One of my customers had many problems with stability of his server. Service stopped under medium web traffic and the only resolution was server restart. I was asked for optimisation of core components (MySQL database / Apache2 web server) but the result was not very satisfactory. I managed to limit swapping by limitting web server concurrency but it was not real problem solution.
Finally I traced problem source to PHP application run on this server. Joomla (especially forum script) caused system to crawl. One process needed above 60 MB of RSS memory. Imagine system handling 20 concurrent requests. The problems became visible when database size exceeded 200 MB. Source code analysis located the problem: loading big recordsets (whole tables) into memory. One PHP process without memory limit might “swap out” whole server.
My customer decided to rewrite from scratch source code, you will find below results of this optimisation (mostly Munin graphs). New system was installed on 27th March.
Here’s number of slow queries recorded by MySQL, you can see they almost disappeared:
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In Agile world there are no immutable constraints. Your requirements may change, libraries used may be replaced during development, application may outgrown your current server setup etc. I’ll show you how to make web application migration between servers as fast as possible: with minimum downtime and data consistency preserved (techniques also apply to hosting providers environment).
Known Problems
You may say: moving a site? No problem: just copy your files, database and voila! Not so fast. There are many quirks you may want to handle properly:
- DNS propagation time
- Database consistency
- Preserve logs
- Preserve external system configuration
- Environment change impact integration tests
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Generics are very useful Java language feature introduced in Java 1.5. Starting from 1.5 you can statically declare expected types of objects inside collections and compiler will enforce this assumption during compilation:
Map<String, BankAccount> bankAccounts = new HashMap<String, BankAccount>();
bankAccounts.put("a1", bankAccount);
bankAccounts.add("a2", "string"); <-- compilation error
Integer x = bankAccounts.get("a1"); <-- compilation error
bankAccounts.put(new Integer("11"), bankAccount); <-- compilation error
Many projects, however, keep 1.4 compatibility mode for many reasons. I think 1.5 is mature enough (ok, let’s say that: old) so it may be used safely.
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I bet everyone knows how to enable SQL logging for Hibernate. If you add this parametr to Hibernate configuration:
<property name="hibernate.show_sql">true</property>
you will see queries like this in log file:
select roles0_.EntityKey as Entity9_1_, roles0_.ENTITYKEY as ENTITY9_168_0_
from USERROLE roles0_
where roles0_.EntityKey=?
but wait: what value is passed as parameter by Hibernate? (parameters are marked as “?” in JDBC) You have configure TRACE level for some log categories, here’s example for simplelog.properties:
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Creating software projects is composed of many activities. One of them (not very appreciated by typical developer) is building process. The activity should create executables or libraries from source code. You may say now: hey, you forget about documentation, generated API specification, installation, deployment, …! As you can see there are many task realised by this activity.
I’ll review most popular building tools and will point out their strengths and weaknesses:
- Make
- Ant
- Maven
- IDE-based builders
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