CBCache Component

Properties   Methods   Events   Config Settings   Errors  

The CBCache component allows applications to easily cache remote file data locally.

Syntax

TcbcCBCache

Remarks

The CBCache component provides a local file-caching implementation that helps applications easily cache the contents of files stored in some external storage, such as a cloud storage service. Once an application has informed the CBCache component of the external storage's file transfer capabilities, the hard work of reading data into and flushing data out of the cache is handled automatically.

Getting Started

Applications typically need only a single instance of the CBCache component, because it can handle caching and accessing multiple files at once in parallel. Each file in the cache is identified by its file Id, and all cached file data are stored in a single cache storage file, whose name and location are determined by the CacheFile and CacheDirectory properties, respectively.

In terms of application architecture, the CBCache component (or, more generally, the cache itself) should reside between application code that needs random access to files in the external storage, and the code that provides access to said external storage. The former makes use of the CBCache component's properties and methods, while the latter is "connected" through the component's ReadData and WriteData events.

Here's how to get up and running:

1. Implement the ReadData and WriteData events so that they provide appropriate access to the external storage. The ReadData event is mandatory. The WriteData event is required only if the CBCache component is to be in charge of flushing changed data back to external storage. Refer to these events' documentation for more information about how to correctly implement them.
2. "Describe" the reading, writing, and resizing capabilities of the external storage to the CBCache component by setting the ReadingCapabilities, WritingCapabilities, and ResizingCapabilities properties. This information is used by the CBCache component to formulate a strategy for interacting with the external storage through the events mentioned in step 1.
3. Open the cache by setting the CacheDirectory and CacheFile properties, and then calling the OpenCache method.
4. When the component opens an existing cache vault, it is recommended that applications use the EnumerateCachedFiles method to search for any Orphan Files (i.e., files that are not currently open have unflushed data). If any such files are found, applications may wish to open, flush, or delete them.
5. To create or open a file in the cache (i.e., a cached file), call the FileOpen method.
6. While a cached file is open, it can be read from or written to using the FileRead and FileWrite methods.

Property List

---

The following is the full list of the properties of the component with short descriptions. Click on the links for further details.

Method List

---

The following is the full list of the methods of the component with short descriptions. Click on the links for further details.

Event List

---

The following is the full list of the events fired by the component with short descriptions. Click on the links for further details.

Config Settings

---

The following is a list of config settings for the component with short descriptions. Click on the links for further details.

Active Property (CBCache Component)

This property describes whether the file cache is open.

Syntax

property Active: Boolean read get_Active;

Default Value

false

Remarks

This property reflects whether the component has opened a file cache; it will be True once the OpenCache method has been called successfully.

This property is read-only and not available at design time.

AutoFlush Property (CBCache Component)

This property specifies whether automatic flushing is enabled.

Syntax

property AutoFlush: Boolean read get_AutoFlush write set_AutoFlush;

Default Value

true

Remarks

This property specifies whether automatic flushing is enabled. When this property is enabled (default), automatic flushing will take place both according to the FlushPolicy setting, and when cached files are closed (after any delay specified by the FlushAfterCloseDelay property).

If this property is disabled, automatic flushing will never occur, regardless of the FlushPolicy setting; however, the FileFlush method can still be used to explicitly flush a cached file.

Typically, applications should disable this property only in cases in which they need to suspend background write operations for some reason (e.g., if connectivity with the external storage is temporarily lost).

CacheDirectory Property (CBCache Component)

This property includes the directory in which the cache's storage file is located.

Syntax

property CacheDirectory: String read get_CacheDirectory write set_CacheDirectory;

Default Value

''

Remarks

This property specifies the path of the directory in which the cache's storage file is located. The name of the cache's storage file is derived from the CacheFile property.

Note: This property cannot be changed when Active is True.

CacheFile Property (CBCache Component)

This property includes the name of the file cache.

Syntax

property CacheFile: String read get_CacheFile write set_CacheFile;

Default Value

''

Remarks

This property specifies the name of the file cache. This name is used to create the cache's storage file in the directory specified by the CacheDirectory property.

Note: This property cannot be changed when Active is True.

CacheSizeLimit Property (CBCache Component)

This property includes the maximum size of the cache's storage file.

Syntax

property CacheSizeLimit: Integer read get_CacheSizeLimit write set_CacheSizeLimit;

Default Value

0

Remarks

This property specifies the maximum size, in megabytes, that the cache's storage file is allowed to grow to. If this limit is reached, the cache will initiate a background cleanup operation automatically, purging file data according to the PurgePolicy and shrinking the cache storage file afterward. By default, this property is set to 0, which means "no limit".

Note: Any purging and shrinking attempts caused by this property's value are made on a "best effort" basis, which can be affected by the following:

• The amount of unflushed data present in the cache (as indicated by the Stats property): if it is greater than the specified size limit, the cache will not be able to purge said data or shrink its storage file until enough data have been flushed.
• The PurgePolicy property: certain values can make it impossible for the cache to immediately perform the required purging.

Note: This property cannot be changed when Active is True.

Compressed Property (CBCache Component)

This property specifies whether or not the cache's storage file should be compressed.

Syntax

property Compressed: Boolean read get_Compressed write set_Compressed;

Default Value

false

Remarks

This property specifies whether or not the cache's storage file should be compressed.

If the StoreDataInFiles configuration setting is enabled, the Compressed property affects only the main cache storage file and not the files used to store individual files' data.

Note: This property cannot be changed when Active is True.

FlushAfterCloseDelay Property (CBCache Component)

This property includes the number of seconds to delay flushing after a file is closed.

Syntax

property FlushAfterCloseDelay: Integer read get_FlushAfterCloseDelay write set_FlushAfterCloseDelay;

Default Value

30

Remarks

This property specifies the number of seconds that flushing will be delayed after a file with unflushed blocks is closed. A value of 0 will cause such files to be flushed immediately upon close.

Flush-after-close delays are intended to prevent excessive flushing operations in cases in which a file is opened, written to, and closed many times in succession within the delay period.

Note: This property cannot be changed when Active is True.

FlushAfterSize Property (CBCache Component)

This property includes the amount of data that must be changed for flushing to begin.

Syntax

property FlushAfterSize: Integer read get_FlushAfterSize write set_FlushAfterSize;

Default Value

1

Remarks

When the FlushPolicy property is set to fpSize, fpSizeOrTime, or fpSizeAndTime, this property specifies how many megabytes of a file's data must be changed for flushing to begin.

Changed data thresholds are intended to prevent excessive flushing in cases in which data are written to the same range of a file many times in succession.

Note: This property cannot be changed when Active is True.

FlushAfterTime Property (CBCache Component)

This property includes the inactivity timeout that must elapse for flushing to begin.

Syntax

property FlushAfterTime: Integer read get_FlushAfterTime write set_FlushAfterTime;

Default Value

30

Remarks

When the FlushPolicy property is set to fpTime, fpSizeOrTime, or fpSizeAndTime, this property specifies how many seconds of inactivity (i.e., time with no writes) must elapse for a file before flushing can begin.

Inactivity delays are intended to prevent excessive flushing in cases in which data are written to a file many times in succession within the inactivity period.

Note: This property cannot be changed when Active is True.

FlushPolicy Property (CBCache Component)

This property includes the file flushing strategy that the cache should use.

Syntax

property FlushPolicy: TcbcCacheFlushPolicies read get_FlushPolicy write set_FlushPolicy;

TcbcCacheFlushPolicies = (
  fpNone,
  fpSize,
  fpTime,
  fpSizeOrTime,
  fpSizeAndTime
);

Default Value

fpTime

Remarks

This property specifies the strategy that the cache should use for flushing-changed file data of the opened file out to external storage. It must be set to one of the following values:

Note: This property cannot be changed when Active is True.

HasOrphans Property (CBCache Component)

This property notes whether or not the cache contains any orphan files.

Syntax

property HasOrphans: Boolean read get_HasOrphans;

Default Value

false

Remarks

This property specifies whether or not the cache contains any Orphan Files, returning True if so or False otherwise.

Note: This property returns a valid value only when Active is True.

This property is read-only and not available at design time.

MaxCachedFileSize Property (CBCache Component)

This property includes the maximum amount of data that should be cached for any given file.

Syntax

property MaxCachedFileSize: Int64 read get_MaxCachedFileSize write set_MaxCachedFileSize;

Default Value

0

Remarks

This property specifies the maximum amount of data, in bytes, that the cache should store for any given file. When a file reaches this limit, the cache will start dropping its least-recently accessed unchanged blocks.

The default value of 0 means "no limit", causing all file data to be kept in the cache at least until the file is closed (and possibly longer, depending on how the cache is configured).

Mode Property (CBCache Component)

This property includes the mode in which the cache operates in regard to access to the external storage.

Syntax

property Mode: TcbcCacheModes read get_Mode write set_Mode;

TcbcCacheModes = (
  cmNormal,
  cmShadow,
  cmPassthrough,
  cmMirror
);

Default Value

cmNormal

Remarks

This property specifies the mode of operations that the cache should use for reading and writing the data. Possible values are as follows:

Note: This property cannot be changed when Active is True.

Offline Property (CBCache Component)

This property specifies whether or not the cache works in Offline mode.

Syntax

property Offline: Boolean read get_Offline write set_Offline;

Default Value

false

Remarks

This property specifies whether or not the cache works in Offline mode. When in this mode, no events are fired to read or write data from/to the external storage.

This property is different from AutoFlush: AutoFlush prevents automated flushing but allows reading and manual flushing, whereas the Offline mode prevents both reading and writing, which means that application-initiated flushing does nothing as well.

PurgeAfterCloseDelay Property (CBCache Component)

This property includes the number of seconds to delay purging for after a file is closed.

Syntax

property PurgeAfterCloseDelay: Integer read get_PurgeAfterCloseDelay write set_PurgeAfterCloseDelay;

Default Value

30

Remarks

When the PurgePolicy property is set to ppFileClose, this property specifies the number of seconds that purging will be delayed for after a file without any unflushed blocks is closed. A value of 0 will cause such files to be purged immediately on close.

Purge-after-close delays are intended to prevent excessive fetching of data from external storage in cases in which a file is opened and closed many times in succession within the delay period.

Note: This property cannot be changed when Active is True.

PurgeAfterTime Property (CBCache Component)

This property includes the inactivity timeout that must elapse for purging to begin.

Syntax

property PurgeAfterTime: Integer read get_PurgeAfterTime write set_PurgeAfterTime;

Default Value

2592000

Remarks

When the PurgePolicy property is set to ppTime, this property specifies the number of seconds that a file data block must remain unchanged for before it is purged from the cache. Because the cache is intended for longer-time data storage and all data survive cache restart, the default value is 30 days.

Note: This property cannot be changed when Active is True.

PurgePolicy Property (CBCache Component)

This property includes the file block purging strategy that the cache should use.

Syntax

property PurgePolicy: TcbcCachePurgePolicies read get_PurgePolicy write set_PurgePolicy;

TcbcCachePurgePolicies = (
  ppNone,
  ppFileClose,
  ppCacheClose,
  ppTime
);

Default Value

ppTime

Remarks

This property specifies the strategy that the cache should use for purging unmodified blocks of file data from the cache. It must be set to one of the following values:

Note: This property cannot be changed when Active is True.

ReadBlockSize Property (CBCache Component)

This property includes the block size to use when reading file data from external storage.

Syntax

property ReadBlockSize: Integer read get_ReadBlockSize write set_ReadBlockSize;

Default Value

1048576

Remarks

This property specifies the block size, in bytes, that the component should use when reading data from external storage through the ReadData event.

Note: In some cases (e.g., when reading the last part of a file), the amount of requested data may be less than the value specified by this property.

Applications should always set this property to a reasonable value (or use the default value), even if the ReadingCapabilities property specifies that the external storage supports only whole-file reading.

This property may be overridden by the corresponding parameter of the FileOpen and FileSetSize methods.

Changing this property causes the BlockSize configuration setting's value to be recalculated.

Note: This property cannot be changed when Active is True.

ReadingCapabilities Property (CBCache Component)

This property includes the reading capabilities supported by the external storage.

Syntax

property ReadingCapabilities: Integer read get_ReadingCapabilities write set_ReadingCapabilities;

Default Value

1048

Remarks

This property specifies any and all behaviors of the external storage that are relevant when reading data from it. The component will automatically alter its behavior based on this property's "description" of how the external storage is able to provide file data.

This property should be set by ORing together zero or more of the flags defined in the following. These flags are grouped by name to specify the operations to which they apply.

Reading and Writing Capability Flags (Position Related)

Reading and Writing Capability Flags (Size Related)

Reading and Writing Capability Flags (Write Related)

Note: This property cannot be changed when Active is True.

ResizingCapabilities Property (CBCache Component)

This property includes the file resizing capabilities supported by the external storage.

Syntax

property ResizingCapabilities: Integer read get_ResizingCapabilities write set_ResizingCapabilities;

Default Value

51

Remarks

This property specifies any and all behaviors of the external storage that are relevant when resizing files. The component will automatically alter its behavior based on this property's "description" of how the external storage is able to resize files.

This property should be set by ORing together zero or more of the following flags:

Note: This property cannot be changed when Active is True.

Serialization Property (CBCache Component)

This property specifies whether nonintersecting operations against the same file or cache should execute serially or in parallel.

Syntax

property Serialization: TcbcSerializationModes read get_Serialization write set_Serialization;

TcbcSerializationModes = (
  smNone,
  smGlobal,
  smPerFileAllOperations,
  smPerFileReadingWritingConcurrent,
  smPerFileReadingConcurrent
);

Default Value

smPerFileReadingWritingConcurrent

Remarks

This property specifies whether the component should force nonintersecting operations against a single file or whole cache to execute serially, or allow them to execute in parallel.

Note: This property cannot be changed when Active is True.

Stats Property (CBCache Component)

This property includes cache statistics.

Syntax

property Stats: TcbcCBCacheStats read get_Stats;

Remarks

This property holds various statistics that are related to the cache.

This property is read-only.

Please refer to the CBCacheStats type for a complete list of fields.

Tag Property (CBCache Component)

This property stores application-defined data specific to a particular instance of the component.

Syntax

property Tag: Int64 read get_Tag write set_Tag;

Default Value

0

Remarks

This property can be used to store data specific to a particular instance of the component.

WriteBlockSize Property (CBCache Component)

This property includes the block size to use when writing file data to external storage.

Syntax

property WriteBlockSize: Integer read get_WriteBlockSize write set_WriteBlockSize;

Default Value

1048576

Remarks

This property specifies the block size, in bytes, that the component should use when writing data to external storage via the WriteData event.

Note: In some cases (e.g., when writing the last part of a file), the amount of data being written may be less than the value specified by this property.

Applications should always set this property to a reasonable value (or use the default value), even if the WritingCapabilities property specifies that the external storage only supports whole-file writing.

The property may be overridden by the corresponding parameter of the FileOpen and FileSetSize methods.

Changing this property causes the BlockSize configuration setting's value to be recalculated.

Note: This property cannot be changed when Active is True.

WritingCapabilities Property (CBCache Component)

This property includes the writing capabilities supported by the external storage.

Syntax

property WritingCapabilities: Integer read get_WritingCapabilities write set_WritingCapabilities;

Default Value

9256

Remarks

This property specifies any and all behaviors of the external storage that are relevant when writing data to it. The component will automatically alter its behavior based on this property's "description" of how the external storage is able to write file data.

This property should be set by ORing together zero or more of the flags defined as follows. The flags are grouped by name to specify the operations to which they apply.

Reading and Writing Capability Flags (Position Related)

Reading and Writing Capability Flags (Size Related)

Reading and Writing Capability Flags (Write Related)

Note: This property cannot be changed when Active is True.

CancelCleanup Method (CBCache Component)

This method cancels a background cleanup operation, if necessary.

Syntax

procedure CancelCleanup();

Remarks

This method cancels any cleanup operation currently being performed in the background. If no such operation is running, this method does nothing.

CheckAndRepair Method (CBCache Component)

This method checks a cache vault's consistency and repairs it as necessary.

Syntax

procedure CheckAndRepair(EncryptionPassword: String; Flags: Integer);

Remarks

This method checks the consistency of a cache vault and attempts to repair it as necessary. Please refer to the Cache Corruption topic for more information.

Applications should call this method if a vault has become corrupted (if a cache operation fails with a ERROR_INTERNAL_VAULT_CORRUPTED error). Be sure to make a vault backup before calling this method, as its repair efforts may cause data loss in cases of severe corruption.

The Flags parameter is used to specify additional options and should be set by OR-ing together zero or more of the following flags:

Note: This method cannot be called when Active is True, and it cannot be called within events.

Clear Method (CBCache Component)

This method removes files from the cache.

Syntax

procedure Clear(RemoveOrphans: Boolean; RemoveLocals: Boolean);

Remarks

This method removes all files from the cache that are fully flushed and currently closed. Depending on the value passed for RemoveOrphans and RemoveLocals, Orphan Files and/or Local Files also may be removed as well.

The operation is performed synchronously, so this method returns once all applicable files have been removed. During the operation, the Progress event will fire periodically to indicate progress.

CloseCache Method (CBCache Component)

This method closes the cache.

Syntax

procedure CloseCache(FlushAction: Integer; PurgeAction: Integer);

Remarks

This method closes the cache, optionally performing file flushing and purging operations before doing so.

The FlushAction parameter specifies what kind of flushing to perform, if any. It must be one of the following values:

The PurgeAction parameter specifies what kind of purging to perform, if any. It must be one of the following values:

CloseEnumeration Method (CBCache Component)

This method closes the given file enumeration.

Syntax

procedure CloseEnumeration(EnumerationHandle: Int64);

Remarks

This method closes the file enumeration identified by EnumerationHandle and releases any associated resources.

Config Method (CBCache Component)

Sets or retrieves a configuration setting.

Syntax

function Config(ConfigurationString: String): String;

Remarks

Config is a generic method available in every component. It is used to set and retrieve configuration settings for the component.

These settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the component, access to these internal properties is provided through the Config method.

To set a configuration setting named PROPERTY, you must call Config("PROPERTY=VALUE"), where VALUE is the value of the setting expressed as a string. For boolean values, use the strings "True", "False", "0", "1", "Yes", or "No" (case does not matter).

To read (query) the value of a configuration setting, you must call Config("PROPERTY"). The value will be returned as a string.

DeleteCache Method (CBCache Component)

This method deletes the cache completely.

Syntax

procedure DeleteCache();

Remarks

This method deletes the cache specified by the CacheFile property completely, removing its storage file from the directory specified by the CacheDirectory property. If the cache is currently open, it is closed first.

EnumerateCachedFiles Method (CBCache Component)

This method enumerates the files in the cache.

Syntax

function EnumerateCachedFiles(EnumerationMode: Integer): Int64;

Remarks

This method initiates a enumeration operation for cached files that match the specified EnunmerationMode, which must be take of the following values:

This method returns a handle that can be passed to the GetNextEnumeratedFile method to retrieve the results of the enumeration. If no matching files are found, -1 is returned instead.

When an application is finished enumerating files, it must call the CloseEnumeration method to release the associated resources.

FileChangeId Method (CBCache Component)

This method changes the Id of a cached file.

Syntax

procedure FileChangeId(FileId: String; NewFileId: String);

Remarks

This method changes the Id of the cached file identified by FileId to NewFileId. Applications that use file paths as Ids may need to use this method if a path element is renamed.

This method fails with an error if no file with the specified FileId is present in the cache, or if another file is already present in the cache with NewFileId.

FileClose Method (CBCache Component)

This method closes a cached file.

Syntax

procedure FileClose(FileId: String);

Remarks

This method closes the cached file identified by FileId, performing any of the flushing and/or purging operations specified by the component's other properties. (To override the configured flushing and purging behaviors, use the FileCloseEx method instead.)

This method returns immediately, and any flushing and/or purging operations are performed in the background.

FileCloseEx Method (CBCache Component)

This method closes a cached file, specifying flushing and purging behaviors explicitly.

Syntax

procedure FileCloseEx(FileId: String; FlushAction: Integer; PurgeAction: Integer);

Remarks

This method closes the cached file identified by FileId, performing any flushing and/or purging operations specified by the FlushAction and PurgeAction parameters.

The FlushAction parameter must be passed by one of the following values:

Note: Passing FLUSH_NONE will cause all background reading and writing threads for the file (if any) to be stopped.

The PurgeAction parameter must be passed by one of the following values:

This method returns immediately, and any flushing and/or purging operations are performed in the background.

FileDelete Method (CBCache Component)

This method deletes a file from the cache.

Syntax

procedure FileDelete(FileId: String);

Remarks

This method deletes the cached file identified by FileId, removing all of its data from the cache. If the specified file is currently open, it is closed first.

Note: The specified file is not flushed to external storage before being deleted; any unflushed data will be lost.

FileExists Method (CBCache Component)

This method checks if a file with the given Id is present in the cache.

Syntax

function FileExists(FileId: String): Boolean;

Remarks

This method checks to see whether a file with the specified FileId is present in the cache; returning True if so or False otherwise.

FileFlush Method (CBCache Component)

This method flushes the specified file's modified blocks out to external storage.

Syntax

function FileFlush(FileId: String; FlushMode: Integer; Delay: Integer; FileContext: Pointer): Integer;

Remarks

This method flushes any modified data blocks for the file specified by FileId out to external storage via the WriteData event (and possibly, the ReadData event, if the cache is forced to retrieve data from external storage before writing it back). The FlushMode parameter specifies what kind of flushing should be performed. It must take one of the following values:

The Delay parameter may be passed a non-zero number of seconds to delay the flushing operation.

The FileContext parameter may be passed any application-defined value. If the flushing operation causes the file to be opened, the specified value will be passed through to the WriteData and ReadData events' corresponding parameter. (If the file is already open, the value passed to FileOpen takes precedence).

This method will return one of the following values:

FileGetPinned Method (CBCache Component)

This method returns the pinned state of a cached file.

Syntax

function FileGetPinned(FileId: String): Boolean;

Remarks

This method indicates whether the cached file identified by FileId is pinned (or reports an error if no such file is present in the cache).

Pinned files are the files whose data are not purged automatically. If you need to remove the local data of a pinned file, use the FilePurge method.

FileGetSize Method (CBCache Component)

This method gets the "real size" of a cached file.

Syntax

function FileGetSize(FileId: String): Int64;

Remarks

This method returns the application-reported File Sizes for the cached file identified by FileId (or reports an error if no such file is present in the cache).

Note: The size value returned by this method is the one previously set by the application through the FileOpen or FileSetSize method; it does not necessarily correspond to the actual amount of data present in the cache for the file.

FileGetStatus Method (CBCache Component)

This method gets the status of a cached file.

Syntax

function FileGetStatus(FileId: String; StatusKind: Integer): Boolean;

Remarks

This method returns the status of the cached file identified by FileId (or reports an error if no such file is present in the cache). StatusKind must be one of the following:

FileOpen Method (CBCache Component)

This method opens the specified cached file, creating it if necessary.

Syntax

procedure FileOpen(FileId: String; RealFileSize: Int64; PrefetchSize: Int64; FileContext: Pointer);

Remarks

This method opens the cached file identified by FileId, creating it if necessary.

The RealFileSize parameter informs the cache of the File Sizes (in bytes) of the real file (i.e., the one in external storage); this size can be changed later using the FileSetSize method. For existing cached files, -1 may be passed to instruct the cache to use the previously set real file size instead.

The PrefetchSize parameter specifies how many bytes of data the cache should prefetch, from external storage, for the file via the ReadData event (which will fire on a background thread). Pass PREFETCH_ALL to prefetch the entire fire, or PREFETCH_NOTHING to prefetch none of it.

The FileContext may be passed to any application-defined value; this value will then be passed through to any ReadData and WriteData events fired for the file.

FileOpenEx Method (CBCache Component)

This method opens the specified cached file, creating it if necessary.

Syntax

procedure FileOpenEx(FileId: String; RealFileSize: Int64; IsLocal: Boolean; ReadBlockSize: Integer; WriteBlockSize: Integer; PrefetchSize: Int64; FileContext: Pointer);

Remarks

This method opens the cached file identified by FileId, creating it if necessary.

The RealFileSize parameter informs the cache of the File Sizes (in bytes) of the real file (i.e., the one in external storage); this size can be changed later using the FileSetSize method. For existing cached files, -1 may be passed to instruct the cache to use the previously set real file size instead.

The IsLocal parameter specifies whether the file is a local to the cache and does not exist in external storage; refer to the Local Files topic for more information.

ReadBlockSize and WriteBlockSize parameters make it possible for the application to configure the size of the block for reading and writing, respectively, thus overriding the values specified in the ReadBlockSize and WriteBlockSize properties for a specific file. Set the parameter to 0 to make the component use the value from the corresponding property. The values of the parameters will be used when ReadingCapabilities or WritingCapabilities properties indicate that reading or writing should be performed in blocks of a specific size (or when the whole file should be read or written using sequential block operations).

The PrefetchSize parameter specifies how many bytes of data the cache should prefetch, from external storage, for the file through the ReadData event (which will fire on a background thread). Pass PREFETCH_ALL to prefetch the entire fire, or PREFETCH_NOTHING to prefetch none of it. This parameter is ignored if IsLocal is True.

The FileContext may be passed to any application-defined value; this value will then be passed through to any ReadData and WriteData events fired for the file.

FilePurge Method (CBCache Component)

This method purges unmodified data from the cache.

Syntax

procedure FilePurge(FileId: String);

Remarks

This method removes unmodified data from the cache.

Use FilePurge with pinned files to free space or after the pinned attribute has been removed and you need to remove the data immediately.

FileRead Method (CBCache Component)

This method reads the specified part of a cached file.

Syntax

function FileRead(FileId: String; Position: Int64; Buffer: TBytes; Index: Integer; Count: Integer): Boolean;

Remarks

This method reads data from the cached file identified by FileId into the specified Buffer. The cached file must be opened using FileOpen before it can be read from.

Count bytes of data will be read from the cached file, starting from the specified Position, into the given Buffer. The data are written to Buffer starting at the given Index.

The method returns False if any errors happened during reading and returns True if everything went fine.

FileSetPinned Method (CBCache Component)

This method sets the pinned state of a cached file.

Syntax

procedure FileSetPinned(FileId: String; Pinned: Boolean);

Remarks

This method specifies whether the cached file identified by FileId should be treated as pinned.

Pinned files are the files whose data are not purged automatically. If you need to remove the local data of a pinned file, use the FilePurge method.

FileSetSize Method (CBCache Component)

This method sets the "real size" of a cached file.

Syntax

procedure FileSetSize(FileId: String; NewSize: Int64; FetchMissingData: Boolean);

Remarks

This method changes the "real size" of the cached file identified by FileId to the number of bytes specified by NewSize. The cached file must be opened using FileOpen before its real size can be changed.

ReadBlockSize and WriteBlockSize parameters make it possible for the application to configure the size of the block for reading and writing, respectively, thus overriding the values specified in the ReadBlockSize and WriteBlockSize properties for a specific file. Set the parameter to 0 to make the component use the value from the corresponding property. The values of the parameters will be used when ReadingCapabilities or WritingCapabilities properties indicate that reading or writing should be performed in blocks of a specific size (or when the whole file should be read or written using sequential block operations).

The FetchMissingData parameter controls how the cache initializes new file space in cases in which the file's real size is increased. If FetchMissingData is True, the new file space is filled with real data fetched from external storage. Otherwise, the new file space is marked as zero-filled (although no actual data writing occurs).

FileSetSizeEx Method (CBCache Component)

This method sets the "real size" of a cached file.

Syntax

procedure FileSetSizeEx(FileId: String; NewSize: Int64; ReadBlockSize: Integer; WriteBlockSize: Integer; FetchMissingData: Boolean);

Remarks

This method changes the "real size" of the cached file identified by FileId to the number of bytes specified by NewSize. The cached file must be opened using FileOpen before its real size can be changed.

ReadBlockSize and WriteBlockSize parameters make it possible for the application to configure the size of the block for reading and writing, respectively, thus overriding the values specified in the ReadBlockSize and WriteBlockSize properties for a specific file. Set the parameter to 0 to make the component use the value from the corresponding property. The values of the parameters will be used when ReadingCapabilities or WritingCapabilities properties indicate that reading or writing should be performed in blocks of a specific size (or when the whole file should be read or written using sequential block operations).

The FetchMissingData parameter controls how the cache initializes new file space in cases in which the file's real size is increased. If FetchMissingData is True, the new file space is filled with real data fetched from external storage. Otherwise, the new file space is marked as zero-filled (although no actual data writing occurs).

FileTagDelete Method (CBCache Component)

This method deletes a file tag.

Syntax

procedure FileTagDelete(FileId: String; TagId: Integer);

Remarks

This method deletes the file tag identified by TagId from the cached file specified by FileId.

Please refer to the File Tags topic for more information.

Note: This method can be called only when Active is True, and it cannot be called within events.

FileTagExists Method (CBCache Component)

This method checks whether or not a file has a specific tag associated with it.

Syntax

function FileTagExists(FileId: String; TagId: Integer): Boolean;

Remarks

This method returns True if a file identified by FileId has an associated tag identified by TagId; otherwise, it returns False.

The value passed for TagId must be in the range from 0x0001 to 0xCFFF (inclusive).

Please refer to the File Tags topic for more information.

Note: This method can be called only when Active is True, and it cannot be called within events.

FileTagGet Method (CBCache Component)

This method retrieves the binary data held by a file tag attached to the specified cached file.

Syntax

function FileTagGet(FileId: String; TagId: Integer): TBytes;

Remarks

This method retrieves the binary data stored in a file tag, identified by TagId attached to the file identified by FileId. If a file tag with the specified TagId is not attached to the specified file, this method raises an exception.

The value passed for TagId must be in the range from 0x0001 to 0xCFFF (inclusive).

Please refer to the File Tags topic for more information.

Note: This method can be called only when Active is True, and it cannot be called within events.

FileTagGetSize Method (CBCache Component)

This method retrieves the size of a file tag attached to the specified cached file.

Syntax

function FileTagGetSize(FileId: String; TagId: Integer): Integer;

Remarks

This method retrieves the size of the binary data stored in a file tag, identified by TagId, attached to the cached file identified by FileId. If a file tag with the specified TagId is not attached to the specified file, this method returns 0 as the tag size.

The value passed for TagId must be in the range from 0x0001 to 0xCFFF (inclusive).

Please refer to the File Tags topic for more information.

Note: This method can be called only when Active is True.

FileTagSet Method (CBCache Component)

This method attaches a file tag with binary data to the specified cached file.

Syntax

procedure FileTagSet(FileId: String; TagId: Integer; Data: TBytes);

Remarks

This method attaches a file tag with binary data to the cached file identified by FileId using the specified TagId. If a file tag with the specified TagId is already attached to the specified file, it is replaced.

The value passed for TagId must be in the range from 0x0001 to 0xCFFF (inclusive).

The Data parameter specifies the raw binary data to store in the file tag; it may be up to 65531 bytes in length.

Please refer to the File Tags topic for more information.

Note: This method can be called only when Active is True, and it cannot be called within events.

FileTouch Method (CBCache Component)

This method touches a range of data in a cached file.

Syntax

procedure FileTouch(FileId: String; Position: Int64; Count: Integer; Flush: Boolean);

Remarks

This method touches a range of data in the cached file identified by FileId, thereby marking it as changed and in need of flushing. The range of data to be touched starts at the specified Position and extends for Count bytes.

The cache will automatically attempt to read in any unavailable data in the touched range via ReadData, and then will write out all data in the touched range via WriteData. If the Flush parameter is True, these operations will take place synchronously and block return of this method; otherwise, they will take place in the background and this method will return immediately.

FileWrite Method (CBCache Component)

This method writes the specified part of a cached file.

Syntax

function FileWrite(FileId: String; Position: Int64; Buffer: TBytes; Index: Integer; Count: Integer): Boolean;

Remarks

This method writes data to the cached file identified by FileId from the specified Buffer. The cached file must be opened using FileOpen before it can be written to.

Count bytes of data will be read from the given Buffer, starting at the given Index, and written into the cached file. The data are written to the cached file starting at the given Position.

If any amount of data is written beyond the known file size, such writing causes the file to grow unless the RWCAP_WRITE_NOT_BEYOND_EOF flag is specified. When the file grows, the known file size value for the file is updated and all the data are eventually flushed (written) to the external storage via the WriteData event. The RWCAP_WRITE_NOT_BEYOND_EOF flag indicates that no writing should occur beyond the known file size, thus preventing the file from growth.

The method returns False if any errors happened during writing and returns True if everything went fine.

GetNextEnumeratedFile Method (CBCache Component)

This method returns the next file in the list of enumeration results.

Syntax

function GetNextEnumeratedFile(EnumerationHandle: Int64): String;

Remarks

This method returns the Id of the next file in the list of enumeration results identified by the EnumerationHandle parameter. If the list does not have any more files, an empty string is returned.

When an empty string is returned because the end of the list has been reached, the component will close the enumeration automatically by calling CloseEnumeration. If the application finishes enumeration without reaching the end of the list, it must call the CloseEnumeration method to release any associated resources.

OpenCache Method (CBCache Component)

This method opens the cache.

Syntax

procedure OpenCache(EncryptionPassword: String; CaseSensitive: Boolean);

Remarks

This method opens the cache specified by the CacheFile property; creating a storage file for it in the directory specified by the CacheDirectory property, if necessary.

To encrypt the cache, pass a nonempty string for the EncryptionPassword parameter the first time it is created. The same value will need to be passed any time the cache is opened again in the future.

If the StoreDataInFiles configuration setting is enabled, encryption affects only the main cache storage file and not the files that are used to store individual files' data.

The CaseSensitive parameter specifies whether the cache should treat its File Ids as case-sensitive or case-insensitive.

RefreshStatistics Method (CBCache Component)

This method forces a refresh of the cache's statistics.

Syntax

procedure RefreshStatistics();

Remarks

This method forces the cache to immediately refresh its statistics. The refreshed statistics will be reported through the Status event and will be used to update the Stats property.

This method needs to be called only if the applications needs to update the cache's statistics sooner than the end of the current StatsRefreshInterval.

ResetErrorState Method (CBCache Component)

This method resets any outstanding errors for a file or external storage.

Syntax

procedure ResetErrorState(FileId: String);

Remarks

If FileId is specified, this method resets any outstanding file errors, related to the cached file identified by FileId, previously reported via ReadData and/or WriteData events, thus allowing operations on the file to continue. If there are no outstanding errors for the specified file, this method does nothing.

If FileId is empty, this method resets any outstanding external-storage-related error previously reported via a ReadData or WriteData event, thus allowing external I/O operations to continue. If there are no outstanding external storage errors, this method does nothing.

StartCleanup Method (CBCache Component)

This method starts a background cleanup operation to remove unused files from the cache.

Syntax

procedure StartCleanup(MaxAge: Integer);

Remarks

This method starts a cleanup operation to remove unused files from the cache. Files used earlier than MaxAge hours ago will be removed (passing 0 will cause all files to be removed).

The cleanup operation is performed asynchronously in a background thread, so this method returns immediately. To cancel a cleanup operation, use the CancelCleanup method.

BeforeFlush Event (CBCache Component)

This event fires to report the start of the asynchronous file flush procedure.

Syntax

type TBeforeFlushEvent = procedure (
  Sender: TObject;
  const FileId: String;
  FileContext: Pointer;
  LastUseTime: TDateTime;
  var PostponeTime: Integer
) of Object;

property OnBeforeFlush: TBeforeFlushEvent read FOnBeforeFlush write FOnBeforeFlush;

Remarks

This event fires when the asynchronous file flush procedure starts for the cached file identified by FileId. This event does not fire if the flush procedure is initiated by the application using FileFlush, FileClose, or FileCloseEx methods.

Note: This event will fire in the context of a dedicated worker thread that performs cache maintenance. Keep handling of this event as quick as possible.

Use the PostponeTime parameter to specify time in seconds to postpone automatic flush operation for this file.

BeforePurge Event (CBCache Component)

This event fires to report the start of the asynchronous file purge procedure.

Syntax

type TBeforePurgeEvent = procedure (
  Sender: TObject;
  const FileId: String;
  FileContext: Pointer;
  LastUseTime: TDateTime;
  var PostponeTime: Integer
) of Object;

property OnBeforePurge: TBeforePurgeEvent read FOnBeforePurge write FOnBeforePurge;

Remarks

This event fires when the asynchronous file purge procedure starts for the cached file identified by FileId. This event does not fire if the purge procedure is initiated by the application using the FileFlush, FileClose, or FileCloseEx methods.

Note: This event will fire in the context of a dedicated worker thread that performs cache maintenance. Keep handling of this event as quick as possible.

Use the PostponeTime parameter to specify time in seconds to postpone automatic purge operation for this file.

EndCleanup Event (CBCache Component)

This event fires to report the beginning of the asynchronous cleanup procedure.

Syntax

type TEndCleanupEvent = procedure (
  Sender: TObject
) of Object;

property OnEndCleanup: TEndCleanupEvent read FOnEndCleanup write FOnEndCleanup;

Remarks

This event fires when the asynchronous cleanup procedure ends.

Note: This event will fire in the context of a dedicated worker thread that performs cache maintenance. Keep handling of this event as quick as possible.

Error Event (CBCache Component)

This event fires if an unhandled error occurs during an event.

Syntax

type TErrorEvent = procedure (
  Sender: TObject;
  ErrorCode: Integer;
  const Description: String
) of Object;

property OnError: TErrorEvent read FOnError write FOnError;

Remarks

This event fires if an unhandled error occurs during another event. Developers can use this information to track down unhandled errors in an application's event handlers.

IMPORTANT: Not everything is possible or allowed in the event handlers. Please see the Event Handling topic for details.

Log Event (CBCache Component)

This log reports events that occur in the component.

Syntax

type TLogEvent = procedure (
  Sender: TObject;
  Level: Integer;
  const Message: String
) of Object;

property OnLog: TLogEvent read FOnLog write FOnLog;

Remarks

This event fires when the component needs to notify an application or developers about something that happened internally. The verbosity is controlled by the LogLevel setting.

LogLevel indicates the level of the message. Possible values are as follows:

Message is the log entry.

Note: This event will fire in the context of a dedicated worker thread. Keep handling of this event as quick as possible.

Progress Event (CBCache Component)

This event fires to report the progress of an operation.

Syntax

type TProgressEvent = procedure (
  Sender: TObject;
  Operation: Integer;
  Current: Int64;
  Total: Int64;
  var Interrupt: Boolean
) of Object;

property OnProgress: TProgressEvent read FOnProgress write FOnProgress;

Remarks

This event fires when the cache wants to report the progress of the operation specified by Operation. Possible values are as follows:

The Current and Total parameters specify the number of the item being processed, and the total number of items being processed, respectively. For some operations, these parameters may be 0 or -1.

The Interrupt parameter can be set to True to stop the operation.

ReadData Event (CBCache Component)

This event fires when the cache needs to read file data from external storage.

Syntax

type TReadDataEvent = procedure (
  Sender: TObject;
  const FileId: String;
  Size: Int64;
  Position: Int64;
  Flags: Integer;
  Buffer: Pointer;
  BytesToRead: Integer;
  var BytesRead: Integer;
  FileContext: Pointer;
  var ResultCode: Integer
) of Object;

property OnReadData: TReadDataEvent read FOnReadData write FOnReadData;

Remarks

This event fires when the cache needs to read data for the cached file identified by FileId from an external storage.

The Size parameter reflects the specified file's "real size" as it is currently known by the cache.

The Position parameter reflects the byte offset in the real file (i.e., the one in the external storage) from which the handler should start reading data.

The Flags parameter provides supplementary information about the nature of the request. This parameter, along with the Size and Position parameters, helps the event handler to determine, which part of the file Buffer represents and whether additional ReadData events should be expected. It will include a combination of one or more of the following flags:

To handle this event property, a handler must read BytesToRead bytes of data from the real file in external storage, starting at the specified Position, into Buffer.

Please see the Buffer Parameters topic for more information on how to work with memory buffer event parameters.

Note: If Flags contains RWEVENT_CANCELED, the transfer is being canceled and no data should be read; the Buffer is not provided and both Position and BytesToRead parameters are set to 0.

When reading is complete, a handler must set BytesRead to reflect the actual number of bytes read from the real file. A handler must not attempt to copy more than BytesToRead bytes of data into Buffer.

The FileContext parameter reflects the application-defined value passed when FileOpen was called.

The ResultCode parameter will always be 0 when the event is fired. If the event cannot be handled in a "successful" manner for some reason (e.g., a resource is not available, security checks failed), set it to one of the following values to report an appropriate error:

Please refer to the Error Handling topic for more information.

StartCleanup Event (CBCache Component)

This event fires to report the beginning of the asynchronous cleanup procedure.

Syntax

type TStartCleanupEvent = procedure (
  Sender: TObject
) of Object;

property OnStartCleanup: TStartCleanupEvent read FOnStartCleanup write FOnStartCleanup;

Remarks

This event fires when the asynchronous cleanup procedure starts.

Note: This event will fire in the context of a dedicated worker thread that performs cache maintenance. Keep handling of this event as quick as possible.

Status Event (CBCache Component)

This event fires to report the latest cache statistics.

Syntax

type TStatusEvent = procedure (
  Sender: TObject;
  CacheSize: Int64;
  TotalData: Int64;
  UnflushedData: Int64;
  UnflushedFiles: Integer;
  CurrentOperation: Integer;
  const CurrentFileId: String
) of Object;

property OnStatus: TStatusEvent read FOnStatus write FOnStatus;

Remarks

This event fires once every StatsRefreshInterval to provide the application with the latest cache statistics, as well as information about any in-progress cache operation.

The CacheSize parameter reflects the current size of the cache, in bytes.

The TotalData parameter reflects the total amount of data present in the cache, in bytes.

The UnflushedData parameter reflects the amount of unflushed data present in the cache, in bytes.

The UnflushedFiles parameter reflects the number of unflushed files present in the cache.

The CurrentOperation parameter reflects the cache's current operations; it will take one of the following values:

When CurrentOperation is CACHE_OP_READ or CACHE_OP_WRITE, the CurrentFileId parameter will reflect the Id of the cached file being processed.

Note: This event can fire in the context of any thread.

WriteData Event (CBCache Component)

This event fires when the cache needs to write file data to external storage.

Syntax

type TWriteDataEvent = procedure (
  Sender: TObject;
  const FileId: String;
  Size: Int64;
  Position: Int64;
  Flags: Integer;
  Buffer: Pointer;
  BytesToWrite: Integer;
  var BytesWritten: Integer;
  FileContext: Pointer;
  var ResultCode: Integer
) of Object;

property OnWriteData: TWriteDataEvent read FOnWriteData write FOnWriteData;

Remarks

This event fires when the cache needs to write data for the cached file identified by FileId to external storage.

The Size parameter reflects the specified file's "real size" as it is currently known by the cache.

The Position parameter reflects the byte offset in the real file (i.e., the one in external storage) at which the application should start writing data.

The Flags parameter provides supplementary information about the nature of the request. This parameter, along with the Size and Position parameters, helps the application to determine which part of the file Buffer represents, and whether additional WriteData events should be expected. It will include a combination of one or more of the following flags:

To handle this event properly, applications must write BytesToWrite bytes of data from Buffer to the real file in external storage, starting at the specified Position.

Please see the Buffer Parameters topic for more information on how to work with memory buffer event parameters.

Note: If Flags contains RWEVENT_CANCELED, the transfer is being canceled and no data should be written; the Buffer is not provided and both Position and BytesToWrite parameters are set to 0.

When writing is complete, applications must set BytesWritten to reflect the actual number of bytes written to the real file. Applications must not attempt to copy more than BytesToWrite bytes of data from Buffer.

The FileContext parameter reflects the application-defined value passed when FileOpen was called.

The ResultCode parameter will always be 0 when the event is fired. If the event cannot be handled in a "successful" manner for some reason (e.g., a resource is not available, security checks failed), set it to one of the following values to report an appropriate error:

Please refer to the Error Handling topic for more information.

CBCacheStats Type

This type holds cache statistics.

Remarks

This type is used to hold various cache statistics.

Fields

Config Settings (CBCache Component)

The component accepts one or more of the following configuration settings. Configuration settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the component, access to these internal properties is provided through the Config method.

CBCache Config Settings

Base Config Settings

Trappable Errors (CBCache Component)

Cache errors Errors