EVENT

An Event in DeQL represents an immutable fact — something that has already happened. Events are the fundamental unit of truth in a DeQL system. They are immutable values.

Purpose

Events serve as:

• The source of truth — all state is derived exclusively from events (SELECT * from <<AggregateName>>$Events)
• The output of decisions — decisions emit events as outcomes (EMIT AS SELECT EVENT)
• The input to aggregates — aggregate state is derived by replaying events (<Aggregate>$Agg)
• The input to projections — read models derive their views by querying events (<Aggregate>$Events)

Syntax

CREATE EVENT <Name> (
    <field> <TYPE>,
    ...
);

Use CREATE OR REPLACE to overwrite an existing definition:

CREATE OR REPLACE EVENT <Name> (
    <field> <TYPE>,
    ...
);

Example: Employee Events

CREATE EVENT EmployeeHired (name STRING, grade STRING);
CREATE EVENT EmployeePromoted (new_grade STRING);

Example: Banking Events

CREATE EVENT AccountOpened (initial_balance DECIMAL(12,2));
CREATE EVENT Deposited (amount DECIMAL(12,2));

Example: E-Commerce Events

CREATE EVENT ItemAdded (
    sku      STRING,
    quantity INT,
    price    DECIMAL(10,2)
);

CREATE EVENT CartCheckedOut (
    total_amount DECIMAL(12,2)
);

CREATE EVENT ItemRemoved (
    sku STRING
);

How Events Are Emitted

Events are produced inside decisions using the EMIT AS SELECT EVENT clause. Field assignment uses the := operator:

CREATE DECISION DepositFunds
FOR BankAccount
ON COMMAND Deposit
STATE AS
    SELECT initial_balance AS balance
    FROM DeReg."BankAccount$Agg"
    WHERE aggregate_id = :account_id
EMIT AS
    SELECT EVENT Deposited (
        amount := :amount
    )
    WHERE balance >= :amount;

The :amount comes from the command, and balance comes from the STATE AS query.

How Events Are Queried

Events are queried using the $Events suffix under the DeReg schema with quoted identifiers. The stream_id column identifies which aggregate instance the event belongs to:

-- Query raw events
SELECT stream_id, event_type, seq, data
FROM DeReg."BankAccount$Events"
ORDER BY stream_id, seq;

-- Use in projections (stream_id for grouping)
CREATE PROJECTION AccountBalance AS
SELECT
    stream_id AS aggregate_id,
    LAST(data.initial_balance) AS balance
FROM DeReg."BankAccount$Events"
GROUP BY stream_id;

The data. prefix accesses the event’s payload fields.

Event Metadata

Every event automatically carries metadata beyond its declared fields:

Field	Description
stream_id	The aggregate instance this event belongs to
event_type	The name of the event (e.g., 'Deposited')
data.*	The declared payload fields
seq	Position in the aggregate’s event stream
timestamp	When the event was recorded

Key Properties

Property	Description
Immutable	Once emitted, an event never changes
Ordered	Events within a stream have a defined sequence
Self-describing	Each event carries its type and payload
Replayable	Any state can be rebuilt by replaying events
Queryable	Accessed via DeReg."<Name>$Events" in projections