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commandhandler.go
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commandhandler.go
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// Copyright (c) 2014 - Max Ekman <max@looplab.se>
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package aggregate
import (
"context"
"errors"
eh "github.com/looplab/eventhorizon"
)
// ErrNilAggregateStore is when a dispatcher is created with a nil aggregate store.
var ErrNilAggregateStore = errors.New("aggregate store is nil")
// ErrAggregateAlreadySet is when an aggregate is already registered for a command.
var ErrAggregateAlreadySet = errors.New("aggregate is already set")
// CommandHandler dispatches commands to registered aggregates.
//
// The dispatch process is as follows:
// 1. The handler receives a command.
// 2. An aggregate is created or loaded using an aggregate store.
// 3. The aggregate's command handler is called.
// 4. The aggregate stores events in response to the command.
// 5. The new events are stored in the event store.
// 6. The events are published on the event bus after a successful store.
type CommandHandler struct {
store eh.AggregateStore
aggregates map[eh.CommandType]eh.AggregateType
}
// NewCommandHandler creates a new CommandHandler.
func NewCommandHandler(store eh.AggregateStore) (*CommandHandler, error) {
if store == nil {
return nil, ErrNilAggregateStore
}
h := &CommandHandler{
store: store,
aggregates: make(map[eh.CommandType]eh.AggregateType),
}
return h, nil
}
// SetAggregate sets an aggregate as handler for a command.
func (h *CommandHandler) SetAggregate(aggregateType eh.AggregateType, cmdType eh.CommandType) error {
// Check for already existing handler.
if _, ok := h.aggregates[cmdType]; ok {
return ErrAggregateAlreadySet
}
// Add aggregate type to command type.
h.aggregates[cmdType] = aggregateType
return nil
}
// HandleCommand handles a command with the registered aggregate.
// Returns ErrAggregateNotFound if no aggregate could be found.
func (h *CommandHandler) HandleCommand(ctx context.Context, cmd eh.Command) error {
err := eh.CheckCommand(cmd)
if err != nil {
return err
}
aggregateType, ok := h.aggregates[cmd.CommandType()]
if !ok {
return eh.ErrAggregateNotFound
}
aggregate, err := h.store.Load(ctx, aggregateType, cmd.AggregateID())
if err != nil {
return err
} else if aggregate == nil {
return eh.ErrAggregateNotFound
}
if err = aggregate.HandleCommand(ctx, cmd); err != nil {
return err
}
if err = h.store.Save(ctx, aggregate); err != nil {
return err
}
return nil
}