Safe transport of unknown record types in distributed systems.
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README.md

incognito

Different Clojure(Script) serialization protocols like edn, fressian or transit offer different ways to serialize custom types. In general they fall back to maps for unknown record types, which is a reasonable default in many situations. But when you build a distributed data management system parts of your system might not care about the record types while others do. This library safely wraps unknown record types and therefore allows to unwrap them later. It also unifies record serialization between fressian and transit as long as you can express your serialization format in Clojure's default datastructures.

The general idea is that most custom Clojure datatypes (either records or deftypes) can be expressed in Clojure datastructures if you do not need a custom binary format, e.g. for efficiency or performance. With incognito you do not need custom handlers for every serialization format. But you can still provide them of course, once you hit efficiency problems. Incognito is at the moment not supposed to provide serialization directly to storage, so you have to be able to serialize your custom types in memory.

Incognito falls back to a pr-str->read-string roundtrip which is a reasonable, but inefficient and will only work if the type has proper Clojure print+read support.

We use it for instance to carry the custom type of a datascript db in topiq. You don't need to provide a write handler for incognito except for efficiency reasons or if your type is not pr-strable (in which case you should make it then first).

Usage

Add this to your project dependencies: Clojars Project

Include all serialization libraries you need, e.g. for edn support only:

[org.clojure/data.fressian com.cognitect/transit-clj "0.8.297"]
[io.replikativ/incognito "0.2.5"]

In general you can control serialization by write-handlers and read-handlers:

(defrecord Bar [a b])

(def write-handlers (atom {'user.Bar (fn [bar] bar)}))
(def read-handlers (atom {'user.Bar map->Bar}))

To handle custom non-record types you have to transform it into a readable data-structure. You can test the roundtrip directly with incognito-reader and writer:

(require '[clj-time.core :as t])
(require '[clj-time.format :as tf])

(incognito-writer 
  ;; read-handlers
  {'org.joda.time.DateTime (fn [r] (str r))}

  (t/now))

(incognito-reader 
  ;; write-handlers
  {'org.joda.time.DateTime (fn [r] (t/date-time r))}

  {:tag 'org.joda.time.DateTime, :value "2017-04-17T13:11:29.977Z"})

NOTE: The syntax quote for the read handler is necessary so you can deserialize unknown classes.

A write-handler has to return an associative datastructure which is internally stored as an untyped map together with the tag information.

Plugging incognito into your serializer

You need to wrap the base map handler in the different serializers so incognito can wrap the serialization for its own handlers.

(Extracted from the tests):

edn

(require '[incognito.edn :refer [read-string-safe]])

(let [bar (map->Bar {:a [1 2 3] :b {:c "Fooos"}})]
  (= bar (->> bar
              pr-str
              (read-string-safe {})
              pr-str
              (read-string-safe read-handlers))))

transit

(require '[incognito.transit :refer [incognito-write-handler incognito-read-handler]]
         '[cognitect.transit :as transit])

(let [bar (map->Bar {:a [1 2 3] :b {:c "Fooos"}})]
  (= (assoc bar :c "banana")
     (with-open [baos (ByteArrayOutputStream.)]
       (let [writer (transit/writer baos :json
                                    {:handlers {java.util.Map
                                                (incognito-write-handler
                                                 write-handlers)}})]
         (transit/write writer bar)
         (let [bais (ByteArrayInputStream. (.toByteArray baos))
               reader (transit/reader bais :json
                                      {:handlers {"incognito"
                                                  (incognito-read-handler read-handlers)}})]
           (transit/read reader))))))

fressian

(require '[clojure.data.fressian :as fress]
         '[incognito.fressian :refer [incognito-read-handlers
                                      incognito-write-handlers]])

(let [bar (map->Bar {:a [1 2 3] :b {:c "Fooos"}})]
  (= (assoc bar :c "banana")
     (with-open [baos (ByteArrayOutputStream.)]
       (let [w (fress/create-writer baos
                                    :handlers
                                    (-> (merge fress/clojure-write-handlers
                                               (incognito-write-handlers write-handlers))
                                        fress/associative-lookup
                                        fress/inheritance-lookup))] ;
         (fress/write-object w bar)
         (let [bais (ByteArrayInputStream. (.toByteArray baos))]
           (fress/read bais
                       :handlers
                       (-> (merge fress/clojure-read-handlers
                                  (incognito-read-handlers read-handlers))
                           fress/associative-lookup)))))))

ClojureScript

For dashed namespace names you need a custom printer to be ClojureScript conform.

(defmethod print-method some_namespace.Bar [v ^java.io.Writer w]
  (.write w (str "#some-namespace.Bar" (into {} v))))

TODO

  • move serialization dependencies into dev profile

Changelog

0.2.5

  • ClojureScript fressian support (for konserve filestore on node.js). Thanks to Ferdinand Kühne!

License

Copyright © 2015-2019 Christian Weilbach, Ferdinand Kühne

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.