client_body_buffer_size - Body Larger Than Buffer Size Is Omitted

[shalhevetm@…]

I set client_body_buffer_size to 4MB and client_max_body_size to 150MB.
When I send a request with a 6 MB body (6MB sized argument), in access_by_lua_block, the body appears as empty.

access_by_lua_block {

ngx.req.read_body()
local body_data = ngx.req.get_body_data()

if body_data then

ngx.log(ngx.ERR, "Request body: ", body_data)

else

ngx.log(ngx.ERR, "No request body found")

end

}

I see the log "No request body found", even though the body was clearly not empty.

client_body_in_single_buffer is default (wasn't set).

When I increased client_body_buffer_size to 10MB, I had no issues with the 6MB body.

Why is that so? Why does the buffer size cause the body to appear as empty?