What is a Cron
Expression?
Many of you are probably already familiar with the unix cron program. For
those that aren't, cron is a daemon process that allows users to execute
commands or scripts automatically at user-configurable dates and times. The
important part as far as OSCache is concerned is the cron expression syntax that
allows users to dictate when commands should be executed - you can now use the
same syntax to expire content in OSCache! A cron expression is a simple text
string that specifies particular dates and/or times that are matched
against.
How
Does OSCache Match Against an Expression?
OSCache uses cron expressions in a manner that might seem 'backwards'
to what you might initially expect. When using a cron expression to test if a
cache entry is stale, OSCache finds the date and time (prior to the current
time) that most recently matches the supplied expression. This date/time is used
as the expiry time - entries that were placed in the cache prior to this expiry
time are considered stale and result in a NeedsRefreshException being
thrown.
As an example, suppose you specify a cron expiry that matches every hour, on
the hour ("0 * * * *"). If the current time is 10:42pm, then any content that
was placed in the cache prior to 10:00pm would be considered stale.
What
is the Difference Between the Refresh Period and a Cron Expression?
The difference between the refresh period and a cron expression is that the
refresh period specifies the maximum allowable age of a cache entry, whilst a
cron expression specifies specific expiry times, regardless of how old an entry
is. Eg imagine caching an object at 10:29am. With a refresh period of 30 minutes
that entry would expire at 10:59am. With a cron expression of "0,30 * * * *"
that entry would expire at 10:30am.
The Cron Expression
Syntax
A cron expression consists of the following 5 fields:
-
Minute - specifies what minute of the hour to expire content on. It
is a number between 0 and 59.
-
Hour - determines what hour of the day content will expire on. It is
specified using the 24-hour clock, so the values must be between 0 (midnight)
and 23 (11pm).
-
DOM - the Day of the Month. This is a number from 1 to 31. It
indicates what day the content should expire on. For example, to expire content
on the 10th of every month, set this field to 10.
-
Month - month of the year to expire the content. This can be
specified either numerically (1 through 12), or by using the actual month name
(eg 'January'). Month names are case-insensitive and only the first three
characters are taken into account - the rest are ignored.
-
DOW - The Day of the Week that the content should be expired on. This
can be a numeric value (0-6, where 0 = Sunday, 1 = Monday, ..., 6 = Saturday),
or you can use the actual day name. As is the case with month names, DOW names
are case-insensitive and only the first three characters matter.
If you don't want to specify a value for a particular field (ie you want the
cron expression to match all values for that field), just use a * character for
the field value.
As an example, an expression that expired content at 11:45pm each day during
April would look like this: "45 23 * April *".
OSCache also allows you to optionally specify lists, ranges and
intervals (or even a combination of all three) within each field:
-
Lists - items in a list are delimited using the ',' character.
Content expiry times will be matched against all values in the list for that
field. For example, "0,15,30,45 * * * *" will expire content every quarter-hour
on the quarter hour.
-
Ranges - ranges are specified using the '-' character. A range will
include all values from the start to the end value (inclusive). For example, "*
* * Jan-June *" will expire content every minute only during the first 6 months
of the year.
-
Intervals - an interval is specified using the '/' character. The
value to the left of the '/' character indicates either the starting point or
the range of values that should be incremented over, while the value to the
right of the '/' specifies the interval or step size. Some examples -
To have a look at further examples of both valid and invalid syntax, it is
suggested you take a look at the JUnit test cases in the
com.opensymphony.oscache.util.TestFastCronParser class. This class is located
under the src/core/test directory. For examples of how to specify cron expiry
times using the taglibs.
Notes
- You can specify both a cron expression and a refresh interval at the same
time if you like. This is useful in cases where you always want to expire
content at midnight, but you also never want it to be more than 6 hours old.
- Specifying out of range values, such as a 13 in the month field, will cause
a ParseException to be thrown.
- If a DOM is specified that cannot exist given the allowable months, a
ParseException will be thrown. For example, "* * 31 Feb *" will fail because no
date will ever match the 31st February!
- The DOM and DOW fields cannot both be specified at the same time. One must
always be set to '*' otherwise a ParseException will be thrown.
- Leap years and local daylight savings time are taken into account. Eg "0 0
29 Feb *" will match midnight on the 29th February, ie only once every 4 years.
- Currently the time used to match the cron expression against is always based
on the local time on the server. If there is demand support for specifying an
alternate timezone may
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