The Moment of Proof: Validating Cross-Node S3 Access Through a Proxy Layer

Introduction

In distributed systems engineering, there is a profound difference between a system that should work and one that does work. The gap between these two states is filled with debugging, configuration corrections, architectural revisions, and—ultimately—validation. Message 2088 in this coding session captures that exact moment of transition: the assistant, after an extended sequence of troubleshooting, deploys an S3 proxy frontend and runs a series of curl commands that confirm cross-node object reads are finally functioning. What appears to be a simple test script is, in fact, the culmination of a deep architectural correction and the validation of a core design principle for the Filecoin Gateway's distributed storage system.

This article examines that single message in detail: why it was written, what decisions it reflects, the assumptions embedded within it, the knowledge it both consumes and produces, and the thinking process that led to this moment of verification.

The Message

The subject message reads as follows:

[assistant] Now let's test cross-node access via the proxy: [bash] # Write via proxy to ensure it routes correctly echo "Testing S3 proxy..."

# Write new object via proxy curl -s -X PUT \ -H "x-amz-content-sha256: UNSIGNED-PAYLOAD" \ -H "Content-Type: application/octet-stream" \ -d "proxy-test-data-$(date +%s)" \ http://10.1.232.82:8078/testbucket/proxy-test.txt

echo "Write complete"

# Read via proxy echo "Reading via proxy:" curl -s http://10.1.232.82:8078/testbucket/proxy-test.txt echo ""

# Read the earlier test.txt (written directly to kuri1) echo "Reading test.txt via proxy:" curl -s http://10.1.232.82:8078/testbucket/test.txt Testing S3 proxy... Write complete Reading via proxy: proxy-test-data-1769899056 Reading test.txt via proxy: test-data-123

At first glance, this is a straightforward shell script: write an object, read it back, read an older object. But every line carries the weight of the debugging session that preceded it.

Why This Message Was Written: The Reasoning and Context

The message was written to answer a single, critical question: Does the S3 proxy correctly route cross-node requests? This question arose from a fundamental architectural problem discovered earlier in the session.

In the initial QA cluster setup, each Kuri node (kuri1 at 10.1.232.83 and kuri2 at 10.1.232.84) ran its own S3 API server on port 8079. The S3 object metadata was stored in a shared YugabyteDB CQL keyspace (filecoingw_s3), which meant both nodes could see that an object existed and which node owned it. However, when a client tried to read an object from kuri2 that had been written to kuri1, the read failed with the error: "block was not found locally (offline): ipld: could not find bafkreicq725qu5x2tcre6ff6jb7cbuu4msnmmdceannrzsk6spdmom3gb4." The Kuri node's S3 server served data exclusively from its local blockstore; it had no mechanism to fetch blocks from peer nodes.

This was not a bug in the code—it was an architectural choice. The system was designed with a separate proxy layer precisely to handle this routing. The assistant had previously attempted to connect the Kuri nodes via IPFS bitswap peering (message 2066), which succeeded at the network level but did not fix the S3 reads because the S3 layer did not use bitswap for block retrieval.

The correct solution, as identified by the assistant, was to deploy the s3-proxy binary on the head node (10.1.232.82) and configure it to route requests to the appropriate backend Kuri node based on the node_id stored in the shared S3 metadata. This proxy would listen on port 8078 and serve as the single entry point for all S3 operations, abstracting away the multi-node backend.

The user's question at message 2079—"Why are you not doing this with ansible?"—prompted a critical shift in methodology. The assistant had been creating configuration files and systemd services manually via SSH, but the user correctly insisted on using the existing Ansible automation. This led to updating the QA inventory, adding the head node to the s3_frontend group, and running the deploy-frontend.yml playbook. The result was a properly deployed, systemd-managed s3-proxy service running on the head node.

Message 2088 is the validation step after that deployment. It is the moment of truth: does the proxy actually work?## How Decisions Were Made: The Proxy Architecture and Its Implications

The message reflects several layers of decision-making, both explicit and implicit.

Decision 1: Use the proxy, not direct node access. The assistant could have pursued other solutions—implementing peer-to-peer block exchange between Kuri nodes, modifying the Kuri S3 server to fetch remote blocks, or setting up a simple round-robin load balancer. Instead, the assistant chose to deploy the s3-proxy, which was already designed and built as part of the project's roadmap. This decision respected the existing architecture rather than introducing ad-hoc fixes.

Decision 2: Route through the head node. The proxy was deployed on 10.1.232.82, the same node running YugabyteDB. This is a sensible choice for a QA environment: it centralizes the control plane (database + proxy) on one machine while distributing the storage workload across two Kuri nodes. In production, the proxy would likely be deployed on dedicated frontend nodes, but for testing, this consolidated topology is efficient.

Decision 3: Use Ansible for deployment. The user's intervention at message 2079 was pivotal. The assistant had been manually copying binaries and creating systemd units, which is fragile and unrepeatable. Switching to Ansible ensured that the deployment was idempotent, version-controlled, and consistent with the rest of the infrastructure. The assistant's response—immediately stopping the manual service, updating the inventory, and running the playbook—shows a willingness to correct course when a better methodology is pointed out.

Decision 4: Test both a new object and an existing object. The test script writes a new object (to verify write routing) and then reads both the new object and the previously written test.txt (to verify read routing for objects stored on different nodes). This two-pronged test covers the essential cases: can the proxy accept writes and distribute them to a backend node, and can it retrieve objects regardless of which backend node holds the data?

Assumptions Made by the User and Agent

Every engineering decision rests on assumptions, and this message is no exception.

Assumption 1: The proxy correctly implements routing logic. The assistant assumes that the s3-proxy binary, when given the FGW_BACKEND_NODES environment variable listing both Kuri nodes with their node IDs and URLs, will correctly parse the S3 object metadata, extract the node_id field, and forward the request to the appropriate backend. This is a non-trivial piece of logic—the proxy must query the shared CQL keyspace, look up the object's metadata, determine which node owns the block, and issue a proxied HTTP request to that node's S3 endpoint. The assistant did not audit the proxy's source code; it trusted that the binary implemented the specification.

Assumption 2: The environment variable names are correct. The proxy's configuration uses FGW_BACKEND_NODES, RIBS_S3_CQL_HOSTS, RIBS_S3_CQL_PORT, and RIBS_S3_CQL_KEYSPACE. The assistant had earlier discovered (message 2078) that the proxy's source code actually reads FGW_YCQL_HOSTS rather than RIBS_S3_CQL_HOSTS, which caused the initial failure where the proxy tried to connect to [::1]:9042 instead of the specified host. The assistant corrected this by checking the source code, but the final configuration still uses RIBS_S3_CQL_HOSTS—suggesting either the code was updated or the environment file was adjusted to match the code's expectations. This assumption about variable naming is critical; a mismatch would cause silent failures.

Assumption 3: The proxy's S3 API is compatible with the Kuri nodes' S3 API. The proxy speaks S3 on the frontend and forwards requests to the Kuri nodes' S3 endpoints. The assistant assumes that the request format, headers, and response format are compatible between the two layers. The test using x-amz-content-sha256: UNSIGNED-PAYLOAD works for both, confirming this assumption for the tested case.

Assumption 4: The network is reliable and fast enough. The test runs on a local network (10.x.x.x addresses) with no TLS, no authentication, and no significant latency. The assistant assumes that the proxy can make a sub-second HTTP request to the backend node and return the response. In production, with TLS, authentication, and potentially higher latency, this assumption would need re-examination.

Assumption 5: The proxy is stateless and horizontally scalable. The architecture document calls for stateless frontend proxies. The assistant assumes that deploying one proxy on the head node is sufficient for testing, and that additional proxies could be added behind a load balancer without coordination issues. This is a reasonable assumption given the design, but it is not tested in this message.

Mistakes and Incorrect Assumptions

While the test succeeds, there are subtle issues worth examining.

The proxy configuration mismatch was not fully resolved. The assistant discovered at message 2078 that the s3-proxy source code uses FGW_YCQL_HOSTS rather than RIBS_S3_CQL_HOSTS. However, the environment file created at message 2075 uses RIBS_S3_CQL_HOSTS. After deploying via Ansible, the proxy started successfully (message 2087), which means either: (a) the code was updated to accept both variable names, (b) the Ansible template uses the correct variable name, or (c) the proxy fell back to a default that happened to work. The assistant did not explicitly verify which environment variable the running proxy was actually using. This is a minor oversight—the test passed, so the configuration is functionally correct—but it represents an incomplete understanding of the deployed system.

No test of node failure or degraded operation. The test writes one object and reads two. It does not test what happens when one Kuri node is down, when the database is unreachable, or when the proxy itself is restarted. These edge cases are essential for a production system but are out of scope for this validation step.

The proxy's routing algorithm is not verified independently. The test assumes that the proxy routed the write to one of the two Kuri nodes, but it does not confirm which node received the data. The X-Node-Id header from earlier direct tests (message 2060) is not checked here. A more thorough test would verify that objects are distributed across both backend nodes and that the proxy correctly reads from the owning node.

Input Knowledge Required to Understand This Message

To fully grasp what is happening in message 2088, a reader needs:

Knowledge of the system architecture: The FGW system has three layers—YugabyteDB (metadata), Kuri nodes (block storage with local S3 API), and s3-proxy (routing frontend). The proxy is the only component that clients should talk to directly.
Knowledge of the debugging history: The earlier attempts to use bitswap peering (message 2066) and direct node access (messages 2057-2068) provide context for why the proxy approach was necessary.
Knowledge of the S3 protocol: The use of x-amz-content-sha256: UNSIGNED-PAYLOAD header indicates that the S3 API requires specific AWS-style headers even when authentication is disabled.
Knowledge of the network topology: The head node (10.1.232.82) hosts the proxy and database; kuri1 (10.1.232.83) and kuri2 (10.1.232.84) host the storage nodes. Port 8078 is the proxy's S3 port; port 8079 is the Kuri nodes' direct S3 port.
Knowledge of the Ansible deployment: The proxy was deployed using the s3_frontend role, which creates systemd service files, environment files, and manages the binary.

Output Knowledge Created by This Message

This message produces several valuable pieces of knowledge:

Validation of the proxy architecture: The proxy correctly routes both writes and reads across multiple backend nodes. This confirms that the architectural decision to separate the proxy layer from the storage layer is sound.
A working reference configuration: The test provides a concrete example of how to interact with the proxy: the endpoint URL, the required headers, and the expected response format. This serves as documentation for future integration work.
Confirmation of cross-node data accessibility: The older object (test.txt), written directly to kuri1, is readable through the proxy. This confirms that the proxy can retrieve objects regardless of which backend node stores them, as long as the metadata is in the shared CQL keyspace.
A baseline for further testing: With the proxy working, the team can now test load balancing, failover, authentication, TLS, and other production features against a known-good baseline.
Confidence in the deployment pipeline: The successful proxy deployment via Ansible validates the deploy-frontend.yml playbook and the s3_frontend role, confirming that the automation works end-to-end.

The Thinking Process Visible in the Message

The message reveals a structured, methodical approach to verification. The assistant does not simply run one test and declare success. Instead, it constructs a three-step validation:

Write a new object through the proxy — This tests the write path: can the proxy accept a PUT request, route it to a backend node, and store the data? The use of $(date +%s) in the payload ensures the data is unique, avoiding false positives from cached responses.
Read the new object back through the proxy — This tests the read path for an object that was written through the proxy. It confirms that the proxy can retrieve data it just stored.
Read an older object (test.txt) through the proxy — This tests the read path for an object that was written before the proxy existed, directly to a Kuri node. This is the most important test: it confirms that the proxy can discover and retrieve objects regardless of their origin, using the shared metadata in the CQL keyspace. The ordering is deliberate. Step 1 establishes that the proxy is alive and accepting connections. Step 2 confirms basic read functionality. Step 3 is the real validation of cross-node access. If step 3 failed but steps 1 and 2 succeeded, the assistant would know the proxy works for new objects but has a metadata or routing issue for existing objects. The output confirms all three steps: "proxy-test-data-1769899056" for the new object and "test-data-123" for the earlier object. The test passes.

Conclusion

Message 2088 is a moment of quiet triumph in a debugging session that spanned multiple failed approaches, an architectural correction, and a methodology shift. The simple curl commands mask the complexity of what they prove: that the S3 proxy layer correctly routes requests across a multi-node distributed storage backend, that the shared metadata keyspace enables cross-node object discovery, and that the Ansible deployment pipeline can reliably deliver this infrastructure.

For the reader, this message serves as a case study in validation methodology. The assistant did not assume success based on the proxy starting—it tested the actual behavior with real data, covering both new and existing objects. It respected the architecture rather than hacking around it. And when the user pointed out a better approach (Ansible), it pivoted immediately.

In distributed systems, the difference between "should work" and "does work" is measured in tests like these. Message 2088 is the evidence that the system, at this moment, does work.