Search monitors


A search monitor is an observer of the resolver. It gives user access before and after executing each main step of the solving process:

  • initialize: when the solving process starts and the initial propagation is run,

  • open node: when a decision is computed,

  • down branch: on going down in the tree search applying or refuting a decision,

  • up branch: on going up in the tree search to reconsider a decision,

  • solution: when a solution is got,

  • restart search: when the search is restarted to a previous node, commonly the root node,

  • close: when the solving process ends,

  • contradiction: on a failure,

With the accurate search monitor, one can easily observe with the resolver, from pretty printing of a solution to learning nogoods from restart, or many other actions.

The interfaces to implement are:

  • IMonitorInitialize,

  • IMonitorOpenNode,

  • IMonitorDownBranch,

  • IMonitorUpBranch,

  • IMonitorSolution,

  • IMonitorRestart,

  • IMonitorContradiction,

  • IMonitorClose.

Most of them gives the opportunity to do something before and after a step. The other ones are called after a step.

Simple example to print every solution:

Solver s = model.getSolver();
s.plugMonitor(new IMonitorSolution() {
    public void onSolution() {
        System.out.println("x = "+x.getValue());

In Java 8 style:

Solver s = model.getSolver();
s.plugMonitor((IMonitorSolution) () -> {System.out.println("x = "+x.getValue());});


A Settings object is attached to each Model. It declares default behavior for various purposes: from general purpose (such as the welcome message), modelling purpose (such as enabling views) or solving purpose (such as the search binder).

Default settings can be accessed through DefaultSettings. This class can be extended to provide more settings and set to modified the default values.

Settings are declared in a Model constructor. Settings are not immutable but modifying value after Model construction can lead to unexpected behavior.

Extensions of Choco


choco-parsers is an extension of Choco 4. It provides a parser for the FlatZinc language, a low-level solver input language that is the target language for MiniZinc. This module follows the flatzinc standards that are used for the annual MiniZinc challenge. It only supports integer variables. You will find it at https://github.com/chocoteam/choco-parsers


choco-graph is a Choco 4 module which allows to search for a graph, which may be subject to graph constraints. The domain of a graph variable G is a graph interval in the form [G_lb,G_ub]. G_lb is the graph representing vertices and edges which must belong to any single solution whereas G_ub is the graph representing vertices and edges which may belong to one solution. Therefore, any value G_v must satisfy the graph inclusion “G_lb subgraph of G_v subgraph of G_ub”. One may see a strong connection with set variables. A graph variable can be subject to graph constraints to ensure global graph properties (e.g. connectedness, acyclicity) and channeling constraints to link the graph variable with some other binary, integer or set variables. The solving process consists of removing nodes and edges from G_ub and adding some others to G_lb until having G_lb = G_ub, i.e. until G gets instantiated. These operations stem from both constraint propagation and search. The benefits of graph variables stem from modeling convenience and performance.

This extension has documentation. You will find it at https://github.com/chocoteam/choco-graph


choco-gui is an extension of Choco 4. It provides a Graphical User Interface with various views which can be simply plugged on any Choco Model object. You will find it at https://github.com/chocoteam/choco-gui

Last modified 16.03.2020: Update doc (2a51b22)