Storing recurring time periods in Oracle database - oracle

Question

I'm writing monitoring software, where most of the logic will be in Oracle databasen & pl/sql.
When my monitoring is called it should alert about problems. For example, it should alert about problem if
1. There are less than 2 operation, in every minute, on Friday from 22:00 till 23:00
2. There are less than 5 operation, in every minute, on 31 of January from 22:00-23:00
3. There are less than 3 operation, in every minute, every day from 10:00 till 12:00
If my monitoring is called on 22:30, 31 of January I should compare my operation number to 5.
4. If there are less than 5 operation, in every minute, from Friday 22:00 till Monday 15:00
I was thinking about saving data periods with cron expression format in database. In this case I have to compare SYSDATE (current call date of monitoring function) to cron expression saved in the database.
My questions:
1. How can I find out if SYSDATE falls under cron expression?
2. Is it correct to use cron expressions in this case, at all? Can you suggest any other way of saving periods of time.

Answer 1

Don't do it
I am completely with SpaceTrucker: Don't do it in SQL or PL/SQL, do it in Java with either Java 8 date API or JodaTime.
How to do it nevertheless
But even when you should't do it, there might still be some good reason to do it. So here is how:
Table for each instant you want to check
First let's create a table for each second or minute in the interval you want to check. The granularity and the length of your interval depends on the cron expressions you want to allow. Usually one second for a whole week should be sufficient (about 100'000 rows). If you want to check a whole year, use minutes as granularity (about 500'000 rows). Both amount or rows are nothing for a modern database. On my notebook, according queries return instantly.
CREATE TABLE week AS
SELECT
running_second,
ts,
EXTRACT(SECOND FROM ts) as sec,
EXTRACT(MINUTE FROM ts) as min,
EXTRACT(HOUR FROM ts) as h,
to_char(ts, 'Day') as dow
FROM (
SELECT
level as running_second,
TO_TIMESTAMP_TZ('2015-09-05 00:00:00 0:00',
'YYYY-MM-DD HH24:MI:SS TZH:TZM') +
NUMTODSINTERVAL(level-1, 'SECOND') AS ts
FROM dual CONNECT BY level<=60*60*24*7
)
;
Query for each filter expression
Next, you convert each cron expression to a query. You can either use PL/SQL to transform each cron expression to a where clause, or you can use a generic where clause.
You should get something like this:
SELECT
*
FROM
week
WHERE
h =5
AND min=0
AND sec=0;
or in a generic version:
SELECT
filter_expression.name, week.ts
FROM
week, filter_expressions
WHERE
(fiter_hour is null or h = filter_hour)
AND (filter_min is null or min = filer_min)
AND (filter_sec is null or sec = filter_sec);
(given your filters are stored in a table filter_expressions, that has a column for each constraint type, and each row has either a parameter for the constraint or NULL if the constraint is not applicable).
Store the result in a global temporary table cron_startpoints.
Check for violations
Group the table cron_startpoints to check for constraint violations. You can count, how many matches are there for Friday or midnight or whatever and can check, whether that number is OK for you or not.

Answer 2

It depends on how much flexibility you want. For the examples you provided such structure would be enough:
CREATE TABLE monitoring_periods (
id INTEGER NOT NULL PRIMARY KEY,
monit_month VARCHAR2(2),
monit_day VARCHAR(2),
monit_day_of_week VARCHAR(3),
monit_time_from INTERVAL DAY TO SECOND,
monit_time_to INTERVAL DAY TO SECOND,
required_ops INTEGER
);
Here are some examples to store the periods and checking against sysdate. I would avoid storing the cron expression literally as a string, as it would require parsing it at query time. However, the more complex your expressions are (kind of '5 4,15,22 */2 * 1-5') the more complicated the structure to store it - you need to think carefully of your requirements.

Answer 3

I once had the task to write difficult date calculations with recurring periods and time windoes for 10g. Among those were things like "Tuesday of the second week of the month every 2 months between 8 AM and 2 PM". We decided to use java stored procedures for this (also because they were already in use for other purposes).
Depending on your oracle version, you can choose a joda-time version, which can be run within the oracle database jvm. Also note that joda-time 1.6 can be compiled with java 1.3 (which we had to use).
If you are looking for cron expressions explicitly, than you might also do well with using another java library within the oracle database jvm. For example here is one:
CronExpression expression = CronExpression.parser()
.withSecondsField(true)
.withOneBasedDayOfWeek(true)
.allowBothDayFields(false)
.parse("0 15 10 L * ?");
assert expression.matches(dateTime);
However i think cron is not suited for your task at hand. Cron is a way to specify when to run jobs. However you need to observe what happend. So for your requirement There are less than 2 operation, in every minute you could have operations at the 1st and 2nd second or at the 1st 31st second and both are valid, but their cron expressions are very different.
When it's about saving the time periods, you could also look at ISO 8601 recurinng intervals stored as varchars:
P1Y2M10DT2H30M
In any case you will need to apply calculations on every row you would like to match. Depending on how many lines that are, you might need to use some heuristics to sort out results which are far away from meeting your criteria.
Thinking a bit more outside the box:
you should question your architecture. The requirements you listed ca be represented by state machines. You can feed them with the events that occured in chronological order. If a state machine reaches some unwanted state you can just report that. However I doubt that this can be easily done in pure pl/sql.