# Tips and Guidelines for Using the Google Template System

(as of 1 September 2006)

The basic rules of the template system are enough to use it, but over time, we at Google have developed some tips, guidelines, and best practices that make it easier to use templates effectively, and to avoid common template errors.

## Program Design Considerations

### Template naming and versioning

Early in Google's use of templates, we noticed a problem: if a binary that uses a template and its corresponding template were both modified, particularly if the change were such that the old binary could not work with the new template or the new binary cannot work with the old template, then somehow they both had to be deployed at the same instant to not present errors to our users. This was hard to do. The solution was to adopt a template naming and versioning convention. The procedure to use it follows:

- Each template name ends with `_postYYYYMMDD.tpl`, where YYYMMDD is the date of this version's initial creation.
- Before making (non-backward-compatible) modifications to a template, copy the template to a new name, incorporating a later date than the original one being copied.
- Edit the new file, and push it to the production server.
- Finally, update the code to refer to the new template-name (ideally, using the `RegisterTemplateFilename` idiom), and push the new executable to the production server.

When this convention is followed, the new template file does not overwrite the old one when it is deployed, because it is a new file with a new name. The old template file is still there to be used as long as the old binary is still in production and the new template file just sits there being ignored. Then when the new binary finally gets deployed, it immediately starts using the new template file, because it is coded (in `RegisterTemplateFilename`) to do so. After that, it is the old template file that continues to sit there ignored.

The `make_tpl_varnames_h` utility knows about the "_postYYYYMMDD" naming convention, so it is important that you use that convention exactly if you use the `make_tpl_varnames_h`.

### Processing Phases

Typically a program using the Google Template System will perform the following phases, usually in this order:

1. Retrieve and prepare the data used to fill a dictionary.
2. Build the data dictionary, including all its sub-dictionaries, that will supply the values to the designated template object, its sections, and its included templates.
3. Retrieve the top-level template object required to format the data. (This may or may not involve reading and parsing a template file, depending on whether the requested file has already been read and parsed by the running program or whether that file has been marked "reload if changed" and was in fact changed.)
4. Expand the template object into an output buffer using the completed data dictionary.
5. Output the buffer.
6. Clean up: Destroy the top-level data dictionary whenever it is no longer needed.
7. Optionally, clear the cache at the end of program execution.

### One template / One procedure call

Most of the code of the program will be in Phases 1 and 2. Clearly, Phase 1 is outside the scope of the template system. But in designing the code for Phase 2 (building the data dictionary), it is wise to have the structure of the program reflect the structure of the templates being used. Specifically, there should be a single procedure call to build the dictionary for a single template. That procedure call should take parameters that include all the data required to populate the data dictionary for that template and all the templates it includes. Following this "one template/one procedure call" guideline further, for each included template, another procedure should be called to populate the (or each) data dictionary for that included template. This maintains the "one template/one procedure call" principle in a nested fashion that reflects the nesting of the templates.

This is not to imply that the "one procedure call" for a template should not be modularized into sub-procedures for readability and maintainability, or that it should not call other auxilliary procedures for such things as formatting the data and converting it to the appropriate strings, etc. But it does mean that there should be one entry point for building the dictionary tree for one template and that entry point should show the data dependencies of that template through its parameter list. This code for populating the data dictionary should NOT be intermingled with data gathering code that should have been done in Phase 1.

(Inside Google, the convention has been used to name the dictionary building procedure using the pattern `fill_..._dictionary` where the dots are related to the name of the template the data is being prepared for. For instance, the data for the template named one_search_result.tpl might be placed in a dictionary via a function named `fill_one_search_result_dictionary`.)

## Tips, Idioms, and Conventions

1. Choose template names to create unique constant prefixes.

   Template names should contain at least two words to avoid constant prefix clashes (e.g. `kxy_` instead of `kx_` ) The name of a new template should be checked against the existing names before proceeding. If your new template name produces a prefix that conflicts with an already existing template, you should change the name of your new template, even though it may be the only perfect name you can come up with. You'll have to use a less than perfect name in that case. (See "Template Syntax Checker and Header File Generator" below for more explanation about constant prefixes.)

2. Use SetFormattedValue discriminately.

   This method should never be used to sneak HTML into the executable as in

            dictionary->SetFormattedValue(kxy_VAR,
                                          "<b>%s</b>",
                                          some_const_char_string);

   In that case, the `<b>` and `</b>` should be moved into the template.

3. Never have a section encompass an entire template.

   If the first line of a template is a start section marker and the last line is its matching end section marker, then those markers are unnecessary in almost all cases. They are usually put there to allow the entire template to be hidden or iterated, but since it encompasses the entire file, the section may be hidden by not expanding the file (or by hiding the template-include section that includes the file) and it may be iterated by iterating the template-include marker of the including template. (The only exception might be if the entire page is to be iterated, but this seems a bit of a stretch.)

4. An included template is just a section whose contents are located in a separate file. You may iterate over it just like you do sections.

   For example, if your template has the following template-include marker:

            {{>MY_INCLUDED_TEMPLATE}}

   you may call

            google::TemplateDictionary *child_dict =
               dictionary->AddIncludeDictionary(kxy_MY_INCLUDED_TEMPLATE);

   to iterate that section. (Note: Make sure you call `child_dict->SetFilename()`! If your included template is not showing in the output, this is the first thing you should check.)

5. The recommended idiom to fill an include-template dictionary is like this:

               fill_include_template_dictionary(dict->AddIncludeDictionary(name), ...);

   But what do you do if you decide, in `fill_include_template_dictionary`, that you don't want to display anything for this include-template after all? It seems like it's too late: you've already created the sub-dictionary. The solution is simple: just be sure that `fill_include_template_dictionary()` doesn't call `SetFilename()` in that case.

6. Never have a section which only contains another section.

   For example, don't do this:

            {{#OUTER_SECTION}}
               {{#INNER_SECTION}}
               section contents here
               {{/INNER_SECTION}}
            {{/OUTER_SECTION}}

   or this equivalent template code (see the previous item):

            {{#OUTER_SECTION}}
               {{>INCLUDED_SECTION}}
            {{/OUTER_SECTION}}

   This is usually done because the developer thinks the outer section must be used to hide the section when the inner section, intended for iteration, has no iterations. In both cases, you should only have one section (either `INNER_SECTION` or `INCLUDED_SECTION` in the examples) and iterate that section either 0 times or more than 0 times. It's the wonder of the dual use of sections, i.e. that they may be conditional or iterative or, in this case, both.

   A related suggestion: Do not have a section whose entire contents is one variable marker with nothing else, unless you need to iterate over that section with multiple values of that variable. You don't need the surrounding section just to hide the marker. A variable marker that is not set, does not produce output. By convention, we set such variables to the empty string. But in neither case do you need to hide it by hiding a surrounding section that contains nothing else.

7. Use this hide/show idiom for `if-else` blocks.

   Since sections are hidden by default, you can use represent if-else logic in your code via `ShowSection`. For example:

            if ( my_test ) {
               dict->ShowSection(kxyz_TRUE_BLOCK);
               [ more code to fill the values for that section]
            } else {
               dict->ShowSection(kxyz_FALSE_BLOCK);
               [ more code to fill the values for that section]
            }

8. `Write...` vs. `Fill...Dictionary` methods - Observe the proper division of labor, don't mix them.

   The output (or write) function should create the top level template dictionary, call one or more fill-dictionary routines with it, then get the template and expand it. It should not call dictionary modifying methods, like `ShowSection` and `SetValue`. By keeping these separated into their own fill-dictionary routine, the code is more modular and lends itself to template re-use. If you maintain the proper division of labor, the template you are filling and outputting may be filled and included in a larger template by someone else.

9. Use `AddSectionDictionary` only when you want to iterate over a section or, secondarily, if you need to avoid name conflicts.

   Sometimes developers get the idea that every section requires its own child dictionary created by an `AddSectionDictionary` call. Because of variable inheritence, this isn't usually so. The intended purpose of `AddSectionDictionary` is to enable iteration over a section. Secondarily, if the section contains generic names that may conflict with the same name in other parts of the template, it may be safer to call `AddSectionDictionary` to create a separate namespace. In any case, do not assume you must call `AddSectionDictionary` just because you are working within a section. The main dictionary can be used for all levels of conditional sections as long as you avoid name conflicts by keeping the marker names unique.

10. Do not place `RegisterTemplateFilename` statements in header (`.h`) files.

    `RegisterTemplateFilename` is a macro that instantiates a `TemplateNamelist` object. If you place it in a header file, a different object will get created each time it is included in another `.cc` file.

    The `RegisterTemplateFilename` statement and its associated `#include` of the `varnames.h` file should occur only in the `.cc` file that implements the fill-dictionary routine for that template. You should never have more than one `RegisterTemplateFilename` for a single template and you should try hard not to copy the `#include` file to other files as well. The template versioning makes this more important because a developer may not know that the template name with included version number needs to be updated in more than one file when versioning occurs. [Also see above for more information about what routine uses the filename declared by the `RegisterTemplateFilename` statement.]

11. Never reference more than one template in a fill...dictionary method.

    Each template should have its own fill-dictionary routine. That routine should only reference marker names defined in that template. If this convention is followed, then all the prefixes in a fill-dictionary routine will be the same. [Note that an implication of this convention is that if the template includes another template, via a template-include marker, then containing template's fill-dictionary routine should call the included template's fill-dictionary routine (being careful to observe the convention described above). But then, this is merely a restatement of "One template / One procedure call".]

12. Have fill...dictionary call `SetFilename` even if the dictionary is never used for a template-include.

    SetFilename() is required when a dictionary is created via `AddIncludeDictionary()`. However, it's safe to set all the time. By setting it always, you make the code work properly if this dictionary ever changes to be template-included after all. Even if not, by saying what template file the dictionary is intended to go with, you are self-documenting your code.

13. Do not call `c_str()` on strings to pass them to `TemplateDictionary` methods.

    Note that all the TemplateDictionary methods are defined to take `TemplateString` objects. These are created automatically from both strings and char*'s (and can be created manually if you have a char* and a length). So if you have a string, it's safe and efficient to just pass it in directly; you do not need to extract the const char * from your string object to pass it to these methods. For some reason, this is a common error of noviced template coders.

    The one exception to this rule is when using the method `SetFormattedValue`. When calling that method, you must call `c_str()` on strings that are to be inserted into the format string, just as you would when providing data for any other printf format string.

14. Do not use `SetGlobalValue` when you could use `SetValue`.

    `SetGlobalValue` should be used quite rarely, for constants that really are consistent across all your templates. It's slower to look up a value in the global dictionary than it is in the template-specific dictionary.

15. Do not use `TemplateFromString` unless you have a specific need for its non-file-based attributes.

    `TemplateFromString` was created for use in highly constrained cases where file I/O may be impaired or undesirable, for instance to produce a server error message where there may be disk problems or to produce formatted output where there are processes that do not have a facility for updating data files dynamically. It is not recommended for ordinary use as it is limited in functionality in at least the following ways:

    - It can neither include nor be included from another template.
    - It cannot be updated dynamically via a data-push; changes always require a binary push.
16. Use the appropriate variable-modifiers (eg `{{VAR:h}}`) to prevent Cross-Site-Scripting security vulnerabilities.

    Apply the appropriate variable-modifiers liberally and omit them only in those (usually rare) cases where there is a specific reason the template variable should not be escaped, for example:

    - The template variable contains HTML markup that should be interpreted by the browser. In this case you must be very careful to ensure that the variable can in no case contain "harmful" HTML. Also, keep in mind the above recommendation on the use of `SetFormattedValue` and consider moving the HTML markup into the template.
    - The variable is known to be already escaped at the point it is inserted into the template (for example, the value might be kept in escaped form in a storage backend). Here, escaping again via a variable-modifier would result in "double escaping". You must ensure that the variable has been escaped with the appropriate escape function for the HTML context into which it will be inserted into the template (i.e., HTML-escaping versus javascript-escaping).

    Applying the modifier even if you don't expect the variable to contain (malicious) HTML markup keeps you on the safe side. It also serves to self-document the template by making it obvious that no XSS can result from the template variable in question. It is recommended to comment uses of modifier-less template variables accordingly, for example

             {{#SNIPPET1_SECTION}}
                {{! SNIPPET1 is HTML-escaped in SnippetGenerator::getSnippetForResult }}
                <br>{{SNIPPET1}}
             {{/SNIPPET1_SECTION}}

    For situations where you need to provisionally escape, or use an escape routine other than the built-in ones, the `Escaped` versions of the set-value methods are useful utility functions to use.

17. Do not leave an extra space when using `{{BI_SPACE}}`

    The built-in template variable `BI_SPACE` is itself replaced by a single space. It is used where you need to make sure a space is preserved at the end of a line. It is a common mistake to leave an extra space before this marker, which results in not one, but two, spaces created in the document.

    Incorrect:

        <table border=0 {{BI_SPACE}}
               align=center>

    Correct:

        <table border=0{{BI_SPACE}}
               align=center>

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