The Configuration That Hid the Bug: A Debugging Crossroads in Distributed S3 Architecture
Introduction
In the middle of a complex debugging session for a horizontally scalable S3-compatible storage system, there comes a moment when every obvious check has been exhausted and the investigator must descend into the details of configuration files. Message 854 in this coding session captures exactly such a moment. The user has reported a puzzling observation: traffic is visible on the Frontend Proxies table in the cluster monitoring dashboard, but all of it flows exclusively to kuri-1, with none reaching kuri-2. The assistant has already verified that both backend nodes report healthy via their /healthz endpoints, reviewed the round-robin routing code and found it logically correct, and checked the S3 proxy logs only to find them silent beyond startup messages. Now, at this juncture, the assistant pivots from examining code and runtime behavior to inspecting the raw configuration files that govern how each node operates. This message is the turning point where the investigation shifts from "is the routing logic broken?" to "is the node actually configured to receive traffic?"
The Message in Full
The assistant writes:
Both backends are healthy. Let me check what port kuri-2 is actually listening on for S3:
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``bash cat /data/fgw2/config/kuri-2/settings.env ``
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``` # Kuri Node 2 Configuration # Generated on Sat Jan 31 02:38:13 PM CET 2026
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# Core settings export RIBS_DATA="/data/ribs" export RIBS_MAX_LOCAL_GROUP_COUNT="4" export RIBS_FILECOIN_API_ENDPOINT="[REDACTED]"
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# Database settings export RIBS_YUGABYTE_CQL_HOSTS="yugabyte" export RIBS_YUGABYTE_CQL_PORT="9042" export RIBS_YUGABYTE_CQL_KEYSPACE="filecoingw" export RIBS_YUGABYTE_SQL_HOST="yugabyte" export RIBS_YUGABYTE_SQL_PORT="5433" export RIBS_YUGABYTE_SQL_DB="filecoingw"
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#... ```
The output is truncated with #..., showing only the first portion of the configuration file. The assistant's stated intent is to check "what port kuri-2 is actually listening on for S3," revealing a hypothesis that perhaps kuri-2 is not properly serving the S3 API despite passing its health check.
The Reasoning Process: A Hypothesis Under Test
The assistant's thinking is visible in the transition from the previous messages to this one. Having confirmed that both kuri-1 and kuri-2 respond "OK" to health checks from within the s3-proxy container, the assistant has eliminated one class of problems: the backends are reachable and self-report as healthy. The round-robin selection code in backend_pool.go was reviewed and appeared correct. The proxy logs showed only the startup message "Backend nodes: 2 configured" with no errors or routing information.
This leaves a narrower set of possibilities. The assistant's next hypothesis is that kuri-2 might not be listening on the S3 port at all—that the health check might be succeeding on a different endpoint (perhaps a general HTTP server) while the S3-specific handler is not bound to the expected port. This is a reasonable inference: in a system where each Kuri node runs multiple services (a LocalWeb endpoint for CAR files on port 7002, a web UI, internal APIs), it is plausible that the S3-compatible API endpoint is not enabled or is bound to a different port than expected.
The choice to inspect the configuration file directly—cat /data/fgw2/config/kuri-2/settings.env—rather than querying the running process or checking Docker port mappings, reveals an assumption that the configuration generation script may have produced incorrect settings. This is a deeper investigative level than checking runtime state; it goes to the source of truth from which the container's behavior is derived.## Context: The Architecture Being Investigated
To understand why this message matters, one must grasp the architecture under test. The system is a horizontally scalable S3-compatible storage layer for a Filecoin Gateway. It follows a three-tier design: a stateless S3 frontend proxy (listening on port 8078) accepts all client requests and routes them to one of several Kuri storage nodes. Each Kuri node is an independent storage backend with its own dedicated database keyspace in YugabyteDB (e.g., filecoingw_kuri1 and filecoingw_kuri2). The frontend proxy uses round-robin selection for write operations (PUT requests) and YCQL-based object routing for reads (GET requests), consulting a shared S3 objects table that records which node holds each object.
The test cluster, orchestrated via Docker Compose, includes two Kuri nodes, a shared YugabyteDB instance, an nginx-based web UI exposing both nodes' dashboards on ports 9010 and 9011, and the S3 frontend proxy itself. The entire system had been built, debugged, and deployed over the course of the session, with numerous bugs already fixed—HTTP route conflicts, JSON case mismatches between frontend and backend, missing database columns, and a fundamental architectural error where Kuri nodes were initially configured as direct S3 endpoints rather than having a separate stateless proxy layer.
The user's observation that traffic reaches only kuri-1 despite both nodes appearing healthy is therefore a critical regression. If the round-robin is working but all writes land on kuri-1, the system is not actually distributing storage load. If kuri-2 is not receiving traffic at all, it may as well not exist for purposes of horizontal scaling.
Assumptions Embedded in the Message
The assistant's approach carries several implicit assumptions worth examining. First, there is the assumption that the configuration file is the authoritative source of truth about what services a node runs. This is reasonable in a Docker Compose environment where configuration is injected via environment files, but it overlooks the possibility that the binary itself might have hardcoded defaults or that the Docker entrypoint script might override these settings.
Second, the assistant assumes that the health check endpoint (/healthz) tests the S3 API specifically. In the previous messages, the assistant ran wget -q -O- http://kuri-2:8078/healthz from within the s3-proxy container and got "OK." But the health check might be a generic handler registered on the HTTP server regardless of whether the S3-specific routing logic is fully initialized. A "200 OK" response does not guarantee that PUT and GET operations will be processed correctly.
Third, the assistant assumes that the configuration file for kuri-2 was generated correctly by the gen-config.sh script. The file header shows it was generated on "Sat Jan 31 02:38:13 PM CET 2026," which is the same session. But the generation script had been modified multiple times during the session to fix various issues (per-node keyspace segregation, independent settings files). It is plausible that a bug in the script produced a configuration for kuri-2 that is subtly different from kuri-1 in a way that affects S3 API availability.
Fourth, there is an assumption that the port mapping in Docker Compose is correct. The S3 proxy is configured with FGW_BACKEND_NODES=kuri-1:http://kuri-1:8078,kuri-2:http://kuri-2:8078, meaning it expects both nodes to serve S3 on port 8078 internally. If kuri-2's Docker container maps port 8078 differently, or if the node's internal service is bound to a different port, the proxy would connect successfully to port 8078 on kuri-2's container but might be talking to a different service entirely (perhaps the LocalWeb endpoint or the web UI).
The Knowledge Inputs Required
To fully understand this message, a reader needs several pieces of background knowledge. One must understand the three-tier architecture and the distinction between the stateless S3 proxy and the stateful Kuri storage nodes. One must know that each Kuri node runs multiple services (S3 API, LocalWeb for CAR file serving, web UI for monitoring, internal RPC endpoints) and that these are distinguished by port numbers. One must be familiar with the Docker Compose orchestration pattern used in the test cluster, including how environment files are injected into containers. One must understand the health check mechanism and its limitations—that a "200 OK" response confirms TCP-level connectivity and basic HTTP handling but not full service readiness. Finally, one must know the history of the session: that the configuration generation script had been rewritten to produce per-node independent settings files after the architectural correction, and that this script is a potential source of bugs.
The Knowledge Output Created
This message produces several important pieces of knowledge for the ongoing investigation. First, it establishes that the configuration file for kuri-2 exists and is populated with plausible values—database connection parameters, data directory, API endpoint. This rules out the hypothesis that the configuration file was never generated or is empty. Second, the truncation of the output (the #... at the end) signals that the full configuration was not displayed, which means the assistant may have missed critical settings further down in the file—settings related to S3-specific configuration, port bindings, or feature flags that could explain why S3 traffic is not being handled. Third, the act of inspecting the configuration file shifts the investigation's focus from runtime behavior to static configuration, opening a new line of inquiry: comparing kuri-2's settings.env against kuri-1's to find discrepancies.
What Follows: The Debugging Trajectory
The subsequent messages (855 through 865) show the assistant pursuing this configuration-focused line of investigation. The assistant adds debug-level logging to the S3 proxy to track round-robin selections, rebuilds the Docker image, restarts the containers, generates test traffic, and checks the logs—only to find that the new log messages are not appearing. This leads to yet another hypothesis: the log level is set to info and the newly added logging may be at debug level. The assistant checks RIBS_LOGLEVEL=ribs=info and realizes the logging infrastructure needs adjustment. The debugging deepens, but the original question—why does all traffic go to kuri-1?—remains unresolved at this point in the conversation.
Conclusion
Message 854 is a classic debugging pivot point. When all the obvious runtime checks pass—both backends respond to health checks, the routing code looks correct, the proxy starts without errors—the investigator must go deeper, examining the raw configuration that defines each node's behavior. The assistant's decision to inspect kuri-2's settings.env file represents a shift from verifying the system's dynamic behavior to auditing its static configuration. This is the kind of methodical, hypothesis-driven debugging that distinguishes effective troubleshooting from random trial and error. The message is brief—a single bash command and its truncated output—but it carries the weight of an entire investigative thread: the suspicion that kuri-2 may not be properly configured to serve S3 traffic despite appearing healthy. Whether that suspicion proves correct or leads to another dead end, the process of forming and testing it is the essence of distributed systems debugging.