Writing a Sequence of Documents to Elasticsearch

I’m writing logs during work. I keep them in text files, and I’ve been writing them for many years. Here’s the format:

- some project
  - some task
    - looking at recent changes in X
    I need to modify Y
- some other project
  - some task
    - doing Z

So obvious idea for a project came up: let’s parse those logs, put them in a database and analyze. The database of choice would be Elasticsearch, because it takes unformatted data and provides full text search, as well as Kibana analysis tool.

Here’s the parser API:

case class Project(name: String, log: List[String])

case class Workday(date: DateTime, projects: List[Project])

object LogParser extends RegexParsers {
  def apply(input: String): List[Workday]

Project is a list of lines in text files that were logged after a project name. On a given day there might be several sections logged for different projects, so Workday consists of a list of projects.

For inserting records to Elasticsearch, I decided that I’ll use a different model:

case class ProjectDay(date: DateTime, name: String, log: List[String])

So I added a def toProjectDays: Seq[ProjectDay] method to Workday. Data inserted into the database would come in Seq[ProjectDay] form, flatmapped from all workdays.

Here comes first approach at inserting ProjectDays into Elasticsearch index, with use of elastic4s library. Let’s look at ElasticDao importData method:

class ElasticDao extends JsonProtocol {
  val uri = ElasticsearchClientUri("elasticsearch://localhost:9300")
  val client = ElasticClient.transport(uri)

  implicit object ProjectDayIndexable extends Indexable[ProjectDay] {
    override def json(t: ProjectDay): String = t.toJson.toString

  def importData(dbData: Seq[ProjectDay]): Future[Seq[IndexResult]] = {
    client.execute { create index "log"  }

    Future.traverse(dbData) { projectDay =>
      client.execute {
        index into "log" / "days" source projectDay

It works fine for small examples. However, for a real-life work log, accumulated over several years, with around 2600 ProjectDay records, following exception occured:

EsRejectedExecutionException[rejected execution of 
org.elasticsearch.transport.TransportService$4@6e6db429 on 
EsThreadPoolExecutor[index, queue capacity = 200, 
Running, pool size = 8, active threads = 8, queued tasks = 200, completed tasks = 0

What’s the matter? Probably the concurrent write was executed too quickly, so some internal queue (of 200) filled up and crashed the client.

Then I tried to write it all sequentially:

  def importData(dbData: Seq[ProjectDay]): Seq[IndexResult] = {
    Await.result(client.execute { create index "log"  }, Duration.Inf)

    dbData.map { projectDay =>
      Await.result(client.execute {
        index into "log" / "days" source projectDay
      }, Duration.Inf)

Sequential approach works, but reaches quite long execution time: around 20 seconds for our 1700 records. It’s slow.

What could work then? Elastic4s has additional module offering reactive streams API (it needs to be added as separate dependency to build.sbt). I tried it:

  def importData(dbData: List[ProjectDay]) = {
    implicit val system = ActorSystem()
    implicit val materializer = ActorMaterializer()

    val source = Source[ProjectDay](dbData)

    implicit val builder = new RequestBuilder[ProjectDay] {
      def request(projectDay: ProjectDay): BulkCompatibleDefinition =  index into "log" / "days" source projectDay
    val completionFn: () => Unit = { () => system.terminate() }
    val subscriber = client.subscriber[ProjectDay](batchSize = 100, concurrentRequests = 1, completionFn = completionFn)
    val sink = Sink.fromSubscriber(subscriber)


First, val source is defined as a Source from in-memory sequence of ProjectDays. Next, thanks to import com.sksamuel.elastic4s.streams.ReactiveElastic._ and implicit builder with record indexing method, ElasticClient gives us a subscriber to write to. One of its parameters is completionFn closure with system.terminate call - without it importData would hang after completion. It’s then easy to transform the subscriber into akka-streams Sink, combine it with our Source and run the stream.

It loads all data successfully, and does it in just few seconds (including parsing, and stream initialization). Why does it work? Maybe because of using batches (parameter batchSize suggests data is loaded not one record at a time). Maybe because of backpressure. I haven’t researched it in depth.

There is a simpler solution - using bulk operations on sequence of index statements. It wasn’t obvious for me at first, because elastic4s bulk operations docs doesn’t show bulk use on Scala Seq. I received a hint from my work colleagues though - thanks! Here’s the code:

  def importData(dbData: Seq[ProjectDay]): Future[BulkResult] = {
    client.execute { create index "log"  }

    client.execute {
      bulk(dbData.map(index into "log" / "days" source _))

It loads the data as quickly as stream implementation.

Here’s the full source code of my small application