Read preference tags deprecation cause and timeline

[k4y3ff]

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Issue

Hello! 👋

I'm responsible for a codebase that uses a primary read preference for some queries and secondaryPreferred for others. We are adding an analytics node to our MongoDB cluster, however, and thus need to specify read preference tags when using secondaryPreferred to ensure that production application queries are not served by the analytics node.

I've looked in the MongoDB Node.js driver, and read preference tags appear to be supported with no caveats (GitHub / API documentation / documentation). However, reading the Mongoose documentation, it appears that read preference tags are marked as DEPRECATED in version 6+.

Since it is not a breaking change, only a deprecation, this does not seem to be mentioned in the "Migrating from 5.x to 6.x" guide. The PR that added DEPRECATED in the documentation (#12140) does not include a detailed description, either. People seem to be recommending using read preference tags in open issues as late as this March, too.

Could anyone provide more information on why read preference tags are considered deprecated and what the timeline is for sunsetting them?

Thanks in advance.

[vkarpov15]

It looks like this "DEPRECATED" line was unintentionally added, I missed this during code review. @hasezoey what was the reason behind marking Aggregate.prototype.read tags param as deprecated here: 437b03a ?

Even without tags param, you can always use .read(new mongoose.mongo.ReadPreference(pref, tags)).

[hasezoey]

i dont know exactly why i added this anymore, but it may have something to do with typescript leading me to mquery when doing a Go to Definition on the Query.prototype.read, which has the following code:

https://github.com/mongoosejs/mquery/blob/e7a8a3099ef282a2d9af36a2880981871adfe9fd/lib/mquery.js#L1603-L1610
(snippit because github only does snippits for same repository):

Query.prototype.read = Query.prototype.setReadPreference = function(pref) {
  if (arguments.length > 1 && !Query.prototype.read.deprecationWarningIssued) {
    console.error('Deprecation warning: \'tags\' argument is not supported anymore in Query.read() method. Please use mongodb.ReadPreference object instead.');
    Query.prototype.read.deprecationWarningIssued = true;
  }
  this.options.readPreference = utils.readPref(pref);
  return this;
};

TL;DR: i likely just didnt properly check that i landed in the mongoose definition instead of the mquery definition of Query.prototype.read