/ * We need to transfer data from a source to a consumer. The source sends data in small batches (~ten records), while the consumer works more efficiently with large batches (~thousand records). A real example is transferring data from Kafka queues to a Clickhouse database. Source: - Conditionally infinite. - The source never returns more than MaxItems records per call to Next. - During one "session" (one call to Pipe), the source returns new data each time Next is called. - After restart, the source resumes from the last "confirmed" position, set by cookie. Therefore, *each* value of cookie returned by Next must be fixed after data is saved in the receiver, by calling Commit in the same order they were returned by Next. Receiver: - Cannot process more than MaxItems at once. Basic level: Implement a function func Pipe(p Producer, c Consumer) error that reads data from the source, groups it into a buffer of size no more than MaxItems, and saves it to the receiver, then fixes progress in the source. Complexity: Methods Next, Process, and Commit involve network calls and can take a long time. To speed up the transfer process, processes of reading, writing, and confirming progress should be parallelized. So that, when calling Process or Commit, reading from the source and forming a new buffer continue. * / const MaxItems = 9999 type Producer interface { // Next returns: // - batch of items to be processed // - cookie to be commited when processing is done // - error Next() (items []any, cookie int, err error) // Commit is used to mark data batch as processed Commit(cookie int) error } type Consumer interface { Process(items []any) error } func Pipe(p Producer, c Consumer) error { // TODO }

To implement the Pipe function with parallel processing of reading, writing, and progress acknowledgment, channels and goroutines can be used. The idea:

• One goroutine reads data from the source (Producer.Next), forming batches of no more than MaxItems.
• A second goroutine processes the batches (Consumer.Process).
• A third goroutine confirms progress (Producer.Commit) strictly in the order of receiving cookies.

To maintain the order of confirmation, a queue or a buffered channel can be used, where cookies are stored in the order they are received.

Example implementation:

func Pipe(p Producer, c Consumer) error {
    type batch struct {
        items  []any
        cookie int
    }

    batches := make(chan batch, 10)      // Channel for passing batches for processing
    commitCh := make(chan int, 10)       // Channel for passing cookies for confirmation
    errCh := make(chan error, 3)         // Channel for errors
    done := make(chan struct{})           // Channel for stopping

    // Reading data
    go func() {
        defer close(batches)
        for {
            items, cookie, err := p.Next()
            if err != nil {
                errCh <- err
                return
            }
            if len(items) == 0 {
                // Source is empty, can finish
                return
            }

            // Form batches no larger than MaxItems
            for start := 0; start < len(items); start += MaxItems {
                end := start + MaxItems
                if end > len(items) {
                    end = len(items)
                }
                batches <- batch{items: items[start:end], cookie: cookie}
            }
        }
    }()

    // Processing data
    go func() {
        defer close(commitCh)
        for b := range batches {
            if err := c.Process(b.items); err != nil {
                errCh <- err
                return
            }
            commitCh <- b.cookie
        }
    }()

    // Confirming progress
    go func() {
        for cookie := range commitCh {
            if err := p.Commit(cookie); err != nil {
                errCh <- err
                return
            }
        }
        close(done)
    }()

    select {
    case err := <-errCh:
        return err
    case <-done:
        return nil
    }
}

In this example:

• Reading and forming batches occurs in the first goroutine.
• Batch processing is in the second.
• Cookie confirmation is in the third.

Channels ensure data flow and confirmation order. Errors are transmitted via errCh, allowing the process to be interrupted upon an error.

This approach enables parallel reading, processing, and confirming data, increasing overall performance.