Why the limit is per route
Different routes accept different things. One global limit is a limit that is wrong for most of them.
The symptom of no limit
A process that grows and is killed. The symptom names the input, and the input names the route.
Reading a body safely
The body is not a size, it is a stream. Reading without a limit is reading a number someone else chose, and the number can be larger than memory.
What you will see
One line with a reason. A limit with a reason is a limit that survives review.
About 1 min of reading
Try It
Enforce a body limit
1
Write the route limit
printf '/api/v1/ping 4096\n' > /tmp/l.txt; awk '{print $2}' /tmp/l.txt
Before you run it, say out loud what it will print. Then open the answer.
What it prints
4096. The limit lives with the route, not in a global.
2
Attempt a larger body
printf 'bytes 90000\n' | awk '{print ($1>4096) ? "refuse" : "read"}'
Before you run it, say out loud what it will print. Then open the answer.
What it prints
refuse. The refusal is a 413, and the 413 is the limit speaking.
3
Write the reason for the limit
printf 'limit 4096 reason=one config object\n' > /tmp/r.txt; cat /tmp/r.txt
Before you run it, say out loud what it will print. Then open the answer.
What it prints
One line with a reason. A limit with a reason is a limit that survives review.
Data Flow
A stream to a bounded read
1
The route declares its limit
↓
2
The stream is read up to the limit
↓
3
One extra byte is checked for
↓
4
The read is refused or accepted
Why this works
Examples
Content length is a claim
It is a header written by the client. Trusting it is trusting the input, and the input is the thing being defended.
The limit is read before the body
The count is checked while reading. A check after the read is a check that already spent the memory.
The limit is a policy
It has a number and a reason. A limit with no reason is a limit that will be raised in review.
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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A handler is a function