What is a mainframe and why do banking and the public sector still rely on it?

September 1, 2026

Every day, millions of people use bank cards, make transfers, check their account balances or receive salaries and pensions. These transactions happen in seconds and so seamlessly that it is easy to associate them solely with the technologies we see: mobile applications, digital platforms or cloud services.

However, behind many critical infrastructures, there is still a technology with several decades of evolution: the mainframe.

Far from being merely a legacy of the past, the mainframe continues to play a central role in organisations that process large volumes of transactions and data, where availability, security and data consistency are critical requirements.

What is a mainframe?

A mainframe is a high-performance computer designed to process large volumes of data and transactions simultaneously, with high levels of availability, security, reliability and fault tolerance.

Unlike other computing infrastructures, a mainframe is particularly well suited to large-scale transactional workloads, where thousands of operations need to be processed quickly, consistently and securely.

It is precisely because of these characteristics that mainframes remain present in sectors such as banking, insurance, telecommunications, transport and Public Administration.

In our daily lives, we may interact with services supported by mainframe systems without even realising it.

ATMs and payments

When we withdraw cash, carry out certain banking operations or pay for a purchase by card, there is a complex infrastructure responsible for authenticating, validating and recording those transactions. In many financial institutions, mainframe systems remain part of the technological core that supports these types of operations.

Transfers and banking operations

Financial systems need to process and record large volumes of transactions while ensuring data integrity and consistency. It is precisely in this type of transactional processing that mainframes continue to stand out.

Social Security, tax authorities and public services

In Public Administration, critical systems need to process and cross-reference large amounts of information over very long periods. Contributions, pensions, taxes and administrative records are examples of areas where continuity and data integrity are essential.

Why does the banking sector still use mainframes in 2026?

In an era dominated by Artificial Intelligence and the cloud, it may seem contradictory that some of the world's largest financial institutions remain dependent on systems whose origins date back to the last century.

The explanation is less technological than it might seem: when it comes to critical systems, replacing a technology that continues to meet business requirements effectively does not always offer an advantage.

High reliability and availability

A bank cannot simply stop processing operations because a system is unavailable.

Modern mainframes deliver availability levels commonly known as "five nines" (99.999%) or higher, which equates to around five minutes of unplanned downtime per year. Many have been running for decades without a single interruption to their core service.

In environments where an interruption can affect millions of transactions and users, this capability is particularly important.

Processing large volumes of transactions

The financial sector requires strict compliance with ACID properties - Atomicity, Consistency, Isolation and Durability - which are essential to ensure that a transaction is completed correctly or, in the event of a failure, rolled back without compromising data integrity.

Mainframes were specifically designed to support this type of large-scale transactional processing.

The risk and cost of replacing critical systems

Migrating the core system of a large financial institution is far from being a simple technology upgrade.

In addition to the required investment, a migration of this scale involves risks related to business continuity, data integrity, security, interoperability and regulatory compliance.

For this reason, many organisations choose to modernise their mainframe systems progressively rather than replace them entirely.

And the public sector? The same principle, a different context

In the public sector, the priority is not profit margins, but rather the continuity of public services and data sovereignty. Systems such as Social Security, public finances and civil registries manage records spanning entire generations of citizens. An outage of just a few hours in tax infrastructure or pension payments can disrupt the economy and have a severe social impact.

There is another challenge: many of these systems have been developed and refined over decades, accumulating highly complex business rules. Replacing them involves not only migrating technology and data but also understanding and accurately reproducing thousands of processes and dependencies built up over time.

This is why, in many cases, the question is no longer simply “how do we replace the mainframe?” but rather “how do we modernise it without compromising what already works?”

Is mainframe synonymous with obsolete technology?

Not necessarily. Although many mainframes run applications developed several decades ago - often associated with technologies such as COBOL - the platforms themselves continue to evolve and can integrate with modern architectures and technologies.

Today, these platforms can operate within hybrid architectures, where the mainframe core remains responsible for managing transactional data and processing large volumes of information, while modern layers of APIs, microservices and cloud platforms connect these systems to mobile applications, web banking services and citizen service portals.

In this way, organisations can combine the robustness, reliability and processing capacity of the mainframe with the flexibility and user experience provided by modern technologies. The result is an architecture that keeps critical systems in the back end while delivering fast, modern digital experiences in the front end.

What does this mean for those managing critical systems?

For IT leaders and risk managers in the financial and public sectors, the continued use of mainframes leads to an important conclusion: modernisation does not mean replacing technology simply because a newer alternative exists.

Assess risk before modernising

Modernisation does not necessarily mean discarding existing code. In many cases, the most effective strategy involves refactoring, exposing APIs and integrating with the cloud while preserving the core architecture.

The importance of specialised expertise

Managing and modernising these mixed environments requires partners with in-depth knowledge of both worlds, who understand both the languages and logic of legacy systems and modern development paradigms.

In short, the mainframe remains relevant not through inertia, but on merit. In a context where millions of transactions and critical services depend on secure and available systems, the question is not how old the technology is, but whether it can continue to meet the demands of the present.