I have to come up a block that prevents ceratin tasks from running
simultaneously, for instance:

- if latest sales data is being loaded, no reports involving recent
sales should run, but it is OK to run any reports against historic
data.
- if the big boss needs some report ASAP, we are gonna let the
already running tasks finish up, but no new tasks should start until
thi big boss's task is completed

and so on

I also need a "what's going on" view, which tasks are running, which
could be started, which task are being prevented from starting and
why.

I'm thinking of something like this:

CREATE TABLE POSSIBLE_TASK(
TASK_PK NOT NULL PRIMARY KEY,
TASK_DESCRIPTION
);

CREATE TABLE RUNNING_OR_BLOCKED_TASK(
TASK_PK NOT NULL PRIMARY KEY,
IS_RUNNING NOT NULL, /* 1 - RUNNING, 0 - BLOCKED FROM RUNNING*/
BLOCKED_BY NOT NULL FOREIGN KEY REFERENCES POSSIBLE_TASK(TASK_PK),
INSERTED_TIME DATE,
PROCESS_ID
);

then I would be inserting one or more rows, for example, starting to
load recent sales:

INSERT INTO RUNNING_OR_BLOCKED_TASK
SELECT
TASK_PK,
CASE WHEN TASK_PK = 'LOAD RECENT SALES' THEN 1 ELSE 0 END,
'LOAD RECENT SALES'
FROM POSSIBLE_TASK WHERE TASK_ID LIKE '%RECENT SALES%'

or for CEO's report:

INSERT INTO RUNNING_OR_BLOCKED_TASK
SELECT
TASK_PK,
CASE WHEN TASK_PK = 'REPORT FOR CEO' THEN 1 ELSE 0 END,
'REPORT FOR CEO'
FROM POSSIBLE_TASK WHERE TASK_ID NOT IN(SELECT TASK_ID FROM
RUNNING_OR_BLOCKED_TASK)

I don't care about concurrency related issues here, only one process
is ever modifying RUNNING_OR_BLOCKED_TASK

I'd appreciate any suggestions