The Moment of Recognition: Debugging Cross-Node Stats in a Distributed S3 Cluster

In the life of a distributed systems engineer, few moments are as valuable as the instant when a confusing symptom snaps into focus and reveals its true cause. Message 2111 in this extended debugging session captures exactly such a moment. The assistant, having initially declared the cluster topology "working correctly," is confronted with contradictory evidence from the user and must re-examine the data. What follows is a masterclass in diagnostic reasoning—a careful parsing of output, a recognition of pattern, and the beginning of a source-code investigation that will ultimately uncover a subtle port-mismatch bug.

The Setup: A Distributed Storage Cluster Under Test

The context for this message is a QA deployment of FGW (Filecoin Gateway), a horizontally scalable distributed storage system that provides an S3-compatible API on top of Filecoin's decentralized storage network. The test cluster consists of three physical nodes: a head node running YugabyteDB and an S3 proxy, plus two Kuri storage nodes (kuri_01 and kuri_02) that handle the actual data storage and retrieval. The S3 proxy on the head node routes incoming requests to the appropriate backend Kuri node based on a hash of the object key.

Earlier in the session, the assistant had successfully deployed this cluster, run load tests (achieving ~112 writes/sec and ~105 reads/sec with zero data corruption), and verified that the S3 proxy was routing traffic to both backends. The cluster topology web UI appeared to be functioning—both nodes showed up in the "Storage Nodes" table. However, the user noticed something wrong: the table didn't show traffic or stats from the other node on each node's view. The assistant initially misunderstood, responding that "the cluster topology is rendering correctly now" and summarizing both nodes' stats as if cross-node visibility was working.

The User's Correction

The user's reply (message 2110) was precise and unambiguous: "I still don't see kuri_1 stats from 2, and don't see kuri_1 stats from kuri_2." This correction is crucial—it forces the assistant to look more carefully at the actual topology output rather than assuming everything is fine based on superficial indicators.

The Subject Message: A Careful Re-examination

Message 2111 is the assistant's response to this correction. It begins with the word "Ah"—a small but significant signal of recognition. The assistant has gone back to look at the raw topology data and now sees what it missed before.

The message quotes the exact topology output from both nodes' perspectives, formatted as a side-by-side comparison:

From kuri_01's perspective:

The Diagnostic Pivot

What makes this message noteworthy is not just the recognition of the bug, but the immediate pivot to investigation. The assistant doesn't speculate about possible causes or ask for more information. Instead, it formulates a concrete next step: "Let me look at how ClusterTopology gathers remote node stats."

The assistant then executes two shell commands in sequence. The first is a targeted grep across the Go source code to find files that reference the relevant symbols (ClusterTopology, storageNodes, StorageNode). The results narrow the search to four files:

Assumptions and Reasoning

Several assumptions underpin the assistant's reasoning in this message:

  1. The topology data is correct at the source: The assistant trusts that the /api/stats endpoint on each node returns accurate local data. The problem must be in how that data is fetched from remote nodes, not in how it's generated.
  2. The regression is in the cross-node fetching logic: Since each node sees its own stats correctly but not the other node's, the bug must be in the code path that reaches out to peer nodes to collect their stats.
  3. The fix will be in the Go source code, not in configuration: The assistant immediately goes to the source code rather than checking environment variables or deployment settings. This is a reasonable assumption given that the topology endpoint is implemented in Go and the cross-node communication is a programmatic concern.
  4. The git history is relevant: By checking git log, the assistant implicitly assumes that this is a regression introduced by a recent change, not a long-standing bug. This is a reasonable heuristic in an active development environment.

What the Message Reveals About the Thinking Process

The structure of message 2111 reveals a clear diagnostic methodology. The assistant:

  1. States the observed symptom in precise, quantitative terms (the exact storageUsed and groupsCount values from both perspectives)
  2. Formulates a hypothesis ("the nodes aren't fetching stats from each other")
  3. Identifies the next investigative step (looking at the ClusterTopology implementation)
  4. Executes the investigation (grep for relevant files, check git history) This is textbook debugging behavior: observe → hypothesize → investigate. The assistant doesn't jump to conclusions or try random fixes. It systematically narrows the search space.

Input Knowledge Required

To fully understand this message, the reader needs:

  1. Knowledge of the cluster architecture: That there are two Kuri storage nodes, each with its own web UI on port 9010, and that the ClusterTopology RPC endpoint is supposed to aggregate stats from all nodes.
  2. Understanding of the FGW_BACKEND_NODES configuration: This environment variable tells each Kuri node the URLs of its peers, which is how cross-node communication is supposed to work.
  3. Familiarity with the Go source code structure: The assistant knows that rbstor/diag.go is the likely location for diagnostic/topology logic, and that integrations/web/rpc.go handles the RPC layer.
  4. Context from the previous debugging: The assistant had earlier added FGW_BACKEND_NODES to fix an empty topology issue, and now a different aspect of the same feature is broken.

Output Knowledge Created

This message produces several important outputs:

  1. A confirmed bug diagnosis: The cross-node stat fetching is broken, not the local stat generation or the web UI rendering.
  2. A narrowed search space: The investigation is now focused on the Go source files that implement the ClusterTopology logic, specifically rbstor/diag.go and integrations/web/rpc.go.
  3. A clear next step: The assistant will read the source code to understand how remote stats are fetched and find the bug.
  4. Documentation of the exact symptom: The formatted topology output serves as a permanent record of the bug's manifestation, which can be used to verify the fix later.

What Happens Next

The subsequent messages (2112–2123) show the payoff of this investigation. Reading rbstor/diag.go reveals the bug on line 263:

statsURL := strings.Replace(nodeURL, ":8078", ":9010", 1) + "/api/stats"

The code assumes the node URL contains :8078 (the S3 proxy port) and replaces it with :9010 (the web UI port). But the Kuri nodes' URLs in FGW_BACKEND_NODES use port :8079 (the internal Kuri S3 port), not :8078. So the string replacement fails silently, and the code tries to hit http://10.1.232.83:8079/api/stats—a path that doesn't exist on the Kuri node's S3 endpoint.

The fix is straightforward: replace :8079 instead of :8078, or better yet, use a more robust approach. The assistant applies the fix, rebuilds the binary, and redeploys to both Kuri nodes.

Broader Implications

This bug is a classic example of a "magic number" or "hardcoded assumption" problem in distributed systems. The code assumed that all backend node URLs would use port 8078—the S3 proxy's port—when in fact the Kuri storage nodes expose their own S3-compatible API on port 8079. This assumption was baked into the stat-fetching logic and silently caused cross-node visibility to fail.

The bug also illustrates a common pitfall in distributed systems debugging: the difference between "the feature appears to work" and "the feature actually works correctly." The assistant initially thought the topology was fine because both nodes appeared in the UI, but the user's closer inspection revealed that the cross-node data was missing. This is a valuable lesson about the importance of verifying distributed system behavior from multiple perspectives.

Conclusion

Message 2111 is a turning point in the debugging session—the moment when a confusing symptom is correctly interpreted and a systematic investigation begins. The assistant's careful re-examination of the topology data, its clear formulation of the problem, and its immediate pivot to source-code investigation exemplify the disciplined approach required to debug distributed systems. The root cause—a hardcoded port assumption in a string replacement—is a small bug with outsized consequences, silently breaking cross-node visibility in a way that superficial testing might miss. The message stands as a testament to the value of precise observation, clear communication, and methodical investigation in software engineering.