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I shall be presenting this at DDD East Anglia in Cambridge (UK) on 16th September.
Pop by and say hi if you are there 
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Meetup 4: Introduction to Event Sourcing - Domain Driven Design Edinburgh (Edinburgh, Scotland)| Meetup[^]
Monday, July 10, 2017, 7:00 PM
People's Postcode Lottery, Edinburgh
"Introduction to Event Sourcing" is a beginner level introduction to event sourcing, a data storage idea that works on capturing the change history of the system as a sequence of events and recreating the state from this history, and I will also show how it fits into the larger concepts of CQRS and up to DDD - with some discussion on how to host and use event sourcing in microservices. Beginner's level talk , no code but mostly diagrams, interactive discussion and Lego.
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This time I am down in the true capital of Ireland: Cork for rebel: con 2017[^]
The third time I've trotted this CQRS / Event Sourcing talk out - the plot to convert the world one developer at a time continues.
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Planning on going to this DDD [^] conference in London - should be a good one.
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After a 10 year furlough / absence I was awarded the Microsoft MVP again this year (this time for Microsoft Azure).
Am going to look for Champagne
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The CQRS on Azure modelling tool now includes creating the project files (.csproj or .vbproj) in the code generation. A milestone that was previously a millstone around my neck
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Getting the projection code generation (which is probably the hardest part) nearer completion now. Yay me!
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It really comes to something when you have method signatures that won't fit in a tweet:-
Function EvaluateProjection(Of TProjection As IProjection(Of MockAggregate, String))(projection As TProjection) As IClassifierEventHandler.EvaluationResult Implements IClassifier(Of MockAggregate, String).EvaluateProjection
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..with the code to allow you to specify the backing storage to use fro any given aggregate class in the application config - so much refactoring is needed.
Maybe I'd better take a break from it for now...
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I'm doing a talk for the Dublin MicroServices user group[^] using my CQRS on Azure project to demonstrate the concepts of event sourcing (and also CQRS) on Oct 28th.
(Hopefully the software is nearer ready by then)
modified 14-Jun-17 14:37pm.
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The MacBook air is going to the farm upstate (where it will have a happy life but we won't be able to visit) and I'm thinking I might replace it with an ASUS ZenBook UX303 variant. Still playing with the spec list though - like a child in a toy shop.
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I got an HP Envy in the end with B&O speakers - currently testing it with a bit of Fields[^]..
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One of the design goals I am looking at with regard to the CQRS on Azure project is that it should allow the solution to any performance issue to be "just throw more hardware at it" (or, as this is to be hosted on Azure the more realistic solution would be "just spin up more virtual machines").
This means that you have to take considerable care that this adding of new hardware does not cause any performance issues of its own - therefore it must not create nor further tax any bottlenecks.
To achieve this I have based the architecture for handling the event streams (and running projections over them as well as maintaining the "identity groups") on a peer-to-peer mesh and used asynchronous message passing to communicate between hosts.
Hosts may pass any request they receive onwards to another host if they are under too high a load. The request message has a reference to the originator so that when a host is found that can respond, that response can be sent straight back without having to unwind the path it took to get there.
This does, of course, mean that requests must have an unique identifier and since I don't want any bottleneck in the production of this uniqueness I use a GUID for each message as it is created.
There are also notifications that hosts can send each other - such as "new host joining", "host shutting down" etc. and also an acknowledgement message can be sent by a host that receives a request to allow the sending host to know that it has been received. If a sending host does not get an acknowledgement response in a given time frame then it can send the request to a different target to try that instead.
Actual data updating commands are a trickier proposition as you either have to ensure that the command is idempotent so if it is applied more than once it doesn't cause a problem or you have to have some sort of way of knowing if a command has been executed or not. (This latter solution would introduce a bottleneck.)
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In order to get the most out of the highly distributed and highly parallelized nature of the Azure back end used for the Event Stream (in particular when using either append blobs or file based storage) it makes sense to have a number of instances performing operations on those event streams simultaneously.
In my project I refer to these as "Hosts" but process or server is equally valid.
These hosts are arranged in a peer-to-peer mesh where no one host is either a bottleneck nor a single point of failure risk. There is also the requirement for a host to be allowed to pass a request onwards if it is too busy (or not best suited) to service it.
Public Enum RequestCategories
NotSet = 0
ExecuteCommand = 1
ExecuteQuery = 2
GetIdentityGroupMembers = 3
RunProjection = 4
RunClassifier = 5
End Enum
The response messages can indicate acknowledgement, or an error state return or can return the data requested. ( In the case of the command there is no data requested but a "command complete" return is needed).
There are also notifications needed for the maintenance of the peer-to-peer mesh network: host starting, host stopping, host high throughput notification and so on.
Any feedback before I bake this in to the CQRS on Azure project?
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I have made the IProjection interface support the concept of caching the state of the projection as at any given point in time so that the running of a projection over a very large event stream can be speeded up - especially in situations where multiple queries are essentially asking the same question.
Public Overridable Sub LoadFromSnapshot(snapshotToLoad As IProjectionSnapshot(Of TAggregate, TAggregateKey))
Implements IProjection(Of TAggregate, TAggregateKey).LoadFromSnapshot
If (snapshotToLoad IsNot Nothing) Then
m_CurrentSequenceNumber = snapshotToLoad.Sequence
If (snapshotToLoad.AsOfDate > m_CurrentAsOfDate) Then
m_CurrentAsOfDate = snapshotToLoad.AsOfDate
End If
m_currentValues.Clear()
m_currentValues.AddRange(snapshotToLoad.Values)
End If
End Sub
This can be done by storing the projection values as name:value pairs or by explicitly hard wiring the snapshot values to the projection properties - I prefer the former but make no exclusion.
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LegacySitePackage failed for package [Microsoft.VisualStudio.Editor.Implementation.EditorPackage]
Source: 'mscorlib' Description: An item with the same key has already been added. System.ArgumentException: An item with the same key has already been added. at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) at Microsoft.VisualStudio.ExtensibilityHosting.VsExportProviderFactory.<>c__DisplayClass54_0.<CreateAssemblyCatalogsAsync>b__6(KeyValuePair`2 namedCatalog) at System.Threading.Tasks.Dataflow.ActionBlock`1.ProcessMessage(Action`1 action, KeyValuePair`2 messageWithId) at System.Threading.Tasks.Dataflow.ActionBlock`1.<>c__DisplayClass5.<.ctor>b__0(KeyValuePair`2 messageWithId) at System.Threading.Tasks.Dataflow.Internal.TargetCore`1.ProcessMessagesLoopCore()
--- End of stack trace from previous location where exception was thrown
--- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.VisualStudio.ExtensibilityHosting.VsExportProviderFactory.<CreateAssemblyCatalogsAsync>d__54.MoveNext()
--- End of stack trace from previous location where exception was thrown
--- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task) at Microsoft.VisualStudio.ExtensibilityHosting.VsExportProviderFactory.<GetCurrentAssemblyCatalogsAsync>d__31.MoveNext()
--- End of stack trace from previous location where exception was thrown
--- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.VisualStudio.ExtensibilityHosting.VsExportProviderFactory.<GetExportProviderFactoryAsync>d__23.MoveNext()
--- End of stack trace from previous location where exception was thrown
--- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.VisualStudio.ComponentModelHost.ComponentModel.<GetMEFV3ExportProviderInternalAsync>d__49.MoveNext()
--- End of stack trace from previous location where exception was thrown
--- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult() at Microsoft.VisualStudio.ComponentModelHost.ComponentModel.<GetMEFV3ExportProviderWrapperAsync>d__48.MoveNext()
--- End of stack trace from previous location where exception was thrown
--- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.VisualStudio.Threading.JoinableTask.CompleteOnCurrentThread() at Microsoft.VisualStudio.Threading.JoinableTask`1.CompleteOnCurrentThread() at Microsoft.VisualStudio.Threading.JoinableTaskFactory.Run[T](Func`1 asyncMethod, JoinableTaskCreationOptions creationOptions) at Microsoft.VisualStudio.Threading.JoinableTaskFactory.Run[T](Func`1 asyncMethod) at Microsoft.VisualStudio.ComponentModelHost.ComponentModel.GetLazyValue[T](AsyncLazy`1 lazy) at Microsoft.VisualStudio.ComponentModelHost.ComponentModel.GetService[T]() at Microsoft.VisualStudio.Editor.Implementation.EditorParts.get_ContentTypeRegistryService() at Microsoft.VisualStudio.Editor.Implementation.LanguageServiceToContentTypeMapper.MakeLanguageServiceContentTypes(SettingsStore settingsStore) at Microsoft.VisualStudio.Editor.Implementation.LanguageServiceToContentTypeMapper.InitLanguageServiceToContentTypeMapper(IServiceProvider serviceProvider) at Microsoft.VisualStudio.Editor.Implementation.EditorPackage.Initialize() at Microsoft.VisualStudio.Shell.Package.Microsoft.VisualStudio.Shell.Interop.IVsPackage.SetSite(IServiceProvider sp)
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I think every project has to go through that phase where I'm listening to "The Reptile House E.P." while programming - CQRS on Azure just reached that point.
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Every morning that I wake out of Africa I am in exile.
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I'm doing a very short splash-and-dash demo of the CQRS designer project at a Dublin meet up[^] on Thursday.
Looking forward to some feedback.
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..and finally got the code generating attributes to classify the events..
<CQRSAzure.EventSourcing.DomainNameAttribute(domainNameIn:="TennisTournament"), _
CQRSAzure.EventSourcing.Category(categoryNameIn:="Match Status")> _
Partial Public Class GameFinished
Quote: )»Ŧ»)
modified 8-Apr-16 8:32am.
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Started trying to split the TFS project and started wake[^] at the same time.
Got to the fix at 47 minutes - spooky, eh?
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Definitely the aggregate needs to control writing to the event stream but maybe there are scenarios where the event stream can be read without the aggregate?
(Hmm - philosophers hat needed)
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Option Strict Off
Option Explicit On
Imports CQRSAzure
Imports CQRSAzure.Aggregation
Imports CQRSAzure.EventSourcing
Namespace Football_League.Game
Partial Public Class Game
Inherits Object
Implements IGame
Private _Key As System.Guid
Private m_eventStream As IEventStream(Of IGame)
Sub New()
MyBase.New
End Sub
Sub New(ByVal Key_In As Object)
MyBase.New
_Key = Key_In
End Sub
Public Function GetAggregateIdentifier() As String Implements IAggregationIdentifier.GetAggregateIdentifier
Return _Key.ToString
End Function
Public Sub SetKey(ByVal Key_In As System.Guid) Implements CQRSAzure.EventSourcing.IAggregationIdentifier(Of System.Guid).SetKey
_Key = Key_In
End Sub
Public Sub AddEvent(ByVal eventToAdd As IEvent(Of IGame)) Implements IEventStream(Of IGame).AddEvent
m_eventStream.Add(eventToAdd)
End Sub
Public Sub Scheduled(ByVal eventToAdd As IScheduled)
End Sub
Public Sub Rescheduled(ByVal eventToAdd As IRescheduled)
End Sub
Public Sub Started(ByVal eventToAdd As IStarted)
End Sub
End Class
End Namespace
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