Squid Web Cache wiki

Squid Web Cache documentation

πŸ”— Storage Manager

πŸ”— Introduction

The Storage Manager is the glue between client and server sides. Every object saved in the cache is allocated a StoreEntry structure. While the object is being accessed, it also has a MemObject structure.

Squid can quickly locate cached objects because it keeps (in memory) a hash table of all StoreEntry. The keys for the hash table are MD5 checksums of the objects URI. In addition there is also a storage policy such as LRU that keeps track of the objects and determines the removal order when space needs to be reclaimed. For the LRU policy this is implemented as a doubly linked list.

For each object the StoreEntry maps to a cache_dir and location via sdirn and sfilen. For the β€œufs” store this file number (sfilen) is converted to a disk pathname by a simple modulo of L2 and L1, but other storage drivers may map sfilen in other ways. A cache swap file consists of two parts: the cache metadata, and the object data. Note the object data includes the full HTTP replyβ€”headers and body. The HTTP reply headers are not the same as the cache metadata.

Client-side requests register themselves with a StoreEntry to be notified when new data arrives. Multiple clients may receive data via a single StoreEntry. For POST and PUT request, this process works in reverse. Server-side functions are notified when additional data is read from the client.

πŸ”— Object storage

To be written…

πŸ”— Object retrieval

To be written…

πŸ”— Storage Interface

πŸ”— Introduction

Traditionally, Squid has always used the Unix filesystem (UFS) to store cache objects on disk. Over the years, the poor performance of UFS has become very obvious. In most cases, UFS limits Squid to about 30-50 requests per second. Our work indicates that the poor performance is mostly due to the synchronous nature of open() and unlink() system calls, and perhaps thrashing of inode/buffer caches.

We want to try out our own, customized filesystems with Squid. In order to do that, we need a well-defined interface for the bits of Squid that access the permanent storage devices. We also require tighter control of the replacement policy by each storage module, rather than a single global replacement policy.

πŸ”— Build structure

The storage types live in squid/src/fs/ . Each subdirectory corresponds to the name of the storage type. When a new storage type is implemented configure.in must be updated to autogenerate a Makefile in squid/src/fs/$type/ from a Makefile.in file.

configure will take a list of storage types through the –enable-store-io parameter. This parameter takes a list of space seperated storage types. For example, –enable-store-io=”ufs coss” .

Each storage type must create an archive file in squid/src/fs/$type.a . This file is automatically linked into squid at compile time.

Each storefs must export a function named storeFsSetup_$type(). This function is called at runtime to initialise each storage type. The list of storage types is passed through store_modules.sh to generate the initialisation function storeFsSetup(). This function lives in store_modules.c.

An example of the automatically generated file:

/* automatically generated by ./store_modules.sh ufs coss
 * do not edit
#include "squid.h"

extern STSETUP storeFsSetup_ufs;
extern STSETUP storeFsSetup_coss;
void storeFsSetup(void)
                storeFsAdd("ufs", storeFsSetup_ufs);
                storeFsAdd("coss", storeFsSetup_coss);

πŸ”— Initialization of a storage type

Each storage type initializes through the storeFsSetup_$type() function. The storeFsSetup_$type() function takes a single argument - a storefs_entry_t pointer. This pointer references the storefs_entry to initialise. A typical setup function is as follows:

storeFsSetup_ufs(storefs_entry_t *storefs)
        storefs->parsefunc = storeUfsDirParse;
        storefs->reconfigurefunc = storeUfsDirReconfigure;
        storefs->donefunc = storeUfsDirDone;
        ufs_state_pool = memPoolCreate("UFS IO State data", sizeof(ufsstate_t));
        ufs_initialised = 1;

There are five function pointers in the storefs_entry which require initializing. In this example, some protection is made against the setup function being called twice, and a memory pool is initialised for use inside the storage module.

Each function will be covered below.

πŸ”— done

typedef void

This function is called whenever the storage system is to be shut down. It should take care of deallocating any resources currently allocated.

typedef void
STFSPARSE(SwapDir *SD, int index, char *path);

typedef void
STFSRECONFIGURE(SwapDir *SD, int index, char *path);

These functions handle configuring and reconfiguring a storage directory. Additional arguments from the cache_dir configuration line can be retrieved through calls to strtok() and GetInteger().

STFSPARSE has the task of initialising a new swapdir. It should parse the remaining arguments on the cache_dir line, initialise the relevant function pointers and data structures, and choose the replacement policy. STFSRECONFIGURE deals with reconfiguring an active swapdir. It should parse the remaining arguments on the cache_dir line and change any active configuration parameters. The actual storage initialisation is done through the STINIT function pointer in the SwapDir.

struct _SwapDir {
        char *type;                             /* Pointer to the store dir type string */
        int cur_size;                           /* Current swapsize in kb */
        int low_size;                           /* ?? */
        int max_size;                           /* Maximum swapsize in kb */
        char *path;                             /* Path to store */
        int index;                              /* This entry's index into the swapDir array */
        int suggest;                            /* Suggestion for UFS style stores (??) */
        size_t max_objsize;                     /* Maximum object size for this store */
        union {                                 /* Replacement policy-specific fields */
        #ifdef HEAP_REPLACEMENT
                struct {
                        heap *heap;
                } heap;
                struct {
                        dlink_list list;
                        dlink_node *walker;
                } lru;
        } repl;
        int removals;
        int scanned;
        struct {
                unsigned int selected:1;        /* Currently selected for write */
                unsigned int read_only:1;       /* This store is read only */
        } flags;
        STINIT *init;                           /* Initialise the fs */
        STNEWFS *newfs;                         /* Create a new fs */
        STDUMP *dump;                           /* Dump fs config snippet */
        STFREE *freefs;                         /* Free the fs data */
        STDBLCHECK *dblcheck;                   /* Double check the obj integrity */
        STSTATFS *statfs;                       /* Dump fs statistics */
        STMAINTAINFS *maintainfs;               /* Replacement maintainence */
        STCHECKOBJ *checkob;                    /* Check if the fs will store an object, and get the FS load */
        /* These two are notifications */
        STREFOBJ *refobj;                       /* Reference this object */
        STUNREFOBJ *unrefobj;                   /* Unreference this object */
        STCALLBACK *callback;                   /* Handle pending callbacks */
        STSYNC *sync;                           /* Sync the directory */
        struct {
                STOBJCREATE *create;            /* Create a new object */
                STOBJOPEN *open;                /* Open an existing object */
                STOBJCLOSE *close;              /* Close an open object */
                STOBJREAD *read;                /* Read from an open object */
                STOBJWRITE *write;              /* Write to a created object */
                STOBJUNLINK *unlink;            /* Remove the given object */
        } obj;
        struct {
                STLOGOPEN *open;                /* Open the log */
                STLOGCLOSE *close;              /* Close the log */
                STLOGWRITE *write;              /* Write to the log */
                struct {
                        STLOGCLEANOPEN *open;   /* Open a clean log */
                        STLOGCLEANWRITE *write; /* Write to the log */
                        void *state;            /* Current state */
                } clean;
        } log;
        void *fsdata;                           /* FS-specific data */

πŸ”— Operation of a storage module

Squid understands the concept of multiple diverse storage directories. Each storage directory provides a caching object store, with object storage, retrieval, indexing and replacement.

Each open object has associated with it a storeIOState object. The storeIOState object is used to record the state of the current object. Each storeIOState can have a storage module specific data structure containing information private to the storage module.

struct _storeIOState {
        sdirno swap_dirn;               /* SwapDir index */
        sfileno swap_filen;             /* Unique file index number */
        StoreEntry *e;                  /* Pointer to parent StoreEntry */
        mode_t mode;                    /* Mode - O_RDONLY or O_WRONLY */
        size_t st_size;                 /* Size of the object if known */
        off_t offset;                   /* current _on-disk_ offset pointer */
        STFNCB *file_callback;          /* called on delayed sfileno assignments */
        STIOCB *callback;               /* IO Error handler callback */
        void *callback_data;            /* IO Error handler callback data */
        struct {
                STRCB *callback;        /* Read completion callback */
                void *callback_data;    /* Read complation callback data */
        } read;
        struct {
                unsigned int closing:1; /* debugging aid */
        } flags;
        void *fsstate;                  /* pointer to private fs state */

Each SwapDir has the concept of a maximum object size. This is used as a basic hint to the storage layer in first choosing a suitable SwapDir. The checkobj function is then called for suitable candidate SwapDirs to find out whether it wants to store a given StoreEntry. A maxobjsize of -1 means β€˜any size’.

The specific filesystem operations listed in the SwapDir object are covered below.

πŸ”— initfs

typedef void STINIT(SwapDir *SD);

Initialise the given SwapDir. Operations such as verifying and rebuilding the storage and creating any needed bitmaps are done here.

πŸ”— newfs

typedef void

Called for each configured SwapDir to perform filesystem initialisation. This happens when β€˜-z’ is given to squid on the command line.

πŸ”— dumpfs

typedef void
STDUMP(StoreEntry *e, SwapDir *SD);

Dump the FS specific configuration data of the current SwapDir to the given StoreEntry. Used to grab a configuration file dump from th cachemgr interface.

Note: The printed options should start with a space character to separate them from the cache_dir path.

πŸ”— freefs

typedef void
STFREE(SwapDir *SD);

Free the SwapDir filesystem information. This routine should deallocate SD->fsdata.

πŸ”— doublecheckfs

typedef int
STDBLCHECK(SwapDir *SD, StoreEntry *e);

Double-check the given object for validity. Called during rebuild if the β€˜-S’ flag is given to squid on the command line. Returns 1 if the object is indeed valid, and 0 if the object is found invalid.

πŸ”— statfs

typedef void
STSTATFS(SwapDir *SD, StoreEntry *e);

Called to retrieve filesystem statistics, such as usage, load and errors. The information should be appended to the passed StoreEntry e.

πŸ”— maintainfs

typedef void

Called periodically to replace objects. The active replacement policy should be used to timeout unused objects in order to make room for new objects.

πŸ”— callback

typedef void

This function is called inside the comm_select/comm_poll loop to handle any callbacks pending.

πŸ”— sync

typedef void
STSYNC(SwapDir *SD);

This function is called whenever a sync to disk is required. This function should not return until all pending data has been flushed to disk.

πŸ”— parse/reconfigure

πŸ”— checkobj

typedef int
STCHECKOBJ(SwapDir *SD, const StoreEntry *e);

Called by storeDirSelectSwapDir() to determine whether the SwapDir will store the given StoreEntry object. If the SwapDir is not willing to store the given StoreEntry -1 should be returned. Otherwise, a value between 0 and 1000 should be returned indicating the current IO load. A value of 1000 indicates the SwapDir has an IO load of 100%. This is used by storeDirSelectSwapDir() to choose the SwapDir with the lowest IO load.

πŸ”— referenceobj

typedef void
STREFOBJ(SwapDir *SD, StoreEntry *e);

Called whenever an object is locked by storeLockObject(). It is typically used to update the objects position in the replacement policy.

πŸ”— unreferenceobj

typedef void
STUNREFOBJ(SwapDir *SD, StoreEntry *e);

Called whenever the object is unlocked by storeUnlockObject() and the lock count reaches 0. It is also typically used to update the objects position in the replacement policy.

πŸ”— createobj

typedef storeIOState *
STOBJCREATE(SwapDir *SD, StoreEntry *e, STFNCB *file_callback, STIOCB *io_callback, void *io_callback_data);

Create an object in the SwapDir *SD. file_callback is called whenever the filesystem allocates or reallocates the swap_filen. Note

The IO callback should be called when an error occurs and when the object is closed. Once the IO callback is called, the storeIOState becomes invalid.

STOBJCREATE returns a storeIOState suitable for writing on sucess, or NULL if an error occurs.

πŸ”— openobj

typedef storeIOState *
STOBJOPEN(SwapDir *SD, StoreEntry *e, STFNCB *file_callback, STIOCB *io_callback, void *io_callback_data);

Open the StoreEntry in SwapDir *SD for reading. Much the same is applicable from STOBJCREATE, the major difference being that the data passed to file_callback is the relevant store_client .

πŸ”— closeobj

typedef void
STOBJCLOSE(SwapDir *SD, storeIOState *sio);

Close an opened object. The STIOCB callback should be called at the end of this routine.

πŸ”— readobj

typedef void
STOBJREAD(SwapDir *SD, storeIOState *sio, char *buf, size_t size, off_t offset, STRCB *read_callback, void *read_callback_data);

Read part of the object of into buf. It is safe to request a read when there are other pending reads or writes. STRCB is called at completion.

If a read operation fails, the filesystem layer notifies the calling module by calling the STIOCB callback with an error status code.

πŸ”— writeobj

typedef void
STOBJWRITE(SwapDir *SD, storeIOState *sio, char *buf, size_t size, off_t offset, FREE *freefunc);

Write the given block of data to the given store object. buf is allocated by the caller. When the write is complete, the data is freed through free_func.

If a write operation fails, the filesystem layer notifies the calling module by calling the STIOCB callback with an error status code.

πŸ”— unlinkobj

typedef void
STOBJUNLINK(SwapDir *, StoreEntry *);

Remove the StoreEntry e from the SwapDir SD and the replacement policy.

πŸ”— Store IO calls

These routines are used inside the storage manager to create and retrieve objects from a storage directory.

πŸ”— storeCreate()

storeIOState *
storeCreate(StoreEntry *e, STIOCB *file_callback, STIOCB *close_callback, void * callback_data)

storeCreate is called to store the given StoreEntry in a storage directory.

callback is a function that will be called either when an error is encountered, or when the object is closed (by calling storeClose()). If the open request is successful, there is no callback. The calling module must assume the open request will succeed, and may begin reading or writing immediately.

storeCreate() may return NULL if the requested object can not be created. In this case the callback function will not be called.

πŸ”— storeOpen()

storeIOState *
storeOpen(StoreEntry *e, STFNCB * file_callback, STIOCB * callback, void *callback_data)

storeOpen is called to open the given StoreEntry from the storage directory it resides on.

callback is a function that will be called either when an error is encountered, or when the object is closed (by calling storeClose()). If the open request is successful, there is no callback. The calling module must assume the open request will succeed, and may begin reading or writing immediately.

storeOpen() may return NULL if the requested object can not be openeed. In this case the callback function will not be called.

πŸ”— storeRead()

storeRead(storeIOState *sio, char *buf, size_t size, off_t offset, STRCB *callback, void *callback_data)

storeRead() is more complicated than the other functions because it requires its own callback function to notify the caller when the requested data has actually been read. buf must be a valid memory buffer of at least size bytes. offset specifies the byte offset where the read should begin. Note that with the Swap Meta Headers prepended to each cache object, this offset does not equal the offset into the actual object data.

The caller is responsible for allocating and freeing buf .

πŸ”— storeWrite()

storeWrite(storeIOState *sio, char *buf, size_t size, off_t offset, FREE *free_func)

storeWrite() submits a request to write a block of data to the disk store. The caller is responsible for allocating buf, but since there is no per-write callback, this memory must be freed by the lower filesystem implementation. Therefore, the caller must specify the free_func to be used to deallocate the memory.

If a write operation fails, the filesystem layer notifies the calling module by calling the STIOCB callback with an error status code.

storeUnlink(StoreEntry *e)

storeUnlink() removes the cached object from the disk store. There is no callback function, and the object does not need to be opened first. The filesystem layer will remove the object if it exists on the disk.

πŸ”— storeOffset()

storeOffset(storeIOState *sio)

storeOffset() returns the current ondisk offset. This is used to determine how much of an objects memory can be freed to make way for other in-transit and cached objects. You must make sure that the storeIOState->offset refers to the ondisk offset, or undefined results will occur. For reads, this returns the current offset of successfully read data, not including queued reads.

πŸ”— Callbacks

πŸ”— STIOCB callback

stiocb(void *data, int errorflag, storeIOState *sio)

The stiocb function is passed as a parameter to storeOpen(). The filesystem layer calls stiocb either when an I/O error occurs, or when the disk object is closed.

errorflag is one of the following:

#define DISK_OK                   (0)
#define DISK_ERROR               (-1)
#define DISK_EOF                 (-2)
#define DISK_NO_SPACE_LEFT       (-6)

Once the The stiocb function has been called, the sio structure should not be accessed further.

πŸ”— STRCB callback

strcb(void *data, const char *buf, size_t len)

The strcb function is passed as a parameter to storeRead(). The filesystem layer calls strcb after a block of data has been read from the disk and placed into buf. len indicates how many bytes were placed into buf. The strcb function is only called if the read operation is successful. If it fails, then the STIOCB callback will be called instead.

πŸ”— State Logging

These functions deal with state logging and related tasks for a squid storage system. These functions are used (called) in store_dir.c.

Each storage system must provide the functions described in this section, although it may be a no-op (null) function that does nothing. Each function is accessed through a function pointer stored in the SwapDir structure:

struct _SwapDir {
    STINIT *init;
    STNEWFS *newfs;
    struct {
        STLOGOPEN *open;
        STLOGCLOSE *close;
        STLOGWRITE *write;
        struct {
            STLOGCLEANOPEN *open;
            STLOGCLEANWRITE *write;
            void *state;
        } clean;
    } log;

πŸ”— log.open()


The log.open function, of type STLOGOPEN, is used to open or initialize the state-holding log files (if any) for the storage system. For UFS this opens the swap.state files.

The log.open function may be called any number of times during Squid’s execution. For example, the process of rotating, or writing clean logfiles closes the state log and then re-opens them. A squid -k reconfigure does the same.

πŸ”— log.close()


The log.close function, of type STLOGCLOSE, is obviously the counterpart to log.open. It must close the open state-holding log files (if any) for the storage system.

πŸ”— log.write()

STLOGWRITE(const SwapDir *, const StoreEntry *, int op);

The log.write function, of type STLOGWRITE, is used to write an entry to the state-holding log file. The op argument is either SWAP_LOG_ADD or SWAP_LOG_DEL. This feature may not be required by some storage systems and can be implemented as a null-function (no-op).

πŸ”— log.clean.start()


The log.clean.start function, of type STLOGCLEANSTART, is used for the process of writing β€œclean” state-holding log files. The clean-writing procedure is initiated by the squid -k rotate command. This is a special case because we want to optimize the process as much as possible. This might be a no-op for some storage systems that don’t have the same logging issues as UFS.

The log.clean.state pointer may be used to keep state information for the clean-writing process, but should not be accessed by upper layers.

πŸ”— log.clean.nextentry()

StoreEntry *

Gets the next entry that is a candidate for the clean log.

Returns NULL when there is no more objects to log

πŸ”— log.clean.write()

STLOGCLEANWRITE(SwapDir *, const StoreEntry *);

The log.clean.write function, of type STLOGCLEANWRITE, writes an entry to the clean log file (if any).

πŸ”— log.clean.done()


Indicates the end of the clean-writing process and signals the storage system to close the clean log, and rename or move them to become the official state-holding log ready to be opened.

πŸ”— Replacement policy implementation

The replacement policy can be updated during STOBJREAD/STOBJWRITE/STOBJOPEN/ STOBJCLOSE as well as STREFOBJ and STUNREFOBJ. Care should be taken to only modify the relevant replacement policy entries in the StoreEntry. The responsibility of replacement policy maintainence has been moved into each SwapDir so that the storage code can have tight control of the replacement policy. Cyclic filesystems such as COSS require this tight coupling between the storage layer and the replacement policy.

πŸ”— Removal policy API

The removal policy is responsible for determining in which order objects are deleted when Squid needs to reclaim space for new objects. Such a policy is used by a object storage for maintaining the stored objects and determining what to remove to reclaim space for new objects. (together they implements a replacement policy)

πŸ”— API

It is implemented as a modular API where a storage directory or memory creates a policy of choice for maintaining it’s objects, and modules registering to be used by this API.

πŸ”— createRemovalPolicy()

RemovalPolicy policy = createRemovalPolicy(cons char *type, cons char *args)

Creates a removal policy instance where object priority can be maintained

The returned RemovalPolicy instance is cbdata registered

πŸ”— policy.Free()

policy->Free(RemovalPolicy *policy)

Destroys the policy instance and frees all related memory.

πŸ”— policy.Add()

policy->Add(RemovalPolicy *policy, StoreEntry *, RemovalPolicyNode *node)

Adds a StoreEntry to the policy instance.

datap is a pointer to where policy specific data can be stored for the store entry, currently the size of one (void *) pointer.

πŸ”— policy.Remove()

policy->Remove(RemovalPolicy *policy, StoreEntry *, RemovalPolicyNode *node)

Removes a StoreEntry from the policy instance out of policy order. For example when an object is replaced by a newer one or is manually purged from the store.

datap is a pointer to where policy specific data is stored for the store entry, currently the size of one (void *) pointer.

πŸ”— policy.Referenced()

policy->Referenced(RemovalPolicy *policy, const StoreEntry *, RemovalPolicyNode *node)

Tells the policy that a StoreEntry is going to be referenced. Called whenever a entry gets locked.

node is a pointer to where policy specific data is stored for the store entry, currently the size of one (void *) pointer.

πŸ”— policy.Dereferenced()

policy->Dereferenced(RemovalPolicy *policy, const StoreEntry *, RemovalPolicyNode *node)

Tells the policy that a StoreEntry has been referenced. Called when an access to the entry has finished.

node is a pointer to where policy specific data is stored for the store entry, currently the size of one (void *) pointer.

πŸ”— policy.WalkInit()

RemovalPolicyWalker walker = policy->WalkInit(RemovalPolicy *policy)

Initiates a walk of all objects in the policy instance. The objects is returned in an order suitable for using as reinsertion order when rebuilding the policy.

The returned RemovalPolicyWalker instance is cbdata registered

Note: The walk must be performed as an atomic operation with no other policy actions intervening, or the outcome will be undefined.

πŸ”— walker.Next()

const StoreEntry *entry = walker->Next(RemovalPolicyWalker *walker)

Gets the next object in the walk chain

Return NULL when there is no further objects

πŸ”— walker.Done()

walker->Done(RemovalPolicyWalker *walker)

Finishes a walk of the maintained objects, destroys walker.

πŸ”— policy.PurgeInit()

RemovalPurgeWalker purgewalker = policy->PurgeInit(RemovalPolicy *policy, int max_scan)

Initiates a search for removal candidates. Search depth is indicated by max_scan.

The returned RemovalPurgeWalker instance is cbdata registered

Note: The walk must be performed as an atomic operation with no other policy actions intervening, or the outcome will be undefined.

πŸ”— purgewalker.Next()

StoreEntry *entry = purgewalker->Next(RemovalPurgeWalker *purgewalker)

Gets the next object to purge. The purgewalker will remove each returned object from the policy.

It is the polices responsibility to verify that the object isn’t locked or otherwise prevented from being removed. What this means is that the policy must not return objects where storeEntryLocked() is true.

Return NULL when there is no further purgeable objects in the policy.

πŸ”— purgewalker.Done()

purgewalker->Done(RemovalPurgeWalker *purgewalker)

Finishes a walk of the maintained objects, destroys walker and restores the policy to it’s normal state.

πŸ”— policy.Stats()

purgewalker->Stats(RemovalPurgeWalker *purgewalker, StoreEntry *entry)

Appends statistics about the policy to the given entry.

πŸ”— Source layout

Policy implementations resides in src/repl/<name>/, and a make in such a directory must result in a object archive src/repl/<name>.a containing all the objects implementing the policy.

πŸ”— Internal structures

πŸ”— RemovalPolicy

typedef struct _RemovalPolicy RemovalPolicy;
struct _RemovalPolicy {
    char *_type;
    void *_data;
    void (*add)(RemovalPolicy *policy, StoreEntry *);
    ... /* see the API definition above */

The _type member is mainly for debugging and diagnostics purposes, and should be a pointer to the name of the policy (same name as used for creation)

The _data member is for storing policy specific information.

πŸ”— RemovalPolicyWalker

typedef struct _RemovalPolicyWalker RemovalPolicyWalker;
struct _RemovalPolicyWalker {
    RemovalPolicy *_policy;
    void *_data;
    StoreEntry *(*next)(RemovalPolicyWalker *);
    ... /* see the API definition above */

πŸ”— RemovalPolicyNode

typedef struct _RemovalPolicyNode RemovalPolicyNode;
struct _RemovalPolicyNode {
    void *data;

Stores policy specific information about a entry. Currently there is only space for a single pointer, but plans are to maybe later provide more space here to allow simple policies to store all their data β€œinline” to preserve some memory.

πŸ”— Policy registration

Policies are automatically registered in the Squid binary from the policy selection made by the user building Squid. In the future this might get extended to support loadable modules. All registered policies are available to object stores which wishes to use them.

πŸ”— Policy instance creation

Each policy must implement a β€œcreate/new” function RemovalPolicy * createRemovalPolicy_<name>(char *arguments) This function creates the policy instance and populates it with at least the API methods supported. Currently all API calls are mandatory, but the policy implementation must make sure to NULL fill the structure prior to populating it in order to assure future API compability.

It should also populate the _data member with a pointer to policy specific data.

πŸ”— Walker

When a walker is created the policy populates it with at least the API methods supported. Currently all API calls are mandatory, but the policy implementation must make sure to NULL fill the structure prior to populating it in order to assure future API compatibility.

πŸ”— Design notes/bugs

The RemovalPolicyNode design is incomplete/insufficient. The intention was to abstract the location of the index pointers from the policy implementation to allow the policy to work on both on-disk and memory caches, but unfortunately the purge method for HEAP based policies needs to update this, and it is also preferable if the purge method in general knows how to clear the information. I think the agreement was that the current design of tightly coupling the two together on one StoreEntry is not the best design possible.

It is debated if the design in having the policy index control the clean index writes is the correct approach. Perhaps not. Perhaps a more appropriate design is probably to do the store indexing completely outside the policy implementation (i.e. using the hash index), and only ask the policy to dump it’s state somehow.

The Referenced/Dereferenced() calls is today mapped to lock/unlock which is an approximation of when they are intended to be called. However, the real intention is to have Referenced() called whenever an object is referenced, and Dereferenced() only called when the object has actually been used for anything good.

πŸ”— Forwarding Selection

To be written…

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