The Websocket Verification Trap: A Microcosm of Distributed Systems Debugging

Introduction

In the course of building a horizontally scalable S3 architecture with distributed storage nodes, every assumption must be verified, every endpoint tested, and every integration point proven. Message 686 in this coding session captures a pivotal moment of verification—a moment when the developer attempts to confirm that a newly implemented cluster topology RPC is actually returning data, only to be stopped cold by a missing Node.js module. This single message, seemingly trivial at first glance, encapsulates the iterative, assumption-laden nature of distributed systems debugging and the constant tension between implementation and verification.

The Message

Let us examine the message exactly as it was written:

Good - both web UIs are now exposed on :9010 and :9011.

Let me write a simple test to verify the ClusterTopology is returning data via websocket:

```bash cat > /tmp/test_rpc.js << 'EOF' const WebSocket = require('ws');

const ws = new WebSocket('ws://localhost:9010/rpc/v0');

ws.on('open', function open() { const req = JSON.stringify({ jsonrpc: '2.0', method: 'RIBS.ClusterTopology', params: [], id: 1 }); ws.send(req); });

ws.on('message', function message(data) { console.log('Response:', data.toString()); ws.close(); process.exit(0); });

ws.on('error', function error(err) { console.error('Error:', err.message); process.exit(1); });

setTimeout(() => { console.error('Timeout'); process.exit(1); }, 5000); EOF node /tmp/test_rpc.js 2>&1 node:internal/modules/cjs/loader:1423 throw err; ^

Error: Cannot find module 'ws' Require stack: - /tmp/test_rpc.js at Module._resolveFilename (node:internal/modules/cjs/loader:1420:15) at defaultResolveImpl (node:internal/modules/cjs/loader:1058:19) at resolveForCJSWithHooks (node:internal/modules/cjs/loader:1063:22) at Module._load (node:internal/modules/cjs/loader:1226:37) at TracingChannel.traceSync (node:diagnostics_channel:328:14) at wrapModuleLoad (node:internal/modules/cjs/loader:... ```

This is the complete message. It opens with a confirmation of success—both web UIs are exposed—and then pivots immediately to a verification attempt. The assistant writes a Node.js script to test the websocket-based RPC endpoint, runs it, and hits a module resolution error. The message ends mid-error output, leaving the reader hanging on the outcome.

Context and Background

To understand why this message exists, we must trace the chain of events that led to it. The user had reported that the cluster monitoring page was empty, displaying the message "No cluster nodes configured. Set up FGW_BACKEND_NODES to see cluster topology" (message 654). The assistant had been working on implementing the ClusterTopology RPC method in rbstor/diag.go, which reads the FGW_BACKEND_NODES environment variable and performs health checks against each configured node. This implementation was critical—without it, the cluster monitoring dashboard would remain a stub, showing no data and providing no operational insight into the distributed system.

The assistant had already rebuilt the Docker image, regenerated configuration files, and restarted the cluster (messages 672-674). Both web UIs were confirmed working on ports 9010 and 9011 (message 675). But when the assistant tried to verify the ClusterTopology RPC using curl with an HTTP POST request, the endpoint returned "Bad Request" (messages 676-678). Investigation revealed that the RPC endpoint at /rpc/v0 uses websocket protocol, not plain HTTP (messages 679-682). The assistant confirmed that the storage nodes could reach each other's health endpoints using wget inside the Docker containers (messages 683-684), but the actual RPC data flow remained unverified.

This is the precise moment captured in message 686: the assistant, having discovered that the RPC uses websocket, decides to write a quick Node.js test script to call the endpoint and inspect the response.

The Reasoning: Why This Test Was Necessary

The assistant's decision to write a custom Node.js test script reveals several layers of reasoning. First, the assistant had already attempted to verify the RPC using curl with a POST request to /rpc/v0, which failed with "Bad Request" (message 676). This failure indicated that the endpoint was not a standard HTTP JSON-RPC endpoint—it required a different transport protocol. The assistant then checked the source code and found that the RPC handler was registered using mux.Handle("/rpc/v0", rpc) in integrations/web/ribsweb.go (message 679-680). The MakeRPCServer function, when traced further, would reveal that it uses websocket transport.

The assistant could have used a tool like websocat or written a Python script with the websockets library, but Node.js was chosen. This is a reasonable choice—Node.js has excellent websocket support through the ws package, and the script is concise and readable. The script sends a JSON-RPC request with method RIBS.ClusterTopology and logs the response. It includes error handling and a timeout, showing careful consideration of failure modes.

However, the assistant made an implicit assumption: that the ws Node.js module would be available in the environment. This assumption turned out to be incorrect, as the error message "Cannot find module 'ws'" demonstrates. This is a classic development environment pitfall—the tools we expect to be available often are not, especially when working across different contexts (in this case, the host system where the assistant is running commands versus the Docker containers where the application runs).

The Assumptions at Play

This message is rich with assumptions, both explicit and implicit:

The websocket endpoint is functional. The assistant assumes that the RPC endpoint, once implemented and the cluster restarted, will respond to websocket connections. This is a reasonable assumption given that the code compiles and the containers are running, but it remains unverified.
The ws module is installed. This is the assumption that fails spectacularly. The assistant writes a Node.js script requiring the ws package without first checking if it's available. In many development environments, Node.js is installed with a minimal set of global packages, and ws is not one of them.
The ClusterTopology implementation is correct. The assistant had edited rbstor/diag.go to implement the ClusterTopology function, parsing FGW_BACKEND_NODES and performing health checks. The implementation compiled without errors (after fixing several LSP-reported issues), but its runtime behavior had never been tested.
The network connectivity between containers is correct. The assistant had verified that kuri-1 could reach kuri-2:8078/healthz and vice versa (messages 683-684), but this only tests the health endpoint, not the full RPC data flow.
The frontend will display the data correctly. The entire chain of verification assumes that if the RPC returns data, the React frontend will render it properly. This is a multi-layered assumption spanning the Go backend, the websocket transport, the JSON-RPC protocol, and the JavaScript frontend.

The Knowledge Required

To understand this message fully, one needs knowledge across several domains:

Distributed systems architecture: The concept of a cluster topology RPC that returns information about all nodes in a distributed storage cluster. The FGW_BACKEND_NODES environment variable configures which nodes are part of the cluster.

Websocket transport: Understanding that JSON-RPC can be transported over websocket rather than HTTP POST, and that this affects how clients must connect.

Node.js module system: The CommonJS require function and how module resolution works. The error trace shows Node.js searching for the ws module and failing.

Docker networking: The containers in the test cluster communicate over an internal Docker network. The assistant had already verified inter-container HTTP connectivity using docker exec and wget.

Go implementation details: The ClusterTopology function in rbstor/diag.go parses the FGW_BACKEND_NODES environment variable, splits it by comma, and for each node performs an HTTP health check against its /healthz endpoint. It returns structured data about proxies and storage nodes.

The Output Knowledge Created

Despite failing to execute, this message creates valuable knowledge:

The ws module is not installed. This is a concrete finding that will inform the next steps—either install the module globally, use a different tool (like websocat), or test from within a Docker container where the module might be available.
The websocket endpoint is reachable. The fact that the script ran and attempted to connect (before failing at module resolution) confirms that the Node.js runtime is available and the network path to localhost:9010 is open. The error is not a connection error but a module resolution error.
The test script is correct in structure. The JSON-RPC request format, the websocket connection pattern, and the error handling are all properly written. Once the module dependency is resolved, this script should work.
The verification strategy is sound. The assistant's approach of writing a targeted test for the RPC endpoint is methodologically correct. It isolates the backend behavior from the frontend rendering, allowing independent verification of each layer.

The Thinking Process Visible in the Message

The assistant's reasoning is laid bare in the structure of the message. It begins with a confirmation of success—"Good - both web UIs are now exposed on :9010 and :9011"—which serves as a status update and a transition point. The assistant is satisfied that the immediate goal (exposing both UIs) has been achieved, but immediately recognizes that the deeper question remains unanswered: is the ClusterTopology RPC actually returning data?

The decision to write a Node.js test script rather than use a simpler tool like websocat or a Python one-liner reveals the assistant's programming background. Node.js is a natural choice for a JavaScript developer or a full-stack engineer who works with React frontends. The script is clean, well-structured, and includes proper error handling—it's not a hack but a deliberate testing tool.

The assistant also demonstrates an understanding of the system's architecture. The JSON-RPC request format ({"jsonrpc":"2.0","method":"RIBS.ClusterTopology","params":[],"id":1}) matches the expected protocol. The choice of ws://localhost:9010/rpc/v0 as the endpoint shows awareness that the RPC is served on the web UI port, not the S3 proxy port.

The timeout of 5 seconds indicates an expectation that the RPC should respond quickly. This is a reasonable expectation for a health-check-based topology query that involves HTTP calls to known endpoints.

Mistakes and Incorrect Assumptions

The primary mistake in this message is the failure to verify the environment before writing the test script. A more experienced developer might have first checked if the ws module was available using node -e "require('ws')" or checked the global npm packages with npm list -g --depth=0. Alternatively, the assistant could have used a tool already present in the environment, such as:

websocat: A command-line websocket client written in Rust, often available on development systems.
Python with websockets: Python is typically available on Linux systems, and the websockets library might be installed or installable.
curl with websocket support: Recent versions of curl support websocket connections via the --ws flag.
Docker exec into a container: Running the test from within a container that has Node.js and the ws module installed. The assistant also implicitly assumed that the development environment was set up for Node.js websocket development. This assumption is not unreasonable—many developers have Node.js with common packages installed globally—but it was incorrect in this case. Another subtle assumption is that the RPC endpoint would respond to a websocket connection from the host machine. The Docker containers expose port 9010 to the host, and the nginx reverse proxy should forward websocket connections. However, nginx requires specific configuration to handle websocket upgrades (the Upgrade and Connection headers). If the nginx configuration in the test cluster doesn't properly handle websocket upgrades, the connection would fail even with the correct module installed. The assistant's script would have revealed this issue on the next attempt.

The Broader Implications

This message is a microcosm of a fundamental challenge in distributed systems development: the gap between implementation and verification. Every layer of a distributed system—the Go backend, the websocket transport, the Docker networking, the nginx proxy, the React frontend—introduces potential failure points. Verifying that the system works end-to-end requires testing each layer independently and then together.

The assistant's approach is methodologically sound: implement the feature, restart the system, verify the basic connectivity (health checks), then verify the RPC data flow, and finally check the frontend display. Each step builds on the previous one. The failure at the verification step is not a failure of the implementation but a failure of the testing infrastructure—a missing Node.js module.

This highlights an important lesson for distributed systems development: invest in testing infrastructure early. Having a websocket client available in the development environment, having integration tests that can be run against a live cluster, and having monitoring that can verify data flow without manual intervention—these are not luxuries but necessities.

Conclusion

Message 686 captures a moment of suspended verification. The assistant has implemented a critical feature (the ClusterTopology RPC), restarted the cluster, confirmed basic connectivity, and now stands at the threshold of proving that the feature works. The test script is written, the endpoint is known, the request format is correct—but a missing Node.js module blocks the final step.

This message is a testament to the iterative nature of distributed systems debugging. Every assumption must be tested, every dependency verified, every endpoint proven. The assistant will likely recover quickly—installing the ws module or switching to an alternative tool—but the lesson remains: in distributed systems, verification is never a single step but an ongoing process of probing, testing, and validating across every layer of the stack.

The message also reveals the assistant's thought process: methodical, assumption-aware, and focused on isolating the verification target. The failure is not in the implementation but in the testing toolchain—a distinction that speaks to the assistant's understanding of where the system's boundaries lie.