LLD7 min read

Notification Service

Multi-channel notifications (email/SMS/push) with template, retry and rate-limit decorators.

llddesign

Intro

A notification service must dispatch the right message to the right channel for the right user, with templating, localisation, opt-outs and retries. The clean model: Channel implementations behind a unified interface; pipeline of decorators around the send.

Functional

Send templated messages to user via Email, SMS or Push.
Respect user channel preferences and opt-outs.
Retry with exponential backoff on transient failures.
Throttle per-user to prevent floods.

Non-functional

Asynchronous send (fire and forget).
Idempotent — duplicate dispatch must not double-send.

Components

Notification
Templated message + recipient + variables.
Channel
Interface: EmailChannel (SES), SmsChannel (Twilio), PushChannel (FCM).
TemplateEngine
Renders body with localised strings + variables.
Dispatcher
Picks channel(s) per user preference; submits to queue.
RetryPolicy / RateLimiter / DeduplicationStore
Decorators around Channel.send.

Code

Snippetjava

interface Channel { void send(Message m); }

class RetryingChannel implements Channel {
    private final Channel delegate;
    private final RetryPolicy policy;
    public void send(Message m) {
        for (int attempt = 0; attempt < policy.maxAttempts(); attempt++) {
            try { delegate.send(m); return; }
            catch (TransientFailure e) { sleep(policy.backoff(attempt)); }
        }
        throw new GiveUpException();
    }
}

class IdempotentChannel implements Channel {
    private final Channel delegate;
    private final Set<String> seen;
    public synchronized void send(Message m) {
        if (!seen.add(m.idempotencyKey)) return;
        delegate.send(m);
    }
}

Pitfalls

Embedding retry / rate-limit logic inside each Channel — duplicates code.
Forgetting an idempotency key — webhooks / queues will replay messages.

Intro

Components

Code

Pitfalls

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