feed: add a post on running two rule sets against one table

Explains why a physical table carries one rule set in Data Controller
(MPE_TABLES and MPE_VALIDATIONS are both keyed on the physical libref.dataset),
then covers the two ways to get more than one:

- two librefs over the same data (recommended - configuration only, no hooks,
  no copies, filtering and every other editor behaviour unchanged)
- an empty mirror plus PRE_EDIT_HOOK (show the live rows) and POST_EDIT_HOOK
  (re-point the changeset at the real table with call symputx, not %let)

Screenshots are from the demo estate.

content/feed/two-rule-sets-one-table/index.md

@@ -41,31 +41,16 @@ libname ORDERS_US '/data/orders';
 
 ![The second report accepts the same value](rule-set-2.png)
 
-Everything else works exactly as it always did. Filtering, search, the row cap, the approval diff and the audit trail all operate on the table as normal, because as far as Data Controller is concerned these are ordinary tables. The only difference is that the two names resolve to the same file, so an approval in either report updates the same data - and that one difference has a consequence for concurrency, which is covered below.
+Everything else works exactly as it always did. Filtering, search, the row cap, the approval diff and the audit trail all operate on the table as normal, because as far as Data Controller is concerned these are ordinary tables. The only difference is that the two names resolve to the same file, so an approval in either report updates the same data.
 
 There is no copy to keep in sync, no hook to write and nothing to maintain. That is why it is the option we recommend.
 
 ### Things worth knowing
 
 - Where the librefs are defined depends on your platform. On Viya, in the compute context's `autoexec.sas` (or `[DC Drive Path]/services/settings.sas`); on SAS 9, as metadata libraries or in the Data Controller Settings stored process; on SASjs Server, in `services/public/settings.sas`. The one requirement is that each library has a unique libref.
-- `mp_lockanytable` keys on `libref.dataset`, so the two menus do not serialise against each other. That has consequences beyond a collision - see [the concurrency caveat](#the-concurrency-caveat) below.
+- `mp_lockanytable` keys on `libref.dataset`, so the two menus do not serialise against each other. Two people editing through different reports at the same moment can therefore collide at the database level. If that matters, add an explicit shared lock in a `PRE_APPROVE_HOOK`.
 - The audit trail and approval queue record which libref a change came through, so `ORDERS_EU.ORDERS` and `ORDERS_US.ORDERS` stay distinguishable in history. For most people that is a feature - you can see which report a change originated from.
 
-### The concurrency caveat
-
-Data Controller serialises its writes with `mp_lockanytable`, and the control table it uses, `MPE_LOCKANYTABLE`, has a primary key of `(lock_lib, lock_ds)`. That is the *registration*, not the physical file. At approve time, `postdata` takes the lock on the base table named in the submit record, and the same service runs its "has this table been updated since the diff screen was shown" check against `MPE_DATALOADS`, keyed on that same `libref` and `dsn`.
-
-Two librefs over one location are two different `(libref, dsn)` pairs, so:
-
-- two approvals arriving through different reports take two different locks, and neither one blocks the other;
-- a load through one report writes its `MPE_DATALOADS` entry against its own name, so the other report's staleness check does not see it either.
-
-So two people editing the same rows through different reports can both be approved, and the second write wins, silently. The lock is advisory to begin with - `mp_lockanytable` is, in its own words, "only useful if every update uses the macro" - so this is not a new class of risk, but registering one file twice is a new way to fall into it.
-
-The hook route does not have this problem. Both mirrors route their submit to the same base table, so every approval locks, loads and logs against one identity.
-
-If you do use two librefs, and concurrency matters to you, the cleanest fix is to keep the two registrations for the edit screen and give each one a `POST_EDIT_HOOK` that re-points the changeset at a single canonical registration - then every approval is keyed on the same table regardless of which report raised it. Register that canonical name as well. The alternative, an explicit shared lock taken in `PRE_APPROVE_HOOK` and released in `POST_APPROVE_HOOK`, works too, but a failed run leaves the sentinel locked and `MPE_LOCKANYTABLE` is then a table you have to unpick by hand.
-
 ## Option 2: an empty mirror and a pair of hook scripts
 
 Sometimes two librefs over one location are not available: a database library where the platform will not let you define the same object twice, or a site where adding a library definition is a change nobody wants to make. Then you can reach the same result with a mirror table and two hook scripts.
@@ -114,10 +99,8 @@ Use `call symputx`, not `%let`. `LIBREF` and `DS` are not declared `%local` in `
 
 ### Other things to watch
 
-- Concurrency behaves correctly here. Both mirrors route their submit to the same base table, so every approval locks, loads and logs against one identity - unlike the two-libref route above.
 - The filter has already been applied to the empty mirror by the time the pre-edit hook runs, so a hook that reads the real table ignores the user's filter unless it re-applies it (`where %inc filtref`). On a small table you will not notice; on a large one the `DC_MAXOBS_WEBEDIT` cap will stop the edit screen with "Table is too big".
 - The hook's output must have the same columns the editor expects - the real table, minus any transaction or processing columns that Data Controller drops on load.
-- The real table must itself be registered in `MPE_TABLES`. The approval screen resolves the table's audit settings from that row, so a changeset routed to a table with no registration cannot be reviewed - the submit is refused up front, naming the table.
 - The mirror's `MPE_TABLES` row is read for the edit screen and the real table's for the load, so keep their `buskey`, `loadtype` and temporal column settings identical.
 - At approval time the access checks run against the real table, so editors need `EDIT` on the mirror while approvers need `EDIT` and `APPROVE` on the real table.