sdks/go/pkg/beam/pcollection.go

// Licensed to the Apache Software Foundation (ASF) under one or more
// contributor license agreements.  See the NOTICE file distributed with
// this work for additional information regarding copyright ownership.
// The ASF licenses this file to You 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 beam

import (
	"github.com/apache/beam/sdks/go/pkg/beam/core/graph"
	"github.com/apache/beam/sdks/go/pkg/beam/core/typex"
	"github.com/apache/beam/sdks/go/pkg/beam/internal/errors"
)

// PCollection is an immutable collection of values of type 'A', which must be
// a concrete type, such as int or KV<int,string>. A PCollection can contain
// either a bounded or unbounded number of elements. Bounded and unbounded
// PCollections are produced as the output of PTransforms (including root
// PTransforms like textio.Read), and can be passed as the inputs of other
// PTransforms. Some root transforms produce bounded PCollections and others
// produce unbounded ones.
//
// Each element in a PCollection has an associated timestamp. Sources assign
// timestamps to elements when they create PCollections, and other PTransforms
// propagate these timestamps from their input to their output implicitly or
// explicitly.
//
// Additionally, each element is assigned to a set of windows. By default, all
// elements are assigned into a single default window, GlobalWindow.
type PCollection struct {
	// n is the graph node that PCollection wraps. If there is no node, the
	// PCollection is invalid.
	n *graph.Node
}

// IsValid returns true iff the PCollection is valid and part of a Pipeline.
// Any use of an invalid PCollection will result in a panic.
func (p PCollection) IsValid() bool {
	return p.n != nil
}

// TODO(herohde) 5/30/2017: add name for PCollections? Java supports it.
// TODO(herohde) 5/30/2017: add windowing strategy and documentation.

// Type returns the full type 'A' of the elements. 'A' must be a concrete
// type, such as int or KV<int,string>.
func (p PCollection) Type() FullType {
	if !p.IsValid() {
		panic("Invalid PCollection")
	}
	return p.n.Type()
}

// Coder returns the coder for the collection. The Coder is of type 'A'.
func (p PCollection) Coder() Coder {
	if !p.IsValid() {
		panic("Invalid PCollection")
	}
	return Coder{p.n.Coder}
}

// SetCoder set the coder for the collection. The Coder must be of type 'A'.
func (p PCollection) SetCoder(c Coder) error {
	if !p.IsValid() {
		panic("Invalid PCollection")
	}

	if !typex.IsEqual(p.n.Type(), c.coder.T) {
		return errors.Errorf("coder type %v must be identical to node type %v", c.coder.T, p.n)
	}
	p.n.Coder = c.coder
	return nil
}

func (p PCollection) String() string {
	if !p.IsValid() {
		return "(invalid)"
	}
	return p.n.String()
}