Code I am proud of.

Real pieces of the code behind Laayo, lightly trimmed. Each one has a note on why it is written that way and what it prevents.

First claim wins, and a retry is not a second claim

PHP · app/Services/FoodRescueService.php

When cooked food from a cancelled order goes up for rescue, several nearby customers can tap Claim at the same moment, so I lock the offer row and let the first committed claim win. A second tap from the same customer gets back the claim they already own instead of an error, which covers double taps and app retries on a weak network. If a claim is the first thing to notice that the timer ran out, it records the expiry and commits before reporting the error, because throwing inside the transaction would roll the expiry back and leave a dead offer looking available.

public function claimAtomically(int $offerId, int $userId): OrderRescueOffer
{
    // Set when THIS call is the first to see the expiry; acted on after commit.
    $expiredOffer = null;
 
    $offer = DB::transaction(function () use ($offerId, $userId, &$expiredOffer) {
        $row = OrderRescueOffer::where('id', $offerId)->lockForUpdate()->first();
        if (!$row) {
            throw new \RuntimeException(translate('Rescue offer not found.'));
        }
        // Same user re-claiming: hand back what they already own
        // (double-tap on Place Order, app retry after a flaky network).
        if ($row->status === 'claimed'
            && (int) $row->claimed_by_user_id === $userId) {
            return $row;
        }
        if ($row->status !== 'available') {
            throw new \RuntimeException(translate('This rescue offer has already been claimed.'));
        }
        if (!$row->expires_at || $row->expires_at->isPast()) {
            // Don't throw here: that would roll back the status write.
            $row->status = 'expired';
            $row->save();
            $expiredOffer = $row->fresh();
            return null;
        }
        $row->status = 'claimed';
        $row->claimed_by_user_id = $userId;
        $row->claimed_at = now();
        $row->save();
        return $row;
    });
 
    if ($expiredOffer !== null) {
        // ... run the expiry side effects (tell the rider, settle) in a try/catch
        throw new \RuntimeException(translate('This rescue offer has expired.'));
    }
    return $offer;
}public function claimAtomically(int $offerId, int $userId): OrderRescueOffer
{
    // Set when THIS call is the first to see the expiry; acted on after commit.
    $expiredOffer = null;
 
    $offer = DB::transaction(function () use ($offerId, $userId, &$expiredOffer) {
        $row = OrderRescueOffer::where('id', $offerId)->lockForUpdate()->first();
        if (!$row) {
            throw new \RuntimeException(translate('Rescue offer not found.'));
        }
        // Same user re-claiming: hand back what they already own
        // (double-tap on Place Order, app retry after a flaky network).
        if ($row->status === 'claimed'
            && (int) $row->claimed_by_user_id === $userId) {
            return $row;
        }
        if ($row->status !== 'available') {
            throw new \RuntimeException(translate('This rescue offer has already been claimed.'));
        }
        if (!$row->expires_at || $row->expires_at->isPast()) {
            // Don't throw here: that would roll back the status write.
            $row->status = 'expired';
            $row->save();
            $expiredOffer = $row->fresh();
            return null;
        }
        $row->status = 'claimed';
        $row->claimed_by_user_id = $userId;
        $row->claimed_at = now();
        $row->save();
        return $row;
    });
 
    if ($expiredOffer !== null) {
        // ... run the expiry side effects (tell the rider, settle) in a try/catch
        throw new \RuntimeException(translate('This rescue offer has expired.'));
    }
    return $offer;
}

Cancel the original exactly once, whoever gets there first

PHP · app/Services/FoodRescueService.php

A rescue never cancels the original order until the new buyer is committed, and for an online payment that only happens once the money lands. Two paths can notice the payment, the app when the customer returns and a cron that runs every minute as a backstop, so both call this one function. Only the call that actually flips the original to cancelled goes on to refund and notify that customer, which means a race between them cannot refund twice. If the rider delivered the order in the meantime, I leave it alone.

// Online-paid rescue claims cancel the original order only once the new order
// is paid. Called by the app's release path and by a once-a-minute cron.
public function finalizeRescuePaidOriginal(OrderRescueOffer $offer): void
{
    $cancelledLiveOriginalId = null;
    DB::transaction(function () use ($offer, &$cancelledLiveOriginalId) {
        $original = Order::withoutGlobalScopes()
            ->lockForUpdate()
            ->find($offer->original_order_id);
        if (!$original || $original->order_status === 'canceled') {
            return; // already finalized, or the original row is gone
        }
        // Rider closed the order during the payment window: the food
        // is gone. Leave the original as-is; there's nothing to cancel.
        if (in_array($original->order_status, ['delivered', 'failed', 'refunded'], true)) {
            return;
        }
        $original->order_status = 'canceled';
        $original->canceled = now();
        // ... cancellation reason, canceled_by, rescue_eligible = false
        $original->save();
        if ($original->delivery_man_id) {
            DeliveryMan::where('id', $original->delivery_man_id)
                ->where('current_orders', '>', 0)
                ->decrement('current_orders');
        }
        $cancelledLiveOriginalId = $original->id;
    });
 
    // Post-commit: refund + notify the original customer, but ONLY
    // when THIS call is the one that cancelled the original (so a
    // racing cron/release pair never double-refunds).
    if ($cancelledLiveOriginalId) {
        $orig = Order::withoutGlobalScopes()->find($cancelledLiveOriginalId);
        if ($orig) {
            $this->notifyOriginalCustomerOfRescueCancel($orig);
        }
    }
}// Online-paid rescue claims cancel the original order only once the new order
// is paid. Called by the app's release path and by a once-a-minute cron.
public function finalizeRescuePaidOriginal(OrderRescueOffer $offer): void
{
    $cancelledLiveOriginalId = null;
    DB::transaction(function () use ($offer, &$cancelledLiveOriginalId) {
        $original = Order::withoutGlobalScopes()
            ->lockForUpdate()
            ->find($offer->original_order_id);
        if (!$original || $original->order_status === 'canceled') {
            return; // already finalized, or the original row is gone
        }
        // Rider closed the order during the payment window: the food
        // is gone. Leave the original as-is; there's nothing to cancel.
        if (in_array($original->order_status, ['delivered', 'failed', 'refunded'], true)) {
            return;
        }
        $original->order_status = 'canceled';
        $original->canceled = now();
        // ... cancellation reason, canceled_by, rescue_eligible = false
        $original->save();
        if ($original->delivery_man_id) {
            DeliveryMan::where('id', $original->delivery_man_id)
                ->where('current_orders', '>', 0)
                ->decrement('current_orders');
        }
        $cancelledLiveOriginalId = $original->id;
    });
 
    // Post-commit: refund + notify the original customer, but ONLY
    // when THIS call is the one that cancelled the original (so a
    // racing cron/release pair never double-refunds).
    if ($cancelledLiveOriginalId) {
        $orig = Order::withoutGlobalScopes()->find($cancelledLiveOriginalId);
        if ($orig) {
            $this->notifyOriginalCustomerOfRescueCancel($orig);
        }
    }
}

An AI agent can only order what the customer saw

Go · internal/mcp/checkout.go

This is the tool an AI assistant such as Claude calls to place a real delivery order through our MCP connector. The schema makes customer_confirmed a constant true and asks for the total the customer agreed to, and I re-price the cart on the server and refuse if it moved, so the model cannot place an order at a price the person never saw. The idempotency key comes from the cart contents and a ten-minute window, so a retried tool call replays the same order instead of sending a second rider, and on a timeout I tell the model to check my_orders before trying again.

func (s *Server) tPlaceOrder(ctx context.Context, ss *session, raw json.RawMessage) (any, error) {
	in, err := decode[struct {
		AddressID int64    `json:"address_id"`
		Payment   string   `json:"payment_method"`
		Note      string   `json:"note"`
		Expected  *float64 `json:"expected_total"`
		Confirmed bool     `json:"customer_confirmed"`
	}](raw) // unknown fields are refused
	if err != nil {
		return nil, err
	}
	if !in.Confirmed {
		return nil, userError("Ask the customer to confirm the order first, then call again with customer_confirmed true.")
	}
	// ... payment method check, per-customer rate limit, store open, minimum order
	co, err := s.prepare(ctx, ss, in.AddressID, in.Payment, in.Note)
	if err != nil {
		return nil, err
	}
	b, _, err := s.price(ctx, ss, co)
	if err != nil {
		return nil, err
	}
	if in.Expected == nil || math.Abs(*in.Expected-b.Total) > 1 {
		return nil, userError(fmt.Sprintf("The total is now Rs %.2f, not what the customer confirmed. Show them the new checkout_preview and ask again. Nothing was placed.", b.Total))
	}
	// ... online payment returns a checkout link instead of placing
	// same cart + address + method within a 10 minute window = same key: a retried call replays, never doubles
	sig := ss.g.ID + "|" + in.Payment + "|" + co.body["latitude"].(string) + "|" + co.body["longitude"].(string)
	for _, r := range co.rows {
		sig += fmt.Sprintf("|%d:%d:%d", r.ID, int64(r.ItemID), int64(r.Quantity))
	}
	idem := "lmcp-" + hashHex(sig + "|" + strconv.FormatInt(time.Now().Unix()/600, 10))[:40]
	var out placedOrder
	if err := s.api.Do(ctx, co.w.call(Call{Method: "POST", Path: placeOrderPath, Body: co.body, Idem: idem}), &out); err != nil {
		// ... 409 = still placing; other 4xx = show the API's own message
		return nil, userError("Laayo did not confirm the order in time. Call my_orders before trying again, so it is not placed twice.")
	}
	// ... return order id, restaurant, total and delivery time
}func (s *Server) tPlaceOrder(ctx context.Context, ss *session, raw json.RawMessage) (any, error) {
	in, err := decode[struct {
		AddressID int64    `json:"address_id"`
		Payment   string   `json:"payment_method"`
		Note      string   `json:"note"`
		Expected  *float64 `json:"expected_total"`
		Confirmed bool     `json:"customer_confirmed"`
	}](raw) // unknown fields are refused
	if err != nil {
		return nil, err
	}
	if !in.Confirmed {
		return nil, userError("Ask the customer to confirm the order first, then call again with customer_confirmed true.")
	}
	// ... payment method check, per-customer rate limit, store open, minimum order
	co, err := s.prepare(ctx, ss, in.AddressID, in.Payment, in.Note)
	if err != nil {
		return nil, err
	}
	b, _, err := s.price(ctx, ss, co)
	if err != nil {
		return nil, err
	}
	if in.Expected == nil || math.Abs(*in.Expected-b.Total) > 1 {
		return nil, userError(fmt.Sprintf("The total is now Rs %.2f, not what the customer confirmed. Show them the new checkout_preview and ask again. Nothing was placed.", b.Total))
	}
	// ... online payment returns a checkout link instead of placing
	// same cart + address + method within a 10 minute window = same key: a retried call replays, never doubles
	sig := ss.g.ID + "|" + in.Payment + "|" + co.body["latitude"].(string) + "|" + co.body["longitude"].(string)
	for _, r := range co.rows {
		sig += fmt.Sprintf("|%d:%d:%d", r.ID, int64(r.ItemID), int64(r.Quantity))
	}
	idem := "lmcp-" + hashHex(sig + "|" + strconv.FormatInt(time.Now().Unix()/600, 10))[:40]
	var out placedOrder
	if err := s.api.Do(ctx, co.w.call(Call{Method: "POST", Path: placeOrderPath, Body: co.body, Idem: idem}), &out); err != nil {
		// ... 409 = still placing; other 4xx = show the API's own message
		return nil, userError("Laayo did not confirm the order in time. Call my_orders before trying again, so it is not placed twice.")
	}
	// ... return order id, restaurant, total and delivery time
}

The admin AI proposes, a person confirms

Go · internal/admin/assist_actions.go

Our admin panel has an AI helper that can also make small changes, like closing a store for an hour or switching a banner off. I only hand the model the propose_action tool when the admin's own message reads like a request for a change, and the tool refuses any target the admin did not name in that message, so text inside a page or a tool result cannot steer it toward something else. It can prepare one change per question and nothing happens until a person clicks Confirm, at which point the server re-reads the live value and refuses if it moved since the proposal.

// assistWantsAction: the user's own words ask for a change (not "how do I ...").
func assistWantsAction(msg string) bool {
	return assistActionVerb.MatchString(msg) && !assistHowQuestion.MatchString(msg)
}
 
// toolDefs: the read tools, plus propose_action only when the user's own message asks for a change.
func (r *assistRun) toolDefs() json.RawMessage {
	if r.in.Explain != nil || !assistActionsOn() || !assistWantsAction(r.in.Message) {
		return assistToolDefs
	}
	t := strings.TrimSpace(string(assistToolDefs))
	return json.RawMessage(strings.TrimSuffix(t, "]") + "," + string(assistProposeDef) + "]")
}
 
// propose: the tool. Returns the model-facing result; adds the action (or a notice) card.
func (r *assistRun) propose(ctx context.Context, raw json.RawMessage) any {
	var a assistProposeIn
	if err := json.Unmarshal(raw, &a); err != nil {
		return map[string]any{"error": "bad arguments"}
	}
	// ... explain mode and the ASSIST_ACTIONS switch refuse here too
	if !assistWantsAction(r.in.Message) {
		return map[string]any{"error": "The user did not ask for a change. Show the page instead."}
	}
	for _, nm := range []string{a.Store, a.Item, a.Banner, a.Coupon, a.Rider} {
		if nm != "" && !assistNamed(r.in.Message, nm) {
			return map[string]any{"error": "That target is not in the user's message. Ask them to name it."}
		}
	}
	if r.proposals >= 1 {
		return map[string]any{"error": "One change at a time."}
	}
	// ... resolve the target by name through the read API; the write route's own rules apply
	if res.from == res.to {
		return map[string]any{"note": "Already " + res.to + ". Nothing to change."}
	}
	// ... store {who, target, from, to, exact request} with a short expiry, show an action card
	r.proposals++
	return map[string]any{"ok": true, "shown": "action card", "note": "Nothing changes until the user clicks Confirm. Tell them to check and confirm."}
}// assistWantsAction: the user's own words ask for a change (not "how do I ...").
func assistWantsAction(msg string) bool {
	return assistActionVerb.MatchString(msg) && !assistHowQuestion.MatchString(msg)
}
 
// toolDefs: the read tools, plus propose_action only when the user's own message asks for a change.
func (r *assistRun) toolDefs() json.RawMessage {
	if r.in.Explain != nil || !assistActionsOn() || !assistWantsAction(r.in.Message) {
		return assistToolDefs
	}
	t := strings.TrimSpace(string(assistToolDefs))
	return json.RawMessage(strings.TrimSuffix(t, "]") + "," + string(assistProposeDef) + "]")
}
 
// propose: the tool. Returns the model-facing result; adds the action (or a notice) card.
func (r *assistRun) propose(ctx context.Context, raw json.RawMessage) any {
	var a assistProposeIn
	if err := json.Unmarshal(raw, &a); err != nil {
		return map[string]any{"error": "bad arguments"}
	}
	// ... explain mode and the ASSIST_ACTIONS switch refuse here too
	if !assistWantsAction(r.in.Message) {
		return map[string]any{"error": "The user did not ask for a change. Show the page instead."}
	}
	for _, nm := range []string{a.Store, a.Item, a.Banner, a.Coupon, a.Rider} {
		if nm != "" && !assistNamed(r.in.Message, nm) {
			return map[string]any{"error": "That target is not in the user's message. Ask them to name it."}
		}
	}
	if r.proposals >= 1 {
		return map[string]any{"error": "One change at a time."}
	}
	// ... resolve the target by name through the read API; the write route's own rules apply
	if res.from == res.to {
		return map[string]any{"note": "Already " + res.to + ". Nothing to change."}
	}
	// ... store {who, target, from, to, exact request} with a short expiry, show an action card
	r.proposals++
	return map[string]any{"ok": true, "shown": "action card", "note": "Nothing changes until the user clicks Confirm. Tell them to check and confirm."}
}

Treat the vision model's menu as untrusted text

Go · internal/menuai/extract.go

Restaurants send us menus as phone photos and PDFs, and a vision model turns each batch of pages into JSON items in parallel. I treat that output as untrusted text: strip code fences, cut to the outermost braces, accept either an object or a bare array, and drop any row without a name instead of guessing at it. Batches often overlap, so I merge duplicates on name, price, sorted variants and category, and a later copy may only fill fields the first copy left empty, never overwrite them.

// menuaiParseVision: model output -> item rows. Malformed rows are dropped, not guessed.
func menuaiParseVision(content string) []map[string]any {
	c := menuaiPHPTrim(content)
	c = menuaiFenceOpen.ReplaceAllString(c, "")
	c = menuaiFenceEnd.ReplaceAllString(c, "")
	c = menuaiPHPTrim(c)
	if a, b := strings.Index(c, "{"), strings.LastIndex(c, "}"); a >= 0 && b > a {
		c = c[a : b+1]
	}
	var dec any
	if json.Unmarshal([]byte(c), &dec) != nil {
		return nil
	}
	// ... accept {"items": [...]} or a bare [...]
	// ... per item: skip non-objects and empty names, keep a price only when it is a number
}
 
// menuaiDedupe: pages overlap, so the same dish can come back twice.
func menuaiDedupe(items []map[string]any) []map[string]any {
	by := map[string]map[string]any{}
	var order []string
	for _, it := range items {
		// ... name, price and the sorted "variant:price" list become strings
		key := name + "|" + price + "|" + vsig + "|" + strings.ToLower(cat)
		ex, ok := by[key]
		if !ok {
			by[key] = it
			order = append(order, key)
			continue
		}
		for _, f := range []string{"price", "veg", "category_name", "sub_category_name", "description"} {
			if menuaiEmpty(ex[f]) && !menuaiEmpty(it[f]) {
				ex[f] = it[f]
			}
		}
	}
	// ... rebuild the slice in first-seen order
}// menuaiParseVision: model output -> item rows. Malformed rows are dropped, not guessed.
func menuaiParseVision(content string) []map[string]any {
	c := menuaiPHPTrim(content)
	c = menuaiFenceOpen.ReplaceAllString(c, "")
	c = menuaiFenceEnd.ReplaceAllString(c, "")
	c = menuaiPHPTrim(c)
	if a, b := strings.Index(c, "{"), strings.LastIndex(c, "}"); a >= 0 && b > a {
		c = c[a : b+1]
	}
	var dec any
	if json.Unmarshal([]byte(c), &dec) != nil {
		return nil
	}
	// ... accept {"items": [...]} or a bare [...]
	// ... per item: skip non-objects and empty names, keep a price only when it is a number
}
 
// menuaiDedupe: pages overlap, so the same dish can come back twice.
func menuaiDedupe(items []map[string]any) []map[string]any {
	by := map[string]map[string]any{}
	var order []string
	for _, it := range items {
		// ... name, price and the sorted "variant:price" list become strings
		key := name + "|" + price + "|" + vsig + "|" + strings.ToLower(cat)
		ex, ok := by[key]
		if !ok {
			by[key] = it
			order = append(order, key)
			continue
		}
		for _, f := range []string{"price", "veg", "category_name", "sub_category_name", "description"} {
			if menuaiEmpty(ex[f]) && !menuaiEmpty(it[f]) {
				ex[f] = it[f]
			}
		}
	}
	// ... rebuild the slice in first-seen order
}

Delivery estimates that notice when the city slows down

Go · internal/eta/drift.go

Kitchens and roads slow down for weeks at a time, and a model trained on older data keeps promising the old times. I add the median recent miss of the shown estimate to every quote, capped, and only once enough delivered orders back it up. In training, each order only sees drift from orders delivered before it was placed, so the evaluation cannot peek at the future, and the term is kept only if it costs no more than half a minute of median error on the later half of validation.

// driftAt is the median miss over points finished in the trailing window before t.
func driftAt(pts []driftPoint, t time.Time) (float64, int) {
	var v []float64
	for _, p := range pts {
		if !p.done.After(t) && p.done.After(t.Add(-driftWindow)) {
			v = append(v, p.miss)
		}
	}
	if len(v) < driftMinN {
		return 0, len(v)
	}
	return clamp(quantile(v, 0.5), -driftCap, driftCap), len(v)
}
 
// withDrift (not shown) adds driftAt's value to p25/p50/p80, keeping them ordered.
 
// decideDrift: on the later half of validation, with drift points from all
// earlier samples, does the drift help without raising the median error?
func decideDrift(m *Model, hist, valid []*Sample) (bool, [2]float64, [2]float64) {
	h := len(valid) / 2
	// ... too few samples: keep the model as it is
	pts := m.driftPoints(append(append([]*Sample(nil), hist...), valid...))
	var off, on grader
	for _, s := range valid[h:] {
		q, ok := m.mixed(&s.Snap, s.PrepX, s.RiderX, &s.V1)
		if !ok {
			continue
		}
		off.add(q.P50, s.Truth.TotalMin)
		on.add(withDrift(q, pts, s.O.Created).P50, s.Truth.TotalMin)
	}
	a, b := off.metrics(), on.metrics()
	use := forceDrift || b.MedAbs <= a.MedAbs+0.5
	return use, [2]float64{a.MedAbs, a.Bias}, [2]float64{b.MedAbs, b.Bias}
}// driftAt is the median miss over points finished in the trailing window before t.
func driftAt(pts []driftPoint, t time.Time) (float64, int) {
	var v []float64
	for _, p := range pts {
		if !p.done.After(t) && p.done.After(t.Add(-driftWindow)) {
			v = append(v, p.miss)
		}
	}
	if len(v) < driftMinN {
		return 0, len(v)
	}
	return clamp(quantile(v, 0.5), -driftCap, driftCap), len(v)
}
 
// withDrift (not shown) adds driftAt's value to p25/p50/p80, keeping them ordered.
 
// decideDrift: on the later half of validation, with drift points from all
// earlier samples, does the drift help without raising the median error?
func decideDrift(m *Model, hist, valid []*Sample) (bool, [2]float64, [2]float64) {
	h := len(valid) / 2
	// ... too few samples: keep the model as it is
	pts := m.driftPoints(append(append([]*Sample(nil), hist...), valid...))
	var off, on grader
	for _, s := range valid[h:] {
		q, ok := m.mixed(&s.Snap, s.PrepX, s.RiderX, &s.V1)
		if !ok {
			continue
		}
		off.add(q.P50, s.Truth.TotalMin)
		on.add(withDrift(q, pts, s.O.Created).P50, s.Truth.TotalMin)
	}
	a, b := off.metrics(), on.metrics()
	use := forceDrift || b.MedAbs <= a.MedAbs+0.5
	return use, [2]float64{a.MedAbs, a.Bias}, [2]float64{b.MedAbs, b.Bias}
}

Money writes that are safe to replay

Go · internal/ledger/apply.go

I write every wallet movement as a plan with a key and a hash, and claim the key with an INSERT inside the same transaction as the money rows. A retried request or a duplicate webhook hits the primary key and comes back as 'already applied' instead of paying twice, and a key reused for a different plan is refused loudly. Wallet rows are read with FOR UPDATE in one fixed order, which closed a lost-update race the old read-then-write code had.

var (
	// ErrAlreadyApplied: the same plan (same key, same hash) already
	// committed. Callers treat it as success (replayed request / webhook).
	ErrAlreadyApplied = errors.New("ledger: already applied")
	// ErrKeyConflict: the key was used for a different plan.
	ErrKeyConflict = errors.New("ledger: idempotency key reused with a different plan")
)
 
// Apply runs the plan inside tx (the caller owns BEGIN/COMMIT/ROLLBACK and
// must roll back on any error). Wallet rows are locked with SELECT ... FOR
// UPDATE in the order the plan loads them (admin -> store -> rider, the same
// order the old stack updates them), so the two never lose each other's
// updates and cannot deadlock on each other.
func Apply(ctx context.Context, tx *sql.Tx, s Settings, now time.Time, p *Plan, useKeyTable bool) (*Result, error) {
	b := &sqlBackend{ctx: ctx, tx: tx}
	// ...
	if useKeyTable && p.Key != "" {
		h := p.Hash()
		_, err := tx.ExecContext(ctx, "INSERT INTO ledger_ops (op_key, plan_hash, created_at) VALUES (?, ?, ?)", p.Key, h, MySQLNow(now))
		if err != nil {
			var me *mysql.MySQLError
			if errors.As(err, &me) && me.Number == 1062 { // duplicate key
				var old string
				if err := tx.QueryRowContext(ctx, "SELECT plan_hash FROM ledger_ops WHERE op_key = ?", p.Key).Scan(&old); err != nil {
					return nil, mapErr(err)
				}
				if old == h {
					return nil, ErrAlreadyApplied
				}
				return nil, ErrKeyConflict
			}
			return nil, mapErr(err)
		}
	}
	res, err := Run(b, s, now, p)
	if err != nil {
		return res, mapErr(err)
	}
	return res, nil
}var (
	// ErrAlreadyApplied: the same plan (same key, same hash) already
	// committed. Callers treat it as success (replayed request / webhook).
	ErrAlreadyApplied = errors.New("ledger: already applied")
	// ErrKeyConflict: the key was used for a different plan.
	ErrKeyConflict = errors.New("ledger: idempotency key reused with a different plan")
)
 
// Apply runs the plan inside tx (the caller owns BEGIN/COMMIT/ROLLBACK and
// must roll back on any error). Wallet rows are locked with SELECT ... FOR
// UPDATE in the order the plan loads them (admin -> store -> rider, the same
// order the old stack updates them), so the two never lose each other's
// updates and cannot deadlock on each other.
func Apply(ctx context.Context, tx *sql.Tx, s Settings, now time.Time, p *Plan, useKeyTable bool) (*Result, error) {
	b := &sqlBackend{ctx: ctx, tx: tx}
	// ...
	if useKeyTable && p.Key != "" {
		h := p.Hash()
		_, err := tx.ExecContext(ctx, "INSERT INTO ledger_ops (op_key, plan_hash, created_at) VALUES (?, ?, ?)", p.Key, h, MySQLNow(now))
		if err != nil {
			var me *mysql.MySQLError
			if errors.As(err, &me) && me.Number == 1062 { // duplicate key
				var old string
				if err := tx.QueryRowContext(ctx, "SELECT plan_hash FROM ledger_ops WHERE op_key = ?", p.Key).Scan(&old); err != nil {
					return nil, mapErr(err)
				}
				if old == h {
					return nil, ErrAlreadyApplied
				}
				return nil, ErrKeyConflict
			}
			return nil, mapErr(err)
		}
	}
	res, err := Run(b, s, now, p)
	if err != nil {
		return res, mapErr(err)
	}
	return res, nil
}

Only the newest GPS point matters

Go · internal/realtime/rider_ingest.go

Rider phones send location over a websocket, and the write path behind it can be slower than the phones. I keep one pending slot per rider and at most one queued job per rider, so a burst of points collapses into the latest one instead of growing a backlog. A full queue drops a point rather than blocking the socket reader, because a stale position is worth less than a responsive connection.

// pending: newest unsent point per rider; inflight: rider already queued
// or being posted; queue: riders for the 4 posting workers.
func (ri *RiderIngest) Accept(ctx context.Context, msg []byte) frameResult {
	// ... validate the frame, build the point
	ri.mu.Lock()
	ri.pending[dmID] = riderPoint{form: form}
	queued := ri.inflight[dmID]
	if !queued {
		ri.inflight[dmID] = true
	}
	ri.mu.Unlock()
	if !queued {
		select {
		case ri.queue <- dmID:
		default: // never block the socket reader
			ri.mu.Lock()
			delete(ri.inflight, dmID)
			delete(ri.pending, dmID)
			ri.mu.Unlock()
		}
	}
	return frameAccepted
}
 
// drain posts the rider's newest point, then any point that arrived while
// that request was in flight.
func (ri *RiderIngest) drain(ctx context.Context, dmID int64) {
	for {
		ri.mu.Lock()
		p, ok := ri.pending[dmID]
		delete(ri.pending, dmID)
		if !ok {
			delete(ri.inflight, dmID)
			ri.mu.Unlock()
			return
		}
		ri.mu.Unlock()
		ri.post(ctx, dmID, p)
	}
}// pending: newest unsent point per rider; inflight: rider already queued
// or being posted; queue: riders for the 4 posting workers.
func (ri *RiderIngest) Accept(ctx context.Context, msg []byte) frameResult {
	// ... validate the frame, build the point
	ri.mu.Lock()
	ri.pending[dmID] = riderPoint{form: form}
	queued := ri.inflight[dmID]
	if !queued {
		ri.inflight[dmID] = true
	}
	ri.mu.Unlock()
	if !queued {
		select {
		case ri.queue <- dmID:
		default: // never block the socket reader
			ri.mu.Lock()
			delete(ri.inflight, dmID)
			delete(ri.pending, dmID)
			ri.mu.Unlock()
		}
	}
	return frameAccepted
}
 
// drain posts the rider's newest point, then any point that arrived while
// that request was in flight.
func (ri *RiderIngest) drain(ctx context.Context, dmID int64) {
	for {
		ri.mu.Lock()
		p, ok := ri.pending[dmID]
		delete(ri.pending, dmID)
		if !ok {
			delete(ri.inflight, dmID)
			ri.mu.Unlock()
			return
		}
		ri.mu.Unlock()
		ri.post(ctx, dmID, p)
	}
}

Fan-out where one slow phone cannot stall the rest

Go · internal/realtime/hub.go

Live order tracking fans each rider position out to every customer and staff screen watching that rider. I copy the subscriber list under a read lock and send outside it, and each socket has a small buffered channel with a single goroutine that owns all writes. If a phone on a bad connection cannot keep up, its buffer fills and I close that one socket, so the broadcast never waits on it and memory cannot grow without bound.

// Copy the targets under the read lock, send outside it.
func (h *Hub) SendToTrackers(dmID int64, frame []byte) int {
	h.mu.RLock()
	targets := make([]*Client, 0, len(h.trackers[dmID]))
	for c := range h.trackers[dmID] {
		targets = append(targets, c)
	}
	h.mu.RUnlock()
	for _, c := range targets {
		c.Send(frame)
	}
	return len(targets)
}
 
// A client too slow to drain its buffer is dropped, not waited for.
func (c *Client) Send(frame []byte) {
	// ... return at once if already closed
	select {
	case c.send <- frame:
	default:
		slog.Warn("client send buffer full, closing", "socket_id", c.socketID)
		c.Close()
	}
}
 
// writePump owns all writes: queued frames plus the keep-alive pings.
func (c *Client) writePump() { // ... ticker setup; defer Close
	for {
		select {
		case <-c.closed:
			return
		case frame := <-c.send:
			_ = c.conn.SetWriteDeadline(time.Now().Add(writeWait))
			if c.conn.WriteMessage(websocket.TextMessage, frame) != nil {
				return
			}
		case <-ticker.C: // ... same, with a PingMessage
		}
	}
}// Copy the targets under the read lock, send outside it.
func (h *Hub) SendToTrackers(dmID int64, frame []byte) int {
	h.mu.RLock()
	targets := make([]*Client, 0, len(h.trackers[dmID]))
	for c := range h.trackers[dmID] {
		targets = append(targets, c)
	}
	h.mu.RUnlock()
	for _, c := range targets {
		c.Send(frame)
	}
	return len(targets)
}
 
// A client too slow to drain its buffer is dropped, not waited for.
func (c *Client) Send(frame []byte) {
	// ... return at once if already closed
	select {
	case c.send <- frame:
	default:
		slog.Warn("client send buffer full, closing", "socket_id", c.socketID)
		c.Close()
	}
}
 
// writePump owns all writes: queued frames plus the keep-alive pings.
func (c *Client) writePump() { // ... ticker setup; defer Close
	for {
		select {
		case <-c.closed:
			return
		case frame := <-c.send:
			_ = c.conn.SetWriteDeadline(time.Now().Add(writeWait))
			if c.conn.WriteMessage(websocket.TextMessage, frame) != nil {
				return
			}
		case <-ticker.C: // ... same, with a PingMessage
		}
	}
}

Reload failed screens once when the server comes back

Swift · ios/Core/Recovery.swift

I opened the app while the server was down, the server came back, and the app sat on its error screen until I killed and reopened it. I added one main-actor registry where a failed screen registers a reload closure with a weak owner, and a single probe loop backs off from 3 s to 30 s, pauses in the background and wakes when the network path returns. When the server answers, each failed screen reloads exactly once, and screens that were fine are never touched, so recovery does not turn into a burst of requests.

@MainActor
final class LaayoRecovery {
    static let shared = LaayoRecovery()
    private final class Entry { weak var owner: AnyObject?; let reload: @MainActor () -> Void /* ... */ }
    private var entries: [Entry] = []; private var loop: Task<Void, Never>?
    private var step = 0
    private var probing = false
    private static let delays: [Double] = [3, 6, 12, 24, 30]
 
    private func schedule() {
        guard loop == nil, !probing, !entries.isEmpty else { return }
        let d = Self.delays[min(step, Self.delays.count - 1)]
        loop = Task { @MainActor in
            try? await Task.sleep(nanoseconds: UInt64(d * 1_000_000_000))
            if Task.isCancelled { return }
            let me = LaayoRecovery.shared
            me.loop = nil; me.step += 1
            await me.probe()
        }
    }
 
    private func probe() async {
        entries.removeAll { $0.owner == nil }
        guard !entries.isEmpty, !probing else { return }
        // Background or no network path: those notifications resume us.
        guard UIApplication.shared.applicationState != .background, LaayoNet.shared.isOnline else { return }
        probing = true
        let ok = await LaayoApi.shared.probe()
        probing = false
        if ok { fire() } else { schedule() }
    }
 
    /// The server answers: each failed page reloads once (its skeleton path).
    private func fire() {
        loop?.cancel(); loop = nil
        let list = entries
        entries.removeAll()
        for e in list where e.owner != nil { e.reload() }
    }
}@MainActor
final class LaayoRecovery {
    static let shared = LaayoRecovery()
    private final class Entry { weak var owner: AnyObject?; let reload: @MainActor () -> Void /* ... */ }
    private var entries: [Entry] = []; private var loop: Task<Void, Never>?
    private var step = 0
    private var probing = false
    private static let delays: [Double] = [3, 6, 12, 24, 30]
 
    private func schedule() {
        guard loop == nil, !probing, !entries.isEmpty else { return }
        let d = Self.delays[min(step, Self.delays.count - 1)]
        loop = Task { @MainActor in
            try? await Task.sleep(nanoseconds: UInt64(d * 1_000_000_000))
            if Task.isCancelled { return }
            let me = LaayoRecovery.shared
            me.loop = nil; me.step += 1
            await me.probe()
        }
    }
 
    private func probe() async {
        entries.removeAll { $0.owner == nil }
        guard !entries.isEmpty, !probing else { return }
        // Background or no network path: those notifications resume us.
        guard UIApplication.shared.applicationState != .background, LaayoNet.shared.isOnline else { return }
        probing = true
        let ok = await LaayoApi.shared.probe()
        probing = false
        if ok { fire() } else { schedule() }
    }
 
    /// The server answers: each failed page reloads once (its skeleton path).
    private func fire() {
        loop?.cancel(); loop = nil
        let list = entries
        entries.removeAll()
        for e in list where e.owner != nil { e.reload() }
    }
}

A websocket that ignores its own ghosts

Kotlin · android/core/Realtime.kt

OkHttp delivers socket callbacks on its own threads, and a closed socket can still report a late message or failure after a new one has opened. I hop every callback to the main thread and compare a generation counter, so events from an old socket are ignored instead of tearing down the healthy one. Reconnects back off from 1 s to 30 s, and the socket only stays open while the app is in the foreground and something is subscribed.

/** Bumped on every connect / teardown: callbacks of an old socket are dropped. */
private var generation = 0
private var attempt = 0
 
private fun connect() {
    val u = url ?: return
    if (socket != null) return
    val req = runCatching { Request.Builder().url(u).build() }.getOrNull() ?: return
    val gen = ++generation
    setState(LaayoRealtimeConnection.CONNECTING)
    socket = client.newWebSocket(req, object : WebSocketListener() {
        override fun onMessage(webSocket: WebSocket, text: String) {
            main.post { if (gen == generation) handle(text) }
        }
        override fun onClosed(webSocket: WebSocket, code: Int, reason: String) {
            main.post { if (gen == generation) dropAndRetry() }
        }
        override fun onFailure(webSocket: WebSocket, t: Throwable, response: Response?) {
            main.post { if (gen == generation) dropAndRetry() }
        }
    })
    // ...
}
 
/** Backoff 1, 2, 4, 8, 16, 30, 30 ... seconds; reset on connect. */
private fun dropAndRetry() {
    if (socket == null) return
    val dropped = teardown() // bumps generation, keeps subscriptions
    if (wanted) {
        val delaySec = minOf(30, 1 shl minOf(attempt, 5))
        attempt++
        val r = Runnable { retry = null; evaluate() }
        retry = r
        setState(LaayoRealtimeConnection.CONNECTING)
        main.postDelayed(r, delaySec * 1000L)
    } else {
        setState(LaayoRealtimeConnection.OFFLINE)
    }
    notifyDropped(dropped)
}/** Bumped on every connect / teardown: callbacks of an old socket are dropped. */
private var generation = 0
private var attempt = 0
 
private fun connect() {
    val u = url ?: return
    if (socket != null) return
    val req = runCatching { Request.Builder().url(u).build() }.getOrNull() ?: return
    val gen = ++generation
    setState(LaayoRealtimeConnection.CONNECTING)
    socket = client.newWebSocket(req, object : WebSocketListener() {
        override fun onMessage(webSocket: WebSocket, text: String) {
            main.post { if (gen == generation) handle(text) }
        }
        override fun onClosed(webSocket: WebSocket, code: Int, reason: String) {
            main.post { if (gen == generation) dropAndRetry() }
        }
        override fun onFailure(webSocket: WebSocket, t: Throwable, response: Response?) {
            main.post { if (gen == generation) dropAndRetry() }
        }
    })
    // ...
}
 
/** Backoff 1, 2, 4, 8, 16, 30, 30 ... seconds; reset on connect. */
private fun dropAndRetry() {
    if (socket == null) return
    val dropped = teardown() // bumps generation, keeps subscriptions
    if (wanted) {
        val delaySec = minOf(30, 1 shl minOf(attempt, 5))
        attempt++
        val r = Runnable { retry = null; evaluate() }
        retry = r
        setState(LaayoRealtimeConnection.CONNECTING)
        main.postDelayed(r, delaySec * 1000L)
    } else {
        setState(LaayoRealtimeConnection.OFFLINE)
    }
    notifyDropped(dropped)
}