package irmin-pack

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package irmin-pack
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Abstraction that governs the lifetime of the various files that are part of a pack store (except the branch store).

The file manager (FM) handles the files one by one and makes explicit all the interactions between them (except the index which is handled from a high level API). This allows to gain confidence on SWMR consistency and crash consistency.

Two types of guarantees

Irmin_pack_unix supports the SWMR access scheme. It means that it is undefined for the files to be opened twice in RW mode by 2 FMs. It also means that it is undefined for a FM in RW mode to be used simultaneously from 2 different fibers.

Irmin_pack_unix aims to be (crash) consistent (in the ACID sense). In case of a system failure (e.g. power outage), the files should be left in a consistent state for later uses.

Ensuring SWMR consistency is easier than ensuring crash consistency because the of the OS's shared page cache.

Files mutations

Here are all the moments where the files managed may be mutated:

  • 1. During create_rw.
  • 2. During open_rw if a major version upgrade is necessary.
  • 3. During the flush routines in file_manager.
  • 4. During a GC, from the worker.
  • 5. At the end of a GC, from the RW fiber, in swap.
  • 6. During integrity check routines.

1. 2. and 6. don't support SWMR and leave the store in an undefined state in case of crash.

4. operates on files private to the worker. It doesn't necessitate to worry about crash concistency and SWMR.

3. and 5. are highly critical.

module Io : Io.S

Low level IO abstraction. A typical implementation is unix. This abstraction is meant to be dead simple. Not a lot of documentation is required.

module Control : Control_file.S with module Io = Io

Abstraction for irmin-pack's control file.

module Dict : Append_only_file.S with module Io = Io

Abstraction for irmin-pack's append only files (i.e. suffix and dict).

module Suffix : Append_only_file.S with module Io = Io

Abstraction for irmin-pack's append only files (i.e. suffix and dict).

module Index : sig ... end

An abstraction on top of the index library that exposes an API that better fits the irmin-pack use case.

module Errs : Io_errors.S with module Io = Io
module Mapping_file : Mapping_file.S with module Io = Io
type t

A series of getters to the underlying files managed.

val control : t -> Control.t
val dict : t -> Dict.t
val suffix : t -> Suffix.t
val index : t -> Index.t
val mapping : t -> Mapping_file.t option
val prefix : t -> Io.t option
type create_error := [
  1. | Io.create_error
  2. | Io.write_error
  3. | Io.open_error
  4. | Io.mkdir_error
  5. | `Corrupted_mapping_file of string
  6. | `Not_a_directory of string
  7. | `Index_failure of string
]
val create_rw : overwrite:bool -> Irmin.Backend.Conf.t -> (t, [> create_error ]) result

Create a rw instance of t by creating the files.

Note on SWMR consistency: It is undefined for a reader to attempt an opening before create_rw is over.

Note on crash consistency: Crashing during create_rw leaves the storage in an undefined state.

Note on errors: If create_rw returns an error, the storage is left in an undefined state and some file descriptors might not be closed.

type open_rw_error := [
  1. | `Corrupted_control_file
  2. | `Corrupted_mapping_file of string
  3. | `Double_close
  4. | `Closed
  5. | `File_exists of string
  6. | `Index_failure of string
  7. | `Invalid_argument
  8. | `Invalid_layout
  9. | `Io_misc of Control.Io.misc_error
  10. | `Migration_needed
  11. | `No_such_file_or_directory
  12. | `Not_a_directory of string
  13. | `Not_a_file
  14. | `Read_out_of_bounds
  15. | `Ro_not_allowed
  16. | `Sys_error of string
  17. | `V3_store_from_the_future
  18. | `Only_minimal_indexing_strategy_allowed
  19. | `Unknown_major_pack_version of string
  20. | `Index_failure of string
  21. | `Sys_error of string
  22. | `Inconsistent_store
]
val open_rw : Irmin.Backend.Conf.t -> (t, [> open_rw_error ]) result

Create a rw instance of t by opening existing files.

If the pack store has already been garbage collected, opening with a non-minimal indexing strategy will return an error.

If no_migrate = false in the config, the store will undergo a major version upgrade if necessary.

Note on SWMR consistency: It is undefined for a reader to attempt an opening during an open_rw, because of major version upgrades.

Note on crash consistency: If open_rw crashes during a major version upgrade, the storage is left in an undefined state. Otherwise the storage is unaffected.

Note on errors: If open_rw returns an error during a major version upgrade, the storage is left in an undefined state. Otherwise the storage is unaffected. Anyhow, some file descriptors might not be closed.

type open_ro_error := [
  1. | `Corrupted_control_file
  2. | `Corrupted_mapping_file of string
  3. | `Io_misc of Io.misc_error
  4. | `Migration_needed
  5. | `No_such_file_or_directory
  6. | `Not_a_file
  7. | `Closed
  8. | `V3_store_from_the_future
  9. | `Index_failure of string
  10. | `Unknown_major_pack_version of string
  11. | `Inconsistent_store
  12. | `Invalid_argument
  13. | `Read_out_of_bounds
  14. | `Invalid_layout
]
val open_ro : Irmin.Backend.Conf.t -> (t, [> open_ro_error ]) result

Create a ro instance of t by opening existing files.

Note on SWMR consistency: open_ro is supposed to work whichever the state of the pack store and the writer, with 2 exceptions: 1. the files must exist and create_rw must be over and 2. during a major version upgrade of the files (which occurs during a open_rw).

Note on crash consistency: Crashes during open_ro cause no issues because it doesn't mutate the storage.

Note on errors: The storage is never mutated. Some file descriptors might not be closed.

type close_error := [
  1. | `Double_close
  2. | `Index_failure of string
  3. | `Io_misc of Io.misc_error
  4. | `Pending_flush
  5. | `Ro_not_allowed
]
val close : t -> (unit, [> close_error ]) result

Close all the files.

This call fails if the append buffers are not in a flushed stated. This situation will most likely never occur because the append buffers will contain data only during the scope of a batch function.

type flush_error := [
  1. | `Index_failure of string
  2. | `Io_misc of Io.misc_error
  3. | `Ro_not_allowed
  4. | `Closed
]
type flush_stages := [
  1. | `After_dict
  2. | `After_suffix
]
type 'a hook := 'a -> unit
val flush : ?hook:flush_stages hook -> t -> (unit, [> flush_error ]) result

Execute the flush routine. Note that this routine may be automatically triggered when buffers are filled.

type reload_stages := [
  1. | `After_index
  2. | `After_control
  3. | `After_suffix
]
val reload : ?hook:reload_stages hook -> t -> (unit, [> Errs.t ]) result

Execute the reload routine.

Is a no-op if the control file did not change.

val register_dict_consumer : t -> after_reload:(unit -> (unit, Errs.t) result) -> unit
val register_suffix_consumer : t -> after_flush:(unit -> unit) -> unit
type version_error := [
  1. | `Corrupted_control_file
  2. | `Corrupted_legacy_file
  3. | `Invalid_layout
  4. | `Io_misc of Io.misc_error
  5. | `No_such_file_or_directory
  6. | `Not_a_directory of string
  7. | `Unknown_major_pack_version of string
]
val version : root:string -> (Irmin_pack.Version.t, [> version_error ]) result

version ~root is the version of the pack stores at root.

val swap : t -> generation:int -> right_start_offset:Optint.Int63.t -> right_end_offset:Optint.Int63.t -> (unit, [> Errs.t ]) result

Swaps to using files from the GC generation. The offsets right_start_offset and right_end_offset are used to properly load the suffix. The control file is also updated.

type write_gc_output_error := [
  1. | `Double_close
  2. | `File_exists of string
  3. | `Io_misc of Io.misc_error
  4. | `Ro_not_allowed
  5. | `Closed
]
val write_gc_output : root:string -> generation:int -> (Optint.Int63.t, Errs.t) result -> (unit, [> write_gc_output_error ]) result

Used by the gc process at the end to write its output in store.<generation>.out.

type read_gc_output_error = [
  1. | `Corrupted_gc_result_file of string
  2. | `Gc_process_error of string
]
val read_gc_output_error_t : read_gc_output_error Irmin.Type.t
val read_gc_output : root:string -> generation:int -> (Optint.Int63.t, [> read_gc_output_error ]) result

Used by the main process, after the gc process finished, to read store.<generation>.out.

val readonly : t -> bool
val generation : t -> int
val gc_allowed : t -> bool
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