Archive for category DevOps

How to Change Elixir Log Levels for a Running Application

Logging is essential to any production system, but it’s also terribly easy to make your logs so noisy they become worthless. That’s why I’m a big fan of carefully specifying the criticality level of everything I log. Not only does it allow us to easily search the logs for potential problems, it allows us to quiet our logs down and raise the signal to noise ratio. In production, I don’t want or need debug level logging… until I do. Thankfully, Elixir makes it easy for us to change what level we’re logging at on the fly.

Let’s say we have a Phoenix website and we want to create a log entry every time we serve the site’s index while we’re developing, but not when we go to production.

require Logger

defmodule LogBlogWeb.PageController do
  use LogBlogWeb, :controller

  def index(conn, _params) do
    Logger.log(:debug, "hello there!")
    render conn, "index.html"
  end
end

By default, a Pheonix app runs with debug logging under the dev configuration and info under the prod config.

# config/dev.exs
config :logger, :console, format: "[$level] $message\n"

# config/prod.exs
config :logger, level: :info

This is great! It’s exactly the behavior we want in our production system. At least, that is, until we need to see the debug information because something is wrong in production.

Or, we would be if it wasn’t for the Erlang VM and iex.

In order to be able to connect to our running system, we need to start the process with a node name. Instead of starting our website with mix phx.server like we normally might, we start it with elixir instead.

MIX_ENV=prod PORT=4000 elixir --name myapp@localhost -S mix phx.server

Now that we have a name for our process, we can use iex to connect to the running process via the --remsh option in a separate shell.

iex --sname debug --remsh "myapp@localhost"

Erlang/OTP 20 [erts-9.2]  [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

Interactive Elixir (1.5.3) - press Ctrl+C to exit (type h() ENTER for help)
iex(myapp@localhost)1>

Finally, now that we have an interactive session established, we can change the Logger configuration to output the debug logs.

iex(myapp@localhost)1> Logger.configure(level: :debug)
:ok

If we check the logs now, we’ll see our debug message.

08:56:34.205 [info] GET /
08:56:34.218 [debug] Processing with LogBlogWeb.PageController.index/2
  Parameters: %{}
  Pipelines: [:browser]
08:56:34.218 [debug] hello there!
08:56:34.227 [info] Sent 200 in 22ms

When we’re done gathering the extra information, we can simply set the level back to info.

iex(myapp@localhost)2> Logger.configure(level: :info)
:ok

And there you have it folks. That’s how you change the log level of your Elixir application without ever taking it offline.

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How to See Memory and CPU Usage for All Your Docker Containers on CentOS 6

I run a bunch of Docker containers on a single CentOS 6 server with a limited amount of memory. (I only recently bumped it from 0.5 to 1 whole whopping gig!) Before I bring another container online, I like to check to see how much room I’ve got. Being the newest versions of Docker aren’t available for CentOS 6, I’m running an ancient version, 1.7 or so. On the new versions of Docker, running docker stats will return statistics about all of your running container, but on old versions, you must pass docker stats a single container id. Here’s a quick one-liner that displays stats for all of your running containers for old versions.

$ docker ps -q | xargs  docker stats --no-stream
CONTAINER           CPU %               MEM USAGE/LIMIT     MEM %               NET I/O
31636c70b372        0.07%               130.8 MB/1.041 GB   12.57%              269.7 kB/262.8 kB
8d184dfbeeaf        0.00%               112.8 MB/1.041 GB   10.84%              45.24 MB/32.66 MB
a63b24fe6099        0.45%               50.09 MB/1.041 GB   4.81%               1.279 GB/1.947 GB
fd1339522e04        0.01%               108.2 MB/1.041 GB   10.40%              8.262 MB/23.36 MB

docker ps -q returns the list of running container ids, which we then pipe through xargs and into docker stats. Adding --no-stream gives us just the first result instead of continually updating the stats.

It’s a neat little trick. If anyone knows how to make this return container names instead of ids, please comment below.

Again, this is unnecessary for the newest versions. Just run docker stats and you’ll get nearly identical output.

~Semper Cogitet

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Complete Guide to Setting Up a Secure Website on a VPS with Lighttpd

Last time I talked about setting up a simple web server and site from my phone. Since then, I’ve been spending a little bit of time getting things set up right. That’s been a bit more challenging than simply getting the thing up and running, so I figured I’d take a moment to jot down some notes in case I ever need to do this from scratch again.

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