What is the difference between back up and replication

Put “backup vs. replication” into a search engine and read a few articles. You might be amazed at just how complicated some high tech prose can make these decidedly low tech concepts.

We will begin with an illustration from simpler times. For our purposes we will need:

  • a desk drawer
  • two sheets of plain paper
  • a typewriter
  • some carbon paper
  • a monkey

For those who need the low tech world of yesteryear explained, if we put a piece of carbon paper between two pieces of plain paper and type on it using a typewriter, then everything we type on the first sheet will be replicated to the second sheet.

Note that with replication, if we misspell a word on the first sheet, it will be misspelled on the second sheet. If we scratch out a word on the first sheet it will be scratched out on the second sheet. If we type a masterpiece on the first sheet, we will find the same masterpiece on the second, but if the legendary “monkey with a typewriter” comes and types gibberish over our elegant prose on the first sheet, there will be only gibberish (and no trace of our elegance) on the second sheet.

That’s replication.

Now, at some point in time before we allowed the monkey in the typing pool, suppose we had taken a carbon copy of our masterpiece, replicated right up to the point of maximum elegance, and had placed it in a desk drawer. Later, when we discover the damage our monkey did to our original, we take the carbon copy out of our desk drawer and use it to meet a deadline.

That’s backup.

So, replication is an alternative copy of an original and changes with the original in real time. Backup is a version replicated up to some point in time, then stored for possible later use when an original might be damaged or lost. All backup relies on some form of replication, but the replication used for backup may not be adequate to the needs of, say, a complete failover system. (Complete failover in the case of hardware failure is the ultimate use of replication.) Conversely, and this cannot be overemphasized, replication is not backup, so if you are only replicating, you are not backed up.

The typewriter is now a keyboard, the pieces of paper are now files and server images, the desk drawer is a separate folder or server. But the principles of replication vs. backup remain the same.

And the monkey? His name is Murphy and he obeys his own law. It’s best to backup andreplicate if you don’t like his typing or the way he handles your equipment.

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