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| # Contributing | ||
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| We'd love to accept your code patches! However, before we can take them, we | ||
| have to jump a couple of legal hurdles. | ||
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| ## Contributor License Agreements | ||
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| Please fill out either the individual or corporate Contributor License | ||
| Agreement as appropriate. | ||
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| * If you are an individual writing original source code and you're sure you | ||
| own the intellectual property, then sign an [individual CLA](https://developers.google.com/open-source/cla/individual). | ||
| * If you work for a company that wants to allow you to contribute your work, | ||
| then sign a [corporate CLA](https://developers.google.com/open-source/cla/corporate). | ||
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| Follow either of the two links above to access the appropriate CLA and | ||
| instructions for how to sign and return it. | ||
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| ## Submitting a Patch | ||
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| 1. Sign the contributors license agreement above. | ||
| 2. Decide which code you want to submit. A submission should be a set of changes | ||
| that addresses one issue in the [issue tracker](https://github.com/google/leveldb/issues). | ||
| Please don't mix more than one logical change per submission, because it makes | ||
| the history hard to follow. If you want to make a change | ||
| (e.g. add a sample or feature) that doesn't have a corresponding issue in the | ||
| issue tracker, please create one. | ||
| 3. **Submitting**: When you are ready to submit, send us a Pull Request. Be | ||
| sure to include the issue number you fixed and the name you used to sign | ||
| the CLA. | ||
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| ## Writing Code ## | ||
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| If your contribution contains code, please make sure that it follows | ||
| [the style guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml). | ||
| Otherwise we will have to ask you to make changes, and that's no fun for anyone. |
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| **LevelDB is a fast key-value storage library written at Google that provides an ordered mapping from string keys to string values.** | ||
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| Authors: Sanjay Ghemawat (sanjay@google.com) and Jeff Dean (jeff@google.com) | ||
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| # Features | ||
| * Keys and values are arbitrary byte arrays. | ||
| * Data is stored sorted by key. | ||
| * Callers can provide a custom comparison function to override the sort order. | ||
| * The basic operations are `Put(key,value)`, `Get(key)`, `Delete(key)`. | ||
| * Multiple changes can be made in one atomic batch. | ||
| * Users can create a transient snapshot to get a consistent view of data. | ||
| * Forward and backward iteration is supported over the data. | ||
| * Data is automatically compressed using the [Snappy compression library](http://code.google.com/p/snappy). | ||
| * External activity (file system operations etc.) is relayed through a virtual interface so users can customize the operating system interactions. | ||
| * [Detailed documentation](http://htmlpreview.github.io/?https://github.com/google/leveldb/blob/master/doc/index.html) about how to use the library is included with the source code. | ||
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| # Limitations | ||
| * This is not a SQL database. It does not have a relational data model, it does not support SQL queries, and it has no support for indexes. | ||
| * Only a single process (possibly multi-threaded) can access a particular database at a time. | ||
| * There is no client-server support builtin to the library. An application that needs such support will have to wrap their own server around the library. | ||
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| # Performance | ||
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| Here is a performance report (with explanations) from the run of the | ||
| included db_bench program. The results are somewhat noisy, but should | ||
| be enough to get a ballpark performance estimate. | ||
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| ## Setup | ||
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| We use a database with a million entries. Each entry has a 16 byte | ||
| key, and a 100 byte value. Values used by the benchmark compress to | ||
| about half their original size. | ||
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| LevelDB: version 1.1 | ||
| Date: Sun May 1 12:11:26 2011 | ||
| CPU: 4 x Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz | ||
| CPUCache: 4096 KB | ||
| Keys: 16 bytes each | ||
| Values: 100 bytes each (50 bytes after compression) | ||
| Entries: 1000000 | ||
| Raw Size: 110.6 MB (estimated) | ||
| File Size: 62.9 MB (estimated) | ||
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| ## Write performance | ||
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| The "fill" benchmarks create a brand new database, in either | ||
| sequential, or random order. The "fillsync" benchmark flushes data | ||
| from the operating system to the disk after every operation; the other | ||
| write operations leave the data sitting in the operating system buffer | ||
| cache for a while. The "overwrite" benchmark does random writes that | ||
| update existing keys in the database. | ||
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| fillseq : 1.765 micros/op; 62.7 MB/s | ||
| fillsync : 268.409 micros/op; 0.4 MB/s (10000 ops) | ||
| fillrandom : 2.460 micros/op; 45.0 MB/s | ||
| overwrite : 2.380 micros/op; 46.5 MB/s | ||
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| Each "op" above corresponds to a write of a single key/value pair. | ||
| I.e., a random write benchmark goes at approximately 400,000 writes per second. | ||
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| Each "fillsync" operation costs much less (0.3 millisecond) | ||
| than a disk seek (typically 10 milliseconds). We suspect that this is | ||
| because the hard disk itself is buffering the update in its memory and | ||
| responding before the data has been written to the platter. This may | ||
| or may not be safe based on whether or not the hard disk has enough | ||
| power to save its memory in the event of a power failure. | ||
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| ## Read performance | ||
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| We list the performance of reading sequentially in both the forward | ||
| and reverse direction, and also the performance of a random lookup. | ||
| Note that the database created by the benchmark is quite small. | ||
| Therefore the report characterizes the performance of leveldb when the | ||
| working set fits in memory. The cost of reading a piece of data that | ||
| is not present in the operating system buffer cache will be dominated | ||
| by the one or two disk seeks needed to fetch the data from disk. | ||
| Write performance will be mostly unaffected by whether or not the | ||
| working set fits in memory. | ||
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| readrandom : 16.677 micros/op; (approximately 60,000 reads per second) | ||
| readseq : 0.476 micros/op; 232.3 MB/s | ||
| readreverse : 0.724 micros/op; 152.9 MB/s | ||
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| LevelDB compacts its underlying storage data in the background to | ||
| improve read performance. The results listed above were done | ||
| immediately after a lot of random writes. The results after | ||
| compactions (which are usually triggered automatically) are better. | ||
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| readrandom : 11.602 micros/op; (approximately 85,000 reads per second) | ||
| readseq : 0.423 micros/op; 261.8 MB/s | ||
| readreverse : 0.663 micros/op; 166.9 MB/s | ||
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| Some of the high cost of reads comes from repeated decompression of blocks | ||
| read from disk. If we supply enough cache to the leveldb so it can hold the | ||
| uncompressed blocks in memory, the read performance improves again: | ||
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| readrandom : 9.775 micros/op; (approximately 100,000 reads per second before compaction) | ||
| readrandom : 5.215 micros/op; (approximately 190,000 reads per second after compaction) | ||
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| ## Repository contents | ||
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| See doc/index.html for more explanation. See doc/impl.html for a brief overview of the implementation. | ||
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| The public interface is in include/*.h. Callers should not include or | ||
| rely on the details of any other header files in this package. Those | ||
| internal APIs may be changed without warning. | ||
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| Guide to header files: | ||
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| * **include/db.h**: Main interface to the DB: Start here | ||
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| * **include/options.h**: Control over the behavior of an entire database, | ||
| and also control over the behavior of individual reads and writes. | ||
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| * **include/comparator.h**: Abstraction for user-specified comparison function. | ||
| If you want just bytewise comparison of keys, you can use the default | ||
| comparator, but clients can write their own comparator implementations if they | ||
| want custom ordering (e.g. to handle different character encodings, etc.) | ||
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| * **include/iterator.h**: Interface for iterating over data. You can get | ||
| an iterator from a DB object. | ||
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| * **include/write_batch.h**: Interface for atomically applying multiple | ||
| updates to a database. | ||
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| * **include/slice.h**: A simple module for maintaining a pointer and a | ||
| length into some other byte array. | ||
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| * **include/status.h**: Status is returned from many of the public interfaces | ||
| and is used to report success and various kinds of errors. | ||
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| * **include/env.h**: | ||
| Abstraction of the OS environment. A posix implementation of this interface is | ||
| in util/env_posix.cc | ||
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| * **include/table.h, include/table_builder.h**: Lower-level modules that most | ||
| clients probably won't use directly |
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