My Rails Website Intermittently Returns Empty JSON Responses From an API Endpoint in Production

[joeroot]

Hello Ruby on Rails Community,

I am currently facing one specific problem with my Rails website in production where one of its API endpoints intermittently returns an empty or incomplete JSON response even though the same endpoint normally works correctly. The endpoint is used by the website frontend to retrieve dynamic content, and under normal circumstances it returns a JSON object containing all of the fields required by the frontend. The problem is not a complete application outage because the website itself remains accessible and most requests to the endpoint succeed normally. However, occasional requests return an empty array, an incomplete object, or otherwise do not contain the data that the frontend expects. Refreshing the page or making the same request again will often return the complete response, which makes the behaviour difficult to reproduce consistently and suggests that something intermittent is happening during request processing.

I have verified that the controller action and route are correct because the endpoint works successfully for a large number of requests. The application is running the same deployed code when both successful and unsuccessful responses occur, and I have not found a particular user action that reliably triggers the problem. I have checked the relevant controller, model, and serializer logic and confirmed that the expected records exist in the database when the affected requests are made. When the endpoint returns the expected response, the JSON structure and data are correct. When the issue occurs, however, the response can contain substantially less information than expected even though the underlying records appear to be available. This has made me question whether the problem could involve the Rails request lifecycle, database query timing, caching, connection handling, or some other production-specific behaviour rather than a simple mistake in the endpoint’s application logic.

I have also been comparing application logs from successful and unsuccessful requests using timestamps and request identifiers where available. The affected requests reach the Rails application rather than simply failing at the browser level, and the application does not always produce an obvious exception when an incomplete response is generated. I have added additional non-sensitive logging around the relevant database lookup and response-generation stages so I can determine whether the expected records are returned before serialization takes place. In successful requests, the database query returns the expected records and the serializer produces the complete JSON payload. In affected requests, I am trying to determine whether the query itself produces an unexpected result or whether the data is lost later while the response is being constructed. I do not want to add excessive logging to production without understanding the best Rails-specific approach for diagnosing this type of intermittent behaviour.

The problem appears more frequently when the website is handling several requests close together, although I have not been able to establish a reliable threshold or reproduce it on demand. The application uses a normal production database connection and a web-server/application-server setup suitable for the website’s traffic. I have considered whether a connection-pool issue, stale data, transaction timing, caching behaviour, or concurrent requests could explain why an endpoint occasionally sees a different result from the same query. I am particularly interested in understanding whether Rails can expose useful information about database connection-pool usage or request processing when this happens. I have also considered whether an application-level cache could be returning an empty result, but I do not want to disable caching across the entire application just to test one endpoint unless that is a recommended diagnostic step.

I have tried repeating the exact API request after receiving an incomplete response, and in many cases the second request returns the complete JSON payload without any code or configuration change. I have also tested the endpoint directly rather than relying solely on the website’s JavaScript frontend, which helps confirm that the problem can occur at the API response level itself. The database continues to contain the expected information, and the application process remains available during the affected requests. I am therefore trying to narrow the investigation down to the Rails request, database, serialization, or caching path. If there are recommended Rails debugging techniques for capturing the generated SQL, query results, request lifecycle, cache reads, or connection-pool state for only the affected endpoint, I would appreciate advice on how to collect that information safely in production.

I would appreciate guidance from the Ruby on Rails community on how to systematically troubleshoot this specific issue where a Rails API endpoint intermittently returns an empty or incomplete JSON response even though the underlying data exists and the same endpoint normally returns the complete payload. In particular, I would like to know which Rails logs, Active Record diagnostics, connection-pool metrics, caching information, request instrumentation, or debugging techniques would be most useful for determining exactly where the expected data disappears during an affected request. I would also appreciate suggestions for distinguishing a database/query consistency problem from a Rails serialization or application-level caching problem. My goal is to identify the actual cause of the intermittent response rather than adding retries or workarounds that could simply hide the underlying issue and potentially make the API less predictable under production traffic. Sorry for long post!

[patrickdonahue]

The most useful thing you can do right now is split this into two questions, because “empty array” and “incomplete object” have quite different causes and at the moment you are chasing both at once:

1. The query genuinely returned fewer rows than you expected.
2. The query was fine and the response was truncated on the way out.

Until you know which, every hypothesis stays in play. The cheap way to separate them is to log the record count and the rendered byte size, server side, next to the request id, on every hit to that endpoint:

payload = MySerializer.new(records).as_json
Rails.logger.info("api.content records=#{records.size} bytes=#{payload.to_json.bytesize}")

Then pull the log line for a known-bad request. If it says records=0, the client received exactly what Rails built and this is a query or data problem. If it says records=42 bytes=18000 and the client saw less than that, the problem is downstream of Rails and you want to be looking at middleware, the proxy and timeouts instead.

If it is case 1, check whether that read is hitting a replica.

This is the classic shape of replication lag, and Rails will do it to you automatically if multi-database is configured. ActiveRecord::Middleware::DatabaseSelector routes reads to the reading role once the last write is older than a delay, and that delay defaults to two seconds:

# activerecord/lib/active_record/middleware/database_selector/resolver.rb
SEND_TO_REPLICA_DELAY = 2.seconds

So a request arriving just after a write reads the primary and sees everything, while the identical request a moment later reads the replica and sees only what has replicated so far. That gives you precisely what you described: intermittent, hard to reproduce, cleared by a refresh, and the records provably present whenever you go and look, because you are looking at the primary.

You can confirm or eliminate this with one initializer, since the middleware instruments both branches:

ActiveSupport::Notifications.subscribe(/database_selector/) do |name, *|
  Rails.logger.info("db_selector: #{name}")
end

That emits database_selector.active_record.read_from_primary or ...read_from_replica. Log it beside the record count. If every bad response lines up with read_from_replica you have your answer, and the fix is to raise the delay, pin that endpoint to the primary with connected_to(role: :writing), or address the replication lag itself.

If it is case 2, the usual suspects are a request timeout killing the response mid-render (Rack::Timeout and similar will cheerfully hand the client a truncated body) or something caching a response generated during a boot or deploy window. Worth checking whether the bad responses cluster in time. If they do, look at deploys and restarts rather than at any particular request.

One more thing worth ruling out cheaply, because it is free: if that relation has a limit or pagination but no deterministic order, Postgres is entirely within its rights to hand back different rows on each execution. You then get “incomplete” results that are in fact working exactly as written. Adding ORDER BY on something unique costs nothing and removes a whole category of this.

If you can share the controller action, the serializer, and whether config/database.yml has a replica configured, people will be able to get a lot more specific.