TL;DR:

  • As soon as the crypto library is used check that the target machines OpenSSL can handle it. To be safer, use a VM with identical OS and OpenSSL to the target machine and build the ERTS you bundle with your release on it. kerl helps a lot here
  • Always include SASL in your releases. The extra logging is valuable and it is needed for relups anyway.
  • Included logging enables fast fault determination while supervisors keep the working parts running.
  • Erlang/OTP enables quick and reliable prototyping.

First deployments in a new environment are always scary. So it was only natural that I was afraid of the potential dangers that could have come with the deployment of our new Erlang prototype. This fear was further intensified by the fact that I was not the person who was handling the deployment. All I could do was send a release containing a small Readme file with instructions and hope for the best.

Technically the software was nothing special. A simple web service that distributes data via WebWockets to all connected and subscribed clients. It featured cowboy to handle the websocket part, erlsom to format the data that was sent, and mysql-otp to talk to a database that was used to store simulation data that needed to be replayed to simulate ‘real’ operation. Our build tool of choice was erlang.mk, which in turn uses relx to build releases.

The prototype felt good. Common Tests covered the expected work-flow. Key functions were tested via property testing (triq) and the Supervisor Tree should at least keep everything crawling along if we did miss something.

So I ran the test suite for a last time and bundled the application together with SASL before sending it to my client. It did not take long till I got a reply. Expecting the worst I opened up the email.

The service started up, but upon a client connection things started to fall apart. Thankfully Erlang’s ecosystem had me covered and logged everything I needed to know into the releases log directory.

=ERROR REPORT==== 10-Sep-2015::09:27:02 ===
Unable to load crypto library. Failed with error:
"load_failed, Failed to load NIF library
/home/<user>/<app>/lib/crypto-3.6/priv/lib/crypto: 'libcrypto.so.1.0.0:
cannot open shared object file: No such file or directory'"
OpenSSL might not be installed on this system.

=WARNING REPORT==== 10-Sep-2015::09:27:02 ===
The on_load function for module crypto returned {error, {load_failed,"Failed to
load NIF library /home/<user>/<app>/lib/crypto-3.6/priv/lib/crypto:
'libcrypto.so.1.0.0: cannot open shared object file: No such file or directory'"}}

=ERROR REPORT==== 10-Sep-2015::09:27:02 ===
Error in process <0.201.0> on node 'rsfo@127.0.0.1' with exit value:
{undef,[{crypto,hash,
    [sha, <<"EBRPiRB0U31VUD25oYQhIg==258EAFA5-E914-47DA-95CA-C5AB0DC85B11">>], []},
    {cowboy_websocket,websocket_handshake,3,
    [{file,"src/cowboy_websocket.erl"},{line,151}]}, {cowboy_protocol,execute,4,
    [{file,"src/cowboy_protocol.erl"},{line,470}]}
]}

The ERTS (Erlang RunTime System) I provided with the release was not playing nice with the OpenSSL libraries on the target machine. To resolve the issue I setup a VM with an identical operating system and OpenSSL library to build a compatible ERTS version with kerl. Just to be sure I also enabled dynamic-ssl-libraries for the build.

Maybe an hour after the new version was made available I got another reply informing me that the simulation part was not working. The Supervisor tree kept the prototype running however and my client could confirm that the messaging part worked as expected. This was already great news. Getting them a machine which they could use to write their clients for the service was the key goal.

The simulation could still not be started because the mysql connection could not be established. The sql driver reported old_server_version. At first I blamed the ancient MySQL version on the server, but as it turned out it was a bug in MySQL itself (Github Issue). However, before that could be resolved via an update they moved their database to another server version not affected by the issue and simply edited the sys.config to point to the new location.

Since then we deployed some small fixes and cleanup our release process to have less manual tasks and even enable the build of relups. The core systems code however is still running today.

All in all I am quite happy on how it all went down. Especially the built in logging capabilities and the supervision structure of OTP applications enabled us to pinpoint faults while keeping the system partly functional even during the earliest stages of prototyping. And the best part, those things come automatically via OTP and don’t require any extra effort that would cause additional development time.

- winlu