Thrift Common Lisp Library

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Using Thrift with Common Lisp

Thrift is a protocol and library for language-independent communication between cooperating processes. The communication takes the form of request and response messages, of which the forms are specified in advance throufh a shared interface definition. A Thrift definition file is translated into Lisp source files, which comprise several definitions:

• Three packages, one for the namespace of the implementation operators, and one each for request and response operators.
• Various type definitions as implementations for Thrift typedef and enum definitions.
• DEF-STRUCT and DEF-EXCEPTION forms for Thrift struct and exception definitions.
• DEF-SERVICE forms for thrift service definitions.

Each service definition expands in a collection of generic function definitions. For each op in the service definition, two functions are defined

• op-request is defined for use by a client. It accepts an additional initial protocol argument, to act as the client proxy for the operation and mediate the interaction with a remote process through a Thrift-encoded transport stream.
• op-response is defined for use by a server. It accepts a single protocol argument. A server uses it to decode the request message, invoke the base op function with the message arguments, encode and send the the result as a response, and handles exceptions.

The client interface is one operator

• with-client (variable location) . body : creates a connection in a dynamic context and closes it upon exit. The variable is bound to a client proxy stream/protocol instance, which wraps the base i/o stream - socket, file, etc, with an operators which implement the Thrift protocol and transport mechanisms.

The server interface combines server and service objects

• serve (location service) : accepts connections on the designated port and responds to requests of the service's operations.

Building

The Thrift Common Lisp library is packaged as the ASDF[1] system thrift. It depends on the systems

• puri-ppcre[2] : for the thrift uri class
• closer-mop[3] : for class metadata
• trivial-utf-8[4] : for string codecs

In order to build it, register those systems with ASDF and evaluate

(asdf:load-system :thrift)

This will compile and load the Lisp compiler for Thrift definition files, the transport and protocol implementations, and the client and server interface functions. In order to use Thrift in an application, one must also author and/or load the interface definitions for the remote service.[5] If one is implementing a service, one must also define the actual functions to which Thrift is to act as the proxy interface. The remainder of this document follows the Thrift tutorial to illustrate how to perform the steps

• implement the service
• translate the Thrift IDL
• load the Lisp service interfaces
• run a server for the service
• use a client to access the service remotely

Note that, if one is to implement a new service, one will also need to author the IDL files, as there is no facility to generate them from a service implementation.

Implement the Service

The tutorial comprises serveral functions: add, ping, zip, and calculate. Each translatd IDL corresponds to three packages. In this case, the packages

• :tutorial
• :tutorial-implementation
• :tutorial-response

The first package is for the service implementation.

;; define the base operations

(in-package :tutorial-implementation)

(defun add ( num1 num2)
  (format t "~&Asked to add ~A and ~A." num1 num2)
  (+ num1 num2))

(defun ping ()
  (print :ping))

(defun zip ()
  (print :zip))

(defun calculate (logid task)
  (calculate-op (work-op task) (work-num1 task) (work-num2 task)))

(defgeneric calculate-op (op arg1 arg2)
  (:method :around (op arg1 arg2)
    (let ((result (call-next-method)))
      (format t "~&Asked to calculate: ~d on  ~A and ~A = ~d." op arg1 arg2 result)
      result))

  (:method ((op (eql operation.add)) arg1 arg2)
    (+ arg1 arg2))
  (:method ((op (eql operation.subtract)) arg1 arg2)
    (- arg1 arg2))
  (:method ((op (eql operation.multiply)) arg1 arg2)
    (* arg1 arg2))
  (:method ((op (eql operation.divide)) arg1 arg2)
    (/ arg1 arg2)))

(defun zip () (print 'zip))

Translate the Thrift IDL

IDL files employ the file type thrift. In this case, there are two files to translate

• tutorial.thrift

• shared.thrift As the former includes the latter, one uses it to generate the interfaces:

  $THRIFT/bin/thrift -O ./ --gen cl $THRIFT/tutorial/tutorial.thrift

For the moment, the Lisp backend is present here as #P"THRIFT:compiler;cpp;src;generate;t_cl_generator.cc". In order to use it, copy that file into the analogous location in the Thrift release tree prior to making thrift.

Load the Lisp translated service interfaces

The translator generates two files for each IDL file. For example tutorial-types.lisp and tutorial-vars.lisp. As the parameter definitions may istantiate objects defined in the -types file, the ASDF dependencies must reflect this constraint. For the tutorial, the system could be defined as

(asdf:defsystem :thrift-tutorial
   :depends-on (:thrift)
   :serial t
   :components ((:file "tutorial")
                (:file "tutorial-types")
                (:file "tutorial-vars")))

Run a Server for the Service

The actual service name, as specified in the def-service form in tutorial.lisp, is calculator. Each service definition defines a global variable with the service name and binds it to a service instance whch describes the operations.

In order to start a service, specify a location and the service instance.

(in-package :tutorial)
(serve #u"thrift://127.0.0.1:9091" calculator)

Use a Client to Access the Service Remotely

[in some other process] run the client

(in-package :cl-user)
(use-package :tutorial-request)

(macrolet ((show (form)
             `(format *trace-output* "~%~s =>~{ ~s~}"
                      ',form
                      (multiple-value-list (ignore-errors ,form)))))
  (with-client (protocol #u"thrift://127.0.0.1:9091")
    (show (ping protocol))
    (show (add protocol 1 2))
    (show (add protocol 1 4))

    (show (shared:get-struct protocol 1))

    (let ((task (make-instance 'work
                  :op operation.subtract :num1 15 :num2 10)))
      (show (calculate protocol task))
    
      (setf (work-op task) operation.divide
            (work-num1 task) 1
            (work-num2 task) 0)
      (show (calculate protocol task)))

    (show (zip protocol))))

Status

The initial library version serves as an interface to Cassandra[6] in order to provide access to Datagraph's Cassandra-based RDF store[7]. The code evolved from an initial version which had been submitted to Thift in 2008[[8]].

A demonstration of access through the Cassandra API is among the READMES[[9]].

Issues

optional fields

Where the IDL declares a field options, the def-struct form includes no initform for the slot and the encoding operator skips an unbound slot. This leave some ambiguity with bool fields.

namespace - package equivalence

The IDL specifies a single namespace. The Lisp binding uses three: the implementation, the request interface, and the response interface. The current pattern is:

• namespace : request proxy function, structure types and accessors, exception types, enum types, constants; use :thrift
• namespace-implementation : implementation function, use :thrift, use namespace, but shadow all implementation function names.
• namespace-response : response functions

instantiation protocol :

struct classes are standard classes and exception classes are whatever the implementation prescribes. decoders apply make-struct to an initargs list. particularly at the service end, there are advantages to resourcing structs and decoding with direct side-effects on slot-values

maps:

Maps are now represented as hash tables. As data through the call/reply interface is all statically typed, it is not necessary for the objects to themselves indicate the coding form. Association lists would be sufficient. As the key type is arbitrary, property lists offer no additional convenience: as getf operates with eq a new access interface would be necessary and they would not be available for function application.

[8]: http://markmail.org/thread/4tfa3zbweyg2qwne: thrift jira lisp issue [9]: ./READMES/readme-cassandra.lisp