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The I/O packages provide transforms for reading from and writing to various data sources and sinks.

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textio

Package textio provides transforms for reading and writing text files.

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Read

Reads a set of files matching a glob pattern and returns lines as PCollection<string>. 
func Read(s beam.Scope, glob string, opts ...ReadOptionFn) beam.PCollection
​
s
beam.Scope
The scope to insert the transform into
​
glob
string
File path or glob pattern (e.g., “gs://bucket/.txt”, “/path/to/.log”)
​
opts
...ReadOptionFn
Optional configuration: ReadAutoCompression(), ReadGzip(), ReadUncompressed()
​
Returns
beam.PCollection
PCollection<string> of lines (newlines removed)
Example: 
import (
    "github.com/apache/beam/sdks/v2/go/pkg/beam"
    "github.com/apache/beam/sdks/v2/go/pkg/beam/io/textio"
)

lines := textio.Read(root, "gs://my-bucket/input/*.txt")

// With compression options
gzLines := textio.Read(root, "data/*.gz", textio.ReadGzip())
autoLines := textio.Read(root, "data/*", textio.ReadAutoCompression())

​
ReadAll

Expands and reads filenames from an input PCollection<string> of globs. 
func ReadAll(s beam.Scope, col beam.PCollection, opts ...ReadOptionFn) beam.PCollection
​
s
beam.Scope
The scope to insert the transform into
​
col
beam.PCollection
PCollection<string> of file paths or glob patterns
​
opts
...ReadOptionFn
Optional compression configuration
​
Returns
beam.PCollection
PCollection<string> of all lines from all matched files
Example: 
patterns := beam.Create(root, "data/part-*.txt", "logs/*.log")
allLines := textio.ReadAll(root, patterns)

​
ReadWithFilename

Reads files and returns PCollection<KV<string,string>> of filename and line. 
func ReadWithFilename(s beam.Scope, glob string, opts ...ReadOptionFn) beam.PCollection
​
Returns
beam.PCollection
PCollection<KV<string,string>> where key is filename and value is line content
Example: 
filesAndLines := textio.ReadWithFilename(root, "logs/*.log")

// Process with filename context
func logWithFile(filename, line string) {
    log.Printf("[%s] %s", filename, line)
}

beam.ParDo0(root, logWithFile, filesAndLines)

​
Write

Writes a PCollection<string> to a file as separate lines. 
func Write(s beam.Scope, filename string, col beam.PCollection)
​
s
beam.Scope
The scope to insert the transform into
​
filename
string
Output file path
​
col
beam.PCollection
PCollection<string> to write (each element becomes a line)
Example: 
lines := beam.Create(root, "line 1", "line 2", "line 3")
textio.Write(root, "output.txt", lines)

// GCS path
textio.Write(root, "gs://my-bucket/output.txt", results)

​
Immediate

Reads a local file at pipeline construction time and embeds data into the pipeline. 
func Immediate(s beam.Scope, filename string) (beam.PCollection, error)
​
s
beam.Scope
The scope to insert the transform into
​
filename
string
Local file path to read immediately
​
Returns
(beam.PCollection, error)
PCollection<string> with file contents, or error if read fails
Note: Only use for small files as data is embedded in the pipeline.

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fileio

Package fileio provides lower-level transforms for matching and reading files with more control.

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MatchFiles

Finds all files matching a glob pattern and returns PCollection<FileMetadata>. 
func MatchFiles(s beam.Scope, glob string, opts ...MatchOptionFn) beam.PCollection
​
s
beam.Scope
The scope to insert the transform into
​
glob
string
File path or glob pattern
​
opts
...MatchOptionFn
Options: MatchEmptyAllow(), MatchEmptyDisallow(), MatchEmptyAllowIfWildcard()
​
Returns
beam.PCollection
PCollection<FileMetadata> with file path, size, and last modified time
Example: 
import "github.com/apache/beam/sdks/v2/go/pkg/beam/io/fileio"

matches := fileio.MatchFiles(root, "data/*.json")

// Allow empty results
matches := fileio.MatchFiles(root, "optional/*.txt", fileio.MatchEmptyAllow())

​
MatchAll

Matches files from an input PCollection<string> of glob patterns. 
func MatchAll(s beam.Scope, col beam.PCollection, opts ...MatchOptionFn) beam.PCollection
​
col
beam.PCollection
PCollection<string> of glob patterns
​
Returns
beam.PCollection
PCollection<FileMetadata> of all matching files

​
MatchContinuously

Continuously watches for new files matching a pattern at regular intervals. 
func MatchContinuously(
    s beam.Scope,
    glob string,
    interval time.Duration,
    opts ...MatchContOptionFn,
) beam.PCollection
​
s
beam.Scope
The scope to insert the transform into
​
glob
string
File pattern to watch
​
interval
time.Duration
How often to check for new files
​
opts
...MatchContOptionFn
Options: MatchStart(), MatchEnd(), MatchDuplicateSkip(), MatchDuplicateAllowIfModified(), MatchApplyWindow()
​
Returns
beam.PCollection
Unbounded PCollection<FileMetadata> of matching files over time
Example: 
import "time"

// Check for new files every 30 seconds
newFiles := fileio.MatchContinuously(
    root,
    "gs://bucket/incoming/*.json",
    30*time.Second,
    fileio.MatchDuplicateSkip(),
)

​
ReadMatches

Converts PCollection<FileMetadata> to PCollection<ReadableFile>. 
func ReadMatches(s beam.Scope, col beam.PCollection, opts ...ReadOptionFn) beam.PCollection
​
s
beam.Scope
The scope to insert the transform into
​
col
beam.PCollection
PCollection<FileMetadata> from MatchFiles/MatchAll
​
opts
...ReadOptionFn
Options: ReadAutoCompression(), ReadGzip(), ReadUncompressed(), ReadDirectorySkip()
​
Returns
beam.PCollection
PCollection<ReadableFile> ready for reading
Example: 
matches := fileio.MatchFiles(root, "data/*.gz")
files := fileio.ReadMatches(root, matches, fileio.ReadGzip())

// Custom processing
func processFile(ctx context.Context, file fileio.ReadableFile, emit func(string)) error {
    data, err := file.Read(ctx)
    if err != nil {
        return err
    }
    // Process data...
    emit(string(data))
    return nil
}

results := beam.ParDo(root, processFile, files)

​
FileMetadata

Contains metadata about a matched file. 
type FileMetadata struct {
    Path         string
    Size         int64
    LastModified time.Time
}
​
Path
string
Full path to the file
​
Size
int64
File size in bytes
​
LastModified
time.Time
When the file was last modified

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ReadableFile

Wrapper around FileMetadata providing methods to read file contents. 
type ReadableFile struct {
    Metadata    FileMetadata
    Compression compressionType
}

​
Open

Opens the file for reading. 
func (f ReadableFile) Open(ctx context.Context) (io.ReadCloser, error)
​
ctx
context.Context
Context for the file operation
​
Returns
(io.ReadCloser, error)
Reader for file contents (caller must close) or error
Example: 
func processFile(ctx context.Context, file fileio.ReadableFile) error {
    reader, err := file.Open(ctx)
    if err != nil {
        return err
    }
    defer reader.Close()
    
    scanner := bufio.NewScanner(reader)
    for scanner.Scan() {
        line := scanner.Text()
        // Process line...
    }
    return scanner.Err()
}

​
Read

Reads the entire file into memory. 
func (f ReadableFile) Read(ctx context.Context) ([]byte, error)
​
ctx
context.Context
Context for the file operation
​
Returns
([]byte, error)
Complete file contents or error

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ReadString

Reads the entire file as a string. 
func (f ReadableFile) ReadString(ctx context.Context) (string, error)
​
Returns
(string, error)
File contents as string or error

​
Other I/O Packages

The Go SDK includes I/O connectors for various data sources:

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avroio

Read and write Avro files. 
import "github.com/apache/beam/sdks/v2/go/pkg/beam/io/avroio"

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bigqueryio

Read from and write to Google BigQuery. 
import "github.com/apache/beam/sdks/v2/go/pkg/beam/io/bigqueryio"

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parquetio

Read and write Parquet files. 
import "github.com/apache/beam/sdks/v2/go/pkg/beam/io/parquetio"

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pubsubio

Read from and write to Google Cloud Pub/Sub. 
import "github.com/apache/beam/sdks/v2/go/pkg/beam/io/pubsubio"

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databaseio

Generic database I/O for SQL databases. 
import "github.com/apache/beam/sdks/v2/go/pkg/beam/io/databaseio"

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mongodbio

Read from and write to MongoDB. 
import "github.com/apache/beam/sdks/v2/go/pkg/beam/io/mongodbio"

​
Complete Example

package main

import (
    "context"
    "strings"
    
    "github.com/apache/beam/sdks/v2/go/pkg/beam"
    "github.com/apache/beam/sdks/v2/go/pkg/beam/io/textio"
    "github.com/apache/beam/sdks/v2/go/pkg/beam/register"
    "github.com/apache/beam/sdks/v2/go/pkg/beam/x/beamx"
)

func init() {
    register.Function1x2(extractWords)
    register.Function2x1(sumCounts)
    register.Function1x1(formatOutput)
}

func extractWords(line string) (string, int) {
    words := strings.Fields(line)
    for _, word := range words {
        // Would emit each word; simplified here
    }
    return word, 1
}

func sumCounts(a, b int) int {
    return a + b
}

func formatOutput(word string, count int) string {
    return fmt.Sprintf("%s: %d", word, count)
}

func main() {
    beam.Init()
    
    p := beam.NewPipeline()
    root := p.Root()
    
    // Read input files
    lines := textio.Read(root, "input/*.txt")
    
    // Count words
    wordCounts := beam.ParDo(root, extractWords, lines)
    grouped := beam.CombinePerKey(root, sumCounts, wordCounts)
    
    // Format and write output
    formatted := beam.ParDo(root, formatOutput, grouped)
    textio.Write(root, "output.txt", formatted)
    
    if err := beamx.Run(context.Background(), p); err != nil {
        log.Fatalf("Failed to execute pipeline: %v", err)
    }
}

​
Best Practices

  • Use appropriate URI schemes: gs:// for GCS, s3:// for S3, local paths for local files
  • Glob patterns support * and ** wildcards
  • Always validate file paths before pipeline execution
  • Use ReadAutoCompression() for mixed compression types
  • Explicit compression options are faster (no detection overhead)
  • Gzip is automatically detected from .gz extension
  • textio.Read uses splittable DoFn for parallel processing
  • Large files are automatically split into chunks
  • Use fileio for custom file processing logic
  • Avoid Immediate() for large files (embeds in pipeline)
  • Use MatchContinuously for streaming file ingestion
  • Configure deduplication based on your use case
  • Consider windowing strategies for time-based processing

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