ckydb

THIS IS NOT FOR PRODUCTION USE (IT WILL CHOKE MEMORY). INSTEAD CONSIDER scdb

A simple fast memory-first thread-safe (or goroutine-safe for Go) key-value embedded database that persists data on disk.

It is read as 'skydb'

Implementations

Note: The project is still under heavy development.

All implementations are found in this monorepo in the implementations folder

• Python (Completed -- public API is stable)

pip install ckydb

• Go (Completed -- public API is stable)

go get github.com/sopherapps/ckydb/implementations/go-ckydb

• Rust (Completed -- public API is stable)

Add the following to your Cargo.toml file

[dependencies]
ckydb = { version = "0.0.5" }

• Nodejs (Pending)

npm install ckydb

Under the Hood

• Every key has a TIMESTAMP prefix, added to it on creation. This TIMESTAMPED key is the one used to store data in a sorted way for easy retrieval.
• The actual key known by user, however, is kept in the index. When ckydb is initialized, the index is loaded into memory from the index file (a ".idx" file). The index is basically a map of key: TIMESTAMPED-key
• The TIMESTAMPED-key and its value are stored first in a log file (a ".log" file). This current log file has an in-memory copy we call memtable
• When the current log file exceeds a predefined size, it is converted to a sorted data file (a ".cky" file) and memtable refreshed and a new log file created.
• The names of each ".cky" or ".log" file are the timestamps when they were created. Do note that conversion of ".log" to "cky" just changes the file extension.
• There is always one ".log" file in the database folder. If on initialization, there is no ".log" file, a new one is created.
• There is an in-memory sorted list of ".cky" files called data_files that is kept updated everytime a ".log" file is converted into ".cky".
• The name of the current log (current_log_file) file is also kept in memory, and updated when a new log file is created.
• There is also a ".del" file that holds all the key: TIMESTAMPED-key pairs that have been marked for deletion.
• At a predefined interval, a background task deletes the values from ".cky" and ".log" files corresponding to the key: TIMESTAMPED-key pairs found in the ".del" file. Each deleted pair is then removed from the ".del" file.
• On initial load, any keys in .del should have their values deleted in the corresponding ".log" or ".cky" files

Operations

• On ckydb.set(key, value):

  • the corresponding TIMESTAMPED key is searched for in the index
  • if the key does not exist:
    • a new TIMESTAMPED key is created and added to the index with its user-defined key
    • the user-defined key and its TIMESTAMPED key are then added to the index file (".idx")
    • this TIMESTAMPED key and its value are then added to memtable.
    • this TIMESTAMPED key and its value are then added to the current log file (".log")
    • A check is made on the size of the log file. If the log file is bigger than the max size allowed, it is rolled into a .cky file and a new log file created, and the memtable refreshed.
  • if the key exists:
    • its timestamp is extracted and compared to the current_log file to see if it is later than the current_log file
    • if it is later or equal, memtable and the current log file are updated
    • else the timestamp is compared to cache's "start" and "stop" to see if it lies within the cache
    • if it exists in the cache, then the cache data and its corresponding data file are updated
    • else, the data file in which the timestamp exists is located within the data_files. This is done by finding the two data files between which the timestamp exists when the list is sorted in ascending order. The file to the left is the one containing the timestamp.
      • the key-values from the data file are then extracted and they new key-value inserted
      • the new data is then loaded into the cache
      • the new data is also loaded into the data file
  • If any error occurs on any of these steps, the preceding steps are reversed and the error returned/raised/thrown in the call

• On ckydb.delete(key):

  • Its key: TIMESTAMPED-key pair is removed from the in-memory index.
  • Its key: TIMESTAMPED-key pair is removed from the ".idx" file
  • Its key: TIMESTAMPED-key is added to the ".del" file
  • If any error occurs on any of these steps, the preceding steps are reversed and the error returned/raised/thrown in the call

• On ckydb.get(key):

  • the corresponding TIMESTAMPED key is searched for in the index
  • if the key does not exist, a NotFoundError is thrown/raised/returned.
  • if the key exists, its TIMESTAMP is extracted and checked if it is greater (later) than the name of the current log file.
  • if this TIMESTAMP is later, its value is quickly got from memtable in memory. If for some crazy reason, it does not exist there, a CorruptedDataError is thrown/raised/returned.
  • If this TIMESTAMP is earlier than the name of the current log file, the TIMESTAMP is compared to the range in the memory cache, if it falls there in, its value is got from cache. If the value is not found for some reason, a CorruptedDataError is thrown/raise/returned
  • Otherwise the ".cky" file whose name is earlier than the TIMESTAMP but whose neighbour to the right, in the in-memory sorted data_files list, is later than TIMESTAMP is loaded into an in-memory cache whose range is set to two ".cky" filenames between which it falls.
  • the value is then got from cache's data. If it is not found for some reason, a CorruptedDataError is thrown/raise/returned

• On ckydb.clear():

  • memtable is reset
  • cache is reset
  • index in memory is reset
  • data_files in memory is reset
  • all files in the database folder are deleted
  • A new ".log" file is created

File formats

• The file format of the ".idx" index files is just "key<key_value_separator>TIMESTAMPED-key" separated by a unique token e.g. "{&*/%}" and a key_value_separator e.g. "[><?&(^#]"

goat[><?&(^#]1655304770518678-goat{&*/%}hen[><?&(^#]1655304670510698-hen{&*/%}pig[><?&(^#]1655304770534578-pig{&*/%}fish[><?&(^#]1655303775538278-fish$%#@*&^&

• The file format of the ".del" files is just "TIMESTAMPED-key" separated by a unique token e.g. "{&*/%}"

1655304770518678-goat{&*/%}1655304670510698-hen{&*/%}1655304770534578-pig{&*/%}1655303775538278-fish$%#@*&^&

• The file format of the ".log" and ".cky" files is just "TIMESTAMPED-key<key_value_separator>value" separated by a unique token e.g. "{&*/%}" and a key_value_separator like "[><?&(^#]"

1655304770518678-goat[><?&(^#]678 months{&*/%}1655304670510698-hen[><?&(^#]567 months{&*/%}1655304770534578-pig[><?&(^#]70 months{&*/%}1655303775538278-fish[><?&(^#]8990 months$%#@*&^&

Ideas For Improvement

• Explicitly allow for multiple concurrent reads (e.g. don't lock at all on read)
• Explicitly allow for conditional multiple concurrent writes (e.g. lock on key, not on store)
• Distribute the database across different machines or nodes ( e.g. have multiple backend nodes, and let each node's timestamped key range be recorded on the master/main/gateway node(s). The gateway nodes themselves could be replicated. Clients read/update data through the gateway node)

Multiple Concurrent Reads, Single Writes at a time

• Have no lock on the main routine of ckydb.get. ckydb.get has props index, memtable and cache as its source of truth.
• To avoid using a stale cache and yet also avoid data races between store.set and store.get, both, of old keys, we have a cache_lock lock. This lock is to be obtained by either store.get or store.set both for old keys
• Have the same mut_lock lock on the ckydb.delete and ckydb.set. If you had separate locks, there would be chance for a data race.
• For ckydb.clear, update index first.
• For ckydb.delete, update index last.
• For ckydb.set of a new key (i.e. not an update), update index last.
• For ckydb.set of pre-existing key, update memtable or cache last as index would already be up-to-date.
• For store.vacuum task and store.delete, there will be a del_file_lock within store to avoid conflicts.

Acknowledgments

• We can do nothing without God (John 15: 5). Glory be to Him.
• Some of these ideas were adapted from leveldb. Thanks.

License

Copyright (c) 2022 Martin Ahindura. All implementations are licensed to Licensed under the MIT License