leveldb-haskell-0.6.5: Haskell bindings to LevelDB

Copyright(c) 2012-2013 The leveldb-haskell Authors
Safe HaskellNone




LevelDB Haskell binding.

The API closely follows the C-API of LevelDB. For more information, see: http://leveldb.googlecode.com


Exported Types

data DB Source #

Database handle


Eq DB Source # 


(==) :: DB -> DB -> Bool #

(/=) :: DB -> DB -> Bool #

data BatchOp Source #

Batch operation

newtype Comparator Source #

User-defined comparator

data Options Source #

Options when opening a database




  • blockRestartInterval :: !Int

    Number of keys between restart points for delta encoding of keys.

    This parameter can be changed dynamically. Most clients should leave this parameter alone.

    Default: 16

  • blockSize :: !Int

    Approximate size of user data packed per block.

    Note that the block size specified here corresponds to uncompressed data. The actual size of the unit read from disk may be smaller if compression is enabled.

    This parameter can be changed dynamically.

    Default: 4k

  • cacheSize :: !Int

    Control over blocks (user data is stored in a set of blocks, and a block is the unit of reading from disk).

    If > 0, use the specified cache (in bytes) for blocks. If 0, leveldb will automatically create and use an 8MB internal cache.

    Default: 0

  • comparator :: !(Maybe Comparator)

    Comparator used to defined the order of keys in the table.

    If Nothing, the default comparator is used, which uses lexicographic bytes-wise ordering.

    NOTE: the client must ensure that the comparator supplied here has the same name and orders keys exactly the same as the comparator provided to previous open calls on the same DB.

    Default: Nothing

  • compression :: !Compression

    Compress blocks using the specified compression algorithm.

    This parameter can be changed dynamically.

    Default: Snappy

  • createIfMissing :: !Bool

    If true, the database will be created if it is missing.

    Default: False

  • errorIfExists :: !Bool

    It true, an error is raised if the database already exists.

    Default: False

  • maxOpenFiles :: !Int

    Number of open files that can be used by the DB.

    You may need to increase this if your database has a large working set (budget one open file per 2MB of working set).

    Default: 1000

  • paranoidChecks :: !Bool

    If true, the implementation will do aggressive checking of the data it is processing and will stop early if it detects any errors.

    This may have unforeseen ramifications: for example, a corruption of one DB entry may cause a large number of entries to become unreadable or for the entire DB to become unopenable.

    Default: False

  • writeBufferSize :: !Int

    Amount of data to build up in memory (backed by an unsorted log on disk) before converting to a sorted on-disk file.

    Larger values increase performance, especially during bulk loads. Up to to write buffers may be held in memory at the same time, so you may with to adjust this parameter to control memory usage. Also, a larger write buffer will result in a longer recovery time the next time the database is opened.

    Default: 4MB

  • filterPolicy :: !(Maybe (Either BloomFilter FilterPolicy))


data ReadOptions Source #

Options for read operations




  • verifyCheckSums :: !Bool

    If true, all data read from underlying storage will be verified against corresponding checksums.

    Default: False

  • fillCache :: !Bool

    Should the data read for this iteration be cached in memory? Callers may with to set this field to false for bulk scans.

    Default: True

  • useSnapshot :: !(Maybe Snapshot)

    If Just, read as of the supplied snapshot (which must belong to the DB that is being read and which must not have been released). If Nothing, use an implicit snapshot of the state at the beginning of this read operation.

    Default: Nothing

data Snapshot Source #

Snapshot handle


data WriteOptions Source #

Options for write operations




  • sync :: !Bool

    If true, the write will be flushed from the operating system buffer cache (by calling WritableFile::Sync()) before the write is considered complete. If this flag is true, writes will be slower.

    If this flag is false, and the machine crashes, some recent writes may be lost. Note that if it is just the process that crashes (i.e., the machine does not reboot), no writes will be lost even if sync==false.

    In other words, a DB write with sync==false has similar crash semantics as the "write()" system call. A DB write with sync==true has similar crash semantics to a "write()" system call followed by "fsync()".

    Default: False


Basic Database Manipulations

withDB :: (MonadMask m, MonadIO m) => FilePath -> Options -> (DB -> m a) -> m a Source #

Run an action with a DB.

withDB path opts = bracket (open path opts) unsafeClose

Note that the DB handle will be released promptly when this function exits.

open :: MonadIO m => FilePath -> Options -> m DB Source #

Open a database.

The returned handle has a finalizer attached which will free the underlying pointers once it goes out of scope. Note, however, that finalizers are not guaranteed to run, and may not run promptly if they do. Use unsafeClose to free the handle immediately, but ensure it is not used after that (otherwise, the program will segault). Alternatively, use the Database.LevelDB.MonadResource API, which will take care of resource management automatically.

put :: MonadIO m => DB -> WriteOptions -> ByteString -> ByteString -> m () Source #

Write a key/value pair.

delete :: MonadIO m => DB -> WriteOptions -> ByteString -> m () Source #

Delete a key/value pair.

write :: MonadIO m => DB -> WriteOptions -> WriteBatch -> m () Source #

Perform a batch mutation.

get :: MonadIO m => DB -> ReadOptions -> ByteString -> m (Maybe ByteString) Source #

Read a value by key.

withSnapshot :: (MonadMask m, MonadIO m) => DB -> (Snapshot -> m a) -> m a Source #

Run an action with a Snapshot of the database.

createSnapshot :: MonadIO m => DB -> m Snapshot Source #

Create a snapshot of the database.

The returned Snapshot should be released with releaseSnapshot.

releaseSnapshot :: MonadIO m => DB -> Snapshot -> m () Source #

Release a snapshot.

The handle will be invalid after calling this action and should no longer be used.

Filter Policy / Bloom Filter

data FilterPolicy Source #

User-defined filter policy

data BloomFilter Source #

Represents the built-in Bloom Filter

Administrative Functions

data Property Source #

Properties exposed by LevelDB

getProperty :: MonadIO m => DB -> Property -> m (Maybe ByteString) Source #

Get a DB property.

destroy :: MonadIO m => FilePath -> Options -> m () Source #

Destroy the given LevelDB database.

The database must not be in use during this operation.

repair :: MonadIO m => FilePath -> Options -> m () Source #

Repair the given LevelDB database.

approximateSize :: MonadIO m => DB -> Range -> m Int64 Source #

Inspect the approximate sizes of the different levels.

compactRange :: MonadIO m => DB -> Range -> m () Source #

Compact the underlying storage for the given Range. In particular this means discarding deleted and overwritten data as well as rearranging the data to reduce the cost of operations accessing the data.

version :: MonadIO m => m (Int, Int) Source #

Return the runtime version of the underlying LevelDB library as a (major, minor) pair.