fix: locking mechanism

docs/locking-mechanism.md

@@ -0,0 +1,37 @@
+---
+layout: article
+title: Locking Mechanism
+description: Data Controller uses a locking mechanism to handle parallel updates to the same table by different users. This mechanism is also available to non-DC applications.
+og_image:  https://docs.datacontroller.io/img/locking-mechanism.drawio.png
+---
+
+# Locking Mechanism
+
+In any multi-user application a locking mechanism is necessary to avoid conflicts when the same table is updated by two people or processes.  In Data Controller this is handled using the `DCLIB.MPE_LOCKANYTABLE` table, specifically using the [mp_lockanytable](https://core.sasjs.io/mp__lockanytable_8sas.html) macro.
+
+This macro is invoked whenever a process requires an exclusive lock on a table - first to mark the table as locked, and again to mark it as unlocked.  The logic flow is demonstrated below:
+
+![lockanytable logic](/img/locking-mechanism.drawio.png)
+
+Notice above the red box - this situation can occur in unusual circumstances (such as a system shutdown or OS error).  If you are waiting more than a minute or two for a table to be unlocked, then it is advised to follow the process below.
+
+## Unlock Process
+
+Before proceeding with an unlock, the first step should be to understand why the process is locked.  By opening the [mpe_lockanytable](locking-mechanism) it will be possible to see which user performed the lock, and when.  Simply filter the table where `LOCK_STATUS_CD='LOCKED'`.
+
+![](/img/mpe_lockanytable.png)
+
+If an unlock is deemed necessary, simply open this table in the EDIT menu, and change the value from UNLOCKED to LOCKED.
+
+## Scalability
+
+This locking mechanism does not scale well if the table remains in SAS dataset (.sas7bdat) form.  If you are deploying Data Controller for usage beyond 10-20 users then you are advised to put the control tables (DC library) in a database.
+
+## Third Party Applications
+
+Third party SAS apps may also make use of this locking table.  This is handled by simply:
+
+1. Ensuring the DC library is assigned in the Service
+2. Using the [mp_lockanytable](https://core.sasjs.io/mp__lockanytable_8sas.html) macro to perform the lock
+3. Using the [mp_lockanytable](https://core.sasjs.io/mp__lockanytable_8sas.html) macro to perform the unlock
+4. Performing appropriate error handling to ensure an unlock if there are any issues in the service

docs/tables/mpe_lockanytable.md

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+---
+layout: article
+title: MPE_LOCKANYTABLE
+description: The MPE_LOCKANYTABLE table provides a mechanism for a process to secure a logical 'lock' on an object to avoid conflicts when running the application with multiple users in parallel
+og_image:  https://docs.datacontroller.io/img/mpe_lockanytable.png
+---
+
+# MPE_LOCKANYTABLE
+
+The `MPE_LOCKANYTABLE` table provides a mechanism for a process to secure a logical 'lock' on an object to avoid conflicts when running the application with multiple users in parallel. 
+
+The underlying utility is open source and documented [here](https://core.sasjs.io/mp__lockanytable_8sas.html).
+
+For more information, see the [locking mechanism guide](https://docs.datacontroller.io/locking-mechanism).
+
+![locking](/img/mpe_lockanytable.png)
+
+## Columns
+
+ - 🔑 `LOCK_LIB char(8)`: SAS Libref (8 chars)
+ - 🔑 `LOCK_DS char(32)`: The dataset name
+ - `LOCK_STATUS_CD char(10)`: Either LOCKED or UNLOCKED
+ - `LOCK_USER_NM char(100)`: The logged-in user who performed the lock or unlock
+ - `LOCK_REF char(200)`: Description of the lock purpose
+ - `LOCK_PID char(10)`: The value of the automatic `sysjobid` macro variable 
+ - `LOCK_START_DTTM num`: The timestamp when the record was LOCKED
+ - `LOCK_END_DTTM num`: The timestamp when the record was UNLOCKED. This is set to missing whilst the record is locked.
+

mkdocs.yml

@@ -9,6 +9,7 @@ nav:
     - Data Lineage: dcu-lineage.md
     - File Uploads: dcu-fileupload.md
     - Filter Mechanism: filter.md
+    - Locking Mechanism: locking-mechanism.md
     - Table Viewer: dcu-tableviewer.md
     - Admin Services: admin-services.md
 - Table Guide: