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Object Storage & Blob Storage • Multipart Uploads & Resumable TransfersHard⏱️ ~2 min

Production Implementation Patterns and SLOs

Client Library Architecture
Production multipart upload clients need several components: a chunker that splits files into parts, a scheduler that manages concurrent uploads, a state manager that tracks progress, and a retry handler for failures. Separate these concerns for testability and maintenance.
The chunker reads file segments without loading the entire file into memory. Seek to part offset, read part size bytes. This enables uploading files larger than available RAM. For streaming sources, buffer to temporary storage before uploading each part.
Checksum Strategies
Compute checksums at multiple levels. Per part MD5 or SHA256 validates each part uploaded correctly. Full file checksum before and after validates the file did not change. Server side checksum on assembled object validates server assembled correctly.
Some platforms support content MD5 headers that the server validates. If the header does not match received content, the server rejects the part. This catches network corruption immediately rather than at assembly time.
SLO Design
Define service level objectives for upload performance. Example targets: p50 throughput of 80% of available bandwidth, p99 upload time within 2x of theoretical minimum, success rate above 99.9% for files under 1GB.
Measure actual performance against targets. Track: initiate latency, part upload throughput, part success rate, complete latency, end to end duration. Alert when metrics deviate from SLOs. Debug by correlating with part size, concurrency level, and time of day.
Monitoring and Observability
Log structured events for each phase: upload initiated, part started, part completed, part failed, upload completed, upload aborted. Include upload ID, part number, byte ranges, duration, and error details. This enables debugging individual failed uploads and aggregate analysis.
Dashboard key metrics: uploads in progress, parts in flight, throughput by client type, failure rate by error code, orphaned upload count. Set alerts on failure rate spikes and orphaned upload growth.
✅ Best Practice: Implement checksums at every layer (part, file, server). Log structured events for every phase. Monitor orphaned uploads and failure rates. Define SLOs and measure against them.

💡 Key Takeaways

✓Client architecture: chunker (memory efficient), scheduler (concurrency), state manager, retry handler

✓Checksums at multiple levels: per part MD5/SHA256, full file hash before/after, server side validation

✓Content MD5 header enables server side corruption detection before storing the part

✓SLO targets: 80% bandwidth utilization p50, 2x theoretical minimum p99, 99.9% success rate under 1GB

✓Observability: structured logs per phase, dashboards for throughput and failure rate, alerts on orphaned uploads

📌 Interview Tips

1Describe the chunker implementation. Open file, seek to offset (partNumber - 1) * partSize, read partSize bytes. This uses constant memory regardless of file size. For streaming, buffer each part to temp file before upload.

2Explain layered checksums. Part MD5 catches bit flips in transit. File SHA256 catches source changes. Server checksum catches assembly bugs. Each layer catches different failure modes.

3Define concrete SLOs. On a 100 Mbps link, 1GB should transfer in 80 seconds theoretical. SLO target: p99 under 160 seconds. If p99 exceeds 200 seconds, investigate network issues or throttling.

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