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.
java -jar cron-condenser.jar [-v] [--draw ./path-to-save-visualizations] "cron-1" "cron-2" ... "cron-n"
-vwill show more information about the problematic segments of each cron.
- JDK 11 or later
- graphviz
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 *.
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
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)