Vmod-dbrw
	Database-driven rewrites for Varnish Cache	Sergey Poznyakoff

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vmod-dbrw User Manual

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vmod-dbrw

This edition of the vmod-dbrw User Manual, last updated 10 March 2020, documents vmod-dbrw Version 2.7.

1 Introduction to vmod-dbrw

Vmod-dbrw is a module for Varnish Cache¹ which implements database-driven rewrite rules. These rules may be similar to RewriteRule directives implemented by mod_rewrite² module in Apache or to Redirect directives of its mod_alias module. What distinguishes the vmod-dbrw rules from these, is that they are handled by Varnish, before the request reaches the httpd server, and that they are stored in an SQL database, which makes them easily manageable.

Some web sites implement thousands of rewrite rules. The purpose of this module is to facilitate deploying and handling them.

2 Overview

Rewrite rules are stored in a MySQL or PostgreSQL database. The vmod-dbrw module does not impose any restrictions on its schema. It only needs to know the SQL query which is to be used to retrieve data. This query is supplied to the module, along with the credentials for accessing the database, by calling the config function in the vcl_recv subroutine of the Varnish configuration file.

Once the module is configured, the rewrite function can be called in the appropriate place of the Varnish configuration file. Its argument is a list of variable assignments separated by semicolons, each assignment having the form name=value. When called, rewrite expands the SQL query registered with the prior call to config by replacing each $name construct (a variable reference) with the corresponding value from its argument. Similarly to the shell syntax, the variable reference can also be written as ${name}. This latter form can be used in contexts where the variable reference is immediately followed by a letter, digit or underscore, to prevent it from being counted as a part of the name. Special syntax is available for substituting default values and invoking built-in functions during the expansion of the query. See Expansions, for a detailed description of these.

Having undergone expansions, the query is sent to the database server. If the query returns no records or if an error occured, rewrite returns empty string. In case of error, it also sets the HTTP header ‘X-VMOD-DBRW-Error: 1’. It can be used in VLC code to provide a special handling for such failures.

The returned set of records (if non-empty) is processed depending on the number of fields it contains.

If the returned set has one or two columns, only the first tuple is used and rewrite returns the value of its first column.

Otherwise, if the returned set has three or more columns, the regular expression matching is performed. For the purpose of this discussion, let’s refer to the columns as follows: result, regexp, value and flags. The flags column is optional. Any surplus columns are ignored.

For each returned tuple, the value column undergoes variable expansion, using the same algorithm as when preparing the query, and the resulting string is matched with the regexp column, which is treated as an extended POSIX regular expression. If the value matches the expression, the result column is expanded by replacing backreferences: each occurrence of $digit (where digit stands for a decimal digit from ‘0’ through ‘9’) is replaced by the contents of the digits parenthesized subexpression in regexp. For compatibility with the traditional usage, the \digit notation is also allowed. The resulting value is then returned to the caller.

Optional flags column is a comma-separated list of flags that control the matching algorithm.

‘NC’

‘nocase’

Treat regexp as case-insensitive regular expression.

‘case’

Treat regexp as case-sensitive (default).

‘eq’

Use exact string matching.

‘QSA’

‘qsappend’

Treat the resulting value as URL; append any query string from the original value to it.

‘QSD’

‘qsdiscard’

Treat the resulting value as URL; discard any query string attached to the original value.

‘redirect=code’

‘R=code’

On success, set the ‘X-VMOD-DBRW-Status’ header to code, which must be a valid HTTP status code.

‘regex’

Use regular expression matching. This is the default. This flag is provided for completeness sake, as a counterpart of ‘eq’.

If regexp or value is NULL, strict matching is assumed (see strict matching).

If flags is NULL, it is ignored.

3 Configuration

function: void config (string dbtype, string params, string query)

This function configures the module and provides it with the data necessary to connect and use the database. It is normally called from the vcl_recv subroutine.

Arguments:

dbtype

Type of the database to use. Valid values are ‘mysql’ and ‘pgsql’.

params

Database connection parameters. This is a list of ‘name=value’ assignments separated by semicolons. The value part can be any sequence of characters, excepting white space and semicolon. If value contains any of these, they either must be escaped by prepending them with a backslash, or the entire value must be enclosed in a pair of (single or double) quotes. The following escape sequences are allowed for use in value:

Sequence	Replaced by
\a	Audible bell character (ASCII 7)
\b	Backspace character (ASCII 8)
\f	Form-feed character (ASCII 12)
\n	Newline character (ASCII 10)
\r	Carriage return character (ASCII 13)
\t	Horizontal tabulation character (ASCII 9)
\v	Vertical tabulation character (ASCII 11)

Table 3.1: Backslash escapes

If a backslash is immediately followed by a symbol not listed in the above table, it is removed and the symbol is reproduced verbatim.

Valid parameters are:

‘debug=n’

Set debugging level. Argument is a decimal number.

‘server=host’

Name or IP address of the database server to connect to. If not defined, localhost (‘127.0.0.1’) is assumed. For MySQL databases, if host begins with a slash, its value is taken to be the full pathname of the local UNIX socket to connect to.

‘port=n’

Port number on the ‘server’ to connect to. Default is ‘3306’ for MySQL and 5432 for Postgres.

‘database=name’

The name of the database to use.

‘config=filename’

(MySQL-specific) Read database access credentials and other parameters from the MySQL options file filename.

‘group=name’

(MySQL-specific) Read credentials from section name of the options file supplied with the config parameter. Default section name is ‘client’.

‘cacert=filename’

Use secure connection to the database server via SSL. The filename argument is a full pathname of the certificate authority file.

‘timeout=n’

Sets idle timeout for a single connection. The connection will be closed and opened again if the module is to use it after n or more seconds since its last use. Set ‘timeout=-1’ to disable idle timeout (session will remain open until the SQL server closes it). Set ‘timeout=0’ to close the connection after each request (not recommended).

The default depends on the selected SQL backend. For MySQL, it equals the value of the ‘wait_timeout’ global variable. For PostgreSQL, it is ‘-1’.

‘options=string’

(Postgres-specific) Connection options.

‘user=name’

Database user name.

‘password=string’

Password to access the database.

query

The SQL query to use. It can contain variable references ($name or ${name}), which will be expanded to the actual value of the name argument to the function rewrite. See Expansions, for details.

The example below configures vmod-dbrw to use MySQL database ‘rewrite’, with the user name ‘varnish’ and password ‘guessme’.

import dbrw;

sub vcl_recv {
    dbrw.config("mysql",
           "database=rewrite;user=varnish;password=guessme",
           {"SELECT dest
                FROM redirects
               WHERE host='$host'
                AND url='$url'"});
}

3.1 Expansions

The ‘query’ argument to the dbrw.config function normally contains variable references. A variable reference has the form ‘$variable’ or ‘${variable}’, where variable is the variable name. When the dbrw.rewrite function (see Rewrite) is called, each such reference is expanded to the actual value of variable passed in the argument to that function.

The two forms are entirely equivalent. The form with curly braces is normally used if the variable name is immediately followed by an alphanumeric symbol, which will otherwise be considered a part of it. This form also allows for specifying the action to take if the variable is undefined or expands to an empty value.

During variable expansion, the forms below cause dbrw.rewrite to test for a variable that is unset or null (i.e., whose value is an empty string). Omitting the colon results in a test only for a variable that is unset.

${variable:-word}

Use Default Values. If variable is unset or null, the expansion of word is substituted. Otherwise, the value of variable is substituted.

${variable:=word}

Assign Default Values. If variable is unset or null, the expansion of word is assigned to variable. The value of variable is then substituted.

${variable:?word}

Display Error if Null or Unset. If variable is null or unset, the expansion of word (or a message to that effect if word is not present) is output to the current logging channel. Otherwise, the value of variable is substituted.

${variable:+word}

Use Alternate Value. If variable is null or unset, nothing is substituted, otherwise the expansion of word is substituted.

After expanding variables, the query undergoes command expansion. Syntactically, a command invocation is

$(cmd args)

where cmd is the command name, and args is a list of arguments separated by whitespace. Arguments can in turn contain variable and command references.

During command expansion, each invocation is replaced by the result of the call to function cmd with the supplied arguments.

As of version 2.7 of vmod-dbrw, only one function is declared:

Command: urlprefixes uri

Expands to comma-separated list of path prefixes contained in uri, starting from the longest one (uri itself, with eventual query part stripped off). Single ‘/’ is not included in the list. Each list item is quoted. The expansion can be used in the ‘IN ()’ SQL conditional.

4 Writing Queries

The query supplied to the config function depends on the database schema and on the desired kind of matching (e.g. exact vs. wildcard). To ensure the best performance of the module it is important to design the schema and the query so that the database look up be as fast as possible.

Suppose that you plan to use vmod-dbrw to implement redirection rules based on strict matching (see strict matching).

The simplest database structure for this purpose (assuming MySQL) will be:

CREATE TABLE redirects (
  id INT AUTO_INCREMENT,
  host varchar(255) NOT NULL DEFAULT '',
  url varchar(255) NOT NULL DEFAULT '',
  dest varchar(255) DEFAULT NULL,
  PRIMARY KEY (host,url)
);

The columns and their purpose are:

id

An integer uniquely identifying the row. It is useful for table management purposes (e.g. deleting the row).

host

Host part of the incoming request.

url

URL part of the incoming request.

dest

Destination URL to redirect to.

The rewrite function looks up a row that has ‘host’ and ‘url’ matching the incoming request and, if found, returns the value of its ‘dest’ column. The corresponding query is:

SELECT dest FROM redirects WHERE host='$host' AND url='$url'

The variables ‘host’ and ‘url’ are supposed to contain the actual host and URL parts of the incoming request.

Handling regular expression matches is a bit trickier. Your query should first return such rows that could possibly match the request. Then the vmod-dbrw engine will do the rest, by iterating over the returned set and finding the row that actually matches the request. It will iterate over the rows in the order they were returned by the database server, so it might be necessary to sort them by some criterion beforehand.

The following is an example table structure:

CREATE TABLE rewrite (
  id INT AUTO_INCREMENT,
  host varchar(255) NOT NULL DEFAULT '',
  url varchar(255) NOT NULL DEFAULT '',
  dest varchar(255) DEFAULT NULL,
  value varchar(255) DEFAULT NULL,
  pattern varchar(255) DEFAULT NULL,
  flags char(64) DEFAULT NULL,
  weight int NOT NULL DEFAULT '0',
  KEY source (host,url)
);

The meaning of id, host, and dest is the same as in the previous example. The meaning of url is described below. Other columns are (see regex matching):

value

The value to be compared with the pattern.

pattern

Regular expression to use.

flags

Optional flags.

weight

Relative weight of this row in the set. Rows will be sorted by this column, in ascending order.

The simplest way to select candidate rows is by their ‘host’ column:

SELECT dest,pattern,value,flags
FROM rewrite
WHERE host='$host'
ORDER BY weight

One can further abridge the returned set by selecting only those rows whose url column is the prefix of the requested URL:

SELECT dest,pattern,value,flags
FROM rewrite
WHERE host='$host'
AND LOCATE(url,'$url')==1
ORDER BY weight

Furthermore, the url column can contain a path prefix, which can be matched using the IN conditional:

SELECT dest,pattern,value,flags
FROM rewrite
WHERE host='$host'
AND url IN ($(urlprefixes $url))
ORDER BY weight

Notice the use of the ‘$(urlprefixes $url)’. This invokes the built-in function urlprefixes, which expands to comma-separated list of properly quoted pathname prefixes, constructed from its argument. For example, if ‘$url’ is ‘/local/user/local?a=1’, then the expansion of ‘$(urlprefixes $url)’ is:

'/local/user/local','/local/user','/local'

5 The rewrite Function

function: string rewrite (string args)

This function is the working horse of the module. It rewrites its argument using the database configured in the previous call to config and returns the obtained value.

To do so, it performs the following steps:

• Parameter parsing

  The args parameter must be a list of name=value pairs separated by semicolons. The function parses this string and builds a symbol table.

• Variable expansion

  Using the symbol table built in the previous stage, each occurrence of $name or ${name} is replaced by the actual value of the variable name from the table. Expanding an undefined variable is considered an error.

• Establishing the database connection

  Unless the connection has already been established by a prior call to rewrite, the function establishes it using the parameters supplied earlier in a call to config. If the connection fails, the function returns NULL immediately.

  Database connections are persisting and thread-specific. This means that each thread keeps its own connection to the database and attempts to re-establish it if it goes down for some reason.

• Query execution

  The query is sent to the server and the resulting set collected from it.

• Result interpretation

  The resulting set is interpreted as described in result interpretation. This results in a single value being returned to the caller.

Assuming the database structure similar to the one discussed in the previous chapter, the following example illustrates how to use rewrite to redirect the incoming request.

sub vcl_recv {
    dbrw.config("mysql",
           "database=rewrite;user=varnish;password=guessme",
           {"SELECT dest
                FROM redirects
               WHERE host='$host'
                AND url='$url'"});
    set req.http.X-Redirect-To =
        dbrw.rewrite("host=" + req.http.Host + ";" +
                     "url=" + req.url);
    if (req.http.X-Redirect-To != "") {
        return(synth(301, "Redirect"));
    }
}

The ‘synth’ sub must be provided in order to construct redirection responses:

import std;

sub vcl_synth {
    if (resp.status == 301) {
        set resp.http.Location = req.http.X-Redirect-To;
        if (req.http.X-VMOD-DBRW-Status != "") {
            set resp.status =
               std.integer(req.http.X-VMOD-DBRW-Status, 301);
        }
        return (deliver);
    }
}

The X-VMOD-DBRW-Status header, if set, contains the status code to be returned to the client (see X-VMOD-DBRW-Status). Notice the use of the vmod_std module to cast it to integer.

If an error occured during the rewrite, it is recommended to not cache the response. This way the next request will call rewrite again and eventually complete the rewriting. This can be achieved using the following vcl_backend_response fragment:

sub vcl_backend_response
{
    if (bereq.http.X-VMOD-DBRW-Error == "1") {
        set beresp.uncacheable = true;
        return (deliver);
    }
}

6 How to Report a Bug

Email bug reports to gray@gnu.org.

As the purpose of bug reporting is to improve software, please be sure to include a detailed information when reporting a bug. The minimum information needed is:

• Module version you use.
• A description of the bug.
• Conditions under which the bug appears.
• It is often helpful to send the contents of config.log file along with your bug report. This file is created after running ./configure in vmod-dbrw source root directory.

Appendix A GNU Free Documentation License

Copyright © 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

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   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

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   You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

A.1 ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

  Copyright (C)  year  your name.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.2
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

    with the Invariant Sections being list their titles, with
    the Front-Cover Texts being list, and with the Back-Cover Texts
    being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

Concept Index

This is a general index of all issues discussed in this manual

This document was generated on April 9, 2020 using makeinfo.