bes  Updated for version 3.17.4
BESFileLockingCache Class Reference

Implementation of a caching mechanism for compressed data. This cache uses simple advisory locking found on most modern unix file systems. 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. More...

#include <BESFileLockingCache.h>

Inheritance diagram for BESFileLockingCache:
Inheritance graph

Public Member Functions

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. More...
 
virtual bool create_and_lock (const string &target, int &fd)
 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. More...
 
virtual void dump (ostream &strm) const
 dumps information about this object More...
 
virtual void exclusive_to_shared_lock (int fd)
 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. More...
 
virtual string get_cache_file_name (const string &src, bool mangle=true)
 
virtual unsigned long long get_cache_size ()
 Get the cache size. Read the size information from the cache info file and return it. This methods locks the cache. More...
 
virtual bool get_read_lock (const string &target, int &fd)
 Get a read-only lock on the file if it exists. More...
 
const string getCacheDirectory ()
 
const string getCacheFilePrefix ()
 
virtual bool getExclusiveLock (string file_name, int &ref_fd)
 
virtual void lock_cache_read ()
 
virtual void lock_cache_write ()
 
virtual void purge_file (const string &file)
 Purge a single file from the cache. More...
 
virtual void unlock_and_close (const string &target)
 
virtual void unlock_cache ()
 
virtual void update_and_purge (const string &new_file)
 Purge files from the cache. More...
 
virtual unsigned long long update_cache_info (const string &target)
 Update the cache info file to include 'target'. More...
 

Static Public Member Functions

static bool dir_exists (const string &dir)
 

Protected Member Functions

 BESFileLockingCache (const string &cache_dir, const string &prefix, unsigned long long size)
 Protected constructor that takes as arguments keys to the cache directory, file prefix, and size of the cache to be looked up a configuration file. More...
 
void initialize (const string &cache_dir, const string &prefix, unsigned long long size)
 

Protected Attributes

string d_cache_dir
 
unsigned long long d_max_cache_size_in_bytes
 How many bytes can the cache hold before we have to purge.
 
string d_prefix
 
unsigned long long d_target_size
 

Detailed Description

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 performed atomically. Other methods that operate on the cache info file must only be called when the lock has been obtained.

Definition at line 73 of file BESFileLockingCache.h.

Constructor & Destructor Documentation

◆ BESFileLockingCache()

BESFileLockingCache::BESFileLockingCache ( const string &  cache_dir,
const string &  prefix,
unsigned long long  size 
)
protected

The keys specified are looked up in the specified keys object. If not found or not set correctly then an exception is thrown. I.E., if the cache directory is empty, the size is zero, or the prefix is empty.

Parameters
cache_dirThe directory into which the cache files will be written.
prefixThe prefix that will be added to each cache file.
sizeThe size of the cache in MBytes
Exceptions
BESInternalErrorIf the cache_dir does not exist or is not writable. size is 0, or if cache dir does not exist.

Definition at line 77 of file BESFileLockingCache.cc.

References d_max_cache_size_in_bytes.

Member Function Documentation

◆ cache_too_big()

bool BESFileLockingCache::cache_too_big ( unsigned long long  current_size) const
virtual
Returns
True if the size is too big, false otherwise.

Definition at line 742 of file BESFileLockingCache.cc.

References d_max_cache_size_in_bytes.

◆ create_and_lock()

bool BESFileLockingCache::create_and_lock ( const string &  target,
int &  fd 
)
virtual
Parameters
targetThe name of the file to make/open/lock
fdValue-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
BESBESInternalErrororif any error except EEXIST is returned by open(2) or if fcntl(2) returns an error.

Definition at line 563 of file BESFileLockingCache.cc.

References lock_cache_write(), and unlock_cache().

◆ dir_exists()

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

Does the directory exist?

Parameters
dirThe pathname to test.
Returns
True if the directory exists, false otherwise

Definition at line 980 of file BESFileLockingCache.cc.

◆ dump()

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

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

Parameters
strmC++ i/o stream to dump the information to

Implements BESObj.

Definition at line 994 of file BESFileLockingCache.cc.

References d_max_cache_size_in_bytes.

◆ exclusive_to_shared_lock()

void BESFileLockingCache::exclusive_to_shared_lock ( int  fd)
virtual
Parameters
fdThe file descriptor that is exclusively locked and which, on exit, will have a shared lock.

Definition at line 592 of file BESFileLockingCache.cc.

◆ get_cache_file_name()

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

Returns the fully qualified file system path name for the cache file associated with this particular cache resource.

Note
How names are mangled: ALl occurrences of the characters '<', '>', '=', ',', '/', '(', ')', '"', ''', ':', '?', and ' ' with the '#' character.
Parameters
srcThe source name to cache
mangleIf 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 477 of file BESFileLockingCache.cc.

References BESUtil::assemblePath().

◆ get_cache_size()

unsigned long long BESFileLockingCache::get_cache_size ( )
virtual
Returns
The size of the cache.

Definition at line 754 of file BESFileLockingCache.cc.

References lock_cache_read(), and unlock_cache().

◆ get_read_lock()

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

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
srcsrc file that will be cached eventually
targeta value-result parameter set to the resulting cached file
Returns
true if the file is in the cache and has been locked, false if the file is/was not in the cache.
Exceptions
Errorif the attempt to get the (shared) lock failed for any reason other than that the file does/did not exist.

Definition at line 535 of file BESFileLockingCache.cc.

References lock_cache_read(), and unlock_cache().

◆ getExclusiveLock()

bool BESFileLockingCache::getExclusiveLock ( string  file_name,
int &  ref_fd 
)
virtual

Get an exclusive read/write lock on an existing file.

Parameters
file_nameThe name of the file.
ref_fpif successful, the file descriptor of the file on which we have an exclusive read/write lock.
Returns
If the file does not exist, return immediately indicating failure (false), otherwise block until an exclusive read/write lock can be obtained and then return true.
Exceptions
Erroris thrown to indicate a number of untoward events.

Definition at line 253 of file BESFileLockingCache.cc.

References BESUtil::assemblePath(), d_max_cache_size_in_bytes, and unlock_cache().

◆ lock_cache_read()

void BESFileLockingCache::lock_cache_read ( )
virtual

Get a shared lock on the 'cache info' file.

Definition at line 631 of file BESFileLockingCache.cc.

◆ lock_cache_write()

void BESFileLockingCache::lock_cache_write ( )
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.

Definition at line 616 of file BESFileLockingCache.cc.

◆ purge_file()

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

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.

Todo:
This is a new feature; add to BESCache3
Parameters
fileThe name of the file to purge.

Definition at line 922 of file BESFileLockingCache.cc.

References get_cache_size(), getExclusiveLock(), lock_cache_write(), and unlock_cache().

◆ unlock_and_close()

void BESFileLockingCache::unlock_and_close ( const string &  file_name)
virtual

Unlock the named file.

This does not do any name mangling; it just closes and unlocks whatever is named (or throws BESBESInternalErroror 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_nameThe name of the file to unlock.
Exceptions
BESBESInternalErroror

Definition at line 675 of file BESFileLockingCache.cc.

◆ unlock_cache()

void BESFileLockingCache::unlock_cache ( )
virtual

Unlock the cache info file.

Note
This is intended to be used internally only bt might be useful in some settings.

Definition at line 648 of file BESFileLockingCache.cc.

◆ update_and_purge()

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

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.

Parameters
new_fileThe 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 835 of file BESFileLockingCache.cc.

References cache_too_big(), lock_cache_write(), and unlock_cache().

◆ update_cache_info()

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

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
targetThe name of the file
Returns
The new size of the cache

Definition at line 699 of file BESFileLockingCache.cc.

References lock_cache_write(), and unlock_cache().


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