Summary:
This PR fixes the following compile errors with Clang:
```
.../rocksdb/env/fs_on_demand.cc:184:5: error: no member named 'for_each' in namespace 'std'; did you mean 'std::ranges::for_each'?
184 | std::for_each(rchildren.begin(), rchildren.end(), [&](std::string& name) {
| ^~~~~~~~~~~~~
| std::ranges::for_each
/opt/homebrew/opt/llvm@17/bin/../include/c++/v1/__algorithm/ranges_for_each.h:68:23: note: 'std::ranges::for_each' declared here
68 | inline constexpr auto for_each = __for_each::__fn{};
| ^
.../rocksdb/env/fs_on_demand.cc:188:10: error: no member named 'sort' in namespace 'std'
188 | std::sort(result->begin(), result->end());
| ~~~~~^
.../rocksdb/env/fs_on_demand.cc:189:10: error: no member named 'sort' in namespace 'std'
189 | std::sort(rchildren.begin(), rchildren.end());
| ~~~~~^
.../rocksdb/env/fs_on_demand.cc:193:10: error: no member named 'set_union' in namespace 'std'
193 | std::set_union(result->begin(), result->end(), rchildren.begin(),
| ~~~~~^
.../rocksdb/env/fs_on_demand.cc:221:5: error: no member named 'for_each' in namespace 'std'; did you mean 'std::ranges::for_each'?
221 | std::for_each(
| ^~~~~~~~~~~~~
| std::ranges::for_each
/opt/homebrew/opt/llvm@17/bin/../include/c++/v1/__algorithm/ranges_for_each.h:68:23: note: 'std::ranges::for_each' declared here
68 | inline constexpr auto for_each = __for_each::__fn{};
| ^
.../rocksdb/env/fs_on_demand.cc:226:10: error: no member named 'sort' in namespace 'std'
226 | std::sort(result->begin(), result->end(), file_attr_sorter);
| ~~~~~^
.../rocksdb/env/fs_on_demand.cc:227:10: error: no member named 'sort' in namespace 'std'
227 | std::sort(rchildren.begin(), rchildren.end(), file_attr_sorter);
| ~~~~~^
.../rocksdb/env/fs_on_demand.cc:231:10: error: no member named 'set_union' in namespace 'std'
231 | std::set_union(rchildren.begin(), rchildren.end(), result->begin(),
| ~~~~~^
8 errors generated.
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12588
Reviewed By: jaykorean
Differential Revision: D56656222
Pulled By: ajkr
fbshipit-source-id: 7e94b6250fc9edfe597a61b7622f09d6b6cd9cbd
Summary:
A basic implementation of RocksDB follower mode, which opens a remote database (referred to as leader) on a distributed file system by tailing its MANIFEST. It leverages the secondary instance mode, but is different in some key ways -
1. It has its own directory with links to the leader's database
2. Periodically refreshes itself
3. (Future) Snapshot support
4. (Future) Garbage collection of obsolete links
5. (Long term) Memtable replication
There are two main classes implementing this functionality - `DBImplFollower` and `OnDemandFileSystem`. The former is derived from `DBImplSecondary`. Similar to `DBImplSecondary`, it implements recovery and catch up through MANIFEST tailing using the `ReactiveVersionSet`, but does not consider logs. In a future PR, we will implement memtable replication, which will eliminate the need to catch up using logs. In addition, the recovery and catch-up tries to avoid directory listing as repeated metadata operations are expensive.
The second main piece is the `OnDemandFileSystem`, which plugs in as an `Env` for the follower instance and creates the illusion of the follower directory as a clone of the leader directory. It creates links to SSTs on first reference. When the follower tails the MANIFEST and attempts to create a new `Version`, it calls `VerifyFileMetadata` to verify the size of the file, and optionally the unique ID of the file. During this process, links are created which prevent the underlying files from getting deallocated even if the leader deletes the files.
TODOs: Deletion of obsolete links, snapshots, robust checking against misconfigurations, better observability etc.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12540
Reviewed By: jowlyzhang
Differential Revision: D56315718
Pulled By: anand1976
fbshipit-source-id: d19e1aca43a6af4000cb8622a718031b69ebd97b
