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- <!DOCTYPE html>
- <html>
- <head>
- <link rel="stylesheet" type="text/css" href="doc.css" />
- <title>Leveldb</title>
- </head>
-
- <body>
- <h1>Leveldb</h1>
- <address>Jeff Dean, Sanjay Ghemawat</address>
- <p>
- The <code>leveldb</code> library provides a persistent key value store. Keys and
- values are arbitrary byte arrays. The keys are ordered within the key
- value store according to a user-specified comparator function.
-
- <p>
- <h1>Opening A Database</h1>
- <p>
- A <code>leveldb</code> database has a name which corresponds to a file system
- directory. All of the contents of database are stored in this
- directory. The following example shows how to open a database,
- creating it if necessary:
- <p>
- <pre>
- #include <assert>
- #include "leveldb/include/db.h"
-
- leveldb::DB* db;
- leveldb::Options options;
- options.create_if_missing = true;
- leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
- assert(status.ok());
- ...
- </pre>
- If you want to raise an error if the database already exists, add
- the following line before the <code>leveldb::DB::Open</code> call:
- <pre>
- options.error_if_exists = true;
- </pre>
- <h1>Status</h1>
- <p>
- You may have noticed the <code>leveldb::Status</code> type above. Values of this
- type are returned by most functions in <code>leveldb</code> that may encounter an
- error. You can check if such a result is ok, and also print an
- associated error message:
- <p>
- <pre>
- leveldb::Status s = ...;
- if (!s.ok()) cerr << s.ToString() << endl;
- </pre>
- <h1>Closing A Database</h1>
- <p>
- When you are done with a database, just delete the database object.
- Example:
- <p>
- <pre>
- ... open the db as described above ...
- ... do something with db ...
- delete db;
- </pre>
- <h1>Reads And Writes</h1>
- <p>
- The database provides <code>Put</code>, <code>Delete</code>, and <code>Get</code> methods to
- modify/query the database. For example, the following code
- moves the value stored under key1 to key2.
- <pre>
- std::string value;
- leveldb::Status s = db->Get(leveldb::ReadOptions(), key1, &value);
- if (s.ok()) s = db->Put(leveldb::WriteOptions(), key2, value);
- if (s.ok()) s = db->Delete(leveldb::WriteOptions(), key1);
- </pre>
-
- <h1>Atomic Updates</h1>
- <p>
- Note that if the process dies after the Put of key2 but before the
- delete of key1, the same value may be left stored under multiple keys.
- Such problems can be avoided by using the <code>WriteBatch</code> class to
- atomically apply a set of updates:
- <p>
- <pre>
- #include "leveldb/include/write_batch.h"
- ...
- std::string value;
- leveldb::Status s = db->Get(leveldb::ReadOptions(), key1, &value);
- if (s.ok()) {
- leveldb::WriteBatch batch;
- batch.Delete(key1);
- batch.Put(key2, value);
- s = db->Write(leveldb::WriteOptions(), &batch);
- }
- </pre>
- The <code>WriteBatch</code> holds a sequence of edits to be made to the database,
- and these edits within the batch are applied in order. Note that we
- called <code>Delete</code> before <code>Put</code> so that if <code>key1</code> is identical to <code>key2</code>,
- we do not end up erroneously dropping the value entirely.
- <p>
- Apart from its atomicity benefits, <code>WriteBatch</code> may also be used to
- speed up bulk updates by placing lots of individual mutations into the
- same batch.
-
- <h1>Synchronous Writes</h1>
- By default, each write to <code>leveldb</code> 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 <code>sync</code> 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 <code>fsync(...)</code> or <code>fdatasync(...)</code> or
- <code>msync(..., MS_SYNC)</code> before the write operation returns.)
- <pre>
- leveldb::WriteOptions write_options;
- write_options.sync = true;
- db->Put(write_options, ...);
- </pre>
- 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 <code>sync</code> is false, an update
- is pushed from the process memory into the operating system before it
- is considered done.
-
- <p>
- Asynchronous writes can often be used safely. For example, when
- loading a large amount of data into the database you can handle lost
- updates by restarting the bulk load after a crash. A hybrid scheme is
- also possible where every Nth write is synchronous, and in the event
- of a crash, the bulk load is restarted just after the last synchronous
- write finished by the previous run. (The synchronous write can update
- a marker that describes where to restart on a crash.)
-
- <p>
- <code>WriteBatch</code> provides an alternative to asynchronous writes.
- Multiple updates may be placed in the same <code>WriteBatch</code> and
- applied together using a synchronous write (i.e.,
- <code>write_options.sync</code> is set to true). The extra cost of
- the synchronous write will be amortized across all of the writes in
- the batch.
-
- <p>
- <h1>Concurrency</h1>
- <p>
- A database may only be opened by one process at a time. The <code>leveldb</code>
- implementation acquires a lock from the operating system to prevent
- misuse. Within a single process, the same <code>leveldb::DB</code> object may
- be safely used by multiple concurrent threads.
- <p>
- <h1>Iteration</h1>
- <p>
- The following example demonstrates how to print all key,value pairs
- in a database.
- <p>
- <pre>
- leveldb::Iterator* it = db->NewIterator(leveldb::ReadOptions());
- for (it->SeekToFirst(); it->Valid(); it->Next()) {
- cout << it->key().ToString() << ": " << it->value().ToString() << endl;
- }
- assert(it->status().ok()); // Check for any errors found during the scan
- delete it;
- </pre>
- The following variation shows how to process just the keys in the
- range <code>[start,limit)</code>:
- <p>
- <pre>
- for (it->Seek(start);
- it->Valid() && it->key().ToString() < limit;
- it->Next()) {
- ...
- }
- </pre>
- You can also process entries in reverse order. (Caveat: reverse
- iteration may be somewhat slower than forward iteration.)
- <p>
- <pre>
- for (it->SeekToLast(); it->Valid(); it->Prev()) {
- ...
- }
- </pre>
- <h1>Snapshots</h1>
- <p>
- Snapshots provide consistent read-only views over the entire state of
- the key-value store. <code>ReadOptions::snapshot</code> may be non-NULL to indicate
- that a read should operate on a particular version of the DB state.
- If <code>ReadOptions::snapshot</code> is NULL, the read will operate on an
- implicit snapshot of the current state.
- <p>
- Snapshots typically are created by the DB::GetSnapshot() method:
- <p>
- <pre>
- leveldb::ReadOptions options;
- options.snapshot = db->GetSnapshot();
- ... apply some updates to db ...
- leveldb::Iterator* iter = db->NewIterator(options);
- ... read using iter to view the state when the snapshot was created ...
- delete iter;
- db->ReleaseSnapshot(options.snapshot);
- </pre>
- Note that when a snapshot is no longer needed, it should be released
- using the DB::ReleaseSnapshot interface. This allows the
- implementation to get rid of state that was being maintained just to
- support reading as of that snapshot.
- <p>
- A Write operation can also return a snapshot that
- represents the state of the database just after applying a particular
- set of updates:
- <p>
- <pre>
- leveldb::Snapshot* snapshot;
- leveldb::WriteOptions write_options;
- write_options.post_write_snapshot = &snapshot;
- leveldb::Status status = db->Write(write_options, ...);
- ... perform other mutations to db ...
-
- leveldb::ReadOptions read_options;
- read_options.snapshot = snapshot;
- leveldb::Iterator* iter = db->NewIterator(read_options);
- ... read as of the state just after the Write call returned ...
- delete iter;
-
- db->ReleaseSnapshot(snapshot);
- </pre>
- <h1>Slice</h1>
- <p>
- The return value of the <code>it->key()</code> and <code>it->value()</code> calls above
- are instances of the <code>leveldb::Slice</code> type. <code>Slice</code> is a simple
- structure that contains a length and a pointer to an external byte
- array. Returning a <code>Slice</code> is a cheaper alternative to returning a
- <code>std::string</code> since we do not need to copy potentially large keys and
- values. In addition, <code>leveldb</code> methods do not return null-terminated
- C-style strings since <code>leveldb</code> keys and values are allowed to
- contain '\0' bytes.
- <p>
- C++ strings and null-terminated C-style strings can be easily converted
- to a Slice:
- <p>
- <pre>
- leveldb::Slice s1 = "hello";
-
- std::string str("world");
- leveldb::Slice s2 = str;
- </pre>
- A Slice can be easily converted back to a C++ string:
- <pre>
- std::string str = s1.ToString();
- assert(str == std::string("hello"));
- </pre>
- Be careful when using Slices since it is up to the caller to ensure that
- the external byte array into which the Slice points remains live while
- the Slice is in use. For example, the following is buggy:
- <p>
- <pre>
- leveldb::Slice slice;
- if (...) {
- std::string str = ...;
- slice = str;
- }
- Use(slice);
- </pre>
- When the <code>if</code> statement goes out of scope, <code>str</code> will be destroyed and the
- backing storage for <code>slice</code> will disappear.
- <p>
- <h1>Comparators</h1>
- <p>
- The preceding examples used the default ordering function for key,
- which orders bytes lexicographically. You can however supply a custom
- comparator when opening a database. For example, suppose each
- database key consists of two numbers and we should sort by the first
- number, breaking ties by the second number. First, define a proper
- subclass of <code>leveldb::Comparator</code> that expresses these rules:
- <p>
- <pre>
- class TwoPartComparator : public leveldb::Comparator {
- public:
- // Three-way comparison function:
- // if a < b: negative result
- // if a > b: positive result
- // else: zero result
- int Compare(const leveldb::Slice& a, const leveldb::Slice& b) const {
- int a1, a2, b1, b2;
- ParseKey(a, &a1, &a2);
- ParseKey(b, &b1, &b2);
- if (a1 < b1) return -1;
- if (a1 > b1) return +1;
- if (a2 < b2) return -1;
- if (a2 > b2) return +1;
- return 0;
- }
-
- // Ignore the following methods for now:
- const char* Name() { return "TwoPartComparator"; }
- void FindShortestSeparator(std::string*, const leveldb::Slice&) const { }
- void FindShortSuccessor(std::string*) const { }
- };
- </pre>
- Now create a database using this custom comparator:
- <p>
- <pre>
- TwoPartComparator cmp;
- leveldb::DB* db;
- leveldb::Options options;
- options.create_if_missing = true;
- options.comparator = &cmp;
- leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
- ...
- </pre>
- <h2>Backwards compatibility</h2>
- <p>
- The result of the comparator's <code>Name</code> method is attached to the
- database when it is created, and is checked on every subsequent
- database open. If the name changes, the <code>leveldb::DB::Open</code> call will
- fail. Therefore, change the name if and only if the new key format
- and comparison function are incompatible with existing databases, and
- it is ok to discard the contents of all existing databases.
- <p>
- You can however still gradually evolve your key format over time with
- a little bit of pre-planning. For example, you could store a version
- number at the end of each key (one byte should suffice for most uses).
- When you wish to switch to a new key format (e.g., adding an optional
- third part to the keys processed by <code>TwoPartComparator</code>),
- (a) keep the same comparator name (b) increment the version number
- for new keys (c) change the comparator function so it uses the
- version numbers found in the keys to decide how to interpret them.
- <p>
- <h1>Performance</h1>
- <p>
- Performance can be tuned by changing the default values of the
- types defined in <code>leveldb/include/options.h</code>.
-
- <p>
- <h2>Block size</h2>
- <p>
- <code>leveldb</code> groups adjacent keys together into the same block and such a
- block is the unit of transfer to and from persistent storage. The
- default block size is approximately 4096 uncompressed bytes.
- Applications that mostly do bulk scans over the contents of the
- database may wish to increase this size. Applications that do a lot
- of point reads of small values may wish to switch to a smaller block
- size if performance measurements indicate an improvement. There isn't
- much benefit in using blocks smaller than one kilobyte, or larger than
- a few megabytes. Also note that compression will be more effective
- with larger block sizes.
- <p>
- <h2>Compression</h2>
- <p>
- Each block is individually compressed before being written to
- persistent storage. Compression is on by default since the default
- compression method is very fast, and is automatically disabled for
- uncompressible data. In rare cases, applications may want to disable
- compression entirely, but should only do so if benchmarks show a
- performance improvement:
- <p>
- <pre>
- leveldb::Options options;
- options.compression = leveldb::kNoCompression;
- ... leveldb::DB::Open(options, name, ...) ....
- </pre>
- <h2>Cache</h2>
- <p>
- The contents of the database are stored in a set of files in the
- filesystem and each file stores a sequence of compressed blocks. If
- <code>options.cache</code> is non-NULL, it is used to cache frequently used
- uncompressed block contents.
- <p>
- <pre>
- #include "leveldb/include/cache.h"
-
- leveldb::Options options;
- options.cache = leveldb::NewLRUCache(100 * 1048576); // 100MB cache
- leveldb::DB* db;
- leveldb::DB::Open(options, name, &db);
- ... use the db ...
- delete db
- delete options.cache;
- </pre>
- Note that the cache holds uncompressed data, and therefore it should
- be sized according to application level data sizes, without any
- reduction from compression. (Caching of compressed blocks is left to
- the operating system buffer cache, or any custom <code>Env</code>
- implementation provided by the client.)
- <p>
- When performing a bulk read, the application may wish to disable
- caching so that the data processed by the bulk read does not end up
- displacing most of the cached contents. A per-iterator option can be
- used to achieve this:
- <p>
- <pre>
- leveldb::ReadOptions options;
- options.fill_cache = false;
- leveldb::Iterator* it = db->NewIterator(options);
- for (it->SeekToFirst(); it->Valid(); it->Next()) {
- ...
- }
- </pre>
- <h2>Key Layout</h2>
- <p>
- Note that the unit of disk transfer and caching is a block. Adjacent
- keys (according to the database sort order) will usually be placed in
- the same block. Therefore the application can improve its performance
- by placing keys that are accessed together near each other and placing
- infrequently used keys in a separate region of the key space.
- <p>
- For example, suppose we are implementing a simple file system on top
- of <code>leveldb</code>. The types of entries we might wish to store are:
- <p>
- <pre>
- filename -> permission-bits, length, list of file_block_ids
- file_block_id -> data
- </pre>
- We might want to prefix <code>filename</code> keys with one letter (say '/') and the
- <code>file_block_id</code> keys with a different letter (say '0') so that scans
- over just the metadata do not force us to fetch and cache bulky file
- contents.
- <p>
- <h2>Large Values</h2>
- <p>
- <code>leveldb</code> has special treatment of large values (by default, a value
- of length greater than or equal to 64K is considered large, though a
- field in Options can be used to adjust this threshold). Each such
- large value is placed in a separate operating system file, and the
- normal database blocks just contain pointers to such files.
- <p>
- Furthermore, if the same large value occurs multiple times in a single
- database, it will be stored just once.
- <p>
- <h1>Checksums</h1>
- <p>
- <code>leveldb</code> associates checksums with all data it stores in the file system.
- There are two separate controls provided over how aggressively these
- checksums are verified:
- <p>
- <ul>
- <li> <code>ReadOptions::verify_checksums</code> may be set to true to force
- checksum verification of all data that is read from the file system on
- behalf of a particular read. By default, no such verification is
- done.
- <p>
- <li> <code>Options::paranoid_checks</code> may be set to true before opening a
- database to make the database implementation raise an error as soon as
- it detects an internal corruption. Depending on which portion of the
- database has been corrupted, the error may be raised when the database
- is opened, or later by another database operation. By default,
- paranoid checking is off so that the database can be used even if
- parts of its persistent storage have been corrupted.
- <p>
- If a database is corrupted (perhaps it cannot be opened when
- paranoid checking is turned on), the <code>leveldb::RepairDB</code> function
- may be used to recover as much of the data as possible
- <p>
- </ul>
- <h1>Approximate Sizes</h1>
- <p>
- The <code>GetApproximateSizes</code> method can used to get the approximate
- number of bytes of file system space used by one or more key ranges.
- <p>
- <pre>
- leveldb::Range ranges[2];
- ranges[0] = leveldb::Range("a", "c");
- ranges[1] = leveldb::Range("x", "z");
- uint64_t sizes[2];
- leveldb::Status s = db->GetApproximateSizes(ranges, 2, sizes);
- </pre>
- The preceding call will set <code>sizes[0]</code> to the approximate number of
- bytes of file system space used by the key range <code>[a..c)</code> and
- <code>sizes[1]</code> to the approximate number of bytes used by the key range
- <code>[x..z)</code>.
- <p>
- <h1>Environment</h1>
- <p>
- All file operations (and other operating system calls) issued by the
- <code>leveldb</code> implementation are routed through a <code>leveldb::Env</code> object.
- Sophisticated clients may wish to provide their own <code>Env</code>
- implementation to get better control. For example, an application may
- introduce artificial delays in the file IO paths to limit the impact
- of <code>leveldb</code> on other activities in the system.
- <p>
- <pre>
- class SlowEnv : public leveldb::Env {
- .. implementation of the Env interface ...
- };
-
- SlowEnv env;
- leveldb::Options options;
- options.env = &env;
- Status s = leveldb::DB::Open(options, ...);
- </pre>
- <h1>Porting</h1>
- <p>
- <code>leveldb</code> may be ported to a new platform by providing platform
- specific implementations of the types/methods/functions exported by
- <code>leveldb/port/port.h</code>. See <code>leveldb/port/port_example.h</code> for more
- details.
- <p>
- In addition, the new platform may need a new default <code>leveldb::Env</code>
- implementation. See <code>leveldb/util/env_posix.h</code> for an example.
-
- <h1>Other Information</h1>
-
- <p>
- Details about the <code>leveldb</code> implementation may be found in
- the following documents:
- <ul>
- <li> <a href="impl.html">Implementation notes</a>
- <li> <a href="table_format.txt">Format of an immutable Table file</a>
- <li> <a href="log_format.txt">Format of a log file</a>
- </ul>
-
- </body>
- </html>
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