Atomic Store

Deprecation notice

We stopped using this library in production at Artsy in early December 2017. It served us well for 18 months, but we opted for another approach for managing an atomic event log, relying on row-based database locks and serializing the event record as a JSON list. Feel free to use it if it suits your purposes, but this repository will no longer be maintained.

Meta

• State: deprecated
• Point People: @acjay
• Point People: @bhoggard

Atomic Store is a system for managing persistent streams of atomic events, with strict consistency. It is intended for systems in which only one event can be admitted to a canonical event log at a time, contingent upon past events. It exists to maintain the atomicity of handling of incoming events, but outsources the actual validation logic back to the event originator. In a sense, the idea here is to do as little as possible to meet this goal, but in a way that is as practical as possible.

Philosophy

Atomic Store is built on top of Akka Persistence, which is designed to natively support highly scalable distributed systems with relaxed consistency. A distributed system can maximize its scalability by reducing coupling between its components, and synchronization of state changes is one such coupling. The general approach to relaxed consistency is to take compensatory actions to rectify inconsistencies between distributed components, in retrospect. But this is complex, and not desirable in all situations. Atomic Store is designed for situations where strict consistency is more desirable or appropriate than extreme scalability.

In the actor framework, individual actors are single-threaded and cannot be preempted mid-process. Consequently, consistency is guaranteed in the course of processing a message. This would be perfect for atomic validation+persistence, except for the fact that it's desirable to have validation potentially be asynchronous. Akka, out of the box, does not provide an answer for this. It's pretty easy to achieve this behavior with an actor stashes incoming messages while it is awaiting validation of the current one, and that's exactly what this project accomplishes.

Running

At the moment, the only thing you can do is execute the tests, by running test from the SBT prompt.

Integrating into a project

Include the following line in your build.sbt:

libraryDependencies ++= Seq(
  "net.artsy" %% "atomic-store" % "0.0.6")

Then, in your project, you will want to instantiate an atomic store matching your event types:

object MyEventStore extends AtomicEventStore[MyEventType, MyRejectionReasonType](myTimeoutReason)   

In your start up code, you'll start the Receptionist actor, which serves as the store's entry point:

case class MyEventStore(
  storeTimeout:     FiniteDuration,
  journalPluginId:  String,
  snapshotPluginId: String
)(
  implicit
  system:  ActorSystem,
  ec:      ExecutionContextExecutor
) {
  import MyEventStore._
  
  val receptionist = system.actorOf(receptionistProps(storeTimeout, journalPluginId, snapshotPluginId))
}

At this point, you'll be able to persist events and check the store by sending messages to the receptionist.

To work within a cluster, it's important that only one instance of each event log be alive within the cluster. This can be accomplished by instantiating the receptionist as a cluster singleton. This might look like:

case class MyEventStore(
  storeTimeout:     FiniteDuration,
  journalPluginId:  String,
  snapshotPluginId: String
)(
  implicit
  system:  ActorSystem,
  ec:      ExecutionContextExecutor,
  timeout: Timeout
) {
  import MyEventStore._

  val singletonProps = ClusterSingletonManager.props(
    singletonProps     = receptionistProps(storeTimeout, journalPluginId, snapshotPluginId),
    terminationMessage = PoisonPill,
    settings           = ClusterSingletonManagerSettings(system)
  )
  val manager = system.actorOf(singletonProps, "receptionist")
  
  val path = manager.path.toStringWithoutAddress
  val proxyProps = ClusterSingletonProxy.props(
    singletonManagerPath = path,
    settings             = ClusterSingletonProxySettings(system)
  )
  val receptionist = system.actorOf(proxyProps)
}

In a project, it's likely you will want some sort of server-push mechanism to notify clients of new events. Rather than containing this logic. This code can likely be located within the handler of the result.

You will likely also want to use Protobuf and a custom serializer for high-performance serialization of messages to and from Atomic Store. A sample .proto file:

syntax = "proto2";

package net.artsy.auction.protobuf;

// Lot Event Store messages

message AtomicStoreMessageProto {
    oneof type {
        QueryEventsProto query_event = 1;
        StoreIfValidProto store_if_valid = 2;
        ValidationRequestProto validation_request = 3;
        ValidationResponseProto validation_response = 4;
        ResultProto result = 5;
    }
}

message QueryEventsProto {
    optional string scope_id = 6;
}

message StoreIfValidProto {
    optional bytes event = 7;
}

message ValidationRequestProto {
    optional bytes prospective_event = 8;
    repeated bytes past_events = 9;
}

message ValidationResponseProto {
    optional bool validation_did_pass = 10;
    optional bytes event = 11;
    optional string reason = 12;
    optional MetaProto meta = 13;
}

message ResultProto {
    optional bool was_accepted = 14;
    optional bytes prospective_event = 15;
    repeated bytes stored_event_list = 16;
    optional string reason = 17;
    optional MetaProto meta = 18;
}

message MetaProto {
    // Domain-specific fields
}

In your Akka Serializer implementation, you'll then want to serialize your events themselves to a byte array, perhaps deferring to a separate serializer.

Technology

Atomic Store is built using Scala, the Akka framework, and associated libraries. Specifically, here are the core technologies being used, with links to documentation:

• Scala (see tutorials)
• Akka
  • Actor basics
  • Persistence

Releasing new versions

For testing changes:

1. Bump the version in build.sbt as appropriate, and add -SNAPSHOT to the end of the version number.
2. Update the libraryDependencies line above in anticipation of the next version.
3. Use the sbt publish-signed task to push snapshots to Maven Central.
4. Update the Changelog as noteworthy changes are made.
5. During the testing period, merge new changes into the development branch, so that the master branch on Github always reflects the latest version on Maven Central.

For releasing new versions:

1. Remove the -SNAPSHOT suffix in build.sbt.
2. Publish to Maven Central staging using sbt publish-signed.
3. Follow the Maven Central workflow for releasing the next version, logging in to Maven Central Nexus with an account set up with the privilege to publish to the Open Source Project Repository Atomic Store entry.
4. Merge development into master to update the canonical version on Github.

For reference on this process, you may want to see the following links:

• SBT: Deploying to Sonatype
• SBT-PGP Usage docs
• The Central Repository: Releasing The Deployment
• Complete guide to continuous deployment to maven central from Travis CI

Todos

• Testing of complicated random flows of events, validations, and timeouts.

Changelog

0.0.7

• Bump Akka version to 2.5.4.
• Cross publish for Scala 2.11 and 2.12.
• Clean up build file.

0.0.6

• Bump Akka version to 2.5.1
• Add CircleCI with automated push to Sonatype/Maven Central

0.0.5

• Add meta field to validation process to allow validation code to pass back arbitrary additional information.
• Bump Akka version to 2.4.8

0.0.4

• Remove clustering code (client code may manage Receptionist as a Cluster Singleton if needed)
• Convert nested singleton messages to case classes, for proper deserialization.

0.0.3

• Factor out transient state data from what is persisted. It is unlikely to be of any use upon recovery, anyway. Important: this will break compatibility with any existing data that's stored.
• Set up Akka Cluster Singleton for EventLog actors

0.0.2

• Remove Timestamped. It's not crucial to the logic of this library, so let the client own all of the metadata it wants to associate with its events.
• Allow Akka Persistence plugin to be selected at run-time.
• Upgrade to Scala compiler 2.11.8.

0.0.1

• Initial release.