Online Travel Agencies: Simplifying Travel Planning

Online travel agencies handle millions of transactions daily, sitting between travellers and a fragmented supply chain of airlines, hotels, car rental companies, and tour operators. Building one, or integrating deeply with one, is a legitimate engineering challenge. This post covers how OTA platforms are architected, where the hard problems actually live, and what to watch out for if you're building in this space.

How Does an OTA Actually Work Under the Hood?

At its core, an OTA is an aggregation and booking layer. It pulls inventory from multiple sources, presents it to end users, takes a booking, and then settles with the supplier. That sounds straightforward. It isn't.

Inventory Aggregation

Airlines expose inventory through Global Distribution Systems (GDS) like Amadeus, Sabre, and Travelport. These systems use NDC (New Distribution Capability), the IATA standard for airline retail, alongside legacy EDIFACT-based messaging that dates back to the 1970s. Hotels typically connect through channel managers or directly via OTA Connectivity APIs like Expedia's EQC or Booking.com's XML API.

The data you get back is not clean. Rates come in different currencies. Room type codes vary by property. Flight segments have conditional fare rules buried in 500-character strings. You spend more engineering time normalising this data than you expect.

Search and Pricing

Real-time search across multiple GDS connections is expensive. A single flight search can fan out into 30 or more API calls. Response times vary from 800ms to over 8 seconds depending on the GDS and route complexity. Most production OTAs implement a combination of:

• Live GDS queries for short-haul and popular routes where inventory changes frequently
• Cached fare families for long-haul where the fare structure is more stable
• Pre-computed packages for hotel+flight bundles

Caching introduces stale pricing risk. You need a re-validation step at checkout, and you will occasionally quote a fare that is no longer available. The UX around this matters as much as the technical implementation.

What Makes Payment and Settlement So Difficult?

This is where most OTA projects underestimate complexity. You are not just processing a payment. You are handling a multi-party settlement where the money flows through you, to suppliers, potentially in different currencies, under different settlement windows.

Airlines typically settle via BSP (Billing and Settlement Plan), a monthly process managed by IATA. Hotels may require direct payment at property or virtual card settlement. Car rentals often run on corporate account billing. You end up running four or five different settlement workflows simultaneously.

Fraud is significant. OTAs are high-ticket, which attracts card-testing and account takeover attacks. 3DS2 authentication reduces chargebacks but adds friction. The balance between fraud prevention and conversion rate is a constant tuning exercise.

Multi-Currency and FX

If you operate across markets, you need a strategy for currency. Displaying prices in local currency while settling in USD or EUR means you carry FX exposure between booking and settlement. Most mid-sized OTAs fix the exchange rate at booking time and absorb the spread. Some use FX hedging instruments, but that introduces accounting complexity most engineering teams aren't equipped to own.

How Should You Structure the Booking Flow?

The booking funnel in an OTA has several stages where things can go wrong: search, selection, ancillary add-ons, passenger details, payment, and confirmation. Each stage is a potential drop-off point.

Session and State Management

Holding a user's search state across a multi-step funnel requires careful session design. If a user searches, leaves, and comes back, do you re-run the search or show cached results? Cached results are faster but carry stale pricing risk. Re-running is accurate but slow.

The standard approach is to store the selected itinerary with a timestamp and re-validate pricing at the payment step, not the search step. This keeps the funnel fast and limits the re-validation to one API call at the most critical moment.

Ancillaries

Airlines now generate a material portion of revenue from ancillaries: seat selection, baggage, meals, lounge access. IATA's EMD-S (Electronic Miscellaneous Document, Series) standard governs how these are issued. Not every GDS exposes the full ancillary catalogue for every airline. Some ancillaries can only be added post-ticketing, which means your confirmation flow needs to support a secondary purchase step.

Reliability, Fallbacks, and the Cost of Supplier Downtime

GDS systems go down. Supplier APIs have maintenance windows. Hotel channel managers have known reliability issues during peak booking periods. Your architecture needs to account for partial availability.

A few patterns that matter here:

• Circuit breakers per supplier connection, so one failing API doesn't cascade into total search failure
• Graceful degradation that shows available results from working suppliers rather than an error page
• Async confirmation flows for bookings where the supplier acknowledgement is slow, with polling or webhooks to update booking status

The confirmation latency problem is real. Some low-cost carriers confirm in under 2 seconds via direct connect APIs. Some GDS-connected bookings can take 45 seconds or longer to return a PNR. Users will refresh, close the tab, or assume the transaction failed. Your frontend needs explicit progress states and your backend needs idempotency on the booking call.

Compliance, Data, and What You Cannot Ignore

PCI DSS compliance is non-negotiable if you're storing or transmitting card data. Most OTAs use a tokenisation approach via a PCI-compliant payment gateway and never touch raw card numbers in their own systems. This is the right call. Maintaining your own PCI DSS Level 1 certification as a startup is not worth the cost.

GDPR and similar data protection regulations affect how you store passport numbers, travel document data, and travel history. These are special category data under GDPR Article 9 interpretations in some EU jurisdictions. Your data retention and deletion policies need to be explicit and enforceable.

Airlines require traveller data in specific formats. A passport expiry date field that accepts invalid dates will cause a booking to fail at check-in, not at the time of booking. Validation at input time, not submission time, prevents a large class of support tickets.

Conclusion

OTA platforms are genuinely complex systems. The engineering surface covers real-time aggregation, multi-supplier settlement, fraud prevention, compliance, and reliability under partial supplier failure. If you're scoping a build, the honest advice is to start with a single supplier category, get that working reliably, and expand from there. Trying to solve all of it at once leads to a system that is fragile everywhere.

The next concrete step: map your supplier connections before writing a line of code. Identify which GDS or direct connect APIs you'll integrate, what authentication model they use, and what their sandbox environment looks like. That scoping exercise will surface 60% of your architectural decisions before you've committed to anything.

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FAQ

What is the difference between a GDS and a direct connect in OTA development? A GDS like Amadeus or Sabre aggregates inventory from hundreds of airlines under one API. A direct connect goes to the airline's own reservation system. Direct connects typically offer better fares and more ancillary options, but you manage separate integrations per airline. Most production OTAs use both, routing by airline and route.

How do OTAs handle booking failures mid-flow? The standard approach is optimistic booking with rollback. You call the supplier to hold or confirm inventory, then process payment. If payment fails, you cancel the supplier booking. If the supplier confirmation fails after payment, you trigger a refund and queue the case for manual review. Both paths need automated handling, because volume makes manual review unsustainable.

Is it necessary to integrate with all three major GDS systems? No. Amadeus covers the largest share of European and Middle Eastern inventory. Sabre has stronger North American coverage. Travelport sits in the middle. Start with one based on your target market. Adding a second GDS is a significant integration and operational overhead and should be driven by clear inventory gaps, not as a default architectural decision.

What is NDC and does it replace the traditional GDS? NDC is an IATA XML-based standard that allows airlines to distribute personalised offers directly to travel sellers, bypassing some GDS markup limitations. It does not replace GDS systems entirely. As of 2024, NDC adoption varies widely by airline. British Airways and Lufthansa have aggressive NDC programmes; many regional carriers still rely on legacy EDIFACT. Expect to support both for the foreseeable future.

How do OTAs manage pricing accuracy given how frequently fares change? They don't guarantee it. Fares are quoted at search time and re-validated at checkout. If the fare has changed, the user is shown the updated price before payment is taken. The re-validation window, the time between showing a price and confirming it, is typically kept under 15 minutes to balance accuracy against user drop-off.

Have a project in mind? Contact Sodio Technologies to discuss your requirements and explore the right technology solution for your business.