monolith to micro-services? event sourcing can help Doug

Transcription

1 monolith to micro-services? event sourcing can help Doug

2 legacy Client Culture Amp ( ) Rails App (Murmur) Read-write Query Server Read-only DB

3 Our journey Our meandering path to CQRS & event sourcing Attempt 1 - Asynchronous micro-services Event Log the monolith and introduce query services Attempt 2 - Greenfield systems of record Rebuild part of the monolith as a stand alone command side service Attempt 3 - Refactor the monolith first And start with the least depended upon aggregates

4 Why micro-services? Attempt 1: Asynchronous Too many developers! Micro-services Monolith was becoming too complex. Want teams to control their own assets. Flexibility to introduce fit for purpose technologies. (for example, elastic search for our comments search) Scale different parts of the app independently. (for example, capture has a very different profile)

5 Attempt 1: Asynchronous Micro-services Why asynchronous? Resilience is architected into the platform. If a micro-service is down, all depended micro-services remain available and are eventually consistent. Also protects yourself from self inflicted DOS. Ability to replay history for consuming services Much easier to refactor, or to introduce new services. Decoupled architecture enabling platform flexibility Consumers don t necessarily need to know where the producers are deployed

6 Reporting Client Service Rails App (Murmur) Event Log Read-write Query Server Read-only Reporting Service DB

7 What went wrong Plagued with bad data issues as we tried to convert our current production database to a stream of events Ongoing source-of-truth issues between the monolith persistence layer and the synthesised event log. Events were generated at the wrong business points intended for consumption but ironically problematic for consumers Events were too coarse-grained and slow to process. Some of the events weren't real business events. Multiple event streams - one for each event type - hurt.

8 What we learned Asynchronous micro-services are the right long term aim. But, don t start with event consumers, start with event producers. Event logging is not event sourcing!

9 Add Add Remove Add What is Employee A Employee B Employee A Employee Event Sourcing? Add Add Employee B Employee Store the business events as the source of truth Can answer questions you don t yet know you have. Legal, accounting, traditional professions have used event sourcing for centuries.

10 Every method should either be a command that performs an action, or a query that returns data to the caller, but not both. In other words, asking a question should not change the answer. Bertrand Meyer (designer of Eiffel language) on CQS

11 Command Query Responsibility Segregation What is CQRS? Without CQRS With CQRS EmployeeService + createemployee(employee): void + getemployee(id): Employee + updateemployeename(id, name): void + makeemployeeadmin(id) : void + getemployees(account): EmployeeReadService + getemployee(id): Employee + getemployees(account): EmployeeWriteService + createemployee(employee): void + updateemployeename(id, name): void + makeemployeeadmin(id) : void

12 CQRS & Client event sourcing architecture Command Handler Query Handler Aggregate Projection event store Eventually consistent!

13 Attempt 2: Client Greenfield systems of record Rails App (Murmur) Event Log Employee SoR Query Server Reporting Service Event Store Projector DB

14 What worked well? CQRS and event sourcing helped us be clear about the boundaries of context. The new service owns the validation, so bad data issues in accounts we migrated were found and fixed upfront We could treat the monolith as one massive projection minimising work required on the Query side, and UI. Removed our source of truth problems. Domain models can be deterministically built from events, not vice versa. We could incrementally migrate accounts

15 What didn t work well? Required a leap of faith for the team to agree to try. Slow to develop as we were learning a new paradigm, and rolling our own event sourcing system. Eventual consistency hurts: Upfront we needed to solve how to update the monolith in a reasonable timeframe. We needed to refactor parts of the UI to cope with the eventual consistency. Disaster recovery becomes a distributed problem.

16 With all the troubles of evented architecture - it is a blessing when "shit hits the fan. I don't know if in the past we would even be able to tell the customers affected by the issue. Unsolicited feedback from our Tech Lead for the Employee Service

17 1. Start with the Command Side. Attempt 3: Refactor the monolith first Treat your existing application like one big projection. 2. Tackle a single Aggregate at a time. No need for a big-bang migration. 3. Generate UUIDS in the legacy monolith first. These will help you link the migration scripts back. 4. Start by synchronously updating your existing application. Your legacy UI will thank you for it. 5. When all writes are through CQRS Commands, refactor the UI to use new dedicated asynchronous projections. 6. When nothing still uses the legacy domain model,

18 Identifying * Demographi c aggregates 1 Start Here! Account 1 1 * Employee * Question * 1 1 Survey Response Comment 1 * 1 * * Report 1 * ActionItem

19 Client Monolith phase 1 Monolith Rails Controllers Survey C Handler Participant C Handler Comment C Handler Legacy Domain Models Survey Aggregate Participant Aggregate Comment Aggregate Synchronous update! Event Store Projector Legacy DB

20 Client Client Client Monolith phase 2 Authentication layer Survey C Handler Question C Handler Participant C Handler Query handler Query handler Survey Aggregate Question Aggregate Participant Aggregate Projection Projection Asynchronous updates Projector Projector PostGres

21 Beyond the monolith Employees Service Capture Service Survey Designer Account Admin Message Bus Ratings Analitycs Text Analytics Public APIs Sender Data Warehous Data Labs Experiments

22 Key learnings 1. Event Logging is not event sourcing and just leads to source of truth confusion. 2. CQRS and event sourcing can help you move to monolith micro-services, but start by refactoring the 3. If you choose to event source your monolith, take side. one step at a time, and focus on the command

23 Questions?