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Module Nn_commit

module Nn_commit: sig .. end
Two-phase commit protocol

val commit_e : (unit -> Nn_db.modification Queue.t) ->
(unit -> Nn_db.modification Queue.t) ->
(bool -> unit) -> Nn_state.shared_state_t -> string -> bool Uq_engines.engine
commit_e get_journal get_journal_mon post_commit shared_state debug_name:

The function calls get_journal() and get_journal_mon() to get the list of modifications. See below for the idea of get_journal_mon.

It writes the modifications to the local database and all slaves, and commits the update. The bool result says whether the commit was successful. If false, the transaction is guaranteed to be rolled back. The commit engine may also enter an error state. This is considered as a critical violation of the protocol.

If the queue of modification is empty, the local transaction is committed, and no remote operations are involved. (This means that the user only submitted read requests and no writes.)

If there are modifications, a new revision string is created and appended to the journal before anything is written.

The function invokes post_commit when the commit or rollback is done. The bool arg is the same as in the result.

Commits are partly serialized (using the commit monitor in shared_state). The function get_journal is called before the monitored section is entered. The function get_journal_mon is called twice: Once before the section starts, and once at the beginning of the section. This second call replaces the values coming from the first call. The problem is here that we need the modifications for determining the condition the commit monitor should wait for. However, some of the modifications may already have changed when the monitor allows that the execution flow continues. These changes can be reflected by get_journal_mon.

The monitored code section includes the calls of post_commit.

As side effects, the function may disable slaves, and even (self-)disable the coordinator.

val trans_e : Nn_state.shared_state_t ->
Nn_state.view_t list ->
'a Uq_engines.engine -> string -> 'a Uq_engines.final_state Uq_engines.engine
let e' = trans_e sh vl e debug_name: Runs the engine e until it is finished, and if e was successful, the modifications of the views vl are taken as the modifications to be done in the transaction, and the transaction is committed.

If e is not successful, the views and the db transaction are rolled back.

The returned engine e' is `Done(st) when the transaction is over (after commit or rollback). st reflects the final state of e.

e' reaches an error state only if the commit fails (critical error).

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