Database.LevelDB

LevelDB Haskell binding.

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

Exported Types

data DB

Database handle

Instances

Eq DB

data BatchOp

Batch operation

Constructors

Put ByteString ByteString

Del ByteString

Instances

Eq BatchOp

Show BatchOp

newtype Comparator

User-defined comparator

Constructors

Comparator (ByteString -> ByteString -> Ordering)

data Compression

Compression setting

Constructors

NoCompression

Snappy

Instances

Eq Compression

Show Compression

data Options

Options when opening a database

Constructors

Options

Fields

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

Instances

Default Options

data ReadOptions

Options for read operations

Constructors

ReadOptions

Fields

verifyCheckSums :: !Bool

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

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

Instances

Eq ReadOptions

Default ReadOptions

data Snapshot

Snapshot handle

Instances

Eq Snapshot

type WriteBatch = [BatchOp]

data WriteOptions

Options for write operations

Constructors

WriteOptions

Fields

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

Instances

Eq WriteOptions

Show WriteOptions

Default WriteOptions

type Range = (ByteString, ByteString)

Defaults

defaultOptions :: Options

defaultWriteOptions :: WriteOptions

defaultReadOptions :: ReadOptions

Basic Database Manipulation

withSnapshot :: MonadResource m => DB -> (Snapshot -> m a) -> m a

Run an action with a snapshot of the database.

The snapshot will be released when the action terminates or throws an exception. Note that this function is provided for convenience and does not prevent the Snapshot handle to escape. It will, however, be invalid after this function returns and should not be used anymore.

open :: MonadResource m => FilePath -> Options -> m DB

Open a database

The returned handle will automatically be released when the enclosing runResourceT terminates.

put :: MonadResource m => DB -> WriteOptions -> ByteString -> ByteString -> m ()

Write a key/value pair

delete :: MonadResource m => DB -> WriteOptions -> ByteString -> m ()

Delete a key/value pair

write :: MonadResource m => DB -> WriteOptions -> WriteBatch -> m ()

Perform a batch mutation

get :: MonadResource m => DB -> ReadOptions -> ByteString -> m (Maybe ByteString)

Read a value by key

createSnapshot :: MonadResource m => DB -> m Snapshot

Create a snapshot of the database.

The returned Snapshot will be released automatically when the enclosing runResourceT terminates. It is recommended to use createSnapshot' instead and release the resource manually as soon as possible.

createSnapshot' :: MonadResource m => DB -> m (ReleaseKey, Snapshot)

Create a snapshot of the database which can (and should) be released early.

Administrative Functions

data Property

Properties exposed by LevelDB

Constructors

NumFilesAtLevel Int

Stats

SSTables

Instances

Eq Property

Show Property

getProperty :: MonadResource m => DB -> Property -> m (Maybe ByteString)

Get a DB property

destroy :: MonadResource m => FilePath -> Options -> m ()

Destroy the given leveldb database.

repair :: MonadResource m => FilePath -> Options -> m ()

Repair the given leveldb database.

approximateSize :: MonadResource m => DB -> Range -> m Int64

Inspect the approximate sizes of the different levels

Iteration

data Iterator

Iterator handle

Instances

Eq Iterator

withIterator :: MonadResource m => DB -> ReadOptions -> (Iterator -> m a) -> m a

Run an action with an Iterator. The iterator will be closed after the action returns or an error is thrown. Thus, the iterator will not be valid after this function terminates.

iterOpen :: MonadResource m => DB -> ReadOptions -> m Iterator

Create an Iterator.

The iterator will be released when the enclosing runResourceT terminates. You may consider to use iterOpen' instead and manually release the iterator as soon as it is no longer needed (alternatively, use withIterator).

Note that an Iterator creates a snapshot of the database implicitly, so updates written after the iterator was created are not visible. You may, however, specify an older Snapshot in the ReadOptions.

iterOpen' :: MonadResource m => DB -> ReadOptions -> m (ReleaseKey, Iterator)

Create an Iterator which can be released early.

iterValid :: MonadResource m => Iterator -> m Bool

An iterator is either positioned at a key/value pair, or not valid. This function returns true iff the iterator is valid.

iterSeek :: MonadResource m => Iterator -> ByteString -> m ()

Position at the first key in the source that is at or past target. The iterator is valid after this call iff the source contains an entry that comes at or past target.

iterFirst :: MonadResource m => Iterator -> m ()

Position at the first key in the source. The iterator is valid after this call iff the source is not empty.

iterLast :: MonadResource m => Iterator -> m ()

Position at the last key in the source. The iterator is valid after this call iff the source is not empty.

iterNext :: MonadResource m => Iterator -> m ()

Moves to the next entry in the source. After this call, iterValid is true iff the iterator was not positioned at the last entry in the source.

If the iterator is not valid, this function does nothing. Note that this is a shortcoming of the C API: an iterPrev might still be possible, but we can't determine if we're at the last or first entry.

iterPrev :: MonadResource m => Iterator -> m ()

Moves to the previous entry in the source. After this call, iterValid is true iff the iterator was not positioned at the first entry in the source.

If the iterator is not valid, this function does nothing. Note that this is a shortcoming of the C API: an iterNext might still be possible, but we can't determine if we're at the last or first entry.

iterKey :: MonadResource m => Iterator -> m (Maybe ByteString)

Return the key for the current entry if the iterator is currently positioned at an entry, ie. iterValid.

iterValue :: MonadResource m => Iterator -> m (Maybe ByteString)

Return the value for the current entry if the iterator is currently positioned at an entry, ie. iterValid.

iterGetError :: MonadResource m => Iterator -> m (Maybe ByteString)

Check for errors

Note that this captures somewhat severe errors such as a corrupted database.

mapIter :: MonadResource m => (Iterator -> m a) -> Iterator -> m [a]

Map a function over an iterator, advancing the iterator forward and returning the value. The iterator should be put in the right position prior to calling the function.

Note that this function accumulates the result strictly, ie. it reads all values into memory until the iterator is exhausted. This is most likely not what you want for large ranges. You may consider using conduits instead, for an example see: https://gist.github.com/adc8ec348f03483446a5

iterItems :: MonadResource m => Iterator -> m [(ByteString, ByteString)]

Return a list of key and value tuples from an iterator. The iterator should be put in the right position prior to calling this with the iterator.

See strictness remarks on mapIter.

iterKeys :: MonadResource m => Iterator -> m [ByteString]

Return a list of key from an iterator. The iterator should be put in the right position prior to calling this with the iterator.

See strictness remarks on mapIter

iterValues :: MonadResource m => Iterator -> m [ByteString]

Return a list of values from an iterator. The iterator should be put in the right position prior to calling this with the iterator.

See strictness remarks on mapIter

Re-exports

class (MonadThrow m, MonadUnsafeIO m, MonadIO m, Applicative m) => MonadResource m where

A Monad which allows for safe resource allocation. In theory, any monad transformer stack included a ResourceT can be an instance of MonadResource.

Note: runResourceT has a requirement for a MonadBaseControl IO m monad, which allows control operations to be lifted. A MonadResource does not have this requirement. This means that transformers such as ContT can be an instance of MonadResource. However, the ContT wrapper will need to be unwrapped before calling runResourceT.

Since 0.3.0

Methods

liftResourceT :: ResourceT IO a -> m a

Lift a ResourceT IO action into the current Monad.

Since 0.4.0

Instances

MonadResource m => MonadResource (ListT m)

(MonadThrow m, MonadUnsafeIO m, MonadIO m, Applicative m) => MonadResource (ResourceT m)

MonadResource m => MonadResource (ExceptionT m)

MonadResource m => MonadResource (MaybeT m)

MonadResource m => MonadResource (IdentityT m)

MonadResource m => MonadResource (ContT r m)

(Error e, MonadResource m) => MonadResource (ErrorT e m)

MonadResource m => MonadResource (ReaderT r m)

MonadResource m => MonadResource (StateT s m)

MonadResource m => MonadResource (StateT s m)

(Monoid w, MonadResource m) => MonadResource (WriterT w m)

(Monoid w, MonadResource m) => MonadResource (WriterT w m)

(Monoid w, MonadResource m) => MonadResource (RWST r w s m)

(Monoid w, MonadResource m) => MonadResource (RWST r w s m)

runResourceT :: MonadBaseControl IO m => ResourceT m a -> m a

Unwrap a ResourceT transformer, and call all registered release actions.

Note that there is some reference counting involved due to resourceForkIO. If multiple threads are sharing the same collection of resources, only the last call to runResourceT will deallocate the resources.

Since 0.3.0

resourceForkIO :: MonadBaseControl IO m => ResourceT m () -> ResourceT m ThreadId

Introduce a reference-counting scheme to allow a resource context to be shared by multiple threads. Once the last thread exits, all remaining resources will be released.

Note that abuse of this function will greatly delay the deallocation of registered resources. This function should be used with care. A general guideline:

If you are allocating a resource that should be shared by multiple threads, and will be held for a long time, you should allocate it at the beginning of a new ResourceT block and then call resourceForkIO from there.

Since 0.3.0