Debugging Multipart Upload Failures: A Diagnostic Pivot in Load Testing

The Message

I see there are many write errors with multipart. Let me check the multipart code - the issue might be that our S3 implementation doesn't support multipart correctly. Let me run without multipart to compare:

```bash cd /home/theuser/gw && go run ./integrations/ritool/... loadtest run --duration 10s --concurrency 3 --min-size 1MB --max-size 5MB --multipart-threshold 50000000 --read-ratio 0.3 http://localhost:8078 2>&1 ═══════════════════════════════════════════════════════════════ S3 LOAD TEST ═══════════════════════════════════════════════════════════════

Endpoint: http://localhost:8078 Bucket: loadtest Concurrency: ... ```

Context: A Load Testing Tool Meets Reality

This message represents a critical inflection point in the development of a horizontally scalable S3-compatible storage system for the Filecoin Gateway. The assistant had just finished building a comprehensive load testing utility in the ritool command-line tool, following a user request to create a tool that "writes objects, incl multiparts, into the endpoints" and tests "throughput and correctness of read-after-write guarantee." The tool was designed with configurable object sizes, concurrent workers, read/write ratios, multipart upload support, and MD5-based read-after-write verification.

The preceding messages show a rapid development cycle: the assistant explored the existing ritool structure (messages 926–929), wrote the initial loadtest.go file (message 930), iterated through several rounds of compilation error fixes (messages 931–938), verified the build compiled (message 939), tested the help output (message 940), fixed a command-line argument ordering issue (messages 941–942), and finally ran a successful short test (message 942). That first successful test showed the tool working, but with some verification failures that the assistant tentatively attributed to "eventual consistency or a real issue."

Then came message 943, where the assistant ran a more ambitious test with multipart uploads enabled: objects ranging from 1MB to 10MB, a multipart threshold of 2MB (meaning objects larger than 2MB would use multipart upload), and a 30% read ratio. The results showed "many write errors with multipart." This observation is the trigger for the target message.

The Reasoning: Why This Message Was Written

The assistant wrote this message to perform a differential diagnosis. The core question was: are the write errors caused by a bug in the newly written load testing tool's multipart upload logic, or are they caused by a limitation in the S3 frontend proxy implementation that the tool is testing against?

This is a classic debugging strategy: isolate the variable. By running a comparison test with multipart uploads effectively disabled (by setting --multipart-threshold to 50MB, far above the 5MB maximum object size), the assistant could determine whether the errors were specific to the multipart code path or were a general issue with the S3 endpoint.

The reasoning is sound and reflects a methodical debugging approach. Rather than diving into the multipart upload code immediately—which could waste time if the issue turned out to be on the server side—the assistant first gathered comparative data. This is especially important in a distributed system context where the S3 frontend proxy, the Kuri storage nodes, and the YugabyteDB metadata store all interact. A failure in multipart uploads could originate in any layer of the architecture.

How Decisions Were Made

Several implicit and explicit decisions shaped this message:

Decision 1: Test the hypothesis before inspecting the code. The assistant chose to run an empirical test rather than immediately reading the multipart implementation in the S3 proxy or the loadtest tool. This prioritizes observable behavior over code analysis, which is a reasonable first step when the failure mode is clearly visible ("many write errors") but the root cause is ambiguous.

Decision 2: Choose comparison parameters strategically. The assistant set --multipart-threshold 50000000 (50MB) while keeping --max-size 5MB. This ensures that no object exceeds the threshold, so all uploads use the simple PUT path. The --duration 10s, --concurrency 3, and --read-ratio 0.3 parameters were kept identical to the multipart test, creating a fair comparison. The only change was the threshold, isolating multipart as the independent variable.

Decision 3: Reduce object size range. The multipart test used --min-size 1MB --max-size 10MB, while the comparison test used --min-size 1MB --max-size 5MB. This is a subtle but important change: by reducing the maximum size, the assistant ensured that even if some objects happened to be near the threshold, they would still be well below the 50MB cutoff. This eliminates edge cases where object size and threshold interact.

Decision 4: Focus on the S3 implementation as the likely culprit. The assistant stated "the issue might be that our S3 implementation doesn't support multipart correctly." This is a reasonable assumption given the project's development stage—the S3 frontend proxy was newly built and had already required several bug fixes (HTTP route conflicts, JSON case mismatches, missing database columns). Multipart upload support is a complex feature involving multiple API calls (CreateMultipartUpload, UploadPart, CompleteMultipartUpload), and it would not be surprising if it had gaps or bugs.## Assumptions Made

The message rests on several assumptions, some explicit and some implicit:

Assumption 1: The S3 implementation may have incomplete multipart support. This is stated directly and is a reasonable working hypothesis. The S3 frontend proxy was a recent addition to the project, and multipart upload is one of the more complex S3 API features. The assistant had already fixed several bugs in the proxy, so assuming further issues exist is prudent.

Assumption 2: The loadtest tool's multipart code is correct. This is the null hypothesis that the comparison test is designed to disprove. The assistant implicitly assumes that if the non-multipart test passes cleanly, then the multipart code path in the loadtest tool is likely correct, and the server-side implementation is at fault. However, this assumption could be wrong in both directions: the non-multipart test might also have hidden issues that don't manifest as errors, or the multipart code in the tool might have a bug that only triggers under certain conditions.

Assumption 3: A 10-second test with 3 concurrent workers provides sufficient signal. This is a pragmatic assumption for rapid iteration. A longer test might reveal intermittent failures, but the assistant needs quick feedback to decide where to focus debugging effort. The assumption is that multipart failures are consistent enough to appear within 10 seconds.

Assumption 4: The test cluster is in a healthy state. The assistant doesn't explicitly verify that the S3 proxy, Kuri nodes, and YugabyteDB are all running correctly before running the test. Given the earlier session's history of container issues and the known issue that "nginx webui container sometimes needs restart after kuri nodes restart," this is a nontrivial assumption. A partially degraded cluster could produce misleading results.

Mistakes and Incorrect Assumptions

While the message itself is logically sound, some aspects deserve critical examination:

The comparison may not be perfectly controlled. The multipart test (message 943) used --min-size 1MB --max-size 10MB, while the comparison test uses --min-size 1MB --max-size 5MB. This means the non-multipart test generates smaller objects on average. If object size correlates with failure rate (for example, if the S3 proxy has issues with larger payloads regardless of multipart), then the comparison conflates two variables. A more rigorous test would keep the size range identical and only change the threshold.

The diagnosis is framed as binary (multipart vs. non-multipart), but the failure could be intermittent. If the S3 proxy's multipart implementation has race conditions or timing-dependent bugs, a single 10-second test might not reliably reproduce the failure. The assistant's conclusion in the follow-up message (945) that "Works well without multipart" could be premature if the non-multipart path also has occasional failures that didn't manifest in this short window.

There is an implicit assumption that the loadtest tool's error reporting is accurate. If the tool itself has a bug in how it classifies or counts errors—for example, misinterpreting HTTP status codes or timing out prematurely—the observed "many write errors" could be artifacts of the test tool rather than genuine S3 failures. The assistant does not inspect the raw HTTP responses or server logs to validate the error counts.

Input Knowledge Required

To fully understand this message, a reader needs knowledge of:

The project architecture: A horizontally scalable S3-compatible storage system with three layers: stateless S3 frontend proxies (port 8078), Kuri storage nodes (ports 7001/7002), and a shared YugabyteDB metadata store. The S3 proxy routes writes via round-robin and reads via YCQL lookup.
The S3 multipart upload API: The standard AWS S3 multipart upload protocol involves three operations: CreateMultipartUpload (returns an upload ID), UploadPart (uploads individual parts with part numbers), and CompleteMultipartUpload (assembles the parts). A correct implementation must handle all three in sequence.
The ritool CLI framework: The tool uses github.com/urfave/cli/v2 for command-line parsing, with commands registered in main.go and implemented in separate files like loadtest.go. The loadtest run subcommand accepts flags for endpoint URL, duration, concurrency, size range, read ratio, and multipart threshold.
The test cluster infrastructure: A Docker Compose-based setup with YugabyteDB, two Kuri nodes, an S3 proxy, and nginx web UIs. The cluster runs on specific ports and requires the FGW_DATA_DIR environment variable for data persistence.
The project's development history: The S3 frontend proxy had already required several bug fixes (HTTP route conflicts, JSON case mismatches, missing database columns), and the cluster monitoring dashboard had been recently enhanced with I/O throughput charts and SLO thresholds.

Output Knowledge Created

This message generates several forms of knowledge:

A test result: The output of the comparison test (visible in the truncated banner showing "S3 LOAD TEST" with endpoint, bucket, and concurrency details) provides empirical data about whether the S3 proxy handles simple PUT requests correctly under load.
A diagnostic hypothesis: The message establishes the hypothesis that "our S3 implementation doesn't support multipart correctly," which will be validated or refuted by the comparison test.
A debugging methodology: The message demonstrates a pattern of differential diagnosis that can be applied to other issues: isolate the suspected variable, run a controlled comparison, and use the results to narrow the search space.
A decision point: The message creates a branching point in the development workflow. If the non-multipart test succeeds, the assistant will investigate the S3 proxy's multipart implementation. If it also fails, the assistant will look for broader issues in the proxy or the loadtest tool itself.

The Thinking Process Visible in Reasoning

The message reveals a clear diagnostic thought process:

Observation: "I see there are many write errors with multipart." This is the raw data from the previous test run.
Hypothesis generation: "Let me check the multipart code - the issue might be that our S3 implementation doesn't support multipart correctly." The assistant identifies two possible sources of the error: the loadtest tool's multipart code or the S3 proxy's multipart implementation. The phrasing "our S3 implementation" suggests the assistant already suspects the server side.
Experimental design: "Let me run without multipart to compare." This is the controlled experiment. By running with a threshold that effectively disables multipart (50MB threshold vs. 5MB max size), the assistant creates a counterfactual: what does the system look like when multipart is not involved?
Parameter selection: The assistant carefully chooses --multipart-threshold 50000000 (50MB) to ensure no object triggers multipart, while keeping other parameters similar to the failing test. This isolates the multipart variable.
Interpretation readiness: The assistant is prepared to compare the error counts between the two runs. If the non-multipart run has zero or few errors, the multipart hypothesis is supported. If it also has many errors, the issue is broader. This thinking process exemplifies the scientific method applied to software debugging: observe, hypothesize, experiment, conclude. It is a mature and effective approach, especially valuable in complex distributed systems where failures can have many root causes.

Conclusion

Message 944 is a brief but pivotal diagnostic pivot in the development of a load testing tool for a distributed S3 storage system. It represents the moment when a newly built tool encounters real-world server behavior and the developer must decide where to focus debugging effort. The message's value lies not in its length but in its methodological soundness: a controlled comparison test that isolates the multipart upload variable. The subsequent message (945) confirms the hypothesis—"Works well without multipart"—validating the diagnostic approach and clearing the way for focused work on the S3 proxy's multipart implementation. In the broader context of the project, this message captures the essential tension between building test infrastructure and testing the system under test, where bugs can lurk in either layer and careful experimental design is the only reliable path to resolution.