Provided by: librinci-perl_1.1.104-1_all bug

NAME

       Rinci::Transaction - A transactional system based on functions

SPECIFICATION VERSION

        Rinci 1.1, protocol version 2

VERSION

       This document describes version 1.1.104 of Rinci::Transaction (from Perl distribution Rinci), released on
       2023-09-30.

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>.

SEE ALSO

       Transaction behavior is largely based on PostgreSQL.

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

       Implementations: Perinci::Tx::Manager

AUTHOR

       perlancar <perlancar@cpan.org>

CONTRIBUTING

       To contribute, you can send patches by email/via RT, or send pull requests on GitHub.

       Most of the time, you don't need to build the distribution yourself. You can simply modify the code, then
       test via:

        % prove -l

       If you want to build the distribution (e.g. to try to install it locally on your system), you can install
       Dist::Zilla, Dist::Zilla::PluginBundle::Author::PERLANCAR,  Pod::Weaver::PluginBundle::Author::PERLANCAR,
       and  sometimes  one  or  two other Dist::Zilla- and/or Pod::Weaver plugins. Any additional steps required
       beyond that are considered a bug and can be reported to me.

COPYRIGHT AND LICENSE

       This software is copyright (c) 2023, 2022, 2021, 2020, 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012  by
       perlancar <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.

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.

perl v5.38.2                                       2024-01-20                            Rinci::Transaction(3pm)