Real-time sync between two systems sounds like a simple technical requirement. At real data volume, it's one of the harder problems a systems migration can run into.
Real-time sounds simple until the data volume is real
In a demo environment, real-time sync looks effortless, a handful of test records moving instantly between two systems. At full production volume, with thousands of transactions an hour, that same architecture can buckle in ways that never showed up in testing.
The two failure modes nobody plans for
The first is capacity: the sheer volume of real-time calls simply exceeds what either system was built to sustain. The second is more subtle, a timing mismatch, where two systems briefly disagree about the true state of something because updates arrive in a slightly different order than expected. Both are far more common at scale than most project plans account for.
Hourly isn't a failure, it's a decision
When real-time sync starts to strain under load, stepping back to a scheduled reconciliation, hourly, for example, isn't giving up. It's often the responsible choice while the actual capacity and data-volume problem gets solved properly, rather than pushing forward on a fragile system and letting it fail in production.
What we look for before promising real-time
Before committing to real-time sync as a requirement, it's worth actually modeling expected transaction volume against both systems' real limits, not their advertised ones. That one step catches most of the problems that would otherwise only show up after go-live, when they're far more expensive to fix.
If your migration plan assumes real-time from day one, it's worth stress-testing that assumption before it becomes a live problem.
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