Argthtjtr

I have been having some difficulties differenciation the two recently. More specificly I have browsed stackoverflow and there is a statement that Events can be named in two different ways:
with "ing" or with past tense "ed". This can be seen here Events - naming convention and style
https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/names-of-type-members

At the same time the CQRS states that names need to be in past tense and then following their guidelines the events named above with "ing" form would be commands. This gets me a bit confused? Do events mean different things in depending on the architectural context and movement. Is there a unified view on what an event and command is ?

CQRS you've been reading only had past-tense event names likely because they didn't consider pre-events. A command commands something to happen, and as such is typically formed in imperative ("click!", "fire!", "tickle!"). It makes no sense for a command to be a gerund ("clicking! clicking faster, you! or I fires you!") As it precipitates an action, it will likely trigger one or more notifications (=events) that something of note is about to happen, and afterwards that something of note did happened.

-ing events (e.g. ("Clicking") happen before the event is handled, e.g. in case someone wants to stop it. Sometimes, they are called "before" events (e.g. "BeforeClick"), or "will" events ("WillClick").

-ed events (e.g. "Clicked") happen after the event is handled, e.g. in order to affect dependents. Sometimes, they are called "after" events (e.g. "AfterClick") or "did" events ("DidClick").

Which specific scheme you follow does not really matter, as long as you (and your team, and your potential partners) are consistent about it. Since CQRS (under that name) is largely a Microsoft thing, follow what Microsoft says. Should you code for Mac, the concepts are similar - but you'd do well to go with the Apple guidelines instead.

Is there a unified view on what an event and command is ?

Unified? No, probably not. But if you want an authoritative definition, Gregor Hohpe's Enterprise Integration Patterns is a good place to start.

• Command Message


• Event Message

Within the context of CQRS, you should consider Greg Young's opinion to be authoritative. He is quite clear that command messages should use imperative spellings, where events use spellings of changes that completed in the past.

Names of Commands and Events should be understood to be spelling conventions -- much in the same way that the spellings of URI, or variable names, are conventions. It doesn't matter at all for correctness, and the computer is for the most part not looking at the spelling (for example, we route messages based on the message name, not by looking at the verb tense).

Events describe a change to the state of a model; all events are ModelChanged. However, we prefer to use domain specific spellings for the type of the event, so that they can be more easily discriminated: MouseClicked, ConnectionClosed, FundsTransfered, and so on.

Use of the present progressive tense spelling for an event name is weird, in so far as the message is a description of the domain model at the point of a transaction, where the present tense semantically extends past that temporal point. More loosely, present progressive describes a current state, rather than a past change of state.

That said, finding a good past tense spelling for pre-events can be hard, and ultimately it is just a spelling convention; the work required to find an accurate spelling that is consistent with the past tense convention may not pay for itself compared to taking a natural but incorrect verb tense.