232 lines
6.1 KiB
ReStructuredText
232 lines
6.1 KiB
ReStructuredText
Sharing the Stats
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=================
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**Junior**
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Good morning!
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**Senior**
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Just about time... We are in trouble!
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The report stuff was a complete success, so much so that now *Susan* has hired
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a frontend developer to create a custom dashboard to see the stats in real
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time.
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Now we have to provide the backend for the solution.
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**Junior**
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And what's the problem?
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**Senior**
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We are not developers! What are we doing writing a backend?
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**Junior**
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Just chill out. Can't be that difficult... What do they need, exactly?
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**Senior**
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We have to provide a new endpoint to serve the same data but in `JSON`
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format.
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**Junior**
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So, we have half of the work done already!
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What about this?
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.. code-block:: bash
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kapow route add /capacitystats - <<-'EOF'
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echo "{\"memory\": \"`free -m`\"}" | kapow set /response/body
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EOF
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**Senior**
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For starters, that's not valid `JSON`. The output would be something like:
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.. code-block:: console
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$ echo "{\"memory\": \"`free -m`\"}"
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{"memory": " total used free shared buff/cache available
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Mem: 31967 3121 21680 980 7166 27418
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Swap: 0 0 0"}
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You can't add new lines inside a `JSON` string that way, you have to escape
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the new line characters as ``\n``.
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**Junior**
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Are you sure?
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**Senior**
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See it for yourself.
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.. code-block:: console
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$ echo "{\"memory\": \"`free -m`\"}" | jq
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parse error: Invalid string: control characters from U+0000 through U+001F must be escaped at line 3, column 44
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**Junior**
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:program:`jq`? What is that command?
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**Senior**
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:program:`jq` is a wonderful tool for working with `JSON` data from the command
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line. With :program:`jq` you can extract data from a `JSON` document and it also
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allows you to generate a well-formed `JSON` document.
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**Junior**
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Let's use it, then!
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How can we generate a `JSON` document with :program:`jq`?
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**Senior**
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To generate a document we use the ``-n`` option:
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.. code-block:: console
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$ jq -n '{"mykey": "myvalue"}'
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{
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"mykey": "myvalue"
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}
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**Junior**
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That does not seem very useful. The output is just the same.
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**Senior**
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Bear with me, it gets better. You can add variables to the `JSON` and
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:program:`jq` will escape them for you.
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.. code-block:: console
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$ jq -n --arg myvar "$(echo -n myvalue)" '{"mykey": $myvar}'
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{
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"mykey": "myvalue"
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}
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**Junior**
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Sweet! That's just what I need.
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(hacks away for a few minutes).
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What do you think of this?
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.. code-block:: console
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$ jq -n --arg host "$(hostname)" --arg date "$(date)" --arg memory "$(free -m)" --arg load "$(uptime)" --arg disk "$(df -h)" '{"hostname": $host, "date": $date, "memory": $memory, "load": $load, "disk": $disk}'
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{
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"hostname": "junior-host",
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"date": "Tue 26 Nov 2019 05:27:24 PM CET",
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"memory": " total used free shared buff/cache available\nMem: 31967 3114 21744 913 7109 27492\nSwap: 0 0 0",
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"load": " 17:27:24 up 10:21, 1 user, load average: 0.20, 0.26, 0.27",
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"disk": "Filesystem Size Used Avail Use% Mounted on\ndev 16G 0 16G 0% /dev"
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}
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**Senior**
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That is the output we have to produce, right. But the code is far from
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readable. And you also forgot about adding the endpoint.
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Can we do any better?
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**Junior**
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That's easy:
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.. code-block:: bash
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kapow route add /capacitystats - <<-'EOF'
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jq -n \
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--arg hostname "$(hostname)" \
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--arg date "$(date)" \
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--arg memory "$(free -m)" \
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--arg load "$(uptime)" \
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--arg disk "$(df -h)" \
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'{"hostname": $hostname, "date": $date, "memory": $memory, "load": $load, "disk": $disk}' \
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| kapow set /response/body
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EOF
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What do you think?
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**Senior**
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I'm afraid you forgot an important detail.
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**Junior**
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I don't think so! the `JSON` is well-formed and it contains all the required
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data. And the code is quite readable.
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**Senior**
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You are right, but you are not using ``HTTP`` correctly. You have to set the
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``Content-Type`` header to let your client know the format of the data you are
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outputting.
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**Junior**
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Oh, I see. Let me try again:
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.. code-block:: bash
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kapow route add /capacitystats - <<-'EOF'
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jq -n \
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--arg hostname "$(hostname)" \
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--arg date "$(date)" \
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--arg memory "$(free -m)" \
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--arg load "$(uptime)" \
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--arg disk "$(df -h)" \
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'{"hostname": $hostname, "date": $date, "memory": $memory, "load": $load, "disk": $disk}' \
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| kapow set /response/body
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echo application/json | kapow set /response/headers/Content-Type
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EOF
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**Senior**
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Better. Just a couple of details.
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1. You have to set the headers **before** writing to the body. This is
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because the body can be so big that *Kapow!* is forced to start sending it
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out.
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2. In cases where you want to set a small piece of data (like the header), it
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is better not to use ``stdin``. *Kapow!* provides a secondary syntax
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for these cases:
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.. code-block:: console
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$ kapow set <resource> <value>
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**Junior**
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Something like this?
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.. code-block:: bash
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kapow route add /capacitystats - <<-'EOF'
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kapow set /response/headers/Content-Type application/json
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jq -n \
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--arg hostname "$(hostname)" \
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--arg date "$(date)" \
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--arg memory "$(free -m)" \
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--arg load "$(uptime)" \
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--arg disk "$(df -h)" \
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'{"hostname": $hostname, "date": $date, "memory": $memory, "load": $load, "disk": $disk}' \
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| kapow set /response/body
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EOF
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**Senior**
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That's perfect! Now, let's upload this to the *Corporate Server* and tell the
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frontend developer about it.
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