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# Distributed Places
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The `racket/place/distributed` library provides support for distributed
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programming.
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The example bellow demonstrates how to launch a remote racket node
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instance, launch remote places on the new remote node instance, and
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start an event loop that monitors the remote node instance.
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The example code can also be found in
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`"racket/distributed/examples/named/master.rkt"`.
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```racket
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#lang racket/base
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(require racket/place/distributed
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racket/class
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racket/place
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racket/runtime-path
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"bank.rkt"
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"tuple.rkt")
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(define-runtime-path bank-path "bank.rkt")
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(define-runtime-path tuple-path "tuple.rkt")
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(provide main)
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(define (main)
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(define remote-node (spawn-remote-racket-node
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"localhost"
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#:listen-port 6344))
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(define tuple-place (supervise-place-at
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remote-node
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#:named 'tuple-server
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tuple-path
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'make-tuple-server))
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(define bank-place (supervise-place-at
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remote-node bank-path
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'make-bank))
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(message-router
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remote-node
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(after-seconds 4
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(displayln (bank-new-account bank-place 'user0))
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(displayln (bank-add bank-place 'user0 10))
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(displayln (bank-removeM bank-place 'user0 5)))
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(after-seconds 2
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(define c (connect-to-named-place remote-node
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'tuple-server))
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(define d (connect-to-named-place remote-node
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'tuple-server))
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(tuple-server-hello c)
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(tuple-server-hello d)
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(displayln (tuple-server-set c "user0" 100))
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(displayln (tuple-server-set d "user2" 200))
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(displayln (tuple-server-get c "user0"))
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(displayln (tuple-server-get d "user2"))
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(displayln (tuple-server-get d "user0"))
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(displayln (tuple-server-get c "user2"))
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)
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(after-seconds 8
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(node-send-exit remote-node))
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(after-seconds 10
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(exit 0))))
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```
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Figure 1: examples/named/master.rkt
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The `spawn-remote-racket-node` primitive connects to `"localhost"` and
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starts a racloud node there that listens on port 6344 for further
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instructions. The handle to the new racloud node is assigned to the
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`remote-node` variable. Localhost is used so that the example can be run
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using only a single machine. However localhost can be replaced by any
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host with ssh publickey access and racket. The `supervise-place-at`
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creates a new place on the `remote-node`. The new place will be
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identified in the future by its name symbol `'tuple-server`. A place
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descriptor is expected to be returned by invoking `dynamic-place` with
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the `tuple-path` module path and the `'make-tuple-server` symbol.
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The code for the tuple-server place exists in the file `"tuple.rkt"`.
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The `"tuple.rkt"` file contains the use of `define-named-remote-server`
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form, which defines a RPC server suitiable for invocation by
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`supervise-place-at`.
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```racket
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#lang racket/base
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(require racket/match
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racket/place/define-remote-server)
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(define-named-remote-server tuple-server
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(define-state h (make-hash))
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(define-rpc (set k v)
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(hash-set! h k v)
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v)
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(define-rpc (get k)
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(hash-ref h k #f))
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(define-cast (hello)
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(printf "Hello from define-cast\n")
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(flush-output)))
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```
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Figure 2: examples/named/tuple.rkt
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The `define-named-remote-server` form takes an identifier and a list of
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custom expressions as its arguments. From the identifier a place-thunk
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function is created by prepending the `make-` prefix. In this case
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`make-tuple-server`. The `make-tuple-server` identifier is the
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`place-function-name` given to the `supervise-named-dynamic-place-at`
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form above. The `define-state` custom form translates into a simple
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`define` form, which is closed over by the `define-rpc` form.
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The `define-rpc` form is expanded into two parts. The first part is the
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client stubs that call the rpc functions. The client function name is
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formed by concatenating the `define-named-remote-server` identifier,
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`tuple-server`, with the RPC function name `set` to form
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`tuple-server-set`. The RPC client functions take a destination argument
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which is a `remote-connection%` descriptor and then the RPC function
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arguments. The RPC client function sends the RPC function name, `set`,
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and the RPC arguments to the destination by calling an internal function
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`named-place-channel-put`. The RPC client then calls
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`named-place-channel-get` to wait for the RPC response.
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The second expansion part of `define-rpc` is the server implementation
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of the RPC call. The server is implemented by a match expression inside
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the `make-tuple-server` function. The match clause for
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`tuple-server-set` matches on messages beginning with the `'set` symbol.
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The server executes the RPC call with the communicated arguments and
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sends the result back to the RPC client.
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The `define-cast` form is similar to the `define-rpc` form except there
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is no reply message from the server to client
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```racket
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(module tuple racket/base
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(require racket/place
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racket/match)
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(define/provide
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(tuple-server-set dest k v)
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(named-place-channel-put dest (list 'set k v))
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(named-place-channel-get dest))
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(define/provide
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(tuple-server-get dest k)
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(named-place-channel-put dest (list 'get k))
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(named-place-channel-get dest))
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(define/provide
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(tuple-server-hello dest)
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(named-place-channel-put dest (list 'hello)))
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(define/provide
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(make-tuple-server ch)
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(let ()
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(define h (make-hash))
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(let loop ()
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(define msg (place-channel-get ch))
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(define (log-to-parent-real
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msg
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#:severity (severity 'info))
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(place-channel-put
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ch
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(log-message severity msg)))
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(syntax-parameterize
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((log-to-parent (make-rename-transformer
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#'log-to-parent-real)))
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(match
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msg
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((list (list 'set k v) src)
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(define result (let () (hash-set! h k v) v))
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(place-channel-put src result)
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(loop))
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((list (list 'get k) src)
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(define result (let () (hash-ref h k #f)))
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(place-channel-put src result)
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(loop))
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((list (list 'hello) src)
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(define result
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(let ()
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(printf "Hello from define-cast\n")
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(flush-output)))
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(loop))))
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loop))))
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```
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Figure 3: Expansion of define-named-remote-server
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