JSON

JSON Framing

Use Akka Stream JsonFraming to split a stream of ByteString elements into ByteString snippets of valid JSON objects.

See ScalaDSL JsonFraming or JavaDSL JsonFraming

Akka documentation

JsonFraming.objectScanner(maximumObjectLength: Int): Flow[ByteString, ByteString, NotUsed]

Returns a Flow that implements a “brace counting” based framing stage for emitting valid JSON chunks.

Typical examples of data that one may want to frame using this stage include:

Very large arrays:

[{"id": 1}, {"id": 2}, [...], {"id": 999}]

Multiple concatenated JSON objects (with, or without commas between them):

{"id": 1}, {"id": 2}, [...], {"id": 999}

The framing works independently of formatting, i.e. it will still emit valid JSON elements even if two elements are separated by multiple newlines or other whitespace characters. And of course is insensitive (and does not impact the emitting frame) to the JSON object’s internal formatting.

Streaming of nested structures

The method above is great for a stream of “flat” JSON objects (an array or just a stream of objects) but doesn’t work for the many use-cases that involve a nested structure. A common example is the response of a database, which might look more like this:

{
  "size": 100,
  "rows": [
    {"id": 1, "doc": {}}
    {"id": 2, "doc": {}}
    ...
  ]
}

The JSON reading module offers a flow, which allows to stream specific parts of that JSON structure. In this particular example, only the rows array is interesting for the application, more specifically even: only the doc inside each element of the array.

Project Info: Alpakka JSON Streaming
Artifact
com.lightbend.akka
akka-stream-alpakka-json-streaming
1.0-M2
JDK versions
OpenJDK 8
Scala versions2.12.7, 2.11.12
JPMS module nameakka.stream.alpakka.json.streaming
License
Readiness level
Community-driven
Since 0.19, 2018-05-09
Home pagehttps://doc.akka.io/docs/alpakka/current/
API documentation
Forums
Release notesIn the documentation
IssuesGithub issues
Sourceshttps://github.com/akka/alpakka

Artifacts

sbt
libraryDependencies += "com.lightbend.akka" %% "akka-stream-alpakka-json-streaming" % "1.0-M2"
Maven
<dependency>
  <groupId>com.lightbend.akka</groupId>
  <artifactId>akka-stream-alpakka-json-streaming_2.12</artifactId>
  <version>1.0-M2</version>
</dependency>
Gradle
dependencies {
  compile group: 'com.lightbend.akka', name: 'akka-stream-alpakka-json-streaming_2.12', version: '1.0-M2'
}

The table below shows direct dependencies of this module and the second tab shows all libraries it depends on transitively.

Direct dependencies
OrganizationArtifactVersionLicense
com.github.jsurferjsurfer-jackson1.4.3MIT License
com.github.jsurferjsurfer1.4.3MIT License
com.typesafe.akkaakka-stream_2.122.5.19Apache License, Version 2.0
org.scala-langscala-library2.12.7BSD 3-Clause
Dependency tree
com.github.jsurfer    jsurfer-jackson    1.4.3    MIT License
    com.fasterxml.jackson.core    jackson-databind    2.9.4    The Apache Software License, Version 2.0
        com.fasterxml.jackson.core    jackson-annotations    2.9.0    The Apache Software License, Version 2.0
        com.fasterxml.jackson.core    jackson-core    2.9.4    The Apache Software License, Version 2.0
    com.github.jsurfer    jsurfer-core    1.4.3    MIT License
        org.antlr    antlr4-runtime    4.7.1    The BSD License
com.github.jsurfer    jsurfer    1.4.3    MIT License
com.typesafe.akka    akka-stream_2.12    2.5.19    Apache License, Version 2.0
    com.typesafe.akka    akka-actor_2.12    2.5.19    Apache License, Version 2.0
        com.typesafe    config    1.3.3    Apache License, Version 2.0
        org.scala-lang.modules    scala-java8-compat_2.12    0.8.0    BSD 3-clause
            org.scala-lang    scala-library    2.12.7    BSD 3-Clause
        org.scala-lang    scala-library    2.12.7    BSD 3-Clause
    com.typesafe.akka    akka-protobuf_2.12    2.5.19    Apache License, Version 2.0
        org.scala-lang    scala-library    2.12.7    BSD 3-Clause
    com.typesafe    ssl-config-core_2.12    0.3.6    Apache-2.0
        com.typesafe    config    1.3.3    Apache License, Version 2.0
        org.scala-lang.modules    scala-parser-combinators_2.12    1.1.1    BSD 3-clause
            org.scala-lang    scala-library    2.12.7    BSD 3-Clause
        org.scala-lang    scala-library    2.12.7    BSD 3-Clause
    org.reactivestreams    reactive-streams    1.0.2    CC0
    org.scala-lang    scala-library    2.12.7    BSD 3-Clause
org.scala-lang    scala-library    2.12.7    BSD 3-Clause

Example

To define which parts of the structure you want to stream the module supports JsonPath notation. For example:

  • Stream all elements of the nested rows array: $.rows[*]
  • Stream the value of doc of each element in the array: $.rows[*].doc

To extract the information needed, run a stream through the JsonReader.select flow.

Scala
val results = Source
  .single(ByteString.fromString(baseDocument))
  .via(JsonReader.select("$.rows[*].doc"))
  .runWith(Sink.seq)
Java
final CompletionStage<List<ByteString>> resultStage =
    Source.single(doc)
        .via(JsonReader.select("$.rows[*].doc"))
        .runWith(Sink.seq(), materializer);
Found an error in this documentation? The source code for this page can be found here. Please feel free to edit and contribute a pull request.