Engagement three of four
Network and backup design
Network design means deciding the addressing, the equipment and the layout on purpose instead of by accretion. Accretion is what happens to a network when each change is made by whoever was available that week, and it is readable by nobody after about three years of it.
Addressing chosen once, so a stranger can read it
An addressing scheme is the rule that decides which number each device is given on the network. A scheme chosen deliberately reserves a block for infrastructure, a block for servers, a block for desks and a block for anything handed out automatically, so that the number alone tells an engineer what the device is before anybody opens a cupboard.
The drawing that comes out of this work is the orthogonal one shown in chapter 02 of the front page: the circuit at the top, the switch beneath it, and the server, desks and access points across the bottom, each with its number. The value of drawing it that way is not tidiness. It is that a link drawn as it physically runs can be followed with a torch, and a link drawn as it was intended to run cannot.
Wireless coverage is planned from the shape of the building and the metal inside it, because a warehouse and an office of equal floor area need different numbers of access points, and a single unit mounted at the wrong height over a roller door serves the door and not the yard.
Two figures the client supplies, and what they each buy
Backup design starts from two figures, and both of them belong to the client rather than to the equipment. The first is how much work the business is willing to redo after a failure, which decides how often copies are taken. The second is how long the business is able to stand still while the system is rebuilt, which decides where the copies are kept, because a copy held offsite is safer and slower to bring back than a copy held in the building.
Retention is the third figure and the one that sets the bill: the number of copies kept in each tier before the oldest is overwritten. A daily tier answers the question asked on the same afternoon, a weekly tier answers the one asked after a holiday, and a monthly tier answers the one asked by an accountant in March about a file somebody deleted in November.
The panel in chapter 05 of the front page totals the shelf a given set of counts asks for, treating every copy as a full one. Real products save space by storing only what changed, so the real figure lands lower, and it is settled once a genuine backup has run for a fortnight and reported its own sizes.
The monthlies are the tier everybody discusses and the dailies are the tier that fills the shelf, which is the reverse of most first conversations.
The restore test, and the note it leaves
A backup is a copy that was made. A restore is a copy that was proved to open. The restore test is the step that turns the first into the second, and the note it produces is what a client actually owns afterwards. On an agreed date a copy is taken from the backup, opened on separate equipment, and checked against something the client recognises, such as last month's invoices.
The note is written whichever way the outcome went, because a failed restore discovered on a Tuesday in a test is a note, and the same failure discovered during an incident is a week. The shape of the note is fixed, so that a run of them read together shows whether the restore time is drifting.
Restore-test note — the fields, shown empty
Date and timewhen the copy was opened, not when it was taken
Copy usedwhich tier, and the date the copy itself was made
Opened onthe equipment used, which is not the live system
Checked againstthe file or record the client recognised
Time takenfrom starting the restore to reading the file
Outcomeopened and read, or the point at which it stopped
Signed bythe person who performed it and the client witness
The fields are shown empty because the entries belong to one client's own test and stay in that client's own pack. A copy taken for a test is removed from the test equipment once the outcome is written down, and what survives the step is the note.
Keeping the arrangement true after the engagement
A backup schedule decays in one predictable way: a new folder, a new system or a new laptop arrives and nobody adds it to the copy. The design therefore names the person who owns the schedule at the client and states the one occasion on which they must look at it, which is whenever something new starts holding records.
The restore test has the same problem and the same answer. The design proposes an interval the client is genuinely able to keep, usually tied to something already in the calendar rather than to a date invented for the purpose, and the notes accumulate in the handover pack where the next engineer will find them.
The drawing this page describes is in chapter 02 of the front page, the shelf panel is in chapter 05, and the pack these notes live in is described on the documented handover page.