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SSHKit is a toolkit for running commands in a structured way on one or more servers.

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How might it work?

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"
        execute "node", "socket_server.js"
      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.

Parallel

Notice on the on() call the in: :sequence option, the following will do what you might expect:

on(in: :parallel) { ... }
on(in: :sequence, wait: 5) { ... }
on(in: :groups, limit: 2, wait: 5) { ... }

The default is to run in: :parallel which has 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.

Synchronisation

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 all_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 all_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

The Command Map

It's often a problem that programmatic SSH sessions don't have the same environment variables as interactive sessions.

A problem often arises when calling out to executables expected to be on the $PATH. Under conditions without dotfiles or other environmental configuration, $PATH may not be set as expected, and thus executables are not found where expected.

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 )

By contrast, the following won't modify the command at all:

with path: '/usr/local/bin/rbenv/shims:$PATH' do
  execute 'ruby --version'
end

Will execute, without mapping the environmental variables, or querying the command map:

ruby --version

(This behaviour is sometimes considered confusing, but it has mostly to do with shell escaping: in the case of whitespace in your command, or newlines, we have no way of reliably composing a correct shell command from the input given.)

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

To make clear the environment is being deferred to, the /usr/bin/env prefix is applied to all commands. Although this is what happens anyway when one would simply attempt to execute ruby, making it explicit hopefully leads 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 opportunity 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.

Output Handling

Example Output

By default, the output format is set to :pretty:

SSHKit.config.format = :pretty

However, if you prefer minimal output, :dot format will simply output red or green dots based on the success or failure of operations.

To output directly to $stdout without any formatting, you can use:

SSHKit.config.output = $stdout

Output Verbosity

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 available to override with SSHKit.config.output_verbosity=, and defaults to Logger::INFO.

At present the Logger::WARN, ERROR and FATAL are not used.

Connection Pooling

SSHKit uses a simple connection pool (enabled by default) to reduce the cost of negotiating a new SSH connection for every on() block. Depending on usage and network conditions, this can add up to a significant time savings. In one test, a basic cap deploy ran 15-20 seconds faster thanks to the connection pooling added in recent versions of SSHKit.

To prevent connections from "going stale", an existing pooled connection will be replaced with a new connection if it hasn't been used for more than 30 seconds. This timeout can be changed as follows:

SSHKit::Backend::Netssh.pool.idle_timeout = 60 # seconds

If you suspect the connection pooling is causing problems, you can disable the pooling behaviour entirely by setting the idle_timeout to zero:

SSHKit::Backend::Netssh.pool.idle_timeout = 0 # disabled

SSHKit Related Blog Posts

SSHKit Gem Basics

SSHKit Gem Part 2

Embedded Capistrano with SSHKit