The Moment of Verification: When an RPC Method Goes Missing

A Single Debugging Message in a Distributed S3 Architecture

In the middle of a sprawling coding session to build a horizontally scalable S3-compatible storage system, there is a quiet but revealing moment. Message 842 in the conversation captures a brief verification step where the assistant, having just rebuilt the Docker image, restarted the cluster, fixed a stubborn container startup failure, and confirmed that basic services are operational, turns its attention to the monitoring dashboard's data endpoints. The message is deceptively simple—two commands, one success, one failure—but it encapsulates the entire rhythm of distributed systems debugging: incremental verification, assumption testing, and the inevitable discovery of gaps between what was planned and what was implemented.

The Message in Full

Here is the exact text of the message under analysis:

[assistant] Cluster topology is working and showing both storage nodes. Now let me check the other chart endpoints: [bash] echo '{"jsonrpc":"2.0","method":"RIBS.ThroughputHistory","params":["5m"],"id":1}' | websocat -n1 ws://localhost:9010/rpc/v0 2>&1 | jq '.' { "error": { "code": -32601, "message": "method 'RIBS.ThroughputHistory' not found" }, "id": 1, "jsonrpc": "2.0" }

The message consists of a brief commentary ("Cluster topology is working and showing both storage nodes. Now let me check the other chart endpoints:"), followed by a shell command that sends a JSON-RPC request to the WebSocket endpoint at ws://localhost:9010/rpc/v0, and the resulting JSON error response. There are no secrets or credentials to redact; the message is entirely about protocol-level debugging.

Why This Message Was Written: The Context of Verification

To understand why this message exists, we must reconstruct the state of the system at the moment it was sent. The assistant had just completed a lengthy cycle of rebuilding and redeploying the entire test cluster. The preceding messages (809–841) document a cascade of operations:

  1. Docker image rebuild: The fgw:local Docker image was rebuilt from source, incorporating the latest frontend changes including new monitoring charts (I/O throughput, latency distribution) and the architectural distinction between S3 frontend proxies and Kuri storage nodes.
  2. Cluster restart: The entire test cluster was force-recreated with docker compose up -d --force-recreate, tearing down and rebuilding all containers.
  3. Debugging kuri-2 failure: The second Kuri storage node failed to start with a cryptic error about a missing sp_deal_stats_tmp table. The assistant investigated database schemas, migration versions, and container logs before resolving the issue—apparently by simply restarting the container, suggesting it was a transient initialization race condition.
  4. Service verification: All containers were confirmed running, the S3 health endpoint returned ok, the web UI served HTML, and test traffic was successfully uploaded and downloaded through the S3 proxy.
  5. RPC endpoint testing: The assistant verified that the RIBS.IOThroughput method returned data (showing read/write byte counters) and that RIBS.ClusterTopology returned a complete topology with both storage nodes and their live statistics. Message 842 is the natural continuation of this verification sequence. Having confirmed that the two most critical monitoring endpoints work, the assistant turns to "the other chart endpoints"—specifically RIBS.ThroughputHistory. The phrase "now let me check" reveals the systematic, checklist-driven mindset: the monitoring dashboard was designed with multiple charts (I/O throughput, latency distribution, throughput history), and each one needs its data source verified independently.

The Assumptions Embedded in the Message

This message is built on several layers of assumptions, and the error response exposes where one of them breaks down.

Assumption 1: ThroughputHistory exists as a registered RPC method. The assistant assumes that because the monitoring dashboard was designed to include a throughput history chart, the corresponding backend method RIBS.ThroughputHistory was implemented and registered with the JSON-RPC handler. This is a reasonable assumption given the comprehensive summary of completed work, which explicitly lists "ThroughputHistory" as a struct with JSON tags in iface/iface_ribs.go and mentions it as part of the cluster monitoring implementation.

Assumption 2: The method name follows the same naming convention as other endpoints. The assistant uses the pattern RIBS.ThroughputHistory, consistent with RIBS.IOThroughput (which works) and RIBS.ClusterTopology (which works). The naming convention appears to be RIBS.<MethodName>, so RIBS.ThroughputHistory follows the pattern.

Assumption 3: The WebSocket RPC endpoint is functioning correctly. The same endpoint (ws://localhost:9010/rpc/v0) was used successfully for RIBS.IOThroughput in message 837 and RIBS.ClusterTopology earlier in message 842. The transport layer is verified working.

Assumption 4: The method takes the same parameters as IOThroughput. The assistant passes ["5m"] as the parameters, the same argument used successfully with RIBS.IOThroughput. This assumes that ThroughputHistory, like IOThroughput, accepts a time window string.

The Mistake: A Method That Was Never Registered

The error response is unambiguous: "method 'RIBS.ThroughputHistory' not found" with JSON-RPC error code -32601 (Method not found). This is a standard JSON-RPC 2.0 error code indicating that the server has no handler registered for that method name.

The most likely explanation is that ThroughputHistory was planned and its data structures were defined (the ThroughputHistory struct exists in the interface file with JSON tags), but the actual RPC handler was never registered on the server side. The summary of completed work mentions modifying iface/iface_ribs.go to add JSON tags to ThroughputHistory and other structs, but it does not mention registering a corresponding RPC method in the WebSocket handler. The IOThroughput method, by contrast, was explicitly added in integrations/web/rpc.go as a new RPC method.

This is a classic software engineering pitfall: defining the data structures and the frontend components that consume them, but forgetting to wire up the server-side handler that produces the data. The struct exists, the frontend chart component may reference it, but the bridge between them—the RPC method registration—is missing.

There is also a possibility that the method was intentionally named differently (perhaps RIBS.Throughput or RIBS.GetThroughputHistory) or that it was deprecated in favor of RIBS.IOThroughput, which serves a similar purpose. The fact that IOThroughput already returns timestamped byte counters suggests it may have subsumed the role of ThroughputHistory.

Input Knowledge Required to Understand This Message

A reader needs several pieces of context to fully grasp what is happening in this message:

  1. The architecture: The system uses a three-layer design with stateless S3 frontend proxies routing requests to independent Kuri storage nodes, which share metadata through a YugabyteDB cluster. Monitoring data flows from the storage nodes through RPC endpoints to a React-based web UI.
  2. The JSON-RPC protocol: The assistant communicates with the backend using JSON-RPC 2.0 over WebSockets. The method name follows the pattern RIBS.<MethodName>, parameters are passed as JSON arrays, and errors use standard JSON-RPC codes.
  3. The monitoring dashboard: The web UI includes multiple charts and panels: cluster topology (showing proxies and storage nodes), active requests, I/O throughput, latency distribution, and throughput history. Each chart requires a corresponding RPC endpoint to supply data.
  4. The development workflow: The assistant is working in a tight loop of code modification → Docker build → cluster restart → endpoint verification. This message belongs to the verification phase.
  5. The prior debugging session: The assistant had just resolved a kuri-2 startup failure and confirmed that IOThroughput and ClusterTopology endpoints work. This message is the next step in that verification sequence.

Output Knowledge Created by This Message

The message produces several valuable pieces of information:

  1. A confirmed bug: The RIBS.ThroughputHistory method is not registered on the server. This is a concrete, reproducible finding that will need to be addressed—either by implementing the missing handler, renaming the frontend to use IOThroughput instead, or removing the reference from the dashboard.
  2. A verified working endpoint: The RIBS.ClusterTopology method is confirmed working and returning complete data for both storage nodes. This validates the cluster monitoring infrastructure for the topology view.
  3. A validated testing methodology: The combination of websocat for raw RPC calls and jq for pretty-printing JSON responses is confirmed as an effective way to test monitoring endpoints independently of the web UI.
  4. A boundary in the implementation: The message reveals the gap between the planned feature set (as documented in the summary) and the actually implemented feature set. This is a form of knowledge that helps scope remaining work.

The Thinking Process: Systematic Debugging in Action

The reasoning visible in this message reveals a methodical, almost scientific approach to verification. The assistant is working through a mental checklist of monitoring endpoints, testing each one in sequence. The progression is logical:

  1. Start with the most critical endpoint: RIBS.IOThroughput was tested first (message 837) because it's the newest addition and directly supports the newly added I/O Throughput chart.
  2. Test the topology endpoint: RIBS.ClusterTopology (message 842) is the backbone of the monitoring dashboard—without it, the entire cluster view is blank. Confirming it works establishes that the core monitoring infrastructure is functional.
  3. Proceed to secondary endpoints: With the two primary endpoints verified, the assistant moves to "the other chart endpoints," starting with RIBS.ThroughputHistory. The failure here is caught early, before any time is wasted investigating frontend rendering issues that would ultimately trace back to missing data. The assistant's commentary is minimal but telling. "Cluster topology is working and showing both storage nodes" is a status update that also serves as a confidence checkpoint. Having confirmed that the most complex part of the monitoring system (the topology with live stats from multiple nodes) is functional, the assistant is optimistic enough to proceed with further checks. The phrase "now let me check the other chart endpoints" suggests an expectation that they will also work—an expectation that is immediately contradicted by the error response. Notably, the assistant does not panic or speculate about the cause of the error within this message. The message ends with the raw error output, allowing the data to speak for itself. In the broader conversation, this finding would likely lead to either implementing the missing RPC handler or adjusting the frontend to use the working IOThroughput endpoint instead.

Conclusion: The Value of a Single Verification Step

Message 842 is, on its surface, a mundane debugging interaction: a command is run, an error is returned, the assistant notes the result. But examined closely, it reveals the essence of building distributed systems. Every component must be verified independently because assumptions compound. The ThroughputHistory struct was defined, the frontend chart was built, the Docker image was rebuilt, the cluster was restarted—and yet the endpoint returns "method not found." This is not a failure of any single step but a gap in the chain between them.

The message also demonstrates the importance of incremental verification. Rather than rebuilding the entire frontend and checking only whether the page loads, the assistant tests each data source independently at the RPC level. This isolates problems early, before they become confusing UI bugs. When the throughput history chart eventually appears blank in the web UI, the assistant will already know why: the data source doesn't exist yet.

In the broader narrative of the coding session, this message is a pivot point. It transforms the assistant's task from "verify the deployment" to "fix the missing RPC method." The error is not a setback but information—precisely the kind of information that makes debugging productive. Every "method not found" is a discovery that narrows the gap between the system as imagined and the system as built.