How to deploy to a single specific server using Capistrano

[Pete Hodgson]

Hi Folks,

I have a system in production that has several servers in serveral
roles. I would like to test a new app server by deploying to that
specific server, without having to redeploy to every server in
production. Is there a way to ask Capistrano to deploy to a specific
server? Ideally I'd like to be able to run something like cap
SERVER=app2.example.com ROLE=app production deploy if I just wanted to
deploy to app2.example.com.

I original posted this question on Stack Overflow a few days back [1]
and got a few answers, but neither solution seemed to work for me.
Hope it's not bad form to now post here.

Cheers,
Pete



[1] http://stackoverflow.com/questions/429816/how-to-deploy-to-a-single-specific-server-using-capistrano

[Jamis Buck]

Try the HOSTS environment variable:

cap HOSTS=app2.example.com production deploy

Note that doing this will treat app2 as being in every role, not just
whichever role(s) it happens to be declared in.

If what you want is to do a regular deploy, but only act on app2, and
only as app2 is declared in your recipe file, you can use the HOSTFILTER
variable instead:

cap HOSTFILTER=app2.example.com production deploy

Sorry if the distinction isn't clear there. Let me know if you need more
clarification as to the difference between HOSTS and HOSTFILTER.

- Jamis

[Gerhardus Geldenhuis]

I have found this very usefull, however I do not understand the
difference between HOSTS and HOSTFILTER. It would be great if you
could explain it from another angle

Regards

> > [1]http://stackoverflow.com/questions/429816/how-to-deploy-to-a-single-s...

[Jamis Buck]

On 1/22/09 4:12 AM, Gerhardus Geldenhuis wrote:
> I have found this very usefull, however I do not understand the
> difference between HOSTS and HOSTFILTER. It would be great if you
> could explain it from another angle

I'll give it another try. Consider this concrete example. Suppose your
script defines three servers, A, B, and C. And it defines a task, "foo",
that (by default) wants to run on A and B, but not C. Like this:

role :app, "A", "B"
role :web, "C"

task :foo, :roles => :app do
run "echo hello"
end

Now, if you do "cap foo", it will run the echo command on both A and B.

If you do "cap HOSTS=C foo", it will run the echo command on C,
regardless of the :roles parameter to the task.

If you do "cap HOSTFILTER=C foo", it will not run the echo command at
all, because the intersection of (A B) and (C) is an empty set. (There
are no hosts in foo's host list that match C.)

If you do "cap HOSTFILTER=A foo", it will run the echo command on only
A, because (A B) intersected with (A) is (A).

Lastly, if you do "cap HOSTFILTER=A,B,C foo", it will run the echo
command on A and B (but not C), because (A B) intersected with (A B C)
is (A B).

To summarize: HOSTS completely overrides the hosts or roles declaration
of the task, and forces everything to run against the specified host(s).
The HOSTFILTER, on the other hand, simply filters the existing hosts
against the given list, choosing only those servers that are already in
the tasks server list.

Hope that's clearer,

Jamis

[Gerhardus Geldenhuis]

That is brilliant clear, thanks.

Basically defining a task with a role safeguards you against using the
task on a server where it would'nt make any sense.

I have been thinking of tasks as a bit more abstract. For me copy
files to a apache server is a generic task but specific to webservers.
However my server list is not static and might change 3 times a day
and I have found it strange to include my server lists as part of my
capistrano program. I guess you can import an external file that
contains a list of servers and dynamically build your role members. It
boils down to how you use capistrano.

Regards

[Jamis Buck]

If you're doing a lot with roles that change a lot, you might want to
consider using dynamic roles. See the "&block" section, here:

http://wiki.capify.org/article/Role#.26block

- Jamis

[Pete Hodgson]

Great Jamis, that's exactly what I was looking for. Thanks, and thanks
for a great product!

Hope you don't mind if I add some of this explanation to the Stack
Overflow question so others can find it.

Cheers,
Pete

[Jamis Buck]

By all means, Pete. I'm glad it was helpful, and please feel free to
reuse elsewhere.

- Jamis