Mainframe
A large, highly reliable computer built to run enormous volumes of transactions and batch work for an entire organisation at once.
Also written big iron
A mainframe is a computer designed around one goal: keep processing huge volumes of work, accurately, without stopping. Banks, insurers, airlines, retailers and government departments run their core records on them, and have done for decades.
What makes a mainframe different is not raw speed. It is throughput and reliability. The hardware is built with redundancy throughout, so parts can fail and be replaced while the system keeps running. The work itself is measured in thousands of transactions a second, sustained for years at a time.
The other defining trait is compatibility. A program written in the 1980s will usually still run today, unchanged. That is why so much of the world's financial infrastructure still sits on the platform: the code works, it has been correct for forty years, and there has never been a compelling reason to rewrite it.
The biggest adjustment when you start is not difficulty but unfamiliarity. The vocabulary, the tools and the screens all look different from a laptop. The underlying ideas of files, programs, jobs and permissions are ones you already know.
Related terms
- IBM ZIBM's current family of mainframe hardware, and the brand name you will see on most modern mainframe job adverts.
- z/OSThe main operating system that runs on IBM mainframes, in the same way Windows or Linux runs on a laptop or server.
- Batch processingRunning work in bulk, without anyone watching, usually on a schedule. The overnight run that settles a day of transactions.
- Legacy systemAn older system still doing essential work. On the mainframe, usually code that has been correct and running for decades.