11748
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11748
>             Project: Cassandra
>          Issue Type: Bug
>         Environment: Rolling upgrade process from 1.2.19 to 2.0.17. 
> CentOS 6.6
> Occurred in different C* node of different scale of deployment (2G ~ 5G)
>            Reporter: Michael Fong
>            Assignee: Matt Byrd
>            Priority: Critical
>             Fix For: 3.0.x, 3.11.x, 4.x
> We have observed multiple times when a multi-node C* (v2.0.17) cluster ran 
> into OOM in bootstrap during a rolling upgrade process from 1.2.19 to 2.0.17. 
> Here is the simple guideline of our rolling upgrade process
> 1. Update schema on a node, and wait until all nodes to be in schema version 
> agreemnt - via nodetool describeclulster
> 2. Restart a Cassandra node
> 3. After restart, there is a chance that the the restarted node has different 
> schema version.
> 4. All nodes in cluster start to rapidly exchange schema information, and any 
> of node could run into OOM. 
> The following is the system.log that occur in one of our 2-node cluster test 
> bed
> ----------------------------------
> Before rebooting node 2:
> Node 1: DEBUG [MigrationStage:1] 2016-04-19 11:09:42,326 
> MigrationManager.java (line 328) Gossiping my schema version 
> 4cb463f8-5376-3baf-8e88-a5cc6a94f58f
> Node 2: DEBUG [MigrationStage:1] 2016-04-19 11:09:42,122 
> MigrationManager.java (line 328) Gossiping my schema version 
> 4cb463f8-5376-3baf-8e88-a5cc6a94f58f
> After rebooting node 2, 
> Node 2: DEBUG [main] 2016-04-19 11:18:18,016 MigrationManager.java (line 328) 
> Gossiping my schema version f5270873-ba1f-39c7-ab2e-a86db868b09b
> The node2  keeps submitting the migration task over 100+ times to the other 
> node.
> INFO [GossipStage:1] 2016-04-19 11:18:18,261 Gossiper.java (line 1011) Node 
> / has restarted, now UP
> INFO [GossipStage:1] 2016-04-19 11:18:18,262 TokenMetadata.java (line 414) 
> Updating topology for /
> ...
> DEBUG [GossipStage:1] 2016-04-19 11:18:18,265 MigrationManager.java (line 
> 102) Submitting migration task for /
> ... ( over 100+ times)
> ----------------------------------
> On the otherhand, Node 1 keeps updating its gossip information, followed by 
> receiving and submitting migrationTask afterwards: 
> INFO [RequestResponseStage:3] 2016-04-19 11:18:18,333 Gossiper.java (line 
> 978) InetAddress / is now UP
> ...
> DEBUG [MigrationStage:1] 2016-04-19 11:18:18,496 
> MigrationRequestVerbHandler.java (line 41) Received migration request from 
> /
> …… ( over 100+ times)
> DEBUG [OptionalTasks:1] 2016-04-19 11:19:18,337 MigrationManager.java (line 
> 127) submitting migration task for /
> .....  (over 50+ times)
> On the side note, we have over 200+ column families defined in Cassandra 
> database, which may related to this amount of rpc traffic.
> P.S.2 The over requested schema migration task will eventually have 
> InternalResponseStage performing schema merge operation. Since this operation 
> requires a compaction for each merge and is much slower to consume. Thus, the 
> back-pressure of incoming schema migration content objects consumes all of 
> the heap space and ultimately ends up OOM!

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