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abstract API layer for chicken-leveldb
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README.md

chicken-level

Provides a high-level API to leveldb implementations. Use in combination with an implementation egg (eg, leveldb).

Interface API

This module exposes an interface, which other eggs provide implementations of. The API described below is what the interface provides.

Basic read and write

(db-get db key)

Returns the value of key in db as a string. Causes an exception if the key does not exist.

(db-put db key value #!key (sync #f))

Stores value under key in datbase db. If the sync option can be set to #t to make the write operation not return until the data being written has been pushed all the way to persistent storage. See the Synchronous Writes section for more information.

(db-delete db key #!key (sync #f))

Removes the value associated with key from db. If the sync option can be set to #t to make the write operation not return until the data being written has been pushed all the way to persistent storage. See the Synchronous Writes section for more information.

Atomic updates (batches)

(db-batch db ops #!key (sync #f))

When making multiple changes that rely on each other you can apply a batch of operations atomically using db-batch. The ops argument is a list of operations which will be applied in order (meaning you can create then later delete a value in the same batch, for example).

(define myops '((put "abc" "123")
                (put "def" "456")
                (delete "abc")))

;; apply all operations in myops
(db-batch db myops)

The first item in an operation should be the symbol put or delete, any other value will give an error. The next item is the key and in the case of put the third item is the value.

Apart from its atomicity benefits, db-batch may also be used to speed up bulk updates by placing lots of individual mutations into the same batch.

Range queries (streams)

(db-stream db #!key start end limit reverse (key #t) (value #t) fillcache)

Allows forward and backward iteration over the keys in alphabetical order. Returns a lazy sequence of all key/value pairs from start to end (up to limit). This uses the lazy-seq egg.

  • start - the key to start from (need not actually exist), if omitted starts from the first key in the database
  • end - the key to end on (need not actually exist), if omitted ends on the last key in the database
  • limit - stops after limit results have been returned
  • reverse - iterates backwards through the keys (reverse iteration may be somewhat slower than forward iteration)
  • key - whether to return the key for each result (default #t)
  • value - whether to return the value for each result (default #t)
  • fillcache - whether to fill leveldb's read cache when reading (turned off by default so the bulk read does not replace most of the cached contents)

When both key: #t and value: #t (as default) values are returned as a list with two items, the car being the key and the cadr being the value. When only key: #t or value: #t the keys or values are not returned as a list but as a string representing the single key or value.

(lazy-map display (db-stream db start: "foo:" end: "foo::" limit: 10)))

You can turn the lazy-seq into a list using lazy-seq->list, just be warned that it will evaluate the entire key range and should be avoided unless you know the number of values is small (eg, when using limit).

(db-batch db '((put "foo" "1")
               (put "bar" "2")
               (put "baz" "3")))

(lazy-seq->list (db-stream db limit: 2)) ;; => (("foo" "1") ("bar" "2"))
(lazy-seq->list (db-stream db key: #f value: #t)) ;; => ("1" "2" "3")
(lazy-seq->list (db-stream db key: #t value: #f)) ;; => ("foo" "bar" "baz")

Synchronous Writes

Note: this information is mostly copied from the LevelDB docs

By default, each write to leveldb is asynchronous: it returns after pushing the write from the process into the operating system. The transfer from operating system memory to the underlying persistent storage happens asynchronously. The sync flag can be turned on for a particular write to make the write operation not return until the data being written has been pushed all the way to persistent storage. (On Posix systems, this is implemented by calling either fsync(...) or fdatasync(...) or msync(..., MS_SYNC) before the write operation returns.)

Asynchronous writes are often more than a thousand times as fast as synchronous writes. The downside of asynchronous writes is that a crash of the machine may cause the last few updates to be lost. Note that a crash of just the writing process (i.e., not a reboot) will not cause any loss since even when sync is false, an update is pushed from the process memory into the operating system before it is considered done.

db-batch provides an alternative to asynchronous writes. Multiple updates may be placed in the same batch and applied together using a sync: #t. The extra cost of the synchronous write will be amortized across all of the writes in the batch.

Creating an interface

If you want to provide your own storage impelmentation, import this egg and define the interface as follows:

(use level)

(define myleveldb
  (implementation level-api

    (define (get db key) ...)
    (define (put db key value #!key (sync #f)) ...)
    (define (delete db key #!key (sync #f)) ...)
    (define (batch db ops #!key (sync #f)) ...)

    (define (stream db
                    #!key
                    start
                    end
                    limit
                    reverse
                    (key #t)
                    (value #t)
                    fillcache)
      ..)))

Implementations

  • leveldb - provides the level API to libleveldb
  • sublevel - provides namespaced API access to another implementation
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