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https://github.com/uber-archive/Objective-LevelDB

An Objective-C database library built over Google's LevelDB
https://github.com/uber-archive/Objective-LevelDB

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An Objective-C database library built over Google's LevelDB

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README 

        
## Introduction



An Objective-C database library built over [Google's LevelDB](http://code.google.com/p/leveldb), a fast embedded key-value store written by Google.



## Installation



By far, the easiest way to integrate this library in your project is by using [CocoaPods][1].



1. Have [Cocoapods][1] installed, if you don't already

2. In your Podfile, add the line 



        pod 'Objective-LevelDB'



3. Run `pod install`

4. Make something awesome.



## How to use



#### Creating/Opening a database file on disk



```objective-c

LevelDB *ldb = [LevelDB databaseInLibraryWithName:@"test.ldb"];

```



##### Setup Encoder/Decoder blocks



By default, any object you store will be encoded and decoded using `NSKeyedArchiver`/`NSKeyedUnarchiver`. You can customize this by providing `encoder` and `decoder` blocks, like this:



```objective-c

ldb.encoder = ^ NSData * (LevelDBKey *key, id object) {

  // return some data, given an object

}

ldb.decoder = ^ id (LevelDBKey *key, NSData * data) {

  // return an object, given some data

}

```



#####  NSMutableDictionary-like API



```objective-c

ldb[@"string_test"] = @"laval"; // same as:

[ldb setObject:@"laval" forKey:@"string_test"];



NSLog(@"String Value: %@", ldb[@"string_test"]); // same as:

NSLog(@"String Value: %@", [ldb objectForKey:@"string_test"]);



[ldb setObject:@{@"key1" : @"val1", @"key2" : @"val2"} forKey:@"dict_test"];

NSLog(@"Dictionary Value: %@", [ldb objectForKey:@"dict_test"]);



```

All available methods can be found in its [header file](https://github.com/matehat/Objective-LevelDB/blob/master/Classes/LevelDB.h) (documented).



##### Enumeration



```objective-c

[ldb enumerateKeysAndObjectsUsingBlock:^(LevelDBKey *key, id value, BOOL *stop) {

    // This step is necessary since the key could be a string or raw data (use NSDataFromLevelDBKey in that case)

    NSString *keyString = NSStringFromLevelDBKey(key); // Assumes UTF-8 encoding

    // Do something clever

}];



// Enumerate with options

[ldb enumerateKeysAndObjectsBackward:TRUE

                              lazily:TRUE       // Block below will have a block(void) instead of id argument for value

                       startingAtKey:someKey    // Start iteration there (NSString or NSData)

                 filteredByPredicate:predicate  // Only iterate over values matching NSPredicate

                           andPrefix:prefix     // Only iterate over keys prefixed with something 

                          usingBlock:^(LevelDBKey *key, void(^valueGetter)(void), BOOL *stop) {

                             

    NSString *keyString = NSStringFromLevelDBKey(key);

    

    // If we had wanted the value directly instead of a valueGetter block, we would've set the 

    // above 'lazily' argument to FALSE

    id value = valueGetter();

}]

```

More iteration methods are available, just have a look at the [header section](https://github.com/matehat/Objective-LevelDB/blob/master/Classes/LevelDB.h)



##### Snapshots, NSDictionary-like API (immutable)



A snapshot is a readonly interface to the database, permanently reflecting the state of 

the database when it was created, even if the database changes afterwards.



```objective-c

LDBSnapshot *snap = [ldb newSnapshot]; // You get ownership of this variable, so in non-ARC projects,

                                       // you'll need to release/autorelease it eventually

[ldb removeObjectForKey:@"string_test"];



// The result of these calls will reflect the state of ldb when the snapshot was taken

NSLog(@"String Value: %@", [snap objectForKey:@"string_test"]);

NSLog(@"Dictionary Value: %@", [ldb objectForKey:@"dict_test"]);

```



All available methods can be found in its [header file](https://github.com/matehat/Objective-LevelDB/blob/master/Classes/LDBSnapshot.h)



##### Write batches, atomic sets of updates



Write batches are a mutable proxy to a `LevelDB` database, accumulating updates

without applying them, until you do using `-[LDBWritebatch apply]`



```objective-c

LDBWritebatch *wb = [ldb newWritebatch];

[wb setObject:@{ @"foo" : @"bar" } forKey: @"another_test"];

[wb removeObjectForKey:@"dict_test"];



// Those changes aren't yet applied to ldb

// To apply them in batch, 

[wb apply];

```



All available methods can be found in its [header file](https://github.com/matehat/Objective-LevelDB/blob/master/Classes/LDBWriteBatch.h)



##### LevelDB options



```objective-c

// The following values are the default

LevelDBOptions options = [LevelDB makeOptions];

options.createIfMissing = true;

options.errorIfExists   = false;

options.paranoidCheck   = false;

options.compression     = true;

options.filterPolicy    = 0;      // Size in bits per key, allocated for a bloom filter, used in testing presence of key

options.cacheSize       = 0;      // Size in bytes, allocated for a LRU cache used for speeding up lookups



// Then, you can provide it when initializing a db instance.

LevelDB *ldb = [LevelDB databaseInLibraryWithName:@"test.ldb" andOptions:options];

```



##### Per-request options



```objective-c

db.safe = true; // Make sure to data was actually written to disk before returning from write operations.

[ldb setObject:@"laval" forKey:@"string_test"];

[ldb setObject:[NSDictionary dictionaryWithObjectsAndKeys:@"val1", @"key1", @"val2", @"key2", nil] forKey:@"dict_test"];

db.safe = false; // Switch back to default



db.useCache = false; // Do not use DB cache when reading data (default to true);

```



##### Concurrency



As [Google's documentation states][2], updates and reads from a leveldb instance do not require external synchronization

to be thread-safe. Write batches do, and we've taken care of it, by isolating every `LDBWritebatch` it inside a serial dispatch 

queue, and making every request dispatch *synchronously* to it. So use it from wherever you want, it'll just work.



However, if you are using something like JSONKit for encoding data to JSON in the database, and you are clever enough to 

preallocate a `JSONDecoder` instance for all data decoding, beware that this particular object is *not* thread-safe, and you will

need to take care of it manually.



### Testing



If you want to run the tests, you will need XCode 5, as the test suite uses the new XCTest. 



Clone this repository and, once in it,



```bash

./setup-test.sh

cd Tests && open Objective-LevelDB.xcworkspace

```



Currently, all tests were setup to work with the iOS test suite.



### License



Distributed under the [MIT license](LICENSE)



[1]: http://cocoapods.org

[2]: http://leveldb.googlecode.com/svn/trunk/doc/index.html