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Q: How to access the sequence number of an entry? #325

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nlfiedler opened this issue Sep 3, 2019 · 5 comments · Fixed by #326
Closed

Q: How to access the sequence number of an entry? #325

nlfiedler opened this issue Sep 3, 2019 · 5 comments · Fixed by #326

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@nlfiedler
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Seems like RocksDB tags every entry with a monotonically increasing sequence number, and there might be some means of accessing that. In fact, it looks like getting a snapshot should return the current sequence number. Is this exposed somewhere in rust-rocksb and I just can't find it? Thanks for your help.
@vitvakatu
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Does the RocksDB itself exposes this functionality? rust-rocksdb is just a binding and will not expose anything that is unavailable in a parent library.

@nlfiedler
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Sorry, I should have looked for a reference sooner. Admittedly I'm not that good at reading large C code bases any more, but it looks like there are a couple of functions related to sequence numbers. There's GetLatestSequenceNumber() in include/rocksdb/db.h and another function named rocksdb_get_latest_sequence_number() in db/c.cc that invokes it. There are also numerous tests invoking GetLatestSequenceNumber() so maybe that's the public function? Again, not too good at reading large C code bases in short order.

@vitvakatu
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Thank you for a little investigation. This function is indeed exists in a public API (and Java bindings exports it as well). It would be fairly easy to add. If you want, you can send a PR (or I can do it myself).

@nlfiedler
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You're much better off not letting me touch the code. :)

@vitvakatu vitvakatu mentioned this issue Sep 3, 2019
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@nlfiedler
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Fantastic, thank you. The reason I asked about the sequence number is that I want to implement a secondary index in RocksDB, and the sequence number is a very good way to detect stale index records. The project is here, https://github.com/nlfiedler/mokuroku -- still needs a lot of work though.

BusyJay pushed a commit to BusyJay/rust-rocksdb that referenced this issue Jul 23, 2020
Cherry-pick build and dependency fixes (rust-rocksdb#325)
68467cb 
Cherry-picking the following changes that fixes zlib, jemalloc and gflags dependencies, and fixes a flaky test:
```
6ead62c 2019-07-25 yiwu@pingcap.com     Avoid build require system zlib (rust-rocksdb#303)
9246b9c 2019-07-25 hi@breeswish.org     Skip jemalloc options on specific platform (rust-rocksdb#324)
7a8dd73 2019-07-22 yiwu@pingcap.com     Add jemalloc-sys dependency when jemalloc is enabled (rust-rocksdb#320)
6f2c632 2019-07-23 yiwu@pingcap.com     Not compile with gflags (rust-rocksdb#322)
09b8032 2019-07-22 yiwu@pingcap.com     Fix perf context test (rust-rocksdb#321)
d67f38e 2019-07-21 yiwu@pingcap.com     Add jemalloc feature (rust-rocksdb#319)
```

Also update rocksdb to fix compile error with gcc9:
```
7a03c83ed 2019-07-23 psergey@askmonty.org Fix MyRocks compile warnings-treated-as-errors on Fedora 30, gcc 9.1.1 (#5553)
```
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Add DB::latest_sequence_number method. vitvakatu/rust-rocksdb
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@nlfiedler @vitvakatu