leveldb-haskell-0.3.0: Haskell bindings to LevelDB

Copyright(c) 2012-2013 The leveldb-haskell Authors
LicenseBSD3
Maintainerkim.altintop@gmail.com
Stabilityexperimental
Portabilitynon-portable
Safe HaskellNone
LanguageHaskell2010

Database.LevelDB.MonadResource

Contents

Description

 

Synopsis

Exported Types

data DB Source

Database handle

Instances

Eq DB 

data BatchOp Source

Batch operation

Instances

newtype Comparator Source

User-defined comparator

data Compression Source

Compression setting

Constructors

NoCompression 
Snappy 

data Options Source

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

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

Instances

data ReadOptions Source

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

data Snapshot Source

Snapshot handle

Instances

data WriteOptions Source

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

Defaults

Basic Database Manipulation

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

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 Source

Open a database

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

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

Write a key/value pair

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

Delete a key/value pair

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

Perform a batch mutation

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

Read a value by key

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

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) Source

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

Filter Policy / Bloom Filter

data FilterPolicy Source

User-defined filter policy

Administrative Functions

data Property Source

Properties exposed by LevelDB

Instances

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

Get a DB property

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

Destroy the given leveldb database.

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

Repair the given leveldb database.

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

Inspect the approximate sizes of the different levels

version :: MonadResource m => m (Int, Int) Source

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

Iteration

data Iterator Source

Iterator handle

Note that an Iterator requires external synchronization if it is shared between multiple threads which mutate it's state. See examples/iterforkio.hs for a simple example of how to do that.

Instances

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

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 Source

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) Source

Create an Iterator which can be released early.

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

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 () Source

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 () Source

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 () Source

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 () Source

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 () Source

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) Source

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) Source

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) Source

Check for errors

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

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

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)] Source

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] Source

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] Source

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, MonadIO m, Applicative m, MonadBase IO 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

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