Why the default is immutable
Most of what a server reads is decided at start. Decided at start means never written, which means no race.
The one question to ask
How many writers. Zero writers is free, one writer is a queue, many writers is a merge.
Shared state in a server
Handlers run at the same time. Anything they share has more than one writer, and more than one writer is a decision about who is allowed to write.
What you will see
One line. A mechanism without its count is a mechanism nobody can argue with.
About 1 min of reading
Try It
Count the writers
1
Write the shared values
printf 'config 0\ncounter 1\ncache 3\n' > /tmp/v.txt; awk '{print $1, $2}' /tmp/v.txt
Before you run it, say out loud what it will print. Then open the answer.
What it prints
Three lines. The writer count is the input to the mechanism choice.
2
Choose from the count
awk 'BEGIN {for (w=0;w<3;w++) print w, (w==0 ? "immutable" : w==1 ? "atomic" : "lock")}'
Before you run it, say out loud what it will print. Then open the answer.
What it prints
0 immutable, 1 atomic, 2 lock. The count decides, not the habit.
3
Write the choice with the count
printf 'cache writers=3 mechanism=lock\n' > /tmp/c.txt; cat /tmp/c.txt
Before you run it, say out loud what it will print. Then open the answer.
What it prints
One line. A mechanism without its count is a mechanism nobody can argue with.
Data Flow
A value to a writer count
1
The value is declared
↓
2
The writers are counted
↓
3
The mechanism is chosen from the count
↓
4
The choice is written with the count
Why this works
Examples
Immutable is the default
Config is read and never written. A read that cannot be written has no race to lose.
Atomic is a narrow claim
One counter is atomic because one word is enough. A structure is not one word, and the claim stops there.
A lock is a hold time
The lock is held for the length of the work. The work is the thing that decides the wait.
Words this lesson uses
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.
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