michaeljmarshall commented on code in PR #20748:
URL: https://github.com/apache/pulsar/pull/20748#discussion_r1297939575
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pip/pip-281.md:
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+I can read the design document and understand the problem statement and what
you plan to change *without* resorting to a couple of hours of code reading
just to start having a high level understanding of the change.
+
+THIS COMMENTS
+Please remove them when done.
+-->
+
+# Background knowledge
+
+- Pulsar broker load balancer periodically unloads bundles from overloaded
brokers. During this unload process, previous owner brokers close topic
sessions(e.g. producers, subscriptions(consumers), managed ledgers). When
re-assigned, new owner brokers recreate the topic sessions.
+
+- Pulsar clients request `CommandLookupTopic` to lookup or assign owner
brokers for topics and connect to them.
+
+- PIP-192, the extensible load balancer introduced the bundle state channel
that event-sources this unloading process in a state machine manner, from
`releasing,` `assigned`, to `owned` state order. At `releasing,` the owner
broker "releases" the bundle ownership(close topic sessions).
+
+- PIP-192, the extensible load balancer introduced TransferShedder, a new
shedding strategy, which pre-assigns new owner brokers beforehand.
+
+
+# Motivation
+
+- When unloading closes many topic sessions, then many clients need to request
CommandLookupTopic at the same time, which could cause many lookup requests on
brokers. This unloading process can be further optimized if we can let the
client directly connect to the new owner broker without following
`CommandLookupTopic` requests.
+- In the new load balancer(pip-192), since the owner broker is already known,
we can modify the close command protocol to pass the new destination broker URL
and skip the lookup requests.
+- Also, when unloading, we can gracefully shutdown ledgers -- we always close
old managed ledgers first and then recreate it on the new owner without
conflicts.
+
+# Goals
+- Remove clients' lookup requests in the unload protocol to reduce the publish
latency spike and e2e latency spike during
+unloading and also to resolve bottlenecks (of thundering lookups) when there
are a large number of topics in a cluster.
+- Gracefully shutdown managed ledgers before new owners create them to reduce
possible race-conditions between ledger close and ledger creations during
unloading.
+
+## In Scope
+
+<!--
+What this PIP intend to achieve once It's integrated into Pulsar.
+Why does it benefit Pulsar.
+-->
+
+- This change will be added in the extensible load balancer.
+
+## Out of Scope
+
+<!--
+Describe what you have decided to keep out of scope, perhaps left for a
different PIP/s.
+-->
+
+- This won't change the existing load balancer behavior(modular load manager).
+
+
+
+# High Level Design
+
+<!--
+Describe the design of your solution in *high level*.
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+
+I should be able to finish reading from beginning of the PIP to here
(including) and understand the feature and
+how you intend to solve it, end to end.
+
+DON'T
+* Avoid code snippets, unless it's essential to explain your intent.
+-->
+
+To achieve the goals above, we could modify the bundle transfer protocol by
the following.
+The proposed protocol change is based on the bundle states from PIP-192.
+
+Basically, we could close the ledgers only in the releasing state and finally
disconnect clients in the owned state with destination broker urls. The clients
will directly connect to the pre-assigned destination broker url without
lookups. Meanwhile, during this transfer, any produced messages will be
ignored by the source broker.
+
+Current Unload and Lookup Sequence in Extensible Load Balancer
+```mermaid
+sequenceDiagram
+ participant Clients
+ participant Owner Broker
+ participant New Owner Broker
+ participant Leader Broker
+ Leader Broker ->> Owner Broker: "state:Releasing:" close topic
+ Owner Broker ->> Owner Broker: close broker topic sessions
+ Owner Broker ->> Clients: close producers and consumers
+ Clients ->> Clients: reconnecting (inital delay 100ms)
+ Owner Broker ->> New Owner Broker: "state:Assign:" assign new ownership
+ New Owner Broker ->> Owner Broker: "state:Owned:" ack new ownership
+ Clients ->> Owner Broker: lookup
+ Owner Broker ->> Clients: redirect
+ Clients ->> New Owner Broker: lookup
+ New Owner Broker ->> Clients: return(connected)
+```
+
+Proposed Unload Sequence in Extensible Load Balancer without Lookup
+```mermaid
+sequenceDiagram
+ participant Clients
+ participant Owner Broker
+ participant New Owner Broker
+ participant Leader Broker
+ Leader Broker ->> Owner Broker: "state:Releasing:" close topic
+ Owner Broker ->> Owner Broker: close broker topic sessions(e.g ledgers)
without disconnecting producers/consumers(fenced)
+ Clients -->> Owner Broker: message pubs are ignored
+ Owner Broker ->> New Owner Broker: "state:Assign:" assign new ownership
+ New Owner Broker ->> Owner Broker: "state:Owned:" ack new ownership
+ Owner Broker ->> Owner Broker: close the fenced broker topic sessions
+ Owner Broker ->> Clients: close producers and consumers (with
newOwnerBrokerUrl)
+ Clients ->> New Owner Broker: immediately connect
Review Comment:
> It is hard to say the exact time as this time could be more or less
depending on the cluster size. I observed that p99.99 pub latency was around <
2 secs in a cluster of 10 brokers with 10k topic partitions and publishers
distributed to 1000 bundles at 1000 msgs/s .
This is a really valuable data point! Is the 2 seconds the time for transfer
or just the time from "owner broker sends assign" to "owner broker receives
owned"? Also, do you have any data comparing the current load balancing
interruption to this new topic transfer protocol's interruption? It'd really
help to see more data.
> Could you explain your concerns about the "ignore messages" state?
Ignoring messages is inefficient, and in my view, our goal is to build a
highly optimized message broker. The owner broker is already overloaded--it is
offloading a bundle--so doing something to minimize its network and memory
utilization seems valuable. Further, I want to minimize the tradeoffs
associated with load balancing because too many tradeoffs will lead to less
aggressive load balancing. If we can avoid ignoring messages, I think we should.
It seems to me that the current load balancing hand off protocol is the
naive solution, and we're looking to replace it with a more sophisticated
solution. The solution we choose will be in place for a while.
> The "ignore messages" state is a somewhat optimistic optimization without
additional commands and client-side complexity, considering 1. the transfer
protocol finishes fairly quickly and 2. the ignored messages could be somewhat
limited by the unloading time(also deterministically controlled by
`maxPendingMessages`). During unloading, the source broker anyway cannot ack
messages(ignore messages) after the ledger has been closed. When the ownership
state transfer is complete, the producer will immediately resend the pending
messages to the dst broker within x secs(< 2 secs in the above example). Also,
the number of pending(ignored) messages can be limited by `maxPendingMessages.`
I agree that it will be bound by `maxPendingMessages`, but I think it is
unlikely that most producers will hit their limit in single digit seconds,
which indicates that the amount of unnecessary data transfer for the bundle
will probably be equal to the `bytes/sec` * `duration of transfer`. I also
agree that we're not really talking about ways to decrease the amount of time
that messages are unacked (however, I think that eager connection to the
destination broker could lead to decreased latency).
> I am worried about the complexity of this change, and the gain might be
less significant, as the unloading is mostly momentary. Also, by this PIP we
want to introduce minimal complexity in this round with less error-prone code.
To be clear, this is a complex change. We are replacing a naive solution
that involved an abrupt disconnect followed by client polling to reconnect with
a reactive solution that tells the client where to connect. The previous
"naive" solution was able to avoid sending ignored messages because it was
eagerly disconnected. It is fair to want to avoid overly complex solutions, but
I don't think it would be surprising to add a new protocol message to solve
topic transfer.
In reviewing my earlier diagram, I think we could get away with 1 new field
on the close commands and 1 new protocol message. The first new field is to
tell clients to disconnect without trying to reconnect immediately. The second
is to tell them the new broker owner. This command optimistically assumes the
assigned broker will transition to the owner broker. (Is that a safe
assumption?) The broker code could be updated to make sure that when a topic
(bundle) is being loaded, it doesn't reject producer and consumer create
requests, but holds them on the line to prevent unnecessary retries.
```mermaid
sequenceDiagram
participant Clients
participant Owner Broker
participant New Owner Broker
participant Leader Broker
Leader Broker ->> Owner Broker: "state:Releasing:" close topic
Owner Broker ->> Owner Broker: close broker topic sessions
Owner Broker ->> Clients: close producers and consumers (and indicate
new broker command will come)
Owner Broker ->> New Owner Broker: "state:Assign:" assign new ownership
Owner Broker ->> Clients: tell clients to connect to new owner broker
Clients ->> New Owner Broker: immediately connect and attempt to create
producers/consumers (broker does not reject)
New Owner Broker ->> Owner Broker: "state:Owned:" ack new ownership
New Owner Broker ->> Clients: PRODUCER_SUCCESS and SUBSCRIBE_SUCCESS
commands
```
What do you think? Thanks!
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