The Holy Java

Building the right thing, building it right, fast

Clojure: How To Prevent “Expected Map, Got Vector” And Similar Errors

Posted by Jakub Holý on April 30, 2014

What my Clojure code is doing most of the time is transforming data. Yet I cannot see the shape of data being transformed – I have to know what the data looks like on the input and hold a mental model of how they change at each step. But I make mistakes. I make mistakes in my code so that the data does not correspond anymore to the model it should follow. And I make mistakes in my mental model of what the data currently looks like, leading likely to a code error later on. The end result is the same – a little helpful exception at some later step regarding wrong shape of data. There are two problems here: The error typically provides too little useful information and it usually manifests later than where the code/model mistake actually is. I therefore easily spend an hour or more troubleshooting these mistakes. In addition to that, it is also hard to read such code because a reader lacks the writer’s mental model of the data and has to derive it herself – which is quite hard especially if the shape of the input data is not clear in the first place.

I should mention that I of course write tests and experiment in the REPL but I still hit these problems so it is not enough for me. Tests cannot protect me from having a wrong model of the input data (since I write the [unit] tests based on the same assumptions as the code and only discover the error when I integrate all the bits) and even if they help to discover an error, it is still time-consuming the root cause.

Can I do better? I believe I can.

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How to create and run Gatling 2.0 tests

Posted by Jakub Holý on April 28, 2014

Getting up and running with Gatling perf. tests as I would like so I record this for my future reference.

0. Create a project:

$ mvn archetype:generate \
   -DarchetypeCatalog=http://repository.excilys.com/content/groups/public/archetype-catalog.xml
   -Dfilter=io.gatling:

(The trailing “:” in the filter is important.)

1. Import to IntelliJ

In IntelliJ choose to import an object, instead of “from sources” select “from external model” and then Maven. You will also need to have the Scala plugin installed and, when imported, you will likely need to right-click on pom.xml and Maven – Reimport.

2. Record a simulation

  1. Run the src/test/scala/Recorder.scala (right-click – Run ‘Recorder’)
  2. Set the port it should listen on, f.ex. 8000 (maybe you also need to set port for HTTPS, f.ex. 8001), set the target app (perhaps localhost, <some port>, <some https/dummy port>)
  3. Optionally set the class name of the recorded simulation and the target package (the output goes to src/test/scala/<package>/<name>.scala)
  4. Click [Start !]
  5. Go to your browser and configure it to use the recorder as its HTTP[s] proxy
  6. Browse localhost:8000/your_app as you want for the test
  7. Click [Stop and save] in the Recorder UI

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Kioo: How to Troubleshoot Template Processing

Posted by Jakub Holý on April 8, 2014

So you have created an Om/Reagent/React component using a Kioo template and the result is not as you expected, perhaps just an empty element? Let me share what I have discovered about troubleshooting this (though I am no expert). You will se how to invoke the underlying Clojure function component* manually, how to expand the deftemplate macro, how to call the resulting JavaScript function, and what the intermediate forms look like.

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Kioo: How To Replace The Whole Body

Posted by Jakub Holý on April 8, 2014


This whole post in unnecessary since it is simply possible to use a snippet directly without a template, as kindly explained by Creighton Kirkendall.

Kioo, the enlive-inspired templating library for React.js and derived libs such as Om, normally works by matching selectors against elements inside <body> and transforming the matched elements while also keeping all the other ones. But what if you want to keep just the single matched element and drop all the others? You need some tricks and I will demonstrate one possible way.

Dislaimer: This is a result of my experimentation, not deep knowledge if Kioo.

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Most interesting links of March ’14

Posted by Jakub Holý on March 31, 2014

Recommended Readings

Clojure Corner

  • Timo Mihaljov’s Pimp My REPL (3/2014)- really great tips – user.clj, :dev profile, user-wide config in .lein/profiles.clj, tools.namespace, making funs available everywhere & more via Vinyasa, form println with Spyscope, debug-repl, difform, clj-ns-browser

Tools/Libs

  • clj-ds – Clojure immutable datastructures extracted from Clojure and made easier for use directly in Java

Favourite Quotes

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Framework Joy: Load in Hibernate Updates Data

Posted by Jakub Holý on March 31, 2014

Would you ever guess that this line

// Load Buyer from DB by id using Spring's HibernateTemplate:
final Buyer traveller = (Buyer) hibernateTemplate.load(Buyer .class, new Long(id));

could lead to a constraint-validation exception during a batch update or delete and re-insert rows (loosing all columns Hibernate does not know about)? I was quite surprised.

In particular buyer’s bonusCards get deleted and re-inserted, because Hibernate believes that the collection of cards is dirty, even though it has just loaded it from the DB. I am not exactly sure why (preventing false positives in dirty checks requires some black magic), this is the configuration:

<class name="Buyer" ...>
...
    <set name="bonusCards " table="bonus_cards">
        <key column="buyer_id"/>
        <composite-element class="BonusCard">
            <property name="number" column="number" not-null="false"/>
            <property name="expiredate" column="expdate"/>
            ...
        </composite-element>
    </set>
</class>

Of course Hibernate certainly has good reasons to update and delete+re-insert data upon load and we could certainly get rid of (some of) these updates by configuring Hibernate better. But it still demonstrates nicely the hidden cost of using a complex framework – in this case, it behaves quite unexpectedly and requires extensive knowledge to set up properly and to troubleshoot.

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How To Generate A Valid Credit Card Number For A Bin (First 6 Digits)

Posted by Jakub Holý on March 25, 2014

There is plenty of generators that can produce numbers that are valid credit card numbers according to the Luhn check and specific rules of the individual issuer companies. However I have not found anything that would generate the missing digits given a bin, i.e. the first 6 digits of a credit card (the “bank identification number”). So I created one, reverse-engineering org.apache.commons.validator.routines.CreditCardValidator from common-validator 1.4:

// Groovy:
/** Map RegExp from C.C.Validator to the total length of the CC# */
binReToLen = [
        (~/^(3[47]\d{0,13})$/) : 13+2, // amex
        (~/^30[0-5]\d{0,11}$/) : 11+3, // diners 1
        (~/^(3095\d{0,10})$/) : 10+4, // diners 2
        (~/^(36\d{0,12})$/) : 12+2,     // diners 3
        (~/^|3[8-9]\d{0,12}$/) : 12+2,  // diners 4
        (~/^(5[1-5]\d{0,14})$/) : 14+2, // master
        (~/^(4)(\d{0,12}|\d{15})$/) : 12+1 // visa
        // Discover cards omitted
]

/** Bin is e.g. 123456 */
def completeCCn(String bin) {
    def ccnFill = "1" * 19
    int ccnLen = lenForBin(bin)

    def ccnWithoutCheck = bin + ccnFill[0..<(ccnLen - 6 - 1)] // - bin, - check digit

    def check = computeLuhncheckDigit(ccnWithoutCheck)

    return "$ccnWithoutCheck$check"
}

def lenForBin(String bin) {
    def match = binReToLen.find { it.key.matcher(bin).matches() }
    if (match == null) {
        throw new RuntimeException("Bin $bin does not match any known CC issuer")
    }
    match.value
}

def computeLuhncheckDigit(def ccnWithoutCheck) {
    org.apache.commons.validator.routines.checkdigit.LuhnCheckDigit.LUHN_CHECK_DIGIT.calculate(ccnWithoutCheck)
}

completeCCn('465944') // => 4659441111118

Testing FTW!

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HttpServletRequest: requestURI/requestURL/contextPath/servletPath/pathInfo/queryString

Posted by Jakub Holý on March 24, 2014

I never remember what some of these HttpServletRequest methods return so here is an example:

URL METHODS BREAKDOWN (JavaEE 6):
- requestURL  = http://localhost:8080/myapp/users/profile.xhtml
- requestURI  = /myapp/users/profile.xhtml = contextPath + servletPath + pathInfo
- contextPath = /myapp = the first segment after hostname (unless the app runs and the root app with context /)
- servletPath = /users/profile.xhtml (part after contextPath to the servlet that handled the request)
- pathInfo    = null (what remains after servletPath up to the queryString; would return '/dummy' if the url was '.../profile.xhtml/dummy?id=007')
- queryString = id=007

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Ansible: Best practices for deriving host-level var from a group var

Posted by Jakub Holý on March 19, 2014

I have a cluster and a group variable (in test/staging/prod) holding the hostname of the cluster master. For each host, I want to derive a variable which is set to either “slave” or “master” and use it in templates.

This can be done with set_fact:

---
# group_vars/staging:
jboss_master_host: barad-dur.example.com

 

---
# roles/xxx/tasks/main.yml:
- name: Set default jboss_host_type
  set_fact: jboss_host_type=slave
- name: Set jboss_host_type to master if master
  set_fact: jboss_host_type=master
  when: jboss_master_host == inventory_hostname

We could simplify that by using more advanced Jinja:

---
# roles/xxx/tasks/main.yml:
- name: Set jboss_host_type var
  set_fact: jboss_host_type={{ 'master' if jboss_master_host == inventory_hostname else 'slave' }}

but it is preferred not to use logic coded in Jinja2.

// Ansible 1.5.3

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Frustration-Driven Development – Towards DevOps, Lean, Clojure

Posted by Jakub Holý on March 17, 2014

A post about development practices, speed, and frustration.

My wife has mentioned that she likes my passion for doing things right in software development. That made me thinking, why do I actually care so much and do not just enjoy the coding itself? It boils down to that I am not happy until my code is in production. Seeking the satisfaction of having my code used by and helping people while trying to eliminate all unnecessary mental drain is behind all the practices that I embrace and evangelize. It’s a drug I like to take often, in small doses.

practices = f(max(delivered value), min(mental energy))

So how does this relate to DevOps, Continuous Delivery, testing, single-piece-flow, Lean Startup, Clojure? It is simple.

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