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Adaptive Radix Tree (ART) in OCaml

This is an implementation in OCaml of ART. Adaptive Radix Tree is like a simple Hashtbl with order:

# let tree = Art.make () ;;
# Art.insert tree (Art.key "foo") 42 ;;
# Art.insert tree (Art.key "bar") 21 ;;
# Art.find tree (Art.key "foo")
- : int = 42

Operation like minimum or maximum are available (which don't exist for a simple Hashtbl.t):

# let tree = Art.make () ;;
# Art.insert tree (Art.key "0") 0 ;;
# Art.insert tree (Art.key "1") 1 ;;
# Art.insert tree (Art.key "2") 2 ;;
# Art.minimum tree
- : int = 0
# Art.maximum tree
- : int = 2

If you want the order and the speed of Hashtbl.t, Art is your library:

The function prefix_iter is also available if you want to get a subset of your tree:

# let t = Art.make () ;;
# Art.insert t (Art.key
# Art.insert t (Art.key "Dalton Joe") 0 ;;
# Art.insert t (Art.key "Dalton Jack") 1 ;;
# Art.insert t (Art.key "Dalton William") 2 ;;
# Art.insert t (Art.key "Dalton Averell") 3 ;;
# Art.insert t (Art.key "Rantanplan") 4 ;;
# let dalton = Art.prefix_iter ~prefix:(Art.key "Dalton")
  (fun k _ a -> (k :> string) :: a) [] t ;;
- : string list = [ "Dalton Joe"
                  ; "Dalton Jack"
		  ; "Dalton William"
		  ; "Dalton Averell" ]

Read Optimised Write Exclusion (ROWEX) in OCaml

ROWEX is a second implementation of ART with atomic operations. It's a functor which expects an implementation of atomic operations such as load or store.

Parallelism, atomic operation & OCaml

The current version of OCaml has a global lock for the GC. By this way, it's not possible for us to execute ROWEX operations (find/insert) with true parallelism if we use the same OCaml runtime. Even if you use LWT or ASYNC, you execute jobs concurrently.

However, ROWEX wants to provide an implementation where find/insert can be executed in parallel without any problems (race condition or ABA problem). So ROWEX provides an implementation, persistent, which implements atomic operations on a memory area. Then, we are able, as parmap, to simulate true parallelism as long as each operations are executed into their own fork().

The goal of this library is provide:

  • the most easy way to switch the implementation to ocaml-multicore
  • a baby step to be able to manipulate a file by several processes (consumers/find, producers/insert) in parallel

ROWEX follows two main papers:

  • The initial implementation of ROWEX
  • A derivation of it to be persistent: PART

Tools

The distribution comes with some tools to manipulate an index:

$ opam pin add -y https://github.com/dinosaure/art
$ opam install rowex
$ part.make index.idx
$ ls -lh
-rw-r--r-- 1 user user 8,0M ----- -- --:-- index.idx
prw------- 1 user user    0 ----- -- --:-- index.idx.socket
prw------- 1 user user    0 ----- -- --:-- index.idx-truncate.socket
$ part.insert index.idx foo 1
$ part.find index.idx foo
1

On the OCaml side, a Part module exists which implements these functions:

type 'a t constraint 'a = [< `Rd | `Wr ]

val create : ?len:int -> string -> unit
val insert : [> `Rd | `Wr ] t -> string -> int -> unit
val lookup : [> `Rd ] t -> string -> int

part is Unix dependent (and it need an Unix named pipe). It ensures with explained internal mechanisms to use multiple readers and one writer:

  • The writer can take the exclusive ownership on the index file and its named pipe
  • readers don't need to take the ownership but they must send a signal into the named pipe (to the writer) that they start to introspect the index

For readers, some functions exist to signal their existence to the write:

val append_reader : Ipc.t -> unit
val delete_reader : Ipc.t -> unit

val ipc : _ t -> Ipc.t

Status: experimental

This part of the distribution is experimental - even if the distribution comes with several tests to ensure that the implementation works, ROWEX is fragile! It still need a synchronization mechanism fsync() which is added pervasively in some parts of the code according to outcomes of errors.