C# (CSharp) Lucene.Net.Store Namespace

All files belonging to a segment have the same name with varying extensions. The extensions correspond to the different file formats used by the Codec. When using the Compound File format these files are collapsed into a single .cfs file (except for the LiveDocsFormat, with a corresponding .cfe file indexing its sub-files.

Files:

• .cfs: An optional "virtual" file consisting of all the other index files for systems that frequently run out of file handles.
• .cfe: The "virtual" compound file's entry table holding all entries in the corresponding .cfs file.

Description:

• Compound (.cfs) --> Header, FileData ^FileCount
• Compound Entry Table (.cfe) --> Header, FileCount, <FileName, DataOffset, DataLength> ^FileCount, Footer
• Header --> CodecUtil#writeHeader CodecHeader
• FileCount --> DataOutput#writeVInt VInt
• DataOffset,DataLength --> DataOutput#writeLong UInt64
• FileName --> DataOutput#writeString String
• FileData --> raw file data
• Footer --> CodecUtil#writeFooter CodecFooter

Notes:

• FileCount indicates how many files are contained in this compound file. The entry table that follows has that many entries.
• Each directory entry contains a long pointer to the start of this file's data section, the files length, and a String with that file's name.

{@code DataInput} may only be used from one thread, because it is not thread safe (it keeps internal state like file position). To allow multithreaded use, every {@code DataInput} instance must be cloned before used in another thread. Subclasses must therefore implement #clone(), returning a new {@code DataInput} which operates on the same underlying resource, but positioned independently.

{@code DataOutput} may only be used from one thread, because it is not thread safe (it keeps internal state like file position).

The locking implementation is by default NativeFSLockFactory , but can be changed by passing in a custom LockFactory instance.

Files with the specified extensions are placed in the primary directory; others are placed in the secondary directory. The provided Set must not change once passed to this class, and must allow multiple threads to call contains at once.

Typical use might look like:

 new Lock.With(directory.makeLock("my.lock")) { public Object doBody() { ... code to execute while locked ... } }.run(); 

NOTE: memory mapping uses up a portion of the virtual memory address space in your process equal to the size of the file being mapped. Before using this class, be sure your have plenty of virtual address space, e.g. by using a 64 bit JRE, or a 32 bit JRE with indexes that are guaranteed to fit within the address space. On 32 bit platforms also consult #MMapDirectory(File, LockFactory, int) if you have problems with mmap failing because of fragmented address space. If you get an OutOfMemoryException, it is recommended to reduce the chunk size, until it works.

Due to this bug in Sun's JRE, MMapDirectory's IndexInput#close is unable to close the underlying OS file handle. Only when GC finally collects the underlying objects, which could be quite some time later, will the file handle be closed.

this will consume additional transient disk usage: on Windows, attempts to delete or overwrite the files will result in an exception; on other platforms, which typically have a "delete on last close" semantics, while such operations will succeed, the bytes are still consuming space on disk. For many applications this limitation is not a problem (e.g. if you have plenty of disk space, and you don't rely on overwriting files on Windows) but it's still an important limitation to be aware of.

this class supplies the workaround mentioned in the bug report (see #setUseUnmap), which may fail on non-Sun JVMs. It forcefully unmaps the buffer on close by using an undocumented internal cleanup functionality. #UNMAP_SUPPORTED is true, if the workaround can be enabled (with no guarantees).

NOTE: Accessing this class either directly or indirectly from a thread while it's interrupted can close the underlying channel immediately if at the same time the thread is blocked on IO. The channel will remain closed and subsequent access to MMapDirectory will throw a ClosedChannelException.

• Instances created by LuceneTestCase#newDirectory() are tracked to ensure they are closed by the test.
• When a MockDirectoryWrapper is closed, it will throw an exception if it has any open files against it (with a stacktrace indicating where they were opened from).
• When a MockDirectoryWrapper is closed, it runs CheckIndex to test if the index was corrupted.
• MockDirectoryWrapper simulates some "features" of Windows, such as refusing to write/delete to open files.

this class only uses FileChannel when reading; writing is achieved with FSDirectory.FSIndexOutput.

NOTE: NIOFSDirectory is not recommended on Windows because of a bug in how FileChannel.read is implemented in Sun's JRE. Inside of the implementation the position is apparently synchronized. See here for details.

NOTE: Accessing this class either directly or indirectly from a thread while it's interrupted can close the underlying file descriptor immediately if at the same time the thread is blocked on IO. The file descriptor will remain closed and subsequent access to NIOFSDirectory will throw a ClosedChannelException. If your application uses either Thread#interrupt() or Future#cancel(boolean) you should use SimpleFSDirectory in favor of NIOFSDirectory.

Implements {@link LockFactory} using native OS file locks. Note that because this LockFactory relies on java.nio.* APIs for locking, any problems with those APIs will cause locking to fail. Specifically, on certain NFS environments the java.nio.* locks will fail (the lock can incorrectly be double acquired) whereas {@link SimpleFSLockFactory} worked perfectly in those same environments. For NFS based access to an index, it's recommended that you try {@link SimpleFSLockFactory} first and work around the one limitation that a lock file could be left when the JVM exits abnormally.

The primary benefit of {@link NativeFSLockFactory} is that lock files will be properly removed (by the OS) if the JVM has an abnormal exit.

Note that, unlike {@link SimpleFSLockFactory}, the existence of leftover lock files in the filesystem on exiting the JVM is fine because the OS will free the locks held against these files even though the files still remain.

If you suspect that this or any other LockFactory is not working properly in your environment, you can easily test it by using {@link VerifyingLockFactory}, {@link LockVerifyServer} and {@link LockStressTest}.

Warning: this class is not intended to work with huge indexes. Everything beyond several hundred megabytes will waste resources (GC cycles), because it uses an internal buffer size of 1024 bytes, producing millions of {@code byte[1024]} arrays. this class is optimized for small memory-resident indexes. It also has bad concurrency on multithreaded environments.

It is recommended to materialize large indexes on disk and use MMapDirectory, which is a high-performance directory implementation working directly on the file system cache of the operating system, so copying data to Java heap space is not useful.