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cron-condenser aims to reduce a list of cron expressions down to the minimum expressions needed to describe the same intervals as the original list.

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Cron Condenser

This tool aims to reduce a list of cron expressions down to the minimum expressions needed to describe the same intervals as the original list.

Usage

java -jar cron-condenser.jar [-v] [--draw ./path-to-save-visualizations] "cron-1" "cron-2" ... "cron-n"
  • -v will show more information about the problematic segments of each cron.

Requirements

  • JDK 11 or later
  • graphviz

When can 2 cron expressions be merged?

Two cron expressions can only be merged when they differ in only one segment. For example:

0 0 0 JAN *
1 0 0 JAN *

can be merged into 0,1 0 0 JAN * since they only differ in the minute segment while

0 0 0 JAN *
30 10 0 JAN *

cannot be merged. Merging them would introduce 2 new trigger times that were not present in any of the original expressions. 0,30 0,10 0 JAN * would also trigger at 0 10 0 JAN * as well as 30 0 0 JAN *.

Cron Expression Merge Graph

In order to merge the expressions, a graph is created in which the nodes are the expressions themselves and the edges represent the type of merge that is possible between two expressions. The types define the segment in which they differ. Two expressions may only have 1 edge between them.

The process through which the expressions get condensed is an iterative process that requires rebuilding the entire graph at each step. Every step guarantess that 2 nodes will be removed and 1 new node will be added. The process stops when there are no more edges connecting the nodes.

The nodes picked for each iteration are not random, the node with the least edges is picked as the origin node, and the node with the least edges where at least one is shared with the origin node is the target node. Those are then merged to produce the new node. The idea behind this strategy is to perform the minimally destructive merge at each step in order to maximize the number of merges performed in total.

The figure below shows the resulting graph states after each iteration for the following expressions as input:

0 0 1 JAN 0
0 10 1 JAN 0
30 10 1 JAN 0
30 12 1 JAN 0
0 0 1 FEB 0
0 10 1 FEB 0
30 10 1 FEB 0
30 12 1 FEB 0
0 0 1 MAR 0
0 10 1 MAR 0
30 10 1 MAR 0
30 12 1 MAR 0
0 0 1 APR 0
0 10 1 APR 0
30 10 1 APR 0
30 12 1 APR 0

Cron condensing merge graphs

Support

Currently only standard UNIX cron expressions are supported. The format is

minute hour day month week-day

and supported features include:

  • Named values (JAN, TUE)
  • Step values (*/2)
  • Ranges (1-5)
  • Named Ranges (MON-TUE)

About

cron-condenser aims to reduce a list of cron expressions down to the minimum expressions needed to describe the same intervals as the original list.

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