LevelDB bindings for CHICKEN Scheme 5

  1. LevelDB bindings for CHICKEN Scheme 5
    1. Dependencies
    2. Source Code
    3. API
      1. Iterators
      2. Writebatch
      3. Compactions
      4. Keyword arguments
        1. (finalizer (lambda (x) (set-finalizer! x (some-destroy-proc x))))
        2. (sync #f)
        3. (verify-checksums #f)
        4. (fill-cache #t)
        5. (create-if-missing #t)
        6. (error-if-exists #f)
        7. (paranoid-checks #f)
        8. (write-buffer-size (* 4 1024 1024))
        9. (max-open-files 1000)
        10. (block-size (* 4 1024))
        11. (restart-interval 16)
        12. (max-file-size (* 2 1024 1024))
        13. (compression 'snappy)
    4. Example
    5. Background
    6. TODO

This egg has been tested on LevelDB 1.23, but should probably work on older versions too.

An aim of this project is to expose the LevelDB C API directly with no dependencies except the native LevelDB library. Errors are raised where the (hidden) errptr argument set to a string pointer on return.

Dependencies

No eggs, but the LevelDB shared library must be installed before installing this egg.

Source Code

Hosted here.

API

[procedure] (leveldb-version)

Returns the native LevelDB library version as a list with two

elements: {{(major minor)}}.
[procedure] (leveldb-open name #!key (create-if-missing #t) (error-if-exists #f) (paranoid-checks #f) (write-buffer-size (* 4 1024 1024)) (max-open-files 1000) (block-size (* 4 1024)) (restart-interval 16) (max-file-size (* 2 1024 1024)) (compression 'snappy) finalizer)

Opens database at path name, returning a leveldb-t object. Use this object as the db argument in procedures below. See Keyword arguments for options.

[procedure] (leveldb-close db)

Closes db. Calling this on a db which is already closed has no effect. This does not normally need to be called explicitly as it is the default finalizer specified in leveldb-open.

[procedure] (leveldb-get db key #!key (verify-checksums #f) (fill-cache #t))

Lookup database entry key in db. key must be a string or a chicken.blob. Returns a string. See Keyword arguments page for options.

Note that if you want to scan for a large number of entries, you should probably use an leveldb-iterator.

[procedure] (leveldb-put db key value #!key (sync #f))

Inserts an entry into db. key and value must both be strings or chicken.blobs. See Keyword arguments for the sync option.

Note that if you want to insert a large number of entries, using a leveldb-writebatch may be faster.

[procedure] (leveldb-delete db key #!key (sync #f))

Deletes a single entry in db on key. If the key entry does not exist, this is a no-op. key must be a string or chicken.blob. See Keyword arguments for usage of sync.

Note that if you want to delete a larger number of entries, it is probably better to use a leveldb-writebatch.

Iterators

[procedure] (leveldb-iterator db #!key (finalizer ...) (seek #f) (verify-checksums #t) (fill-cache #t))

Create a leveldb-iterator-t instance which you can use to seek, and read keys and values from db. It is very efficient at moving through keys sequentially using leveldb-iter-next.

seek, if present and not #f, will be passed to a call to leveldb-iter-seek. As with leveldb-iter-seek, you can specify 'first to initialize the iterator to the first entry, for example.

See Keyword arguments for the other options.

[procedure] (leveldb-iter-valid? it)

Returns #t if it is in a valid position (where you can read keys and move it back or forwards) and #f otherwise. A newly created iterator starts before the first entry in the database where leveldb-iter-valid?, leveldb-iter-key and leveldb-iter-value will return #f.

[procedure] (leveldb-iter-seek it key)

Move it to the absolute position specified. If key is a string or chicken.blob, the iterator will be placed on the first entry equal to or after key. key may also be the symbols first and last to seek to the start and the end of the database respectively.

[procedure] (leveldb-iter-next it)
[procedure] (leveldb-iter-prev it)

Move it forward or backward one entry. Calling this when it is invalid has no effect.

[procedure] (leveldb-iter-key it)
[procedure] (leveldb-iter-value it)

Get the current key or value for it at its current position. These procedures will return #f if (leveldb-iter-valid? it) returns #f, or strings otherwise.

The current implementation copies the foreign memory into a CHICKEN string may not be ideal for large values.

[procedure] (leveldb-iter-destroy it)

Free the leveldb_t structure held by this record. Calling this on an iterator that is already closed has no effect. It does normally not need to be called as it's the default finalizer specified in leveldb-iterator.

Writebatch

A leveldb-writebatch-t can be used to apply changes atomically. See write_batch.h for details.

[procedure] (leveldb-writebatch #!key (finalizer leveldb-writebatch-destroy))

Create a new leveldb-writebatch object. A writebatch can hold key-value pairs temporarily, for later to be atomically applied to a database with leveldb-write.

[procedure] (leveldb-writebatch-put wb key value)

Inserts an entry into wb. key and value must be strings or chicken.blobs.

[procedure] (leveldb-writebatch-delete writebatch key)

Mark key as deleted. This works like leveldb-delete. Note that if you call put and delete for the same key, order is significant.

[procedure] (leveldb-writebatch-clear wb)

Remove all entries in wb previously inserted by leveldb-writebatch-put, making it available for re-use.

[procedure] (leveldb-writebatch-destroy wb)

Free the wb object and its foreign memory. Calling it if wb is already destroy has no effect. This does not normally need to be called explicitly as it's the default finalizer specified in leveldb-writebatch.

[procedure] (leveldb-write db wb #!key (sync #f))

Write all the entries of wb into db, persisting them on disk. This is an atomic operation. See Keyword arguments for the sync option.

Compactions

[procedure] (leveldb-compact-range db start limit)

Run a database compaction, hopefully reducing the consumed disk space. start and limit are keys that specify the range of keys to run the compaction for. Both may be #f to specify all keys in the database.

Keyword arguments

With the exception of the finalizer options, these options are mostly a copy-paste from the C API. They apply to all procedures accepting them.

(finalizer (lambda (x) (set-finalizer! x (some-destroy-proc x))))

Procedures accepting a finalizer keyword argument allow manual memory control. It is a procedure of 1 argument, the object potentially needing a finalizer. The defaults call set-finalizer! with the corresponding leveldb-*-destroy or leveldb-*-close procedure. This does not normally need to be specified, but can sometimes be used to tweak performance.

(sync #f)

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()".

(verify-checksums #f)

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

(fill-cache #t)

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

(create-if-missing #t)

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

(error-if-exists #f)

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

(paranoid-checks #f)

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.

(write-buffer-size (* 4 1024 1024))

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 two write buffers may be held in memory at the same time, so you may wish 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.

(max-open-files 1000)

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).

(block-size (* 4 1024))

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.

(restart-interval 16)

Number of keys between restart points for delta encoding of keys. This parameter can be changed dynamically. Most clients should leave this parameter alone.

(max-file-size (* 2 1024 1024))

Leveldb will write up to this amount of bytes to a file before switching to a new one.

Most clients should leave this parameter alone. However if your filesystem is more efficient with larger files, you could consider increasing the value. The downside will be longer compactions and hence longer latency/performance hiccups. Another reason to increase this parameter might be when you are initially populating a large database.

(compression 'snappy)

Compression must be either 'snappy (the default), or #f. 'snappy gives lightweight but fast compression. Typical speeds on an Intel(R) Core(TM)2 2.4GHz:

Note that these speeds are significantly faster than most persistent storage speeds, and therefore it is typically never worth switching it off. Even if the input data is incompressible, the 'snappy compression implementation will efficiently detect that and will switch to uncompressed mode.

Example

 
 
(import leveldb)
(define db (leveldb-open "testing.ldb"))

(leveldb-put db "key1" "value1")
(leveldb-put db "key2" "value2")

(define it (leveldb-iterator db seek: 'first))
(let loop ()
  (when (leveldb-iter-valid? it)
    (print (leveldb-iter-key it) "\t" (leveldb-iter-value it))
    (leveldb-iter-next it)
    (loop)))

Please see the tests folder for more usage.

Background

There is a leveldb egg for CHICKEN 4. This egg, however, is a port of the CHICKEN 5 rocksdb egg, replacing the CHICKEN 4 egg with the permission of the author. This egg remains closer to the C API than the CHICKEN 4 egg.

Rocksdb is a fork of LevelDB by Facebook, which with almost identical C APIs.

For CHICKEN 5, there is also the lmdb egg. My informal tests indicate that lmdb is faster for smaller databases (< 100k entries), whereas leveldb's performance is relatively stable across all database sizes.

TODO