For very big serializable dictionaries with small keys but memory consuming values, one might think of a dictionary that holds its values in a direct accessible file on disk. Within RAM, the dictionary can map the keys to their values by means of intermediate file-pointers. Does such type of dictionary already exists? If not and some people are interested, I am very well willing and prepared to make such a dictionary. I am thinking of the following: Call the dictionary a "da" and its file a "da-file". Opening a da-file opens it for both input and output and reads the mutable top-da containing an index that maps keys onto file-positions. Closing a da-file stores the mutated top-da, then stores the pointer to the new top-da in a manner as atomic as possible. A da-file may contain sub-das. When a program/system crashes without closing a da-file, the logical content of the file will be consistent. In this case new or updated data may be physically present, but will not exists in logical sense. Opening the da-file after a crash shows the same dictionary as when last opened. When a program/system crashes while storing the pointer to the top-da, we may have a problem. However, it will be possible to reconstruct the most recent consistent da-file. The dictionary will contain history info such as to allow going back to a previous state of the da-file. This can be acheived by storing all new data at the end of the file and storing the pointer to the current top-da in the first block of the file. Each da will have a pointer to its predecessor. A cleanup-procedure will be present such as to prepare a new da-file whose physical content corresponds to the logical content according to its current top-da. Procedures: open-da-file -> da close-da-file -> da make-da -> da (for new sub-das) da-ref da-set! da-remove! da-revert! -> da (go back in history for top or sub-da) copy-da-file (cleaning up) da-repair (repairs da-file which is inconsistent because of a failure while storing the pointer to the top-da while closing the da-file) May be closing das and da-files can be made optional by means of a guard, but I do not yet fully understand how to do this. Jos
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