missing docs for setting the readPreference option in a query

[oallouch]

Hi,
Like in my previous issue (where I said I can't find user.getSessionToken() in the docs), I face another documentation problem : this time I'd like to make some queries read from the secondary mongodb server, but I can't find the readPreference in the reference js documentation.

Thx a lot in advance,
Olivier

[davimacedo]

The only way to change the readPreference so far is by using a beforeFind trigger, like in the example below:

Parse.Cloud.beforeFind('MyClass', req => {
  req.readPreference = 'SECONDARY';
});

Note that in this example all queries in MyClass will be changed to the SECONDARY, but you can also do some verifications in the req object in order to check for a specific set of queries and only change for them.

I will open a PR soon in the docs repo explaining this.

[oallouch]

That's odd. Reading the sources, I would think I can do it directly in a Query

parse-server/src/RestQuery.js

Line 112 in 3608b19

case 'readPreference':

and MongoStorageAdapter seems to retrieve it from the Query, next to limit, skip, ...

parse-server/src/Adapters/Storage/Mongo/MongoStorageAdapter.js

Line 595 in 3e003ee

{ skip, limit, sort, keys, readPreference }: QueryOptions

What do you think ?

[dplewis]

@oallouch Good catch surprised query.setReadPerference() was never added. Would you like to do a PR?

[oallouch]

I can try, but it'll be my first. I might need some help.
For instance, when I try npm install , I get many errors like:

npm WARN tar ENOENT: no such file or directory, open 'C:\Users\xxx\Documents\GitHub\Parse-SDK-JS\node_modules.staging\insert-module-globals-cbc4cf8a\test\global\filename.js'

[dplewis]

Which node version are you running and do you have permissions?

[davimacedo]

@dplewis @oallouch when I wrote the readPreference option, my original idea was to include it as an option in the API and also in the JavaScript SDK, but, in that point in time, there was a discussion among some of the maintainers and they thought it was not a good idea to allow the client to choose where the query should run (it is a backend responsibility). I agree with the argument but I still think it would be much easier to have this option in the API. We can also protect it with master key so it could be set only in cloud code and server environments. So I see we have three options:

• keep like it is, only setting it using triggers
• include the readPreference option in the API and SDKs without requiring a master key
• include the readPreference option in the API and SDKs requiring master key

What do you think? @acinader can you please also share your thoughts on this?

[davimacedo]

I've just remembered we actually wrote the PR with the API option for readPreference but it was never approved:
#3963

I think it is a good start point for a new PR on this.

[oallouch]

The second one is very fine with me.

[oallouch]

@davimacedo I think you should write the server and client PRs. All the options (readPreference, includedReadPreference and subqueryReadPreference ) aren't as clear for me as it must be for you.

Btw, when a readPreference is set for a query, shouldn't it also change the default values of includedReadPreference and subqueryReadPreference ?

@dplewis my problem was just a config issue

[oallouch]

@dplewis I'm trying to write a Query.setReadPreference() test in the JS SDK, but I can't find an example of test that uses the output REST call generated from a Query.
I looked in integration/test/ParseQueryTest.js

[oallouch]

Ok, I'll just look at the output of ParseQuery.toJSON() .

[dplewis]

It would be easier to wait for something like #3963 to be implemented. That PR needs rebasing. @davimacedo Would you like to take a crack at it again?

[oallouch]

Yes, that would be good.
#3963 is great, but 'includeReadPreference' and 'subqueryReadPreference' aren't completely treated in MongoStorageAdapter

I tried to modify the server myself.
I added the symlinks (using npm link) so that my project uses the local parse-server repo (with express), but I get "Cannot find module 'parse/node'".
Any ideas ?