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Partial Objet Caching Update
- 2. Partial Object Caching Handle range operations for HTTP. Major change to cache implementation. But still backwards compatible! Other things can be improved because of these cahnges Read while write. Origin connection collapsing in core. Stale while revalidate support in core. Better locking / less polling.
- 3. Request Quantization Requests to origin quantized to fragment sized (default 1MB). Can’t assume origin / upstream supports multi-range. Need to consolidate to single range. Making multiple requests not currently feasible. Trailing requests need expansion to be sufficiently large to have a cacheable portion. May also want to extend toward byte 0 if “close enough” to avoid empty earliest fragment.
- 4. Request Quantization Initial Interstitial Suffix Minimal Quantization Initial Quantization Interstitial Quantiation Suffix Expansion
- 5. Transaction Contention Want to minimize contention between transactions for a cache object. Example: Request for a large object must not block range request for an extended period. Initial request Second Request Long range leading
- 6. Current status Currently in production testing on a 6.2 base. Will be forwarded ported to 7.1. I think this will be the fourth version port Still implementing Stale while revalidate Stale while error Backwards compatibility
- 7. Challenges Finding time to work and test. Still finding out things about how the cache works. Handling edge cases.
- 8. Data structure recap OpenDir bucket OpenDirEntry vector HashTable HTTPInfoVector table Alternates CacheVC next prev Fragment Table CacheVC next prev CacheVC next prev CacheVC next prev Single instance with active request to origin waiting for response headers List of instances that are waiting for object metadata Item writers readers active List of CacheVC with open request to origin. List of CacheVC with pending I/O to cache. List of CacheVC serving to user agent fragments writer waiting_readers Position Size Data sideBuffers Data from origin that could not be cached.
- 10. Problem: Alternate selection for 2nd request Wait for headers Pending request may be alternate desired. Must prevent build up of pending requests. Don’t wait for headers Must not delay requests too long
- 11. Alternate selection / write lock fail Request Alternate Vector No Alt Selected Request Request New alternate Response (Headers) Alternate Select
- 12. Problem: Alternate selection for 2nd request Configuration to select two items: wait time and timeout action Wait time – # ms (or ns) to wait for headers to return. Timeout action Fail – return error code to useragent. Select from existing alternates. Serve selection. Go to origin anyway (disable caching). Fail – return error code to user agent.
- 13. Alternate update second writer OpenDirEntry vector HTTPInfoVector table Alternates writer waiting_readers CacheVC update_key alternate Alternates CacheVC New reader Writer copy override CacheVC alternate Writer update_key override
- 15. Update vector – Trailing readers vs. fast stale & update Updated alternate itself becomes stale while readers still serving. Move reader CacheVC alternate pointers? Technically challenging. Treat as update miss? My preferred option. Treat updated alternate as not stale as long as active readers Must force new readers to alternate in “stale” vector.
- 16. Ongoing work Stale while revalidate, stale while error OpenDirEntry is not updated until write finishes. Stale object remains accessible until then. Stale if error
- 17. Audience requests Put in forward protection – ignore future objects to roll back ATS version. Need this to be able to roll back more easily. Look at configuration/ checks for origins without range support. How to determine if an origin doesn’t support ranges. Miles to the rescue – use header_rewrite to remove Range: header. Test coverage Integration tests Unit tests Try to break up in to smaller commits.