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The impact of an upgrade really depends on what features you use within Pulsar. So, I recommend checking the release notes for all the intermediate versions you are “including” in your upgrade for possible breaking changes. A jump from 2.10.x to 3.0 would include 2.11, and 3.0 changes.

That idea and requirement won't change irrespective of doing a linear upgrade or a direct. So I am assuming that has no real bearing on the recommendation

Is there any known challenge/concern when skipping versions in upgrades? What if I want to go from 2.10.x to 3.2.x directly? It would save me a LOT of testing, and benchmarking cycles. If 2.10.x -> 3.2.x is not feasilbe, whats the shortest path here?

Honestly, we haven't tested upgrading to 3.2.x from 2.10.x. Instead, we have practiced upgrading to 3.0.x from 2.10.x and helped customers upgrade to the LTS version successfully; it requires some configuration changes. We don't see any problem during the upgrading. Here are the required configuration changes from 2.10.x to 3.0.x • Configure proxyRoles in the conf/broker.conf, https://github.com/apache/pulsar/pull/19455 introduces a breaking change, but it is necessary to improve the security. • Pulsar 3.0 has fixed the issue of broker using the wrong tls trust certificates path. This fix may break some deployments. Please make sure use tlsTrustCertsFilePath instead of tlsCertificateFilePath from the broker.conf to specify the tls trust certificates before upgrading to the Pulsar 3.0. • There is some JDK configurations deprecated as follows:

XX:+UseConcMarkSweepGC XX:CMSInitiatingOccupancyFraction=70  XX:+UseCMSInitiatingOccupancyOnly

, If you want to use your existing configuration file, you'd better check if you have these options in your pulsar_env.sh and bkenv.sh It should be similar to upgrading to 3.2.x, but you need to practice on your test environment first.

The documentation says "starting from 3.0.. live upgrades between one LTS and next one is guaranteed" - I am assuming this only means going from 3.0.x (LTS version) to whatever the next LTS version would be (say, 4.0.x) .. but it does not mean that I can go directly from 3.0.x to 3.3.x - is this correct?

This is a good question. It should be part that we need to clarify in the documentation. We need to start a discussion in the community to ensure the strategy and add it to the documentation.

Thanks for the update @Penghui. this is really insightful and helpful for pulsar users trying to upgrade to the latest stack! We will go from 2.10 -> 3.0.x in our case to begin with and then based on the discussion thread, skip 3.1 if needed.

Also zk memory config may have to be bumped up if it is less than 512MB on 2.10.