通过Rails API响应大量对象

Question

I currently have an API for one of my projects and a service that is responsible for generating export files as CSVs, archive and store them somewhere in the cloud.

Since my API is written in Rails and my service in plain Ruby, I use the Her gem in the service to interact with the API. But I find my current implementation less performant, since I do a Model.all in my service, which in turn triggers a request that may contain way too many objects in the response.

I am curious on how to improve this whole task. Here's what I've thought of:

• implement pagination at API level and call Model.where(page: xxx) from my service;
• generate the actual CSV at API level and send the CSV back to the service (this may be done sync or async).

If I were to use the first approach, how many objects should I retrieve per page? How big should a response be?

If I were to use the second approach, this would bring quite an overhead to the request (and I guess API requests shouldn't take that long) and I also wonder whether it's really the API's job to do this.

What approach should I follow? Or, is there something better that I'm missing?

Answer 1

You need to pass a lot of information through a ruby process, that's always not simple, I don't think you're missing anything here.

If you decide to generate CSVs at the API level then what do you get with maintaining the service? You could just ditch the service altogether because replacing your service with an nginx proxy would do the same thing better (if you're just streaming the response from API host)?

If you decide to paginate, there will be a performance reduction for sure, but nobody can tell you exactly how much you should paginate - bigger pages will be faster and consume more memory (reducing throughput by being able to run less workers), smaller pages will be slower and consume less memory but demand more workers because of IO wait times,

exact numbers will depend on the IO response times of your API app and the cloud and your infrastructure, I'm afraid no one can give you a simple answer you can follow without experimentation with a stress test, and once you set up a stress test, you will get a number of your own anyway - better than anybody's estimate.

A suggestion, write a bit more about your problem, constraints you are working under etc and maybe someone can help you with a bit more radical solution. For some reason I get the feeling that what you're really looking for is a background processor like sidekiq or delayed job, or maybe connect your service to the DB directly through a DB view if you are anxoius to decouple your apps, or an nginx proxy for API responses, or nothing at all... but I really can't tell without more information.

Answer 2

I think it really depends how you want do define 'performance' and what your goal for your API is. Do you want to make sure no request to your API takes longer than 20msec to respond, than adding pagination would be a reasonable approach. Especially if the CSV generation is just an edge case, and the API is really built for other services. The number of items per page would then be limited by the speed at which you can deliver them. Your service would not be particularly more performant (even less so), since it needs to call the service multiple times.

Creating an async call (maybe with a webhook as callback) would be worth adding to your API if you think it is a valid use case for services to dump the whole record set.

Having said that, I think strictly speaking it is the job of the API to be quick and responsive. So maybe try to figure out how caching can improve response times, so paging through all the records is reasonable. On the other hand it is the job of the service to be mindful of the amount of calls to the API, so maybe store old records locally and only poll for updates instead of dumping the whole set of records each time.