The Moment of Verification: Querying a Distributed Cluster Topology

In any complex software engineering effort, there comes a pivotal moment when the developer stops writing code and starts asking questions of the system. Message 896 in this coding session captures exactly such a moment: the assistant, having just implemented cross-node metric aggregation for a horizontally scalable S3 storage cluster, reaches out to the running system with a carefully crafted RPC query to see whether the architecture actually works as intended.

The Message Itself

The assistant executed the following command:

echo '{"jsonrpc":"2.0","method":"RIBS.ClusterTopology","params":[],"id":1}' | websocat -n1 ws://localhost:9010/rpc/v0 2>&1 | jq '.'

The response showed a JSON-RPC result containing cluster topology data:

{
  "id": 1,
  "jsonrpc": "2.0",
  "result": {
    "proxies": [
      {
        "id": "kuri-2",
        "address": "localhost",
        "status": "healthy",
        "requestsPerSecond": 0.5,
        "activeConnections": 0,
        "backendPool": null,
        "latencyMs": 0,
        "errorRate": 0
      }
    ],
    "storageNodes": [
      {
        "id": "kuri-1",
        "address": "http://kuri-1:8078",
        "status": "healthy",
        "storageUsed": 19447018394,
        "storageTotal": 0,
        ...
      }
    ]
  }
}

The Motivation: Why This Message Was Written

This message did not appear in a vacuum. It was the culmination of a lengthy debugging session that spanned dozens of previous messages, each one building toward a single goal: making the cluster monitoring dashboard show live, accurate statistics from all nodes in the distributed S3 architecture, not just the local node.

The problem that motivated this query was subtle but critical. Earlier in the session, the assistant had discovered that the ClusterTopology RPC method—the primary data source for the React-based monitoring dashboard—only returned statistics for the local node. When the web UI queried kuri-1's RPC endpoint, it would see kuri-1's storage usage, request rates, and groups count, but kuri-2 would appear as a ghost node with zero values. This made the monitoring dashboard misleading: an operator looking at the UI would see only half the cluster's activity.

The root cause lay in the implementation of ClusterTopology() in rbstor/diag.go. The method parsed the FGW_BACKEND_NODES environment variable to discover peer nodes, but when it encountered a node ID that didn't match the local node's identity, it simply returned a placeholder with empty statistics. There was no mechanism to reach across the network and ask a peer, "What are your metrics right now?"

The assistant's response was to implement a lightweight HTTP stats endpoint (/api/stats) on each Kuri node and modify ClusterTopology() to make HTTP requests to peer nodes, collecting their real-time metrics. Message 896 is the first test of that newly implemented cross-node aggregation—the moment of truth.

Input Knowledge Required

To fully understand this message, one needs to grasp several layers of context:

Architecture knowledge: The system under development is a horizontally scalable S3-compatible storage system with a three-layer architecture. At the top sits a stateless S3 frontend proxy (port 8078) that routes client requests to Kuri storage nodes. Each Kuri node (ports 7001/7002 for data, 9010/9011 for web UI) operates as an independent storage backend with its own RIBS keyspace in YugabyteDB. A shared S3 keyspace holds object routing metadata, enabling the frontend proxy to locate objects across nodes.

RPC infrastructure: The monitoring dashboard communicates with Kuri nodes via JSON-RPC over WebSocket at /rpc/v0. The RIBS.ClusterTopology method returns a structured view of the entire cluster, including proxy nodes, storage nodes, and data flow information. This is the primary data source for the React-based ClusterTopology component.

The debugging history: Prior messages reveal a trail of investigation. The assistant had verified that the round-robin proxy was correctly distributing traffic between kuri-1 and kuri-2 (alternating requests). The assistant had added info-level logging to confirm both backends were in the pool. But the web UI stubbornly showed traffic only on kuri-1, leading the assistant to trace the problem to the ClusterTopology() implementation.

Tooling: The command uses websocat (a WebSocket client) to connect to the RPC endpoint, pipes a JSON-RPC request, and formats the response with jq. This is a standard debugging workflow for systems with WebSocket-based RPC interfaces.

The Thinking Process Visible in This Message

This message is deceptively simple—a single command execution—but it reveals sophisticated reasoning. The assistant is not randomly poking at the system; it is executing a deliberate verification strategy.

The choice of endpoint is significant. The assistant queries port 9010 (kuri-1's web UI) rather than port 9011 (kuri-2's). This tests the cross-node aggregation path: kuri-1 must now reach out to kuri-2 via HTTP to collect its stats. If the implementation is correct, the response should show both nodes with live data.

The timing is also deliberate. Just before this message, the assistant generated test traffic by uploading 10 files (cluster1.bin through cluster10.bin) to the S3 proxy at port 8078. This ensures that requestsPerSecond will show non-zero values, confirming that the metrics pipeline is live and not returning stale or default data.

The response reveals a partial success. The proxies array shows kuri-2 with requestsPerSecond: 0.5, and the storageNodes array shows kuri-1 with storageUsed: 19447018394. Both nodes are present with live data—a marked improvement over the previous state where only the local node had statistics. However, the response is truncated in the conversation log, and the subsequent message (897) reveals that the assistant noticed something curious: querying port 9010 (kuri-1's UI) shows kuri-2 in the proxies list, while querying port 9011 (kuri-2's UI) shows kuri-1 in the proxies list. This asymmetry suggests that the proxy information is being populated from the FGW_BACKEND_NODES environment variable rather than from actual RPC queries to proxy nodes—a subtle distinction that may or may not matter depending on the intended semantics.

Output Knowledge Created

This message produced concrete, actionable knowledge:

  1. Cross-node aggregation works: The ClusterTopology RPC now returns data from multiple nodes, confirming that the HTTP stats endpoint and the aggregation logic in diag.go are functional.
  2. Both nodes show traffic: With requestsPerSecond: 0.5 on both nodes (as confirmed in message 897), the round-robin load distribution is verified to be working correctly, and the metrics pipeline captures this distribution.
  3. Storage metrics are live: The storageUsed value of approximately 19.4 GB on kuri-1 reflects actual data written during testing, confirming that the storage metrics path from the RIBS database through the metrics collector to the RPC endpoint is intact.
  4. A potential asymmetry: The proxy list shows only one proxy (kuri-2 from kuri-1's perspective, and kuri-1 from kuri-2's perspective), suggesting that the proxy discovery logic may have a blind spot or that the environment variable configuration creates a mirror-image view rather than a unified cluster view.

Assumptions and Potential Mistakes

The assistant operates under several assumptions in this message:

That the RPC endpoint is reachable: The command assumes that websocat can connect to ws://localhost:9010/rpc/v0 and that the WebSocket handshake will succeed. This is a reasonable assumption given that the container infrastructure has been running and tested, but it implicitly depends on the nginx proxy configuration, Docker networking, and the Kuri daemon's HTTP server all functioning correctly.

That jq is available: The command pipes through jq for JSON formatting. If jq were not installed, the output would be a raw JSON blob, but the command would still succeed. The assistant assumes a standard development environment.

That the JSON-RPC format is correct: The request uses method RIBS.ClusterTopology with an empty params array. This assumes the RPC dispatcher correctly routes this method name to the ClusterTopology() function and that no authentication or session state is required.

That the metrics are meaningful: The assistant treats requestsPerSecond: 0.5 as evidence of traffic distribution. This is a reasonable inference, but it's worth noting that the value could be an artifact of the metrics sampling window or a default value rather than a true measurement of recent request volume.

The most significant potential mistake is an incorrect assumption about proxy identity. The response shows kuri-2 in the proxies list when querying kuri-1's endpoint. This is because the ClusterTopology() implementation populates the proxies array from the FGW_BACKEND_NODES environment variable, which lists both kuri-1 and kuri-2 as backend nodes. But in the three-layer architecture, the backend nodes are storage nodes, not proxy nodes. The proxies array is intended to show S3 frontend proxy instances (stateless, horizontally scalable), not Kuri storage nodes. This confusion between storage nodes and proxy nodes in the topology output suggests that the environment variable configuration may be conflating two distinct roles, or that the ClusterTopology() method is misinterpreting the node list.

The Deeper Significance

Message 896 is, on its surface, a simple test query. But it represents a critical transition in the engineering workflow: from building to verifying. The assistant has spent dozens of messages implementing cross-node stats aggregation, adding HTTP endpoints, modifying RPC handlers, rebuilding Docker images, and restarting containers. This message is the first moment where the assistant pauses to ask, "Did it work?"

The answer is a qualified yes. The system responds with data from multiple nodes. The metrics are live. The round-robin distribution is visible. But the proxy list anomaly hints at a deeper conceptual confusion that may need to be addressed—are the nodes listed in FGW_BACKEND_NODES storage backends or S3 proxies? The architecture diagram clearly separates these roles, but the implementation may be blurring the line.

This tension between the idealized architecture and the running system is the essence of debugging. Every verification step reveals not just what works, but also what doesn't quite fit. Message 896 captures that moment of discovery: the system works, but it works in a way that reveals new questions.