🚀 Meteor 3.5 is out: Change Streams & Performance improvements

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I opened an issue with production evidence from our 3.5-rc.1 rollout:

Short version: after moving to 3.5-rc.1, we saw a significant method/publication latency regression in production. Pinning Mongo reactivity back to ["oplog", "polling"] and restoring MONGO_OPLOG_URL brought latency back to normal. The issue includes Monti APM before/after data, MongoDB currentOp/profiler evidence, and our follow-up investigation plan.

cc @italojs @nachocodoner

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Another one that could be helpful to check if you are testing Change Streams with MONGO_OPLOG_URL still present: Meteor 3.5-rc.1: Change Streams did not appear to activate until MONGO_OPLOG_URL was removed · Issue #14454 · meteor/meteor · GitHub

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A public reproduction repo is now available at quavedev/meteor-changestream-fanout-repro. It reproduces the Change Streams fanout shape with many distinct publication selectors observing the same hot collection.

cc @nachocodoner @italojs

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My 2 cents in the issue Meteor 3.5-rc.1 Change Streams caused production method/pub latency regression; pinning to oplog/polling restored normal performance · Issue #14453 · meteor/meteor · GitHub
(Doens’t see your repro app @filipenevola before posting mine but nvm :+1: )

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DX comment: it would be nice to be able to simply use oplog instead of CS in development (and CS in production). I already run multiple local MongoDB servers for multiple projects and doing replica sets would be a nightmare.

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Has anyone managed to deploy 3.5 with MUP?!

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Oplog requires a replicatset too, if you run a standalone mongo intance locally, it should to try use CS → fallback to oplog → fallback to pooling
isnt it hapenning for you?

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I haven’t set this and when I started my project I got a lot of errors related to the replica set in the server console and my M5 Macbook Pro started to ventilate like crazy.

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But when you set it up, does it work well? Does this happen with every project or just a specific one?

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I only tried it in one project so far. However, I can manage it now from the settings.json files (one for dev and one for production).
I have this in development mode and nothing in production.

"packages": {
    "mongo": {
      "reactivity": [
        "oplog",
        "polling"
      ]
    }
  }
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hmmmm for that settings.json you shoudnt use changeStream, only oplog or polling
can you share a reproducible repo, please? I would to check it before next release

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@paulishca @italojs I dug into the standalone case — it’s not the fanout (#14453), it’s the replica-set detection.

mongo_connection.js gates Change Streams on Boolean(isMaster.setName || isMaster.ismaster || isMaster.secondary). But isMaster.ismaster is true on a standalone too (it just means the node accepts writes), so a standalone is misdetected as a replica set → Change Streams selected → collection.watch() fails with The $changeStream stage is only supported on replica sets (40573) → the driver retries forever, flooding the log and spiking CPU (that’s the fan, @paulishca). No fallback to oplog/polling.

Only hits a standalone mongod — meteor’s built-in dev mongo is a single-node replica set, so it doesn’t show there; you need an external standalone via MONGO_URL.

Repro (floods within seconds): https://github.com/dupontbertrand/meteor-3.5-changestream-bug · One-line fix: https://github.com/meteor/meteor/pull/14468
Workaround: pin {"packages":{"mongo":{"reactivity":["oplog","polling"]}}}, or use a replica set.

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Meteor 3.5-rc.2 is out! :rocket:

We’ve just released Meteor 3.5-rc.2, the second release candidate for Meteor 3.5. This RC includes several important fixes and improvements to the change streams implementation introduced in rc.1.

To update your app:


meteor update --release 3.5-rc.2

What’s new in rc.2

Fixes

  • Fixed change-stream observer dropping documents matched by ObjectID selectors — a BSON type mismatch at the selector boundary was silently losing documents. (#14461)

  • Fixed change streams being incorrectly selected on standalone MongoDB instances, which caused a 40573 restart loop. (#14468)

  • Fixed a DDP write fence left waiting indefinitely after a change-stream observer was stopped. (#14457)

  • Fixed a race condition in Accounts HttpOnly cookie login that could leave a session unauthenticated. (#14469)

Improvements

  • One MongoDB change stream per collection — previously, each subscription created its own change stream cursor; now all subscriptions on the same collection share a single stream, significantly reducing server-side resource usage. (#14456)

  • Change-stream documents are now translated once per path instead of once per observer, reducing redundant EJSON work under concurrent subscriptions. (#14471)

  • Removed legacy isMeteorPre144 shim and semver dependency from babel-compiler — dead code for Meteor versions before 1.4.4 (released in 2017). (#14382)

Please help us test

This is a release candidate — your feedback is critical before we ship the official release. If you run into any issues, please open a report at github.com/meteor/meteor/issues.

Special thanks :sparkles:

A huge thank you to everyone who contributed to Meteor 3.5, including this RC:

@italojs, @nachocodoner, @dupontbertrand, @mvogttech, @harryadel, @Grubba27, @StorytellerCZ, @vlasky, @9Morello, @alextaaa, @OleksandrChekhovskyi, @Eshaan-byte, @radekmie

Your work makes Meteor better for everyone. :pray:

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Cool. Any ideas on when this will get to release stage?

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The release wil come soon as possible, but first we should wait the community test is enougth, after a few weeks withtout any bug report we can do a officla release :wink:

btw, to speed up this process, please, help us testing it in production :smiley:

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Has anyone got changeStreams uws working with multitenancy ?

I still use oplog sockjs.

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This example I did (pr) didnt work for you? can you bring us more detail about what is happening?

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Meteor 3.5 is out :partying_face: (1st topic post updated)

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