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A URL shortener stores a destination under a compact code, then looks up that code and redirects the visitor to the stored URL. The crucial design rule is that a generated code is only a candidate: truncating a hash does not make it unique. Enforce code uniqueness in storage and handle conflicts when creating links.
What a URL shortener does
The smallest useful design has two operations:
- Create: accept a destination URL, choose a short code, and persist the code-to-destination mapping.
- Resolve: accept a short code, retrieve its mapping, and return an HTTP redirect to the saved destination.
For example, a record might associate k7Pq2 with https://example.org/articles/design. The code is the lookup key; the destination is data stored by the service. The redirect endpoint should retrieve the destination from that record rather than infer it from the code.
How link creation should work
- Parse and validate the destination. Define which schemes are permitted—typically, whether only web URLs such as HTTP and HTTPS are accepted—and reject malformed or disallowed destinations. If users can submit arbitrary external URLs, parse them with a maintained URL parser and apply an explicit policy. Raw prefix or suffix checks can be fooled by misleading hostnames or user-information components.
- Generate a candidate code. A hash, random string, or encoded unique ID can supply a candidate, but only a successful uniqueness check makes it usable.
- Insert the mapping atomically. Store the code and destination together under a unique constraint on the code, or use an equivalent atomic operation. A separate “check whether this code exists” followed by an unprotected insert can race: two requests may both see it as available.
- Resolve conflicts deliberately. If the insert reports a uniqueness conflict, retry with a new candidate, add a disambiguating component, or use a separate unique-ID allocation strategy. Do not overwrite the existing mapping.
- Return the completed short URL. Publish it only after the mapping has been persisted successfully.
Custom aliases need the same uniqueness enforcement. They also need reserved-name handling so an alias cannot collide with service routes or administrative paths.
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Why truncating a hash is not enough
A hash maps input into a finite output space. Keeping only a prefix—or otherwise reducing the output to a short code—makes that space smaller still. Different destination URLs can therefore produce the same retained code.
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Hashing can make a useful candidate, and a deterministic hash can make repeated submissions of the same URL produce the same candidate. Neither property guarantees that different URLs receive different short codes. The storage layer must enforce the invariant that each active code maps to at most one destination, and the creation path must respond to a conflict instead of treating the hash as proof of uniqueness.
There is a separate product choice when the same destination is submitted more than once: should the service reuse one link, or issue a distinct link for each request? A deterministic hash naturally encourages reuse, but that may not fit per-link analytics, ownership, expiration, or campaign tracking. Choose and implement that policy explicitly; it is not implied by hashing.
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Choosing a code-generation strategy
| Approach | Advantage | Trade-off | Design consideration |
|---|---|---|---|
| Unique database ID encoded in Base62 | Encodes an allocated unique ID in a compact, URL-friendly form. | Requires reliable unique-ID allocation. Sequential IDs can be predictable. | Consider whether users guessing codes or inferring link volume is a concern. |
| Hash of the destination, shortened or truncated | Can produce the same candidate for the same input without inherently requiring a central counter. | Truncation permits collisions; deterministic output also raises the question of whether identical destinations should share a link. | Enforce uniqueness and define what the service does after a conflict. |
| Random code from a finite alphabet | Avoids visibly sequential allocation. | Collisions remain possible, so creation needs a uniqueness check and retry behavior. | Choose code length in light of expected volume and acceptable retry behavior; no one length is established as optimal for every workload. |
These are design alternatives, not performance results. A database-generated ID is often straightforward when reliable allocation is available; random or hash-based candidates can suit other requirements, provided collisions are handled. The right choice depends on coordination needs, predictability, product behavior, and expected workload.
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- Receive the short code at the redirect endpoint.
- Look up the mapping by the code, which should be an indexed key.
- If a valid, enabled mapping exists, return a redirect response whose
Locationheader contains the stored destination. - If the code is missing or disabled, return the service’s defined not-found or unavailable response; do not invent a destination.
The code-to-destination mapping is the critical record for this path. A durable indexed store is a sensible starting point. If repeated lookups make backing-store reads or latency a concern, a cache can hold frequently used mappings. Optional click analytics can be handled asynchronously where practical, so analytics work does not unnecessarily delay the redirect.
Choose redirect status and caching behavior deliberately
HTTP redirect codes have different meanings. RFC 9110, published in June 2022, defines 301 Moved Permanently for a permanent URI change and notes that a 301 response is heuristically cacheable unless caching rules say otherwise. It defines 302 Found for a temporary alternative URI; the RFC says the target resource resides temporarily under a different URI.
Use the status that matches the intended lifetime and mutability of the mapping. A permanent response can be cached by clients or intermediaries, which may make a later destination change harder to observe. A temporary response communicates a different expectation. Set caching directives with the status and product behavior in mind; neither status universally improves analytics, because client behavior and caching affect whether every visit reaches the service.
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Security and abuse controls
OWASP’s Unvalidated Redirects and Forwards Cheat Sheet recommends, where possible, accepting a short name, ID, or token that is mapped server-side to a full target URL. That is the core shortener model: the short code selects a stored destination, rather than letting an untrusted request supply an arbitrary redirect target at resolution time.
- Validate the destination when creating the mapping, using a maintained parser and a clear scheme and destination policy.
- Keep the resolved destination tied to the stored record; do not accept an override destination on the redirect request.
- Define controls appropriate to the product, such as rate limits, abuse reporting, takedown handling, or destination scanning. Their exact scope depends on the service and is not determined by the basic architecture.
- Restrict who can create, edit, disable, or delete mappings, and apply the same authorization rules to custom aliases.
What to decide before adding scale features
Without a stated traffic estimate, availability target, geographic distribution, or retention policy, there is no evidence-based reason to prescribe a particular database size, cache topology, or analytics architecture. Start with a durable mapping store and indexed code lookup, then use observed workload and service requirements to decide whether caching, partitioning, or additional processing is warranted. Keep any nonessential click-measurement work off the synchronous redirect path where practical.
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