vmod-dbrw User Manual

Table of Contents


This edition of the vmod-dbrw User Manual, last updated 13 February 2019, documents vmod-dbrw Version 2.5.

vmod-dbrw User Manual:   Section:   Chapter:FastBack: Top   Up: Top   FastForward: Overview   Contents: Table of ContentsIndex: Concept Index

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.

vmod-dbrw User Manual:   Section:   Chapter:FastBack: Intro   Up: Top   FastForward: Configuration   Contents: Table of ContentsIndex: Concept Index

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.


Treat regexp as case-insensitive regular expression.


Treat regexp as case-sensitive (default).


Use exact string matching.


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


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


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


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.

vmod-dbrw User Manual:   Section:   Chapter:FastBack: Overview   Up: Top   FastForward: Query   Contents: Table of ContentsIndex: Concept Index

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.



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


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:

SequenceReplaced by
\aAudible bell character (ASCII 7)
\bBackspace character (ASCII 8)
\fForm-feed character (ASCII 12)
\nNewline character (ASCII 10)
\rCarriage return character (ASCII 13)
\tHorizontal tabulation character (ASCII 9)
\vVertical 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:


Set debugging level. Argument is a decimal number.


Name or IP address of the database server to connect to. If not defined, localhost (‘’) 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 number on the ‘server’ to connect to. Default is ‘3306’ for MySQL and 5432 for Postgres.


The name of the database to use.


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


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


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


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’.


(Postgres-specific) Connection options.


Database user name.


Password to access the database.


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 {
           {"SELECT dest
                FROM redirects
               WHERE host='$host'
                AND url='$url'"});

vmod-dbrw User Manual:   Section:   Chapter:FastBack: Configuration   Up: Configuration   FastForward: Query   Contents: Table of ContentsIndex: Concept Index

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.


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


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


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.


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 2.5 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.

vmod-dbrw User Manual:   Section:   Chapter:FastBack: Configuration   Up: Top   FastForward: Rewrite   Contents: Table of ContentsIndex: Concept Index

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 (
  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:


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


Host part of the incoming request.


URL part of the incoming request.


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 (
  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):


The value to be compared with the pattern.


Regular expression to use.


Optional flags.


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:


vmod-dbrw User Manual:   Section:   Chapter:FastBack: Query   Up: Top   FastForward: Reporting Bugs   Contents: Table of ContentsIndex: Concept Index

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 {
           {"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);

vmod-dbrw User Manual:   Section:   Chapter:FastBack: Rewrite   Up: Top   FastForward: Copying This Manual   Contents: Table of ContentsIndex: Concept Index

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:

vmod-dbrw User Manual:   Section:   Chapter:FastBack: Reporting Bugs   Up: Top   FastForward: Concept Index   Contents: Table of ContentsIndex: Concept Index

Appendix A GNU Free Documentation License

Version 1.2, November 2002
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.

    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 make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.


    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

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    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

    If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.


    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.


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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.
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vmod-dbrw User Manual:   Section:   Chapter:FastBack: Copying This Manual   Up: Top     Contents: Table of ContentsIndex: Concept Index

Concept Index

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

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Index Entry  Section
append query string: Overview
backslash interpretation: Configuration
cacert: Configuration
case: Overview
case-insensitive regular expression: Overview
case-sensitive regular expression: Overview
certificate authority file, SSL: Configuration
command expansion: Expansions
config: Configuration
config: Configuration
configuration: Configuration
database: Configuration
database engines: Overview
database name: Configuration
database password: Configuration
database query: Configuration
database server: Configuration
database server port: Configuration
database user name: Configuration
debug: Configuration
debugging level: Configuration
discard query string: Overview
eq: Overview
escape sequences: Configuration
exact matching: Overview
expansion, command: Expansions
expansion, variable: Expansions
expansions: Expansions
FDL, GNU Free Documentation License: Copying This Manual
flags: Overview
group: Configuration
idle timeout, SQL: Configuration
initialization: Configuration
MySQL options file: Configuration
name, database: Configuration
NC: Overview
nocase: Overview
options: Configuration
options, MySQL: Configuration
options, PostgreSQL: Configuration
password: Configuration
port: Configuration
QSA: Overview
qsappend: Overview
QSD: Overview
qsdiscard: Overview
query: Configuration
query: Query
query string handling: Overview
query string handling: Overview
redirection code: Overview
regex: Overview
regex matching: Overview
regular expression matching: Overview
regular expression, case-insensitive: Overview
regular expression, case-sensitive: Overview
result interpretation: Overview
rewrite: Rewrite
rewrite rules: Intro
rewrite rules: Overview
server: Configuration
SSL connection, MySQL: Configuration
status code: Overview
strict matching: Overview
supported databases: Overview
timeout: Configuration
timeout, idle, SQL: Configuration
urlprefixes: Expansions
user: Configuration
variable expansion: Expansions
Varnish: Intro
vcl_recv: Configuration
vcl_recv: Rewrite
vcl_synth: Rewrite
vmod_std: Rewrite
X-VMOD-DBRW-Error: Overview
X-VMOD-DBRW-Status: Overview
X-VMOD-DBRW-Status: Rewrite
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