Provided by: librinci-perl_1.1.93-1_all bug

NAME

       Rinci::Transaction - A transactional system based on functions

SPECIFICATION VERSION

        Rinci 1.1, protocol version 2

VERSION

       This document describes version 1.1.93 of Rinci::Transaction (from Perl distribution
       Rinci), released on 2019-07-19.

SPECIFICATION

       This document describes a transactional system based on functions, where several function
       calls participate in a single transaction. This transactional system has the following
       properties:

       ·   Client/server architecture

           Transaction can be performed over Riap. Client can start more than one active
           transaction on the server. Each transaction-management request and the function calls
           are requested separately (each one is a separate Riap request).

           For more details on this, see Riap::Transaction.

       ·   Undo/redo

           Committed transactions are still recorded in the database along with its undo
           information. Client can request to undo/redo the transactions. Thus the system is also
           an undo/redo system.

       ·   Relies on the functions for reliability/ACID properties

           Server or framework provides the transaction manager (TM), but each function acts as
           the resource manager (RM). It is the responsibility of the functions to maintain ACID
           properties while modifying resources. For best results, each function should be
           written carefully and tested extensively, and utilize a real, robust RM (like an RDBMS
           to store data or a transactional filesystem layer to read/modify files). In the
           absence of a real RM, some ACID properties like isolation and consistency might be
           compromised. For example: one transaction TX1 modifies a file in an ordinary (i.e.
           non-transactional) filesystem. Another transaction TX2 can see TX1's modification in
           the middle of uncommitted transaction (violates isolation principle).

   How transaction works
       The basic idea is that actions are performed by function calls. For each action, TM will
       call the function twice. First to get undo information, and second to actually perform the
       action. The undo information can be used to perform rollback, undo, and redo. All
       functions performing actions in the transaction must be able to supply undo information.

   Function requirements
       Functions that participate in transaction must declare their "tx" feature in the metadata.
       In addition, function must also be idempotent.

        features => {
            ...
            tx => {v=>2},
            idempotent => 1,
        }

       Function must then follow the transaction protocol, described below.

   Transaction manager
       The transaction manager manages transaction data and performs actions as well as
       transaction management.

       For the sake of examples, our TM stores data in a SQL database (like SQLite) with the
       following tables:

       ·   tx

            id (PK)
            summary
            ctime (creation time)
            commit_time
            status
            last_action_id -- in-progress action ID (for tx with status=i), or last
                           -- processed action (for tx with other transient statuses)

       ·   do_action

            id (PK)
            tx_id (refers to tx(id))
            ctime
            sp (savepoint name, UNIQUE(sp,tx_id))
            f (function name)
            args (arguments, serialized)

       ·   undo_action

            id (PK)
            tx_id (refers to tx(id))
            ctime
            f (function name)
            args (arguments, serialized)

   Transaction status
       A transaction can have one of these statuses. They will be fully explained in the
       following sections. Statuses having lowercase labels are transient statuses.  Statuses
       having uppercase labels are final statuses.

        i (in-progress)
        a (aborted, pending rollback to R)
        R (rolled back)
        C (committed)
        u (committed, undoing)
        v (aborted undoing, pending rollback back to C)
        U (committed, undone)
        d (committed & undone, redoing)
        e (aborted redoing, pending rollback back to U)
        X (unresolvable/error)

   Transaction manager initialization
       User instantiates TM. TM sets up its data directory and performs cleanup and crash
       recovery.

       In cleanup, TM purges unneeded data, like data for rolled back transactions or committed
       transactions that have been around for too long.

       In crash recovery, TM looks at all crashed transactions and resolves them (either by
       performing rollback or roll forward). Crashed transactions are in-progress ("i")
       transactions that have an in-progress action, or transactions having one of these statuses
       (all the other transient statuses): "a", "u", "v", "d", "e". Crash recovery will be
       explained in the following sections below.

       TM also can perform rollback for in-progress transactions that have been around for too
       long without being committed or rolled back.

   Starting transaction
       User invokes "$tm->begin(tx_id => $tx_id)", providing a unique transaction ID $tx_id as
       identifier for the transaction. $tx_id is an arbitrary string with a length between 1 and
       200 characters. User can also supply "summary", a textual description for the transaction.
       It should not be longer than 1024 characters. TM will create an entry for the transaction
       in its journal:

        BEGIN;
        INSERT INTO tx (id,summary,ctime,status) VALUES ($tx_id,$summary,$now,'i')
        COMMIT;

       As can be seen, initial transaction status is "i" (in-progress).

       Upon success, TM must return status 200. If transaction with that ID already exists, TM
       must return status 409, unless when the existing transaction is still on-going, in which
       case TM should just return 200. TM must return 400 if no $tx_id is given. TM can also
       return status 412 if there are already too many transactions being started, either
       globally on the server or for the particular client.

   Performing action
       1) User performs action by invoking "$tm->action(f=>$fname, args=>$args)" one or several
       times. Transaction status must be "i". TM will first check whether function exists and
       supports transaction. If function does not exist, or does not support transaction, TM must
       return status 412.

       2) TM records this action in its journal:

        BEGIN;
        INSERT INTO action (tx_id,ctime,f,args) VALUES
            ($tx_id,$now,$fname,JSON($args)); -- $act_id
        UPDATE tx SET last_action_id=$act_id WHERE id=$tx_id;
        COMMIT;

       3) TM requests state checking and undo information to function, by calling the function
       using the arguments $args and a special argument "-tx_action=>'check_state'". In addition
       TM also passes "-tx_v => N" (the protocol version) and "-tx_action_id => UUID" (a unique
       identifier to link between this call and the 'fix_state' call later).

       There are 3 possible states that the function must decide which we are in:

       ·   fixed

           This is the final, desired state. When we are already in a fixed state, function must
           return status 304 (nothing to do). TM will then skip calling the function the second
           time to fix state, since there is nothing to fix. For example:

            [304, "File $path already exists"]      # e.g., in a create_file() function
            [304, "User $u already does not exist"] # e.g., in a delete_user() function

       ·   fixable

           This is where the final, desired state has not been reached, but it is possible to
           reach it. When we are in this state, function must return status 200 with the result
           metadata "undo_actions". The message should also describe what needs to be fixed.

           For example:

            [200, "Directory $path needs to be created", undef,
             {undo_actions => [ [rmdir => {path=>$path}] ]}]  # e.g. in a mkdir() function
            [200, "User $u should be created with UID $uid", undef,
             {undo_actions => [ [delete_user=>{user=>$u}] ]}] # e.g. in create_user()

       ·   unfixable

           This is where the final, desired state has not been reached, and it is impossible or
           inappropriate for the function to fix into the fixed state. This state is used to
           avoid undoing what was not fixed by the function. If we are in this state, function
           should return status 412 (precondition failed).

           For example:

            [412, "Path $path exists but not a symlink"] # e.g. in setup_symlink()
            [412, "User $u exists but with different UID $cur_uid (needs $uid)"]

       If state is unfixable, or function returns other statuses (assumed as failure), TM stops
       the process and starts a rollback. "$tm->action()" will return with the function's result.

       For example, let us use function "My::setup_unix_user()" which can create a Unix user with
       an empty home directory if the user has not been created. This function utilizes several
       simpler functions: "My::adduser()" to add entry to /etc/passwd and /etc/shadow,
       "My::addgroup" to add entry to /etc/group and /etc/gshadow, "My::mkdir" to create
       directory. Then there are also these functions for the undo actions: "My::deluser" to
       delete user entry in Unix passwd database, "My::delgroup" to delete group entry in Unix
       group database, and "My::rmdir" to remove directory.

       For "My::adduser", the fixable state is that the user does not exist, the fixed state is
       that the user exists. For "My::deluser", the fixable state is that user exists
       (additionally with the same UID as the one created previously), the fixed state is user
       does not exist, the unfixable state is user exists but with different UID. For
       "My::addgroup", the fixable state is that group does not exist, the fixed state is that
       the group exists. For "My::delgroup", the fixable state is that group exists (additionally
       with the same GID as the one created previously), the fixed state is group does not exist,
       the unfixable state is group exists but with different GID. For "My::mkdir", the fixable
       state is path does not exist, the fixed state is directory exists, and unfixable state is
       path exists but is not a directory. For "My::rmdir", the fixable state is directory exists
       and empty, the fixed state is path does not exist, the unfixable state is path exists but
       not a directory or directory is not empty.

       The "undo_actions" must be an array containing action information, in reverse order. Each
       action is a two-element array "[$fname, $args]" where $fname is name of a function (not
       necessarily the same function) and $args its call arguments.

       For example, if user invokes "$tm->action(f=>'My::setup_unix_user', args=>{user=>'bob'})"
       and user "bob" does not exist yet, function will return:

        [200, "OK", undef,
         {undo_actions=>[
             ['My::deluser', {group=>'bob'}],
             ['My::delgroup', {group=>'bob'}],
             ['My::rmdir', {path=>'/home/bob'}],
         ]},
        ]

       4) TM records these undo actions in its journal:

        BEGIN;
        INSERT INTO undo_action (tx_id,ctime,action_id,f,args) VALUES
            ($tx_id,$now,$act_id,'My::deluser','{"group":"bob"}');    -- # $uact_id1
        INSERT INTO undo_action (tx_id,ctime,action_id,f,args) VALUES
            ($tx_id,$now,$act_id,'My::delgroup','{"user":"bob"}');    -- # $uact_id2
        INSERT INTO undo_action (tx_id,ctime,action_id,f,args) VALUES
            ($tx_id,$now,$act_id,'My::rmdir','{"path":"/home/bob"}'); -- # $uact_id3
        COMMIT;

       5) If we are in fixed state, this step is skipped.

       If we are in fixable state, TM calls function the second time, this time with "-tx_action
       => 'fix_state'". TM also passes "-tx_v" and "-tx_action_id" with the same value as the one
       passed previously during the 'check_state' call.  Function must perform action to fix the
       state into the fixed state. In our example, "setup_unix_user()" should create user and
       group "bob", and creates an empty directory "/home/bob".

       Function must return status 200 on success. Other status will be interpreted as failure,
       in which case TM will stop the process and starts rollback. "$tm->action()" will return
       with the function's result.

       Note: During the 'check_state' phase in step 3, function can also optionally return
       "do_actions" in its result metadata, for example:

        [200, "OK", undef,
         {do_actions=>[
             ['My::adduser', {group=>'bob'}],
             ['My::addgroup', {group=>'bob'}],
             ['My::mkdir', {path=>'/home/bob'}],
          ],
          undo_actions=>[
             ['My::deluser', {group=>'bob'}],
             ['My::delgroup', {group=>'bob'}],
             ['My::rmdir', {path=>'/home/bob'}],
          ]},
        ]

       In this case, instead of calling function the second time, TM will just call the actions
       provided by the function, using a nested "$tm->action(actions => $do_actions)". Step 4
       will be skipped since each do action will provide its own undo actions.

       6) If 'fix_state' phase in step 5 succeeds, the action is finished. TM marks this:

        BEGIN;
        UPDATE tx SET last_action_id=NULL WHERE id=$tx_id;
        COMMIT;

       TM is ready to process another action.

       Crash recovery

       Recovery rolls back interrupted in-progress transaction. See "Rollback of in-progress
       (status i) transaction" for more details.

       If crash happens after step 1, transaction will not be marked as crash since
       "last_action_id" has not been set and no recovery is necessary.

       If crash happens after step 2 until 5, recovery will be performed by rollback.  Details of
       rollback is explained in "Rollback of in-progress (status i) transaction".

       If crash happens after step 6, transaction will not be marked as crash since
       "last_action_id" is already unset and no recovery is necessary.

   Commit
       To commit transaction, user invokes "$tm->commit()". Transaction status must be "i" or
       "a". If transaction status is "a", transaction must be rolled back instead.

       TM will mark the transaction status as "C" (committed) and delete all entries in the
       "do_action" table since they are no longer needed:

        BEGIN;
        UPDATE tx SET status='C' WHERE id=$tx_id;
        DELETE FROM do_action WHERE tx_id=$tx_id;
        COMMIT;

       TM still stores the "undo_actions" entries for some time, to allow undo (and redo) of
       transactions.

       If transaction status is "a", transaction should be rolled back instead of committed.

       Transaction status progress:

        i -> C

   Rollback of in-progress (status i) transaction
       If an action fails, or some other error happens, rollback will be performed by TM.
       Rollback can also be started by user using "$tm->rollback". TM marks transaction status to
       "a" (aborted). This will prevent other clients trying to add new actions to this
       transaction, since aborted transaction can longer accept new actions, it can only be
       rolled back.

       TM will then perform undo for each function, in reverse order, using the undo actions
       previously recorded in "undo_action" table. The process is similar to performing action,
       except that:

       ·   After rollback succeeds, transaction status is changed to "R"

           "R" means rolled back. These transactions can be discarded by the next cleanup
           process.

       ·   Undo actions are not recorded

           Since we do not rollback from the rollback process, but continue it. TM still calls
           function twice for each action (check_state + fix_state), but do not bother to record
           the undo actions returned by function in the check_state phase to its database.

       ·   Failure in rollback step will mark transaction status as "X"

           "X" means inconsistent/error. Transactions left in this state are probably half-done
           and thus inconsistent. We give up on these transactions and the next cleanup process
           can discard them.

           (TODO: Should there be an option to continue to the next action anyway? But this is
           not necessarily more robust or correct.)

       Transaction status progress:

        i -> a -> R  # successful rollback
        i -> a -> X  # failed rollback

       Example. Continuing our previous example, in the "setup_unix_user(user=>'bob')" action,
       there are 3 actions involved:

        ['My::adduser', {group=>'bob'}]
        ['My::addgroup', {group=>'bob'}]
        ['My::mkdir', {path=>'/home/bob'}]

       Suppose action 1 and 2 succeed, and the following undo actions have been recorded in
       "undo_action":

        ['My::deluser', {group=>'bob'}]  # recorded with ID $ucall_id1
        ['My::delgroup', {group=>'bob'}] # recorded with ID $ucall_id2

       Suppose action 3 fails with status 500 (e.g. permission denied) and thus rollback is
       started. The following is the steps that happen during rollback.  Actions will be
       processed in reverse order: $ucall_id2, $ucall_id1.

       1) TM marks transaction status to aborted:

        BEGIN;
        UPDATE tx SET status='a', last_action_id=NULL WHERE id=$tx_id;
        COMMIT;

       TM performs action "My::delgroup".

       2a) TM calls "My::delgroup()" the first time with "-tx_action => 'check_state'". TM also
       passes "-tx_is_rollback => 1" for informative purposes (some function can utilize this
       information to behave more robust, for example, to avoid failing the rollback process). TM
       does not record the "undo_actions" metadata returned, but observes the "do_actions".

       If function returns 304, step 2b is skipped and TM moves on to the next action.  If
       function returns 200, TM continues to step 2b. If function returns other statuses, TM
       assumes rollback failure and marks transaction as "X" and ends the rollback process for
       this transaction.

       2b) TM invokes "My::delgroup()" the second time to perform the action, passing "-tx_action
       => 'fix_state'" and "-tx_is_rollback => 1". Function sees that group exists (fixable
       state), deletes it, return status 200.

       2c) TM sets transaction's "last_action_id" to $uact_id1 to mark that this action has been
       processed:

        BEGIN;
        UPDATE tx SET last_action_id=$ucall_id1 WHERE id=$tx_id;
        COMMIT;

       TM then continues to perform action "My::delgroup".

       3a) Just like in step 2, TM invokes "My::deluser()" the first time to check state.

       3b) TM invokes "My::deluser()" to perform the action. Function sees that user exists
       (fixable state), deletes it, return status 200.

       3c) TM sets transaction's "last_action_id" to $uact_id2 to mark that this action has been
       processed:

        BEGIN;
        UPDATE tx SET last_action_id=$uact_id2 WHERE id=$tx_id;
        COMMIT;

       4) TM completes the rollback process by setting transaction status to "R".

        BEGIN;
        UPDATE tx SET status='R' WHERE id=$tx_id;
        COMMIT;

       By now the effect of the transaction has been nullified.

       * Crash recovery

       Recovery continues the interrupted rollback process.

       If crash happens after step 1, recovery will continue the rollback process.  Rollback of
       aborted (status a) transaction is exactly the same as rollback of in-progress (status i)
       transaction, except that "last_action_id" is not reset.

       If crash happens after step 2a-2b, "last_action_id" is still unset, so the process resumes
       at step 2a. TM does not remember whether previously before crash the function has been
       executed (and cannot remember, the progress of the execution inside the function). This is
       the reason why function needs to be idempotent, because it is potentially executed twice
       by TM for the same action.  If function has completed deleting the group before crash,
       "check_state" will return status 304 (fixed) and TM will skip step 2b. If function has not
       deleted the group before crash, "check_state" will return status 200 (fixable) and TM will
       execute step 2b.

       If crash happens after step 2c/3a-3b, "last_action_id" is set to $uact_id1.  Process will
       resume at step 3a, since $uact_id1 has been marked as done.

       If crash happens after step 3c, process will resume at step 4.

       If crash happens after step 4, no recovery is necessary since transaction has been rolled
       back completely.

   Undo
       TM allows undoing committed transaction, so the transaction system also serves as an
       undo/redo system.

       1) User performs undo by invoking "$tm->undo(tx_id => $tx_id)", where $tx_id is the ID of
       a committed transaction. If $tx_id is not supplied, the client's newest committed
       transaction is used. TM will first check that transaction status is indeed "C".

       2) TM sets transaction status to "u" (undoing):

        BEGIN;
        UPDATE tx SET status='u' WHERE id=$tx_id;
        COMMIT;

       TM then performs actions specified in the "undo_action" table. The process is similar to
       performing action, except:

       ·   After undo succeeds, transaction status is changed to "U"

           "U" means committed but undone transaction. These transactions can be redone back to
           status "C".

       ·   Undo actions are recorded in "do_action" table instead of "undo_action"

       ·   Failure in undo step will cause transaction to roll back to status "C"

       Transaction status progress:

        C -> u -> U       # successful undo
        C -> u -> v -> C  # failed undo, rolled back to C

       Continuing our previous example, suppose our "setup_unix_user(user=>'bob')" transaction
       has succeeded and been committed. The "undo_action" table contains these entries:

        ['My::deluser', {group=>'bob'}]    # recorded with ID $uact_id1
        ['My::delgroup', {group=>'bob'}]   # recorded with ID $uact_id2
        ['My::rmdir', {path=>'/home/bob'}] # recorded with ID $uact_id3

       Actions will be processed in reverse order: $uact_id3, $uact_id2, $uact_id1.

       3a) TM invokes "My::rmdir" the first time with "-tx_action => 'check_state'". If directory
       has been filled by files/subdirectories, function will return 412 ("Cannot remove home
       directory, non-empty") and the undo process fails with this status. If directory exists
       and is still empty, function will return 200 (fixable state) and process continues.

       3b) TM records the "undo_actions" result metadata returned by function to "do_action"
       table, for redo information.

        BEGIN;
        INSERT INTO do_action (tx_id,ctime,f,args) VALUES
            ($tx_id,$now,'My::mkdir', '{"path":"/home/bob"}'); # -- $ract_id1
        COMMIT;

       3c) TM invokes "My::rmdir" the second time with "-tx_action => 'fix_state'". Function
       deletes directory and return 200.

       3d) TM updates "last_action_id" to mark that this action has been processed:

        BEGIN;
        UPDATE tx SET last_action_id=$uact_id3 WHERE id=$tx_id;
        COMMIT;

       TM then continue to $uact_id2.

       4a) TM invokes "My::delgroup" the first time with "-tx_action => 'check_state'".

       4b) TM records undo_actions:

        BEGIN;
        INSERT INTO do_action (tx_id,ctime,f,args) VALUES
            ($tx_id,$now,'My::addgroup', '{"group":"bob"}'); # -- $ract_id2
        COMMIT;

       4c) TM invokes "My::addgroup" the second time with "-tx_action => 'fix_state'". Function
       sees that group exists, deletes it, and returns 200.

       4d) TM updates "last_action_id":

        BEGIN;
        UPDATE tx SET last_action_id=$uact_id2 WHERE id=$tx_id;
        COMMIT;

       TM then continue to $uact_id1.

       5a) TM invokes "My::deluser" the first time with "-tx_action => 'check_state'".

       5b) TM records undo_actions:

        BEGIN;
        INSERT INTO undo_action (tx_id,ctime,f,args) VALUES
            ($tx_id,$now,'My::adduser', '{"user":"bob"}'); # -- $ract_id3
        COMMIT;

       5c) TM invokes "My::adduser" the second time with "-tx_action => 'fix_state'". Function
       sees that user exists, deletes it, and returns 200.

       5d) TM updates "last_action_id":

        BEGIN;
        UPDATE tx SET last_action_id=$uact_id1 WHERE id=$tx_id;
        COMMIT;

       6) TM completes the undo process by setting transaction status to "U":

        BEGIN;
        UPDATE tx SET status='U', last_action_id=NULL WHERE id=$tx_id;
        COMMIT;

       Crash recovery

       Recovery rolls back interrupted undoing process so that transaction status is back to "C"
       (committed). For more details, refer to "Rollback of undoing (status u) transaction".

       If crash happens before finishing step 2, no recovery is necessary.

       If crash happens after step 2-3c, recovery resumes from step 3a since "last_action_id" is
       still unset. That is why "My::mkdir" needs to be idempotent and can check state, since it
       is potentially executed (step 3c) twice, before and after recovery.

       If crash happens after step 3d-4c, recovery recovery resumes from step 4a since
       "last_action_id" is set to $uact_id3.

       If crash happens after step 4d-5c, recovery resumes from step 5a since "last_action_id" is
       set to $uact_id2.

       If crash happens after step 5d, recovery resumes from step 6.

   Rolling back the undoing (status u) transaction
       If undo fails in the middle, rollback will happen. TM marks transaction status from "u" to
       "v", this differentiates between an undo process in progress (in which case recovery
       should continue it until status is "U") and a failed undo process (in which case recovery
       should rolls it back to status "C").

       TM will then perform actions from the "do_action" table. The process is similar to
       rollback of in-progress (status i) transaction, except that after rollback succeeds,
       transaction status is set to "C".

       If rollback fails, transaction status is set to "X".

       Transaction status progress:

        u -> v -> C # rollback succeeds
        u -> v -> X # rollback fails

       Crash recovery

       Recovery continues the rollback process.

   Redo
       An undone transaction (status "U") can be redone back to "C". To do this, user invokes
       "$tm->undo(tx_id => $tx_id)", where $tx_id is the ID of an undone transaction. If $tx_id
       is not supplied, the client's newest undone transaction is used. TM will first check that
       transaction status is indeed "U".

       TM will then set transaction status to "d" (redoing):

        BEGIN;
        UPDATE tx SET status='d' WHERE id=$tx_id;
        COMMIT;

       This will prevent other clients trying to redo the same transaction. TM will then process
       actions found in "do_action" table, just like when performing normal action.

       Transaction status progress:

        U -> d -> C

       Crash recovery

       Recovery rolls back the redoing process. See "Rolling back a redoing (status d)
       transaction".

   Rolling back a redoing (status d) transaction
       If redo fails in the middle, rollback will happen. TM marks transaction status from "d" to
       "e" (failed redo). This will differentiate between a redo process in progress (in which
       case recovery should continue it until status is "C") and a failed redo process (in which
       case recovery should rolls it back to status "U").

       TM will perform actions from the "undo_action" table. The process is similar to rollback
       of an in-progress (status i) transaction, except that after rollback succeeds, transaction
       status is set to "U".

       If rollback fails, TM will set transaction status to "X".

       Transaction status progress:

        d -> e -> U # rollback succeeds
        d -> e -> X # rollback fails

       Crash recovery

       Recovery continues the rollback process.

   Cleanup
       Cleanup is done at TM startup and at regular intervals. TM should delete (forget) all C
       and U transactions that are too old, or keep the number of those transactions under a
       certain limit, according to its settings. As soon as those transactions are deleted, they
       can no longer be undone/redone, since the undo actions data has been deleted too.

       The cleanup process also deletes all X transactions, since they cannot be resolved anyway
       (TODO: perhaps some retry mechanism can be applied, if desired?)

       Cleanup process also deletes all R transactions.

       Cleanup process can also roll back any transactions with status "i" that have been going
       for too long without being committed/rolled back.

   Savepoint
       Basically savepoint is just a label in the "do_action" table.

       To mark a savepoint, user invokes "$tm->savepoint(sp_id=>$sp_id)" where $sp_id is an
       arbitrary string from 1-64 characters. It must be unique within the transaction. If the
       same savepoint is used, the old savepoint is replaced by the new one.

       To release (forget) a savepoint, user invokes "$tm->release_savepoint(sp_id=>$sp_id)". It
       just clears the label in the "do_action" table.

       Rollback to a savepoint is just a normal rollback process, except we stop after finishing
       the undo actions of the corresponding action with the savepoint, and transaction status is
       set back to "i". If savepoint is unknown (or marked before any action, which is
       effectively the same), we rollback everything in the transaction.

   Discard
       User can optionally do a cleanup of her transactions by issuing
       "$tm->discard(tx_id=>$tx_id)" or "$tm->discard_all". Transactions that can be discarded
       are those with the final statuses: "C", "U", "X".

FAQ

   Why is this useful?
       The protocol is a pretty generic and simple way to build transactional system, even on
       heterogenous, multiuser environment. If the functions are written carefully, the system
       can be reliable. And even if some of the ACID properties are compromised due to lack of
       real RM, the system is still useful for its undo/redo capability.

   What are the drawbacks?
       The reliability of the system rests on the reliability of each involved function. One
       buggy function can break the transaction.

   What about non-undoable actions?
       Non-undoable actions (like sending an email, permanently deleting files) should be
       executed outside the scope of transaction.

HOMEPAGE

       Please visit the project's homepage at <https://metacpan.org/release/Rinci>.

SOURCE

       Source repository is at <https://github.com/perlancar/perl-Rinci>.

BUGS

       Please report any bugs or feature requests on the bugtracker website
       <https://rt.cpan.org/Public/Dist/Display.html?Name=Rinci>

       When submitting a bug or request, please include a test-file or a patch to an existing
       test-file that illustrates the bug or desired feature.

SEE ALSO

       Transaction behavior is largely based on PostgreSQL.

       Related specifications: Rinci::function, Riap::Transaction

       Implementations: Perinci::Tx::Manager

AUTHOR

       perlancar <perlancar@cpan.org>

COPYRIGHT AND LICENSE

       This software is copyright (c) 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012 by
       perlancar@cpan.org.

       This is free software; you can redistribute it and/or modify it under the same terms as
       the Perl 5 programming language system itself.