Mortgage Server on a Raspberry Pi

In the last post we discussed creating a server to calculate an amortization schedule that takes and returns both protocol buffer messages and JSON. In this post we will discuss hosting this server on a Raspberry Pi. There are some pitfalls, and the story isn’t complete, but it’s still fairly compelling.

What We Will Use:

Hardware:

We will use a Raspberry Pi 3 model B as our server. We will use the stock operating system Raspbian. This SOC has a quad core 64-bit processor with floating point on chip. The operating system itself is 32-bit which makes the processor run on 32-bit mode.

Software:

We will be using SBCL as our Common Lisp, CL-PROTOBUFS as our protocol buffer and JSON library, and Hunchentoot as our web server.

Problems

1. SBCL on a Raspbian

When trying to run the mortgage-info server on Raspbian the first error I got was an inability to load the lisp file generated by protoc. On contacting Doug Katzman he noted I was running an old version of SBCL. The Raspbian apt-get repository has an old version of SBCL. If someone desires to run SBCL on a Raspberry Pi they should follow the binary installation instructions here: http://www.sbcl.org/getting.html.

2. CL-Protobufs on a 32-Bit OS

The cl-protobufs library has been optimized to run on a 64-bit x86 platform. The Raspberry Pi environment is 32-bit arm. As noted before, the 32-bit arm environment is supported by SBCL. I don’t think anyone has attempted to run cl-protobufs on the 32-bit arm environment running SBCL. After modifying cl-protobufs.asd to have float-bits.lisp loaded on SBCL not running in 64-bit we could quickload mortgage-info into a repl.

3. Bugs in the mortgage-info repo  

There were several bugs I fixed in my very limited testing of the mortgage info repo, as well as some bugs that are still existent. 

  1. When trying to set numbers in the proto message structs I had to coerce them to double-float. I’m not sure why… This works on SBCL running on the x86-64 without the coercions.
  2. A division by 0 bug if the entered interest rate is 0.
  3. The possibility of having 0 as the number of repayment periods. I added an assertion so we will return a 500 stating the assertion was hit. We should have a more graceful error message than a stack trace, but this is currently only a proof of concept.
  4. The mortgage.proto file had interest as an integer, but interest is usually a float divisible by .125. 
  5. We have rounding problems if the interest rate is too high (say 99%). We only ever pay interest and the amount never goes down, at least with a 300 payment period. This is most likely due to rounding, we do not accept fractional pennies. This is okay, if the national interest rate went anywhere near 99% we have BIG problems.

CL-protobufs on the Pi

I have cl-protobufs running on SBCL on the Raspberry Pi, but some of the tests don’t pass. I’m not sure if it would work on a 64-bit OS on the Raspberry Pi, I don’t have the inclination to get a 64-bit OS for my Pi. If you do, please tell me what happens!

I wasn’t able to get CCL on arm32 to load cl-protobufs. It gives an error saying it doesn’t have asdf 3.1. Quickloading asdf I get undefined function version<=. If any CCL folk has an idea about what’s going on, please send me a message.

Trying to run ABCL lead me to yet another bug: https://github.com/armedbear/abcl/issues/359

Running Server

My Raspberry Pi is running at: http://65.96.161.53:4242/mortgage-info

Feel free to send either JSON or protobuf messages to the server.

Example JSON:

{
“interest”:3,
“loan_amount”:380000,
“num_periods”:300
}

I don’t know how long I will keep it running. If it goes down and you are interested in sending it messages please send me an email.


Ron, Carl, and Ben edited this post (as usual). Doug provided a great deal of help with SBCL on ARM 32.

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