SSHKit is a toolkit for running commands in a structured way on one or more servers.
The typical use-case looks something like this:
require 'sshkit/dsl'
on %w{1.example.com 2.example.com}, in: :sequence, wait: 5 do
within "/opt/sites/example.com" do
as :deploy do
with rails_env: :production do
rake "assets:precompile"
runner "S3::Sync.notify"
end
end
end
end
One will notice that it's quite low level, but exposes a convenient API, the
as()
/within()
/with()
are nestable in any order, repeatable, and stackable.
When used inside a block in this way, as()
and within()
will guard
the block they are given with a check.
In the case of within()
, an error-raising check will be made that the directory
exists; for as()
a simple call to sudo su -<user> whoami
wrapped in a check for
success, raising an error if unsuccessful.
The directory check is implemented like this:
if test ! -d <directory>; then echo "Directory doesn't exist" 2>&1; false; fi
And the user switching test implemented like this:
if ! sudo su <user> -c whoami > /dev/null; then echo "Can't switch user" 2>&1; false; fi
According to the defaults, any command that exits with a status other than 0
raises an error (this can be changed). The body of the message is whatever was
written to stdout by the process. The 1>&2
redirects the standard output
of echo to the standard error channel, so that it's available as the body of
the raised error.
Helpers such as runner()
and rake()
which expand to execute(:rails, "runner", ...)
and
execute(:rake, ...)
are convenience helpers for Ruby, and Rails based apps.
Notice on the on()
call the in: :sequence
option, the following will do
what you might expect:
on(in: :parallel, limit: 2) { ...}
on(in: :sequence, wait: 5) { ... }
on(in: :groups, limit: 2, wait: 5) { ... }
The default is to run in: :parallel
with no limit, if you have 400 servers,
this might be a problem, and you might better look at changing that to run in
groups, or sequence.
Groups were designed in this case to relieve problems (mass Git checkouts) where you rely on a contested resource that you don't want to DDOS by hitting it too hard.
Sequential runs were intended to be used for rolling restarts, amongst other similar use-cases.
The on()
block is the unit of synchronisation, one on()
block will wait
for all servers to complete before it returns.
For example:
all_servers = %w{one.example.com two.example.com three.example.com}
site_dir = '/opt/sites/example.com'
# Let's simulate a backup task, assuming that some servers take longer
# then others to complete
on servers do |host|
in site_dir do
execute :tar, '-czf', "backup-#{host.hostname}.tar.gz", 'current'
# Will run: "/usr/bin/env tar -czf backup-one.example.com.tar.gz current"
end
end
# Now we can do something with those backups, safe in the knowledge that
# they will all exist (all tar commands exited with a success status, or
# that we will have raised an exception if one of them failed.
on servers do |host|
in site_dir do
backup_filename = "backup-#{host.hostname}.tar.gz"
target_filename = "backups/#{Time.now.utc.iso8601}/#{host.hostname}.tar.gz"
puts capture(:s3cmd, 'put', backup_filename, target_filename)
end
end
It's often a problem that programatic SSH sessions don't share the same environmental variables as sessions that are started interactively.
This problem often comes when calling out to executables, expected to be on
the $PATH
which, under conditions without dotfiles or other environmental
configuration are not where they are expected to be.
To try and solve this there is the with()
helper which takes a hash of variables and makes them
available to the environment.
with path: '/usr/local/bin/rbenv/shims:$PATH' do
execute :ruby, '--version'
end
Will execute:
( PATH=/usr/local/bin/rbenv/shims:$PATH /usr/bin/env ruby --version )
Often more preferable is to use the command map.
The command map is used by default when instantiating a Command object
The command map exists on the configuration object, and in principle is quite simple, it's a Hash structure with a default key factory block specified, for example:
puts SSHKit.config.command_map[:ruby]
# => /usr/bin/env ruby
The /usr/bin/env
prefix is applied to all commands, to make clear that the
environment is being deferred to to make the decision, this is what happens
anyway when one would simply attempt to execute ruby
, however by making it
explicit, it was hoped that it might lead people to explore the documentation.
One can override the hash map for individual commands:
SSHKit.config.command_map[:rake] = "/usr/local/rbenv/shims/rake"
puts SSHKit.config.command_map[:rake]
# => /usr/local/rbenv/shims/rake
Another oportunity is to add command prefixes:
SSHKit.config.command_map.prefix[:rake].push("bundle exec")
puts SSHKit.config.command_map[:rake]
# => bundle exec rake
SSHKit.config.command_map.prefix[:rake].unshift("/usr/local/rbenv/bin exec")
puts SSHKit.config.command_map[:rake]
# => /usr/local/rbenv/bin exec bundle exec rake
One can also override the command map completely, this may not be wise, but it would be possible, for example:
SSHKit.config.command_map = Hash.new do |hash, command|
hash[command] = "/usr/local/rbenv/shims/#{command}"
end
This would effectively make it impossible to call any commands which didn't provide an executable in that directory, but in some cases that might be desirable.
Note: All keys should be symbolised, as the Command object will symbolize it's first argument before attempting to find it in the command map.
The output handling comprises two objects, first is the output itself, by default this is $stdout, but can be any object responding to a StringIO-like interface. The second part is the formatter.
The formatter and output have a strange relationship:
SSHKit.config.output = SSHKit.config.formatter.new($stdout)
The formatter will typically delegate all calls to the output, depending
on it's implementation it will almost certainly override the implementation of
write()
(alias <<()
) and query the objects it receives to determine what
should be printed.
By default calls to capture()
and test()
are not logged, they are used
so frequently by backend tasks to check environmental settings that it
produces a large amount of noise. They are tagged with a verbosity option on
the Command
instances of Logger::DEBUG
. The default configuration for
output verbosity is avaialble to override with SSHKit.config.output_verbosity=
,
and defaults to Logger::INFO
.
At present the Logger::WARN
, ERROR
and FATAL
are not used.
- No handling of slow / timed out connections
- No handling of slow / hung remote commands
No built-in way to background() something (execute and background the process).- No environment handling (sshkit might not need to care)
No arbitraryHost
properties (example storingroles
on servers, or other metadata that might be useful in theon()
block)No log/warning facility (passing Log messages to the output would work) A log object could be made available globally which would emit a LogMessage type object which would be recognised by the formatters that need to care about them.No verbosity control, commands should have aLogger::LEVEL
on them, user-generated should be at a high level, the commands auto-generated from the guards and checks from as() and within() should have a lower level.Decide ifexecute()
(and friends) should raise on non-zero exit statuses or not, perhaps a family of similarly named bang methods should be the ones to raise. (Perhapstest()
should be a way toexecute()
without raising, andexecute()
and friends should always raise)It would be nice to be able to saySSHKit.config.formatter = :pretty
and have that method setter do the legwork of updatingSSHKit.config.output
to be an instance of the correct formatter class wrapping the existing output stream.- No "trace" level debugging for internal stuff, the debug level should be reserved for client-level debugging, with trace being (int -1) used internally for logging about connection opening, closing, timing out, etc.
- No method for uploading or downloading files, or the same for saving/loading a string to/from a remote file.
- No closing of connections, the abstract backend class should include a cleanup method which is empty but can be overriden by other implementations.
- No conncetion pooling, the
connection
method of the NetSSH backend could easily be modified to look into some connection factory for it's objects, saving half a second when running lots ofon()
blocks. - Documentation! (YARD style)
- Wrap all commands in a known shell, that is that
execute('uptime')
should be converted intosh -c 'uptime'
to ensure that we have a consistent shell experience. - There's no suitable host parser that accepts
Host.new('user@ip:port')
, it will decode auser@hostname:port
, but IP addresses don't work. - If Net::SSH raises
IOError
(as it does when authentication fails) this needs to be caught, and re-raised as some kind of ConnectionFailed error.