Crate mtbl

Rust bindings to the mtbl library for dealing with SSTables (immutable sorted map files).

SSTables (String-String Tables) are basically constant on-disk maps from [u8] to [u8], like those used by CDB (which also has Rust bindings), except using sorted maps instead of hashmaps. SSTables are suitable for storing the output of mapreduces or other batch results for easy lookup later.

Version 0.2.0 of this library is a rather literal translation of the mtbl C API. Later versions may change the API to be friendlier and more in the Rust idioms.

Usage

Creating a database

// Create a database, using a Sorter instead of a Writer so we can add
// keys in arbitrary (non-sorted) order.
{
  use mtbl::{Sorter,Write};
  let mut writer = mtbl::Sorter::create("data.mtbl");
  writer.add("key", "value");
  // Data is flushed to file when the writer/sorter is destroyed.
}

Reading from a database

use mtbl::{Read,Reader};
let reader = mtbl::Reader::open("data.mtbl");
// Get one element
let val: Option(Vec<u8>) = reader.get("key");
assert_eq!(val, Option("value".as_bytes()));
// Or iterate over all entries
for (key: Vec<u8>, value: Vec<u8>) in &reader {
    f(key, value);
}

More details about MTBL

Quoting from the MTBL documentation:

mtbl is not a database library. It does not provide an updateable key-value data store, but rather exposes primitives for creating, searching and merging SSTable files. Unlike databases which use the SSTable data structure internally as part of their data store, management of SSTable files -- creation, merging, deletion, combining of search results from multiple SSTables -- is left to the discretion of the mtbl library user.

mtbl SSTable files consist of a sequence of data blocks containing sorted key-value pairs, where keys and values are arbitrary byte arrays. Data blocks are optionally compressed using zlib or the Snappy library. The data blocks are followed by an index block, allowing for fast searches over the keyspace.

The basic mtbl interface is the writer, which receives a sequence of key-value pairs in sorted order with no duplicate keys, and writes them to data blocks in the SSTable output file. An index containing offsets to data blocks and the last key in each data block is buffered in memory until the writer object is closed, at which point the index is written to the end of the SSTable file. This allows SSTable files to be written in a single pass with sequential I/O operations only.

Once written, SSTable files can be searched using the mtbl reader interface. Searches can retrieve key-value pairs based on an exact key match, a key prefix match, or a key range. Results are retrieved using a simple iterator interface.

The mtbl library also provides two utility interfaces which facilitate a sort-and-merge workflow for bulk data loading. The sorter interface receives arbitrarily ordered key-value pairs and provides them in sorted order, buffering to disk as needed. The merger interface reads from multiple SSTables simultaneously and provides the key-value pairs from the combined inputs in sorted order. Since mtbl does not allow duplicate keys in an SSTable file, both the sorter and merger interfaces require a caller-provided merge function which will be called to merge multiple values for the same key. These interfaces also make use of sequential I/O operations only.

Why prefer MTBL over CDB or other constant databases?

• Storing data in sorted order makes merging files easy.
• Compression is built-in (options: zlib and snappy).
• The library code is a little more modern and uses mmapped files to have a properly immutable (and therefore thread-safe) representation -- it doesn't go mucking about with file pointers.

Structs

Fileset	An MTBL reader that watches a "setfile" containing a list of MTBL files to read from.
FilesetOptions	Options for opening an MTBL fileset.
Merger	An MTBL reader that opens and reads from several MTBL files, merging their contents.
Reader	A reader for a single MTBL file.
ReaderOptions	MTBL Reader opening options.
Sorter	A tool to create an MTBL file out of keys in any order.
SorterOptions	Options used to create a Sorter.
Writer	A struct to create an MTBL file from keys and values in sorted order.
WriterOptions	Options for writing an MTBL file.

Enums

CompressionType

Compression method used when writing an MTBL file.

Traits

Read	A trait for objects that can read keys from an MTBL file.
Write	A trait for objects that can write an MTBL file.

Type Definitions

MergeFn

An MTBL merging function: given a key and two values for that key, create a merged value for that key.