abracad

Abracad is a Clojure library for de/serializing Clojure data structures with Avro, leveraging the Java Avro implementation.

Abracad supports: a generic mapping between Avro and Clojure data for arbitrary schemas; customized protocol-based mappings between Avro records and any JVM types; and “schema-less” EDN-in-Avro serialization of arbitrary Clojure data.

Installation

Abracad is available on Clojars. Add this :dependency to your Leiningen project.clj:

[com.damballa/abracad "0.4.13"]

Usage

Example usage follows; detailed API documentation available, generated via codox.

Schemas

Avro schemas may be parsed from JSON (from either strings or input streams), from the Clojure data representation of a JSON schema, or from existing Avro Schema objects.

(require '[abracad.avro :as avro])

(def schema
  (avro/parse-schema
   {:type :record
    :name "LongList"
    :aliases ["LinkedLongs"]
    :fields [{:name "value", :type :long}
             {:name "next", :type ["LongList", :null]}]}))

The parse-schema function may be passed multiple schemas, in which case later schemas may reference types defined in earlier schemas. The result is the schema generated from the final argument.

Basic de/serialization

Abracad provides functions which act as a thin layer over the Java Avro interface, plus Clojure generic datum reader and writer implementations which allow Clojure data structures to be directly de/serialized.

(with-open [adf (avro/data-file-writer "snappy" schema "example.avro")]
  (.append adf {:value 0, :next nil})
  (.append adf {:value 8, :next {:value 16, :next nil}}))

(with-open [adf (avro/data-file-reader "example.avro")]
  (doall (seq adf)))
;;=> ({:value 0, :next nil} {:value 8, :next {:value 16, :next nil}})

The Avro type deserialization mappings are as follows:

Numeric primitives deserialize as their Java counterparts
Strings currently always deserialize as Strings
Enums deserialize as keywords
Arrays currently always deserialize as persistent vectors
Maps deserialize as persistent maps
Fixed values currently always deserialize as primitive byte arrays
Bytes values currently always deserialize as primitive byte arrays
Records deserialize as maps with keyword field names and :type metadata indicating the Avro schema name

The Avro specification allows field names to contain the _ character but disallows the - character. Clojure keywords frequently contain - but rarely contain _. Abracad attempts to work around this difference by mapping _ in Avro field names to - in Clojure symbols and vice-versa. The current implementation of this conversion does not handle keywords containing _ instead, which is probably a bug. This mangling may be disabled by binding abracad.avro.util/*mangle-names* to false.

Record de/serialization tweaking

In addition to the generic map de/serialization, records may also be generically de/serialized as vectors. During serialization, whenever a record is expected and a vector is encountered, the vector will be serialized by matching fields by position, so long as the expected and provided numbers of fields match.

During deserialization, a record schema with the annotation :abracad.reader set to "vector" will be deserialized as a vector, with fields encoded by position.

(let [schema (avro/parse-schema
              {:name "example", :type "record",
               :fields [{:name "left", :type "string"}
                        {:name "right", :type "long"}]
               :abracad.reader "vector"})]
  (->> ["foo" 31337]
       (avro/binary-encoded schema)
       (avro/decode schema)))
;;=> ["foo" 31337"]

Maps serialized as records will be checked to ensure that they do not have any extra entries not encoded by the schema, raising an exception if extra entries are present. This check may be avoided for individual records by including :type metadata matching the schema. The check may be en/disabled recursively for a record and all contained records via the :abracad.avro/unchecked metadata.

Custom record de/serialization

During union and record serialization, Abracad uses a protocol to determine an object's Avro schema name and to access its fields. During deserialization, Abracad uses a facility directly analogous to the Clojure Reader *data-readers* facility to find custom deserialization constructor functions. These may be used to extend Avro de/serialization to arbitrary existing types.

(import 'java.net.InetAddress)

(extend-type InetAddress
  avro/AvroSerializable
  (schema-name [_] "ip.address")
  (field-get [this field] (case field :address (.getAddress this)))
  (field-list [this] #{:address}))

(defn ->InetAddress
  [address] (InetAddress/getByAddress address))

(def schema
  (avro/parse-schema
   {:type :record
    :name 'ip.address
    :fields [{:name :address
              :type [{:type :fixed, :name "IPv4", :size 4}
                     {:type :fixed, :name "IPv6", :size 16}]}]}))

(binding [avro/*avro-readers* {'ip/address #'->InetAddress}]
  (with-open [adf (avro/data-file-writer schema "example.avro")]
    (.append adf (InetAddress/getByName "8.8.8.8"))
    (.append adf (InetAddress/getByName "8::8")))

  (with-open [adf (avro/data-file-reader "example.avro")]
    (doall (seq adf))))
;;=> (#<Inet4Address /8.8.8.8> #<Inet6Address /8:0:0:0:0:0:0:8>)

EDN-in-Avro

Abracad supports expressing EDN data structures as Avro records in the abracad.avro.edn Avro namespace. The new-schema function in the same-named Clojure namespace returns schemas which express a superset of EDN capturing most commonly-used Clojure constructs. These allow using Avro for Clojure data without pre-defining application-specific schemas.

(require '[abracad.avro.edn :as aedn])

(def schema (aedn/new-schema))

(->> {:foo ['bar "baz" 1337]}
     (avro/binary-encoded schema)
     (avro/decode schema))
;;=> {:foo [bar "baz" 1337]}

Hadoop MapReduce integration

Avro 1.7.5 and later supports configurable “data models” for datum reading, writing, and comparison in Hadoop MapReduce jobs. Abracad 0.4.0 and later provides a ClojureData class which can be passed to the AvroJob/setDataModelClass static method in order to map job Avro input and output directly to and from Clojure data structures.

TODO

These are the early days. Still to be done:

Kick the tires on the interface. There may be glaring holes.
Write more exhaustive tests, to cover the full range of types.
Figure out a cleaner way of handling _ vs -.
Dynamically generate schema-specific datum reader/writer implementations. All the speed of generating & compiling de/serialization classes from schemas, but with none of the ahead-of-time hassle.

License

Distributed under your choice of the Eclipse Public License or the Apache License, Version 2.0.

damballa/abracad

damballa

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