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Filtering

Filtering is the term given to reducing the entire set of servers declared in a stage file to a smaller set. There are three types of filters used in Capistrano (Host, Role and Property) and they take effect in two quite different ways because of the two distinct uses to which the declarations of servers, roles and properties are put in tasks:

  • To determine configurations: typically by using the roles(), release_roles() and primary() methods. Typically these are used outside the scope of the on() method.

  • To interact with remote hosts using the on() method

An illustration of this would be to create a /etc/krb5.conf file containing the list of available KDC’s by using the list of servers returned by roles(:kdc) and then uploading it to all client machines using on(roles(:all)) do upload!(file) end

A problem with this arises when filters are used. Filters are designed to limit the actual set of hosts that are used to a subset of those in the overall stage, but how should that apply in the above case?

If the filter applies to both the interaction and configuration aspects, any configuration files deployed will not be the same as those on the hosts excluded by the filters. This is almost certainly not what is wanted, the filters should apply only to the interactions ensuring that any configuration files deployed will be identical across the stage.

So we define two different categories of filter, the interaction ones which are called On-Filters and the configuration ones which are Property-Filters

On-Filtering

On-filters apply only to the on() method that invokes SSH. There are two default types:

In both the above cases, when filters are specified using comma separated lists, the final filter is the union of all of the components. However when multiple filters are declared the result is the intersection.

This means that you can filter by both role and host but you will get the intersection of the servers. For example, lets say you filtered by the role app, then by the hostnames server1 and server2. Capistrano would first filter the available servers to only those with the role app, then filter them to look for servers with the hostname server1 or server2. If only server2 had the role app (server1 has some other role), then in this situation your task would only run on server2.

Custom filters may also be added; see Custom Filters.

Property-Filtering

Property-filters select servers based on the value of their properties alone and are specified by options passed to the roles() method (and implicitly in methods like release_roles() and primary())

An example of that is the ‘no_release’ property and it’s use in the release_roles() method.

See the documentation for details

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