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From Daniel Dekany <ddek...@freemail.hu>
Subject Re: [FM3] Solving FM2 caching problems due to locale etc.
Date Tue, 21 Mar 2017 17:29:09 GMT
Tuesday, March 21, 2017, 3:12:38 PM, Woonsan Ko wrote:

> On Sat, Mar 18, 2017 at 6:47 PM, Daniel Dekany <ddekany@freemail.hu> wrote:
>> The problem in FM2
>> ------------------
>>
>> In FM2, if you write `cfg.getTemplate("foo.ftl", Locale.GERMANY)`,
>> then FreeMarker will return a Template where template.getLocale() is
>> Locale.GERMANY, even if there's no foo_de.ftl or such, and it has just
>> fallen back to the most generic foo.ftl template. In the template
>> cache, we will have an entry like
>>
>>   ("foo.ftl", GERMANY) -> Template where getLocale() returns GERMANY
>>
>> Then if you do `cfg.getTemplate("foo.ftl", Locale.ITALY)`, then you
>> will have yet another cache entry:
>>
>>   ("foo.ftl", ITALY) -> Template where getLocale() returns ITALY
>>
>> So far you loaded (I/O!) and parsed foo.ftl twice, and you also store
>> the parsed template (the AST, which includes all the static text too)
>> in the memory twice. You see the problem; you don't reuse a single
>> Template object, despite there's not foo_de.ftl or foo_it.ftl. It's
>> wasteful, and users has complained about it too (if you use the
>> preferred locale of the browser of the visitor, the bloat can be
>> disastrous).
>>
>> Actually, the are multiple Template properties that have the same
>> problem (cache inefficiency):
>> - locale: See earlier
>> - name: This stores the name used for the lookup (such as for the
>>   localized lookup, but note that the lookup strategy is pluggable).
>>   This is the normalized form of what you pass to
>>   Configuration.getTemplate(...).
>> - customLookupCondition: Used by some custom lookup strategies. Also
>>   passed to Configuration.getTemplate(...)
>> - (There's also `encoding` and `parsed`, but those will get out of the
>>   way according the proposal from 2017-02-06.)
>>
>>
>> Possible solution  in FM3
>> -------------------------
>>
>> We could introduce something like LookupIndependentTemplate, which
>> stores:
>> - The parsed template (the AST)
>> - The sourceName of the template (it's the name used as the parameter
>>   of the TemplateLoader that has actually loaded the template "file",
>>   so this is past lookup)
>> - The template specific configuration settings (provided by
>>   the Configuration.templateConfigurations setting, also by the #ftl
>>   header in the template)
>> - Reference to the Configuration
>>
>> Then there's Template, which is familiar from FM2, but this time it
>> only contains:
>> - The lookup parameters:
>>   - locale used for the lookup
>>   - name (the one used for the lookup)
>>   - customLookupCondition
>> - The LookupIndependentTemplate. Multiple Template-s may use the same
>>   LookupIndependentTemplate.
>>
>> The template cache split to two layers:
>> - Level 1: Looks up the Template with the lookup parameters. So this
>>   is very similar to the FM2 cache.
>> - Level 2: Looks up the UnboundTempalate based solely on the
>>   sourceName (the TemplateLoader parameter).
>
> Is UnboundTemplate the same as LookupIndependentTemplate mentioned above?
>
>>
>> So if you have a level 1 cache miss (or stale entry hit), then during
>> the lookup, we get the templates from the level 2 cache. So in the
>> original example, you will still have two Template-s (one for GERMANY
>> and one for ITALY), but they are quite light and share the single
>> UnboundTemplate that belongs to "foo.ftl". You have two entries in the
>> level 1 cache merely to speed up the lookup. (Though the lookup on top
>> of a level 2 cache that has already warmed up is quite fast, because
>> just as in FM2, we would cache negative results, on both levels. Like
>> we cache the fact that there was no "foo_de_DE.ftl" and "foo_de.ftl".)
>>
>> A tricky corner case is when LookupIndependentTemplate itself stores a
>> non-null Locale (it can, as a TemplateCofiguration can contain all
>> runtime configuration settings), which differs from the Locale for
>> which the template was successfully looked up. I guess then that
>> should take precedence over Template.locale. (A possible use case: you
>> might don't use localized lookup, and instead the template itself
>> dictates the locale. After all, the static text in the template uses
>> some language.)
>>
>> Any thoughts?
>
> Everything sounds good. Your judgement on the tricky corner case makes
> sense, too.
> One consideration might be naming. It sounds like Template 'contains'
> LookupIndependentTemplate. But the names sound like 'inheritance' to
> me. ;-)

I agree, it's confusing.

> So I wonder if there could be a better / more intuitive naming.

I was thinking more about this, and I think that probably we just
shouldn't store information about the lookup in Template, and then,
naturally, we don't need LookupIndependentTemplate. (The two level
caching would still of course remain, only it doesn't add an extra
class to the published API, which is good.)

So how would that work? My idea is that we first create the
Environment first, set the locale and the customLookupCondition in it,
and after that the Environment invokes the TemplateResolver to look up
the main template, with env.locale and env.customLookupCondition as
parameters (in additionally to the template name, of course). That's
the opposite order compared to how FM2 does it; there you look up the
template first, the resulting Template remembers the lookup locale an
the customLookupCondition, and then the Template creates the
Environment, and env.locale defaults to template.locale. If we do it
in the other way around, then the Template doesn't have to remember
the lookup parameters, and so Template is lookup independent. (Also,
in case the TemplateLookupStrategy ignores the locale, or the
customLookupCondition, now you don't have to instantiate a Template
for each permutations of their values.)

But of course, there's at least a catch with this idea... If Template
is lookup independent, then it can only store the `sourceName`, but
not the `name`. (`name` is the lookup parameter after some
normalization, which is then possibly transformed by the
TemplateLookupStrategy to the `sourceName`. `sourceName` is the
parameter to the TemplateLoader invocation that has actually managed
to load the template file.) But we need the `name` to resolve relative
paths in the template. So when a Template is loaded into the
Environment, that information will have to be tracked somehow. Because
the same template can be included for multiple times with different
`names` (for some lookup strategies), it's not entirely trivial. But
it certainly adds less complexity than having a separate
LookupIndependentTemplate and Template class, or at least less
complexity that's visible for the users.

The other thing with this choice is that it changes some of our most
fundamental API-s. In FM2 you do this:
  
    Template t = cfg.getTemplate(name, locale, customLookupCond);
    t.process(dataModel, out);
    
So things happen in the wrong order. Thus we had to change that to
something like:
  
    cfg.processTemplate(name, dataModel, out, locale, customLookupCond);

There would be overloads of processTemplate, where you omit the last
or last two parameters. Also, there should be overloads where instead
of a `name` you specify a Template object, that you have created from
a String yourself with new Template(...), so it couldn't have lookup
information anyway.

Template.process(...) would be removed, as it's not the duty of the
Template to create the Environment anymore.
  
On the level of the more expert Environment API, this translates to
something like this:
  
    Environment env = new Environment(cfg, dataModel, out);
    env.setLocale(aLocale);
    env.setCustomLookupCondition(aLookupCondition);
    ... // Addjust env further
    env.loadMainTemplate(name); // After setting locale and customLookupCond!
    env.process();
    ... // Get back variables from env if you want

You might notice that Environment now stores the
customLookupCondition, while in FM2 the Template stores it. Any
template loading from the Environment will happen with that condition
(and with env.locale, just as in FM2). In FM2 the
customLookupCondition is inherited from the Template that contains the
#import/#include statement, but if you think about it, the end results
is practically the same.

> Regards,
>
> Woonsan
>
>>
>> --
>> Thanks,
>>  Daniel Dekany

-- 
Thanks,
 Daniel Dekany


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