Implementation of a caching mechanism for compressed data. More...

#include <BESFileLockingCache.h>

Inheritance diagram for BESFileLockingCache:

Collaboration diagram for BESFileLockingCache:

Public Member Functions
	BESFileLockingCache (const BESFileLockingCache &)=delete

	BESFileLockingCache (std::string cache_dir, std::string prefix, unsigned long long size)
	Make an instance of FileLockingCache.

bool	cache_enabled () const

virtual bool	cache_too_big (unsigned long long current_size) const
	look at the cache size; is it too large? Look at the cache size and see if it is too big.

virtual bool	create_and_lock (const std::string &target, int &fd)
	Create a file in the cache and lock it for write access.

void	disable ()
	Disable the cache.

void	dump (std::ostream &strm) const override
	dumps information about this object

void	enable ()
	Enable the cache.

virtual void	exclusive_to_shared_lock (int fd)
	Transfer from an exclusive lock to a shared lock.

std::string	get_cache_directory () const

virtual std::string	get_cache_file_name (const std::string &src, bool mangle=true)

std::string	get_cache_file_prefix () const

virtual unsigned long long	get_cache_size ()
	Get the cache size.

virtual bool	get_exclusive_lock (const std::string &target, int &fd)

virtual bool	get_exclusive_lock_nb (const std::string &target, int &fd)

virtual bool	get_read_lock (const std::string &target, int &fd)
	Get a read-only lock on the file if it exists.

void	initialize (const std::string &cache_dir, const std::string &prefix, unsigned long long size)
	Initialize an instance of FileLockingCache.

bool	is_unlimited () const
	Is this cache allowed to store as much as it wants?

BESFileLockingCache &	operator= (const BESFileLockingCache &rhs)=delete

virtual void	purge_file (const std::string &file)
	Purge a single file from the cache.

virtual void	unlock_and_close (const std::string &target)

virtual void	update_and_purge (const std::string &new_file)
	Purge files from the cache.

virtual unsigned long long	update_cache_info (const std::string &target)
	Update the cache info file to include 'target'.

Static Public Member Functions
static bool	dir_exists (const std::string &dir)

Friends
class	BESFileLockingCacheTest

class	cacheT

class	FileLockingCacheTest

Detailed Description

Implementation of a caching mechanism for compressed data.

This cache uses simple advisory locking found on most modern unix file systems. It was originally designed to hold the decompressed versions of compressed files. Compressed files are uncompressed and stored in a cache where they can be used over and over until removed from the cache. Several processes can share the cache with each reading from files. At the same time, new files can be added and the cache can be purged, without disrupting the existing read operations.

How it works. When a file is added to the cache, the cache is locked - no other processes can add, read or remove files. Once a file has been added, the cache size is examined and, if needed, the cache is purged so that its size is 80% of the maximum size. Then the cache is unlocked. When a process looks to see if a file is already in the cache, the entire cache is locked. If the file is present, a shared read lock is obtained and the cache is unlocked.

Methods: create_and_lock() and get_read_lock() open and lock files; the former creates the file and locks it exclusively iff it does not exist, while the latter obtains a shared lock iff the file already exists. The unlock() methods unlock a file. The lock_cache_info() and unlock_cache_info() are used to control access to the whole cache - with the open + lock and close + unlock operations are performed atomically. Other methods that operate on the cache info file must only be called when the lock has been obtained.

Note: The locking mechanism uses Unix fcntl(2) and so is per process. That means that while getting an exclusive lock in one process will keep other processes from also getting an exclusive lock, it will not prevent other threads in the same process from getting another 'exclusive lock.' We could switch to flock(2) and get thread-safe locking, but we would trade off the ability to work with files on NFS volumes.

Definition at line 86 of file BESFileLockingCache.h.

Constructor & Destructor Documentation

◆ BESFileLockingCache()

BESFileLockingCache::BESFileLockingCache	(	std::string	cache_dir,
		std::string	prefix,
		unsigned long long	size )

Make an instance of FileLockingCache.

Instantiate the FileLockingClass, using the given values for the cache directory, item prefix and size. If the cache directory is the empty string, do not initialize the cache, but instead mark the cache as 'disabled.' Otherwise, attempt to build the FileLockingCache object and throw an exception if the directory, ..., values are not valid.

Note: A side effect of this is that the cache_enabled() property will be set to true or false based on whether the cache should be used or note. This ensures that the evaluation of the various parameters (and the BESKeys that are likely used by specializations) will happen only once when the cache is disabled.

Parameters

cache_dir	The directory into which the cache files will be written.
prefix	The prefix that will be added to each cache file.
size	The size of the cache in MBytes

Exceptions

BESInternalError	If the cache_dir does not exist or is not writable. size is 0, or if cache dir does not exist.
BESError	If the parameters (directory, ...) are invalid.

Definition at line 137 of file BESFileLockingCache.cc.

◆ ~BESFileLockingCache()

BESFileLockingCache::~BESFileLockingCache ( )

inlineoverride

Definition at line 143 of file BESFileLockingCache.h.

Member Function Documentation

◆ cache_enabled()

bool BESFileLockingCache::cache_enabled ( ) const

inline

Returns: Is this cache enabled?

Definition at line 200 of file BESFileLockingCache.h.

◆ cache_too_big()

bool BESFileLockingCache::cache_too_big ( unsigned long long current_size ) const

virtual

look at the cache size; is it too large? Look at the cache size and see if it is too big.

Returns: True if the size is too big, false otherwise.

Definition at line 817 of file BESFileLockingCache.cc.

◆ create_and_lock()

bool BESFileLockingCache::create_and_lock	(	const std::string &	target,
		int &	fd )

virtual

Create a file in the cache and lock it for write access.

If the file does not exist, make it, open it for read-write access and get an exclusive lock on it. The locking operation blocks, although that should never happen.

Parameters

target	The name of the file to make/open/lock
fd	Value-result param that holds the file descriptor of the opened file

Returns: True if the operation was successful, false otherwise. This method will return false if the file already existed (the file won't be locked and the descriptor reference is undefined - but likely -1).

Exceptions

BESBESInternalError if any error except EEXIST is returned by open(2) or if fcntl(2) returns an error.

Definition at line 625 of file BESFileLockingCache.cc.

◆ dir_exists()

bool BESFileLockingCache::dir_exists ( const std::string & dir )

static

Does the directory exist?

Note: This is a static method, so it can be called from other static methods like those that build instances of singletons.

Parameters

dir	The pathname to test.

Returns: True if the directory exists, false otherwise

Definition at line 1190 of file BESFileLockingCache.cc.

◆ disable()

void BESFileLockingCache::disable ( )

inline

Disable the cache.

Definition at line 205 of file BESFileLockingCache.h.

◆ dump()

void BESFileLockingCache::dump ( std::ostream & strm ) const

overridevirtual

dumps information about this object

Displays the pointer value of this instance along with information about this cache.

Parameters

strm	C++ i/o stream to dump the information to

Implements BESObj.

Definition at line 1205 of file BESFileLockingCache.cc.

◆ enable()

void BESFileLockingCache::enable ( )

inline

Enable the cache.

Definition at line 210 of file BESFileLockingCache.h.

◆ exclusive_to_shared_lock()

void BESFileLockingCache::exclusive_to_shared_lock ( int fd )

virtual

Transfer from an exclusive lock to a shared lock.

If the file has an exclusive write lock on it, change that to a shared read lock. This is an atomic operation. If the call to fcntl(2) is protected by locking the cache, a dead lock will result given typical use of this class. This method exists to help with the situation where one process has the cache locked and is blocking on a shared read lock for a file that a second process has locked exclusively (for writing). By changing the exclusive lock to a shared lock, the first process can get its shared lock and then release the cache.

Parameters

fd	The file descriptor that is exclusively locked and which, on exit, will have a shared lock.

Definition at line 660 of file BESFileLockingCache.cc.

◆ get_cache_directory()

std::string BESFileLockingCache::get_cache_directory ( ) const

inline

Returns: The directory used for the an instance of BESFileLockingCache

Definition at line 191 of file BESFileLockingCache.h.

◆ get_cache_file_name()

string BESFileLockingCache::get_cache_file_name	(	const std::string &	src,
		bool	mangle = true )

virtual

Returns the fully qualified file system path name for the cache file associated with this particular cache resource. This does not look in the cache to see if the fle is present, it just returns the name that will (or is) be used once/if the file is added to the cache.

Note: Names are mangled: ALl occurrences of the characters '<', '>', '=', ',', '/', '(', ')', '"', ''', ':', '?', and ' ' are replaced with the '#' character.

Parameters

src	The source name to (or in the) cache
mangle	If True, assume the name is a file pathname and mangle it. If false, do not mangle the name (assume the caller has sent a suitable string) but do turn the string into a pathname located in the cache directory with the cache prefix. The 'mangle' param is true by default.

Reimplemented in BESUncompressCache.

Definition at line 472 of file BESFileLockingCache.cc.

◆ get_cache_file_prefix()

std::string BESFileLockingCache::get_cache_file_prefix ( ) const

inline

Returns: The prefix used for items in an instance of BESFileLockingCache

Definition at line 186 of file BESFileLockingCache.h.

◆ get_cache_size()

unsigned long long BESFileLockingCache::get_cache_size ( )

virtual

Get the cache size.

Read the size information from the cache info file and return it. This methods locks the cache.

Returns: The size of the cache.

Definition at line 829 of file BESFileLockingCache.cc.

◆ get_exclusive_lock()

bool BESFileLockingCache::get_exclusive_lock	(	const std::string &	target,
		int &	ref_fd )

virtual

A blocking call to get an exclusive (write) lock on a file in the cache. Because this cache uses per-process advisory locking, it's possible to call this several times from within a single process and get a lock each time.

Note: This is used only by the purge_file() method.

Parameters

file_name	Name of the file to lock
ref_fd	Return value parameter that holds the file descriptor that is locked.

Returns: Return false if the file does not exist, otherwise block until the lock is acquired and then return true.

Exceptions

BESInternalError is thrown on any condition other than the file not existing

Definition at line 1079 of file BESFileLockingCache.cc.

◆ get_exclusive_lock_nb()

bool BESFileLockingCache::get_exclusive_lock_nb	(	const std::string &	target,
		int &	ref_fd )

virtual

A non-blocking call to get an exclusive (write) lock on a file in the cache. Because this cache uses per-process advisory locking, it's possible to call this several times from within a single process and get a lock each time.

Note: This is used only by the update_and_purge() method.

Parameters

file_name	Name of the file to lock
ref_fd	Return value parameter that holds the file descriptor that is locked.

Returns: Return false if the file does not exist or the non-blocking lock acquisition fails, otherwise the lock is acquired and then return true.

Exceptions

BESInternalError is thrown on any condition other than the file not existing or the lock being unavailable.

Definition at line 920 of file BESFileLockingCache.cc.

◆ get_read_lock()

bool BESFileLockingCache::get_read_lock	(	const std::string &	target,
		int &	fd )

virtual

Get a read-only lock on the file if it exists.

Try to get a read-only lock on the file, blocking until we can get it. If the file does not exist, return false.

Note: If this code returns false, that means the file did not exist in the cache at the time of the test. by the time the caller gets the result, the file may have been added to the cache by another process.

Parameters

target	The path of the cached file
fd	A value-result parameter set to the locked cached file. Undefined if the file could not be locked for read access.

Returns: true if the file is in the cache and has been locked, false if the file is/was not in the cache.

Exceptions

Error if the attempt to get the (shared) lock failed for any reason other than that the file does/did not exist.

Definition at line 565 of file BESFileLockingCache.cc.

◆ initialize()

void BESFileLockingCache::initialize	(	const std::string &	cache_dir,
		const std::string &	prefix,
		unsigned long long	size )

Initialize an instance of FileLockingCache.

Initialize and instance of FileLockingCache using the passed values for the cache directory, item prefix and max cache size. This will ignore the value of enable_cache() (but will correctly (re)set it based on the directory, ..., values). This provides a way for clients to re-initialize caches on the fly.

Parameters

cache_dir	The directory into which the cache files will be written.
prefix	The prefix that will be added to each cache file.
size	The size of the cache in MBytes

Exceptions

BESInternalError	If the cache_dir does not exist or is not writable. size is 0, or if cache dir does not exist.
BESError	If the parameters (directory, ...) are invalid.

Definition at line 259 of file BESFileLockingCache.cc.

◆ is_unlimited()

bool BESFileLockingCache::is_unlimited ( ) const

inline

Is this cache allowed to store as much as it wants?

If the size of the cache is zero bytes, then it is allowed to grow with out bounds.

Returns: True if the cache is unlimited in size, false if values will be purged after a preset size is exceeded.

Definition at line 181 of file BESFileLockingCache.h.

◆ purge_file()

void BESFileLockingCache::purge_file ( const std::string & file )

virtual

Purge a single file from the cache.

Purge a single file from the cache. The file might be old, etc., and need to be removed. Don't use this to shrink the cache when it gets too big, use update_and_purge() instead since that file optimizes accesses to the cache control file for several changes in a row.

Parameters

file	The name of the file to purge.

Definition at line 1125 of file BESFileLockingCache.cc.

◆ unlock_and_close()

void BESFileLockingCache::unlock_and_close ( const std::string & target )

virtual

Get an exclusive lock on the 'cache info' file. The 'cache info' file is used to control certain cache actions, ensuring that they are atomic. These include making sure that the create_and_lock() and read_and_lock() operations are atomic as well as the purge and related operations.

Note: This is intended to be used internally only but might be useful in some settings. Unlock the named file.

This does not do any name mangling; it just closes and unlocks whatever is named (or throws BESBESInternalError if the file cannot be closed). If the file was opened more than once, all descriptors are closed. If you need to close a specific descriptor, use the other version of unlock_and_close().

Note: This method assumes that the file was opened/locked using one of read_and_lock() or create_and_lock(). Those methods record the name/file- descriptor pairs so that the files can be properly closed and locks released.

Parameters

file_name The name of the file to unlock.

Exceptions

BESBESInternalError

Definition at line 745 of file BESFileLockingCache.cc.

◆ update_and_purge()

void BESFileLockingCache::update_and_purge ( const std::string & new_file )

virtual

Purge files from the cache.

Purge files, oldest to newest, if the current size of the cache exceeds the size of the cache specified in the constructor. This method uses an exclusive lock on the cache for the duration of the purge process.

Note: If the cache size in bytes is zero, calling this method has no affect (the cache is unlimited in size). Other public methods like update_cache_info() and get_cache_size() still work, however.

Parameters

new_file Do not delete this file. The name of a file this process just added to the cache. Using fcntl(2) locking there is no way this process can detect its own lock, so the shared read lock on the new file won't keep this process from deleting it (but will keep other processes from deleting it).

Definition at line 977 of file BESFileLockingCache.cc.

◆ update_cache_info()

unsigned long long BESFileLockingCache::update_cache_info ( const std::string & target )

virtual

Update the cache info file to include 'target'.

Add the size of the named file to the total cache size recorded in the cache info file. The cache info file is exclusively locked by this method for its duration. This updates the cache info file and returns the new size.

Parameters

target The name of the file

Returns: The new size of the cache

Definition at line 769 of file BESFileLockingCache.cc.

Friends And Related Symbol Documentation

◆ BESFileLockingCacheTest

friend class BESFileLockingCacheTest

friend

Definition at line 134 of file BESFileLockingCache.h.

◆ cacheT

friend class cacheT

friend

Definition at line 132 of file BESFileLockingCache.h.

◆ FileLockingCacheTest

friend class FileLockingCacheTest

friend

Definition at line 133 of file BESFileLockingCache.h.

The documentation for this class was generated from the following files:

Public Member Functions

Static Public Member Functions

Friends

Detailed Description

Constructor & Destructor Documentation

◆ BESFileLockingCache()

◆ ~BESFileLockingCache()

Member Function Documentation

◆ cache_enabled()

◆ cache_too_big()

◆ create_and_lock()

◆ dir_exists()

◆ disable()

◆ dump()

◆ enable()

◆ exclusive_to_shared_lock()

◆ get_cache_directory()

◆ get_cache_file_name()

◆ get_cache_file_prefix()

◆ get_cache_size()

◆ get_exclusive_lock()

◆ get_exclusive_lock_nb()

◆ get_read_lock()

◆ initialize()

◆ is_unlimited()

◆ purge_file()

◆ unlock_and_close()

◆ update_and_purge()

◆ update_cache_info()

Friends And Related Symbol Documentation

◆ BESFileLockingCacheTest

◆ cacheT

◆ FileLockingCacheTest