Commander: Better Distributed Applications through CQRS, Event Sourcing, and Immutable Logs

Talk video: https://www.youtube.com/watch?v=B1-gS0oEtYc
Reference implementation on GitHub: https://github.com/capitalone/cqrs-manager-for-distributed-reactive-services

Have you ever hit a wall with REST? Does modeling your problem domain into CRUD-able entities feel like fitting a square peg into a round hole? Perhaps instead of modeling our services like little databases, we should instead model them like reactors over immutable event streams.

REST APIs are great, but their typical implementation tightly couples various concerns that would be better separated:

* Reads (perception) with writes (action)
* Current state with historical narrative
* Business logic with HTTP request processing, and with operational concerns like metrics and monitoring

Commander is:

1. A pattern for writing distributed applications that de-couples these concerns using ideas from CQRS and Event Sourcing, thereby alleviating common frustrations with CRUD-flavored REST
2. An implementation of the core component in this pattern using Clojure in the HTTP layer, using Java with the new Kafka Streams library in the event stream processing layer, and using Datomic for persistence

The Commander pattern imposes a clear separation of action from perception, and uses immutable values conveyed by Kafka and the Kafka Streams library to separate business logic from HTTP request processing, all while preserving the historical narrative of the entire event stream.

In this talk, I'll discuss the benefits and tradeoffs of applying this pattern in an enterprise context, and both demonstrate and open-source my implementation.

Transcript

1. Commander
   Better Distributed Applications through CQRS, Event
   Sourcing, and Immutable Logs
   

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2. Hi, I’m Bobby
   I’m on the Technology Fellow’s team at
   I dislike accidental complexity
   bobby.calderwood@capitalone.com
   @bobbycalderwood
   https://github.com/bobby
   

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3. Problem Space
   • Provide valuable informational and transactional
   services via Mobile and Web software
   • To lots of customers, with excellent user experience
   • Securely and in compliance with regulations
   • With ability to easily enhance, experiment, monitor,
   maintain, and operate
   • By many participants within a large organization
   

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4. Big Ideas
   • Immutability is central to information systems
   • Data language of system >> Programming language of
   components
   • Action and perception are not the same, and immutability
   facilitates their separation
   • Businesses services are not databases, they’re event
   stream reactors
   • Cross-cutting concerns must be satisfied in the presence
   of Conway’s Law
   

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5. Problematic Architecture
   /[resource](/:id) Database
   Logic Analytics
   Web Services Business Logic Databases and
   Services
   Operations
   Metrics/Monitoring
   Security/Compliance
   Audit
   Afterthoughts
   Other side-effects
   

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6. Immutability is central
   to information systems
   

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7. Event Sourcing Analogy
   Image by Alan Light CC BY-SA 3.0
   

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8. Data Loss by Design
   /[resource](/:id) Database
   Logic Analytics
   Web Services Business Logic Databases and
   Services
   Operations
   Metrics/Monitoring
   Security/Compliance
   Audit
   Afterthoughts
   Other side-effects
   

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9. The (data) language of the System
   >>
   The (runtime) language of each
   component
   

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10. What is system language?
    /[resource](/:id) Database
    Logic Analytics
    Web Services Business Logic Databases and
    Services
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    Afterthoughts
    Other side-effects
    

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11. Action != Perception
    

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12. Writes != Reads
    

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13. Writes Tied to Reads
    /[resource](/:id) Database
    Logic Analytics
    Web Services Business Logic Databases and
    Services
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    Afterthoughts
    Other side-effects
    

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14. Business Services
    are not Databases
    

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15. –John M. Culkin
    “We shape our tools and thereafter our tools
    shape us.”
    

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16. Database Leaking
    /[resource](/:id) Database
    Logic Analytics
    Web Services Business Logic Databases and
    Services
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    Afterthoughts
    Other side-effects
    

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17. Conway’s Law
    Yup, totally a thing
    

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18. –Melvin Conway
    “organizations which design systems ... are
    constrained to produce designs which are
    copies of the communication structures of these
    organizations”
    

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19. Cross-cutting Concerns?
    /[resource](/:id) Database
    Logic Analytics
    Web Services Business Logic Databases and
    Services
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    Afterthoughts
    Other side-effects
    

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20. We can do better!
    

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21. Commander
    • A better architecture for APIs and services using
    REST + Immutable Event Log + Reactive Event
    Stream Processing
    • The write-handling component of that architecture,
    my implementation is in Clojure
    

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22. Commander Architecture
    • Several categories of microservices with structured
    interactions among them
    • REST + CQRS + Event Sourcing + Reactive Event
    Stream Processing
    

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23. Commander Architecture
    /commands(/:id)
    /[query-apis]
    commands
    /updates
    Read-
    optimized
    View
    Command
    Processing
    Analytics
    [other topics]
    Arbitrary
    command
    action
    Arbitrary
    command
    action
    Microconsume
    rs
    Web Services
    Load balancing
    Auth(n|z)
    Input validation
    events
    Business Logic
    Event Log Datastores and
    Materialized Views
    sync
    “Ledger” of events
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    User-facing
    APIs
    Third-party
    Partners
    WS/SSE
    audit
    

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24. Commander Component
    /commands(/:id)
    /[query-apis]
    commands
    /updates
    Read-
    optimized
    View
    Command
    Processing
    Analytics
    [other topics]
    Arbitrary
    command
    action
    Arbitrary
    command
    action
    Microconsume
    rs
    Web Services
    Load balancing
    Auth(n|z)
    Input validation
    events
    Business Logic
    Event Log Datastores and
    Materialized Views
    sync
    “Ledger” of events
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    User-facing
    APIs
    Third-party
    Partners
    WS/SSE
    audit
    

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25. Embrace Immutability
    

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26. Immutable Data Log
    /commands(/:id)
    /[query-apis]
    commands
    /updates
    Read-
    optimized
    View
    Command
    Processing
    Analytics
    [other topics]
    Arbitrary
    command
    action
    Arbitrary
    command
    action
    Microconsume
    rs
    Web Services
    Load balancing
    Auth(n|z)
    Input validation
    events
    Business Logic
    Event Log Datastores and
    Materialized Views
    sync
    “Ledger” of events
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    User-facing
    APIs
    Third-party
    Partners
    WS/SSE
    audit
    

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27. Express actions in
    domain language
    (not in database language)
    

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28. A Command
    {"id": "33bb75db-6e13-48ee-8a54-b3976d3d065b",
    "action": "transfer-money",
    "data": {"from_account": "12345",
    "to_account": "54321",
    "amount": 10000}
    "timestamp": "2016-05-20T14:33:28.902-00:00"}
    

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29. An Event
    {"id": "d435ed18-4ff7-4cae-a21b-3adb7b06fe58",
    "parent": "33bb75db-6e13-48ee-8a54-b3976d3d065b",
    "action": "money-transferred",
    "data": {"id": "a6b903f6-0b9c-4c5b-95fa-afd4cc3bf938",
    "from_account": "12345",
    "to_account": "54321",
    "amount": 10000},
    "timestamp": "2016-05-20T14:33:28.904-00:00"}
    

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30. Separate Action
    from Perception
    

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31. 1 Log => n Data Views
    Read-
    optimized
    View
    Command
    Processing
    Analytics
    [other topics]
    Arbitrary
    command
    action
    Arbitrary
    command
    action
    Microconsume
    rs
    events
    Business Logic
    Event Log Datastores and
    Materialized Views
    “Ledger” of events
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    HDFS
    Elastic
    Search
    Domain-
    specific
    

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32. Commander Architecture
    /commands(/:id)
    /[query-apis]
    commands
    /updates
    Read-
    optimized
    View
    Command
    Processing
    Analytics
    [other topics]
    Arbitrary
    command
    action
    Arbitrary
    command
    action
    Microconsume
    rs
    Web Services
    Load balancing
    Auth(n|z)
    Input validation
    events
    Business Logic
    Event Log Datastores and
    Materialized Views
    sync
    “Ledger” of events
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    User-facing
    APIs
    Third-party
    Partners
    WS/SSE
    audit
    

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33. Exploit Conway’s Law
    

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34. Primary Team Provides
    /commands(/:id)
    /[query-apis]
    commands
    /updates
    Read-
    optimized
    View
    Command
    Processing
    Analytics
    [other topics]
    Arbitrary
    command
    action
    Arbitrary
    command
    action
    Microconsume
    rs
    Web Services
    Load balancing
    Auth(n|z)
    Input validation
    events
    Business Logic
    Event Log Datastores and
    Materialized Views
    sync
    “Ledger” of events
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    User-facing
    APIs
    Third-party
    Partners
    WS/SSE
    audit
    

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35. Enterprise Provides
    /commands(/:id)
    /[query-apis]
    commands
    /updates
    Read-
    optimized
    View
    Command
    Processing
    Analytics
    [other topics]
    Arbitrary
    command
    action
    Arbitrary
    command
    action
    Microconsume
    rs
    Web Services
    Load balancing
    Auth(n|z)
    Input validation
    events
    Business Logic
    Event Log Datastores and
    Materialized Views
    sync
    “Ledger” of events
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    User-facing
    APIs
    Third-party
    Partners
    WS/SSE
    audit
    

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36. Why Commander
    Component?
    • Single writer to commands topic
    • Ensuring schema conformance
    • Indexing all Commands and Events for reads and
    server-push
    • Provides optional illusion of synchrony to clients
    

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37. How to implement
    Reactive Services?
    

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38. Kafka Streams!
    

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39. What is Kafka?
    • Apache Kafka is publish-subscribe messaging
    rethought as a distributed commit log
    • But it’s not really about messaging, that’s just the
    interface
    • Logs > Messages for my domain
    • It provides distributed, immutable logs!
    

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40. What is Kafka Streams?
    • A Java library for building streaming applications on
    top of Kafka, lives in your application
    • Low-level API for building topologies of processors,
    streams, and tables
    • High-level DSL for common patterns like filter, map,
    aggregations, joins, stateful and stateless processing
    • Nice operational characteristics (low latency, elastic,
    fault-tolerant)
    

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41. How to use Kafka Streams
    within Commander
    • Implement Command Processor
    • Implement Event consumers and producers
    • Provide local state management as backend for
    APIs
    

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42. Commander Architecture
    /commands(/:id)
    /[query-apis]
    commands
    /updates
    Read-
    optimized
    View
    Command
    Processing
    Analytics
    [other topics]
    Arbitrary
    command
    action
    Arbitrary
    command
    action
    Microconsume
    rs
    Web Services
    Load balancing
    Auth(n|z)
    Input validation
    events
    Business Logic
    Event Log Datastores and
    Materialized Views
    sync
    “Ledger” of events
    Operations
    Metrics/Monitoring
    Security/Compliance
    Audit
    User-facing
    APIs
    Third-party
    Partners
    WS/SSE
    audit
    

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43. KStreamBuilder builder = new KStreamBuilder();
    KStream commands = builder.stream(commandsTopic);
    KStream customerEvents = commands
    .filter((id, command) -> command.get("action")
    .equals("create-customer"))
    .map((id, command) -> {
    Map userEvent = new HashMap(command);
    userEvent.put("action", "customer-created");
    userEvent.put("parent", id);
    Map userValue = (Map) userEvent.get("data");
    userValue.put("id", UUID.randomUUID());
    return new KeyValue<>(UUID.randomUUID(), userEvent);
    }).through(eventsTopic);
    KStream customers = customerEvents
    .map((id, event) -> {
    Map customer = (Map) event.get("data");
    UUID customerId = (UUID) customer.get("id");
    return new KeyValue(customerId, customer);
    });
    customers.through(customersTopic);
    StateStoreSupplier store = Stores.create("Customers")
    .persistent()
    .build();
    builder.addStateStore(store);
    customers.process(customerStore, "Customers");
    this.kafkaStreams = new KafkaStreams(builder, kafkaStreamsConfig);
    this.kafkaStreams.start();
    

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44. public class CustomerStore implements Processor {
    private KeyValueStore store;
    public List getCustomers() {
    List customers = new ArrayList<>();
    KeyValueIterator iterator = store.all();
    while (iterator.hasNext()) {
    KeyValue entry = iterator.next();
    customers.add(new Customer(entry.value));
    }
    iterator.close();
    return customers;
    }
    public Customer getCustomer(UUID id) {
    return new Customer(store.get(id));
    }
    @Override
    public void init(ProcessorContext processorContext) {
    this.store = (KeyValueStore) context.getStateStore("Customers");
    }
    @Override
    public void process(UUID uuid, Map map) {
    store.put(uuid, map);
    }
    @Override
    public void punctuate(long l) {}
    @Override
    public void close() {}
    }
    

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45. Demo
    

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46. Summary
    • Capture customer intent and business events as
    immutable data in domain language
    • From these action streams, services implement their
    own functionality in this common lingua franca
    • building many independent data views
    • reactively
    • without temporal or organizational coordination
    

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47. Giant Shoulders
    • Immutability: Rich Hickey, Stu Halloway
    • CQRS: Udi Dahan, Martin Fowler, Chris Richardson
    • Kafka Event Stream Reactors: Neha Narkhede,
    Jay Kreps, Martin Kleppmann
    • Organization and Management: Mel Conway,
    Eliyahu Goldratt, Gene Kim, Michael Nygard
    

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48. References
    • https://tinyurl.com/capital-one-cmdr
    • http://www.datomic.com/
    • http://blog.cognitect.com/?tag=NewNormal+Series
    • http://www.confluent.io/blog/event-sourcing-cqrs-stream-
    processing-apache-kafka-whats-connection/
    • https://engineering.linkedin.com/distributed-systems/log-what-
    every-software-engineer-should-know-about-real-time-datas-
    unifying
    • https://www.infoq.com/presentations/Value-Values
    

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