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Horst

A fast key-value store with ACID guarantees written in pure Rust. Heavily inspired by Badger.

This is a work in progress. Use it at your own risk.

Status

The database does not yet support durability. That means your data will be lost after a restart. You also currently have to manually delete the data from the directory. There is also no shapshot creation mechanism yet, which means outdated data is never deleted. Finally there is currently no way to delete a certain key from the database.

Everything else is working correctly and can in theory already be used.

Implementation

The implementation closely follows the way Badger got implemented, as the Dgraph team explained in their blog post. Given so Horst's transactions offer the some ACID guarantees as Badger does.

We maintain the list of pending commits in a sorted Vec containing (commit_ts, finished) elements. If a commit finishes successfully finished gets set to true, if it fails the entry gets removed. Once all transactions up to a certain commit_ts have been marked as finished, they all get removed from the Vec and the oracle's read_ts gets increased to that commit_ts.

The value log is implemented in such a way that each VLog file contains solely the transactions corresponding to a certain interval of commit timestamps which is disjunct with the intervals of all other VLog files. This ensures we can easily detect after the creation of a snapshot which VLog files can be deleted and which must be preserved.

The LSM tree uses a similiar system. Each Slice (or 'rune' as it is called on Wikipedia) contains solely the key-value pairs corresponding to a distinct interval of commit timestamps. The Slices are in that sense sorted, with older Slices from higher levels containing solely older values and newer Slices containing solely newer values. This ensures that once we find the first key-value pair with a commit_ts lower then the transaction's read_ts, we can be sure the older Slices will not contain a newer value and end the search early.

Contrary to Badger which uses byte slices of arbitrary length as keys, we use fixed sized keys of type u128. You can still use others keys, but you would have to convert them to u128 before adding them to the database. You should also make sure there are no key collisions. We will likely add some interface and utility functionalites to assist with that.

TODO

  • Oracle
    • Implement a seperate task which manages the oracle.
    • Let the oracle keep track of commit timestamps for all keys.
      • We use a sorted Vec for quick search capabilites and to keep the memory usage low.
      • A BTreeMap gets used for quickly adding new key/commit_ts pairs.
      • If the BTreeMap gets too large, we merge it with the main Vec store.
      • If a certain commit updates too many keys, it gets directly merged into the main Vec store.
      • Upon conflict detection, both stores are checked for commit_ts higher then the transactions read_ts.
    • Let the oracle keep track of pending transaction commits.
      • Successfully commited transactions get saved in a Vec with both the commit_ts and a finished flag defaulting to false.
      • If a transaction successfully completes a commit, it informs the oracle which will set the finished flag to false.
      • If a transaction fails, the oracle gets also informed and the corresponding entry gets deleted from the store.
      • After each successfull commit the oracle removes all entries starting from the beginning which consecutively(!) have the finished flag set to true. The oracle's read_ts gets updated to the highest commit_ts where there are no pending commits with a lower commit_ts.
    • Provide the interface for letting other tasks and transactions communicate with the oracle.
      • Allow asking for the latest read_ts.
      • Allow asking for commiting a transaction.
        • Send over all keys read or set by the transaction.
        • Perform conflict detection by ensuring none of the keys read or set got modified.
  • Value log
    • Implement a seperate task which manages the value log.
    • Catch incomming transactions and write them in batches for better write performance.
      • Implement a buffer for incomming transactions.
      • Empty the buffer at a configuable interval.
      • Empty the buffer if a configurable size gets exceeded.
      • Make sure the transactions are given to the correct VLogs (s.b.).
    • Add a cache for recently read values for better performance.
    • Implement the management for opening mulitple VLog files.
    • Allow creating VLog files in multiple directories, e.g. on different harddrives.
    • Open multiple VLog files.
      • Implement a seperate taks which manages each Individual VLog file.
      • Allow for concurrent reads from different VLog files.
    • Make each VLog file contain the transactions corresponding to a certain interval of commit_ts which is disjunct with all other VLogs.
      • Let the header of each VLog file maintain the upper and lower bound (inclusive) of the commit timestamps contained therein.
      • The first VLog's lower bound gets initialized to zero.
      • Upon writing transactions to a VLog, maintain an index of the highest commit_ts seen so far on this VLog.
      • Upon closure of the VLog, update the headers upper bound to the highest seen commit_ts. Do also save the upper bounds fixed flag (s.b.).
      • If a VLog exceeds a certain size limit, create a new VLog.
        • The last VLog's upper bound gets marked as fixed.
        • The new VLog's lower bound gets set as the last VLog's upper bound +1.
      • Make sure that all new transactions get written to the correct VLog.
  • Log-Structure-Merge (LSM) tree
    • Implement a seperate task which manages the LSM tree.
    • Implement a seperate task for each Slice and the level 0 storage.
    • Implement level 0 (RAM storage).
      • Store new key-value-pairs in a BTreeMap.
      • Regularely merge the new keys into a sorted Vec for better memory usage.
      • Do also trigger a merge whenever a configurable size gets exceeded.
      • Implement a configurable maximum total number of entries in level 0.
        • All entries get merged into the sorted Vec before further processing.
        • Level 0 asks the oracle for the latest read_ts and splits its currently saved entries into those with a commit_ts above this read_ts, and those whose commit_ts is below or equal to it.
        • Those with a commit_ts above the read_ts stay inside level 0, the rest get added to level 1.
    • Implement higher levels (Disk storage).
      • Implement Slices which are immutable.
      • Ensure we can efficiently read Slices from disk.
        • Make use of mmap. (memmap)
        • Implement a pure rust alternative.
          • Implement a caching file reader.
          • Solve bugs at certain corner edge cases.
      • Make sure we can efficiently search for the latest value (with respect to a certain read_ts) of a certain key.
        • Make sure each Slice contains a distinct interval of commit timestamps.
        • Implement a properly working binary search.
        • Different Slices can be searched concurrently.
  • Durability
    • Add a proper clean shutdown mechanism.
      • Add a flag to all file headers indicating if the file got properly closed.
      • Add a header entry to VLog files taking note of the last known fully commited commit_ts.
    • Reopen all files on restart.
      • Reopen and add all VLog files on restart.
      • Add some validation to the VLog files header parsing to detect possible contract violations or similiar inconsistancies.
      • Reopen and add all Slice files on restart.
      • Add some validation to the Slice files header parsing to detect possible contract violations or similiar inconsistancies.
      • Detect not fully written transactions and truncate the corresponding VLog files.
      • Detect not fully commited transactions and update the Slices as needed.
  • Database functions
    • Implement a snapshot creation function, so outdated data gets removed.
      • Add a function for manually triggering the creation of a snapshot.
      • Define and implement other criteria (time interval, database size) for automatic snapshot creation.
      • Implement a seperate task which manages shapshot creations.
        • Make sure it returns an appropriate error if a snapshot creation is requested even trough one is currently already being created.
      • Let it ask the LSM tree for handles to all Slices saved at level 1 and above plus the current snapshot Slice, if any.
        • This will also give it access to the highest commit_ts currently saved on disk in the LSM tree.
        • Note that by construction that this commit_ts is lower or equal to the oracle's read_ts. In particular we can be sure there are no pending or future transactions which would belong into this snapshot.
      • Perform a merge of all Slices.
        • This will exclude all outdated values in the newly created snapshot Slice.
        • This Slice is only temporary, since it points to the values of the not yet merged VLogs.
      • Give a handle of the new temporary snapshot Slice to the value log and ask it to create a snapshot VLog containing all values referenced by the Slice.
        • Give that VLog a snapshot and finished flag since it will not contain a regular transaction and can not know its total size in advance.
        • Create a new snapshot Slice file.
        • Iterate over the keys in the temporary snapshot Slice, read the values from the old VLog files, add them to the new snapshot VLog and add a corresponding reference to the new snapshot Slice file.
        • Once done, mark the new VLog file as finished,delete the temporary snapshot Slice and give the new snapshot Slice to the snapshot task.
      • Give the new snapshot Slice over to the LSM tree together with a list of the Slices which got merged into it.
        • Let the LSM tree delete all new outdated Slices (including a possible old snapshot one) and add the new snapshot Slice.
        • Once done inform the snapshot task of it.
      • Tell the value log to switch to the new snaphost VLog and delete all VLog files (including a possible old snapshot one) which contain solely values with commit_ts lower or equal to the snapshots max commit_ts.
    • Implement a delete key function by adding a corresponding flag.
    • Implement an async key range iteration function. (Usefull for prefix scanning)
    • Implement key conversion functionalites.
      • Add an interface for using abritrary keys (probably trough a trait).
      • Implement a utility structure which detects and prevents key collisions.