Engagement two of four
Migration planning
A migration is the move of a system from where it runs now to where it will run next. The plan is the part that decides whether the move is dull, and a dull move is the entire objective: nobody is congratulated for an exciting cutover.
The eight steps a plan is written in
The plan is a sequence, not a schedule of days. Each step names who performs it, the check that confirms it worked, and the point at which the step is finished. Step five is the only one at which the move can still be called off cheaply, which is why it is marked.
-
Fix what is moving, and what is staying
The plan opens with a list of the systems in scope and, beside it, the systems deliberately left where they are. A list of what stays is written because half the trouble in a migration comes from a system nobody intended to move that quietly depended on one that did.
-
Measure the volume and the circuit
The size of the data is measured rather than estimated, and the speed of the internet circuit is measured at a working hour rather than taken from the contract. Those two figures set the length of every copy in the plan, and the front page of this site has a panel that turns them into a required transfer rate.
-
Agree the window in hours
The cutover window is the agreed stretch of hours in which the business stops using one system and starts using the next. It is agreed with the people who will lose those hours, in hours, and written into the plan as a start time and an end time on a named evening or weekend day.
-
Rehearse on the real circuit
A rehearsal copies the data into the new system while the old one keeps running, so the copy time is measured rather than predicted and the awkward files surface a fortnight early. Awkward here means the long file names, the files somebody has open, and the folder that turns out to hold eleven years of scanned delivery notes.
-
The go decision, taken in daylight
The rehearsal produces a figure and a list of problems, and the decision to proceed is taken against both, in the office, in daylight, by the client. Calling the move off at this step costs the rehearsal and nothing else, which is precisely why the step exists as a step rather than as an assumption.
-
Write the rollback position down
The rollback position is the state the old system must be left in so that abandoning the move takes minutes, together with the condition that would trigger abandoning it. It is written before the cutover begins, because the version of it held in somebody's head is the version that fails at two in the morning.
-
Cut over inside the window
The final copy runs, the new system is checked against something the client recognises, and the old system is left in the state the rollback position describes rather than switched off. Leaving it running and unreachable by users is usually the cheapest insurance available on the night.
-
Hold the old system, then retire it
The old arrangement is kept in its rollback state for an agreed number of working days, and it is retired on a named date by a named person, after a last copy has been taken and its location recorded in the handover pack.
What shapes the length of a window
Three things decide how many hours a cutover needs, and only one of them is technical. The first is the volume against the circuit, which is arithmetic and is answered on the front page. The second is verification, meaning the decision to read the moved data back and compare it, which doubles the volume crossing the link because the same data travels twice. The third is the number of people who must be told that the system has changed before they use it on Monday.
In practice the third is what makes a window on a Saturday morning preferable to one on a Wednesday night, because a Saturday leaves a whole day in which the people who find something odd are able to ring somebody who is awake.
A plan that fits a spreadsheet but not the working week is not a plan, it is a spreadsheet.
Where the work divides between us
Some clients want the review and the plan, and will move the system themselves with their own staff over a weekend. Others want the move performed and the notes handed over afterwards. Both are ordinary, and the plan document is written to the same standard either way, because a plan that is only usable by the person who wrote it has failed the one test that matters.
Where the client performs the move, the plan is written with the names of their own people in it, and the office is available on the night by telephone under an arrangement agreed in advance. Where the office performs the move, the client still holds the plan, still holds the rollback position, and still takes the go decision at step five.
Before a migration there is usually a systems review; after one there is always a documented handover. The cutover window panel is on the front page, chapter 04.