Keep what a consumer gives up on until a person delivers it again or drops it
Design 25 promised a dead-letter stream that did not exist: a message a consumer gave up on stayed only in its source, which drops it after a week, and its condition cleared when the advisories stopped (hq issue 330, ADR 0264).
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jochen
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package main
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import (
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"context"
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"errors"
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"fmt"
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"strconv"
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"strings"
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"sync/atomic"
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"github.com/nats-io/nats.go"
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"github.com/novox/mesh-controller/internal/broker"
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"github.com/novox/mesh-controller/internal/catalogue"
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"github.com/novox/mesh-controller/internal/conditions"
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"github.com/novox/mesh-controller/internal/link"
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)
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// What a consumer gave up on, answered by the serving controller (novox/hq issue 330).
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//
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// **In this process, on its own connection**: DEAD_LETTERS is read and changed on the bus, and the
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// serving controller is already on it. A verb run as a fresh process would open a connection of its own
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// for each call (novox/hq issue 327).
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// deadLettersOn is the serving controller's bus, set when it starts watching the mesh; nil in a
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// process that serves nothing.
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var deadLettersOn atomic.Pointer[busHandles]
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// busHandles are the serving controller's connection and JetStream handle.
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type busHandles struct {
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conn *nats.Conn
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js nats.JetStreamContext
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}
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// defaultDeadLetters is how many the list says when not asked for more.
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const defaultDeadLetters = 50
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// causeDeadLetter is the cause a delivery or drop gives when the caller gives none.
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const causeDeadLetter = "dead-letter"
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// deadLettersAnswer is what `dead-letters` answers: the list, one whole, or what came of delivering one
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// again or dropping it.
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func deadLettersAnswer(ctx context.Context, a *verbArguments) (any, error) {
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on := deadLettersOn.Load()
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if on == nil {
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return nil, errors.New("this controller is not serving, so it does not read DEAD_LETTERS: ask again, " +
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"and the serving controller answers")
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}
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// The shape first, and every argument it reads; one given beside it is refused before anything is
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// done, as every verb refuses what it would pass over (novox/hq issue 244).
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var deliver, drop, why, cause, idText, consumer, limit string
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switch {
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case a.given["deliver"] != "" || a.given["drop"] != "":
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deliver, drop, why, cause = a.str("deliver"), a.str("drop"), a.str("why"), a.str("cause")
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case a.given["id"] != "":
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idText = a.str("id")
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default:
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consumer, limit = a.str("consumer"), a.str("limit")
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}
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if unused := a.unused(); len(unused) > 0 {
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return nil, fmt.Errorf("dead-letters did not use %s together with %s, and an argument a verb would pass "+
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"over is refused: nothing was done", quoteAll(unused), quoteAll(a.usedGiven()))
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}
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switch {
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case deliver != "" && drop != "":
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return nil, errors.New("dead-letters delivers one again or drops one, not both. Nothing was done")
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case deliver != "" || drop != "":
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act, text := "deliver", deliver
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if drop != "" {
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act, text = "drop", drop
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}
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if strings.TrimSpace(why) == "" {
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return nil, fmt.Errorf("dead-letters %s is a hand act, and says why: why is required and recorded in "+
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"the hand-act log (novox/hq to-be 45 §7). Nothing was done", act)
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}
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id, err := deadLetterID(text)
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if err != nil {
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return nil, err
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}
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return actOnDeadLetter(ctx, on, act, id, why, cause)
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case idText != "":
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id, err := deadLetterID(idText)
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if err != nil {
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return nil, err
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}
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return link.DeadLetterNamed(on.js, id)
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}
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most := defaultDeadLetters
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if limit != "" {
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n, err := strconv.Atoi(limit)
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if err != nil || n <= 0 {
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return nil, fmt.Errorf("limit is a number of dead letters, not %q", limit)
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}
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most = n
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}
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held, total, err := link.DeadLetters(on.js, consumer, most)
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if err != nil {
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return nil, err
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}
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answer := map[string]any{"dead_letters": held, "held": total,
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"note": "newest first; with id, one whole; deliver or drop one with why"}
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if total == 0 {
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answer["note"] = "no consumer gave up on a message that is still kept"
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}
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return answer, nil
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}
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// deadLetterID is a dead letter's id as a caller wrote it.
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func deadLetterID(text string) (uint64, error) {
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id, err := strconv.ParseUint(strings.TrimSpace(text), 10, 64)
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if err != nil || id == 0 {
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return 0, fmt.Errorf("a dead letter's id is its number in %s, as dead-letters lists it, not %q",
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broker.DeadLettersStream, text)
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}
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return id, nil
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}
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// actOnDeadLetter delivers one again or drops it, recorded in the hand-act log before it is done. A log
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// that cannot be written is said, and the act still happens: the log is never the reason a person's act
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// is refused (handacts.go).
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func actOnDeadLetter(ctx context.Context, on *busHandles, act string, id uint64, why, cause string) (any, error) {
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d, err := link.DeadLetterNamed(on.js, id)
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if err != nil {
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return nil, err
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}
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if act == "deliver" {
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// Refused before it is recorded: an act that cannot be done is not an act.
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if _, err := link.AgainTo(d); err != nil {
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return nil, err
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}
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}
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if cause == "" {
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cause = causeDeadLetter
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}
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by := link.CallerIn(ctx)
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if by == "" {
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by = "a seat call whose caller the bus did not name"
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}
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answer := map[string]any{"dead_letter": d.ID, "consumer": d.Who, "subject": d.Subject}
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recorded, logErr := link.RecordHandAct(ctx, on.conn, link.HandAct{Verb: "dead-letters " + act,
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Args: []string{strconv.FormatUint(id, 10), d.Stream + "." + d.Consumer}, Why: why, Cause: cause,
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Condition: conditions.Key(conditions.ScopeBus, d.Stream+"."+d.Consumer, link.AdvisoryMaxDeliveries),
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By: by + ", through the " + catalogue.ControllerSeatName + " seat"})
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if logErr != nil {
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answer["unrecorded"] = "the hand-act log could not be written, and the act was done all the same: " + logErr.Error()
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} else {
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answer["recorded"] = recorded.ID
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}
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switch act {
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case "deliver":
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_, to, err := link.DeliverAgain(on.js, id)
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if err != nil {
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return nil, err
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}
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answer["delivered_on"] = to
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answer["done"] = fmt.Sprintf("dead letter %d was delivered again to %s, and nobody else; it is no longer kept",
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id, consumerWho(d.Stream, d.Consumer))
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case "drop":
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if _, err := link.DropDeadLetter(on.js, id); err != nil {
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return nil, err
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}
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answer["done"] = fmt.Sprintf("dead letter %d, which %s gave up on, was dropped for good", id,
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consumerWho(d.Stream, d.Consumer))
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}
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return answer, nil
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}
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