vmod-dbrw
1 Introduction to vmod-dbrw
2 Overview
3 Configuration
4 Writing Queries
5 The 'rewrite' Function
6 How to Report a Bug
Appendix A GNU Free Documentation License
Concept Index
vmod-dbrw
1 Introduction to vmod-dbrw
2 Overview
3 Configuration
3.1 Expansions
4 Writing Queries
5 The 'rewrite' Function
6 How to Report a Bug
Appendix A GNU Free Documentation License
A.1 ADDENDUM: How to use this License for your documents
Concept Index
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 Cache'(1) which implements
database-driven rewrite rules. These rules may be similar to
'RewriteRule' directives implemented by 'mod_rewrite'(2) 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.
---------- Footnotes ----------
(1) <https://www.varnish-cache.org>
(2) <http://httpd.apache.org/docs/current/rewrite/>
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. *Note 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 (*note 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 '${NAME}'), which will be expanded to the actual
value of the NAME argument to the function 'rewrite'. *Note
Expansions::, for details.
The example below configures 'vmod-dbrw' to use MySQL database
'rewrite', with the user name 'varnish' and password 'guessme'.
import dbrw;
sub vcl_recv {
dbrw.config("mysql",
"database=rewrite;user=varnish;password=guessme",
{"SELECT dest
FROM redirects
WHERE host='$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 (*note 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
$(CMD ARGS)
where CMD is the command name, and ARGS is a list of arguments separated
by whitespace. Arguments can in turn contain variable and command
references.
During command expansion, each invocation is replaced by the result
of the call to function CMD with the supplied arguments.
As of version 2.7 of 'vmod-dbrw', only one function is declared:
-- Command: urlprefixes URI
Expands to comma-separated list of path prefixes contained in URI,
starting from the longest one (URI itself, with eventual query part
stripped off). Single '/' is not included in the list. Each list
item is quoted. The expansion can be used in the 'IN ()' SQL
conditional.
4 Writing Queries
*****************
The query supplied to the 'config' function depends on the database
schema and on the desired kind of matching (e.g. exact vs. wildcard).
To ensure the best performance of the module it is important to design
the schema and the query so that the database look up be as fast as
possible.
Suppose that you plan to use 'vmod-dbrw' to implement redirection
rules based on strict matching (*note strict matching::).
The simplest database structure for this purpose (assuming MySQL)
will be:
CREATE TABLE redirects (
id INT AUTO_INCREMENT,
host varchar(255) NOT NULL DEFAULT '',
url varchar(255) NOT NULL DEFAULT '',
dest varchar(255) DEFAULT NULL,
PRIMARY KEY (host,url)
);
The columns and their purpose are:
id
An integer uniquely identifying the row. It is useful for table
management purposes (e.g. deleting the row).
host
Host part of the incoming request.
url
URL part of the incoming request.
dest
Destination URL to redirect to.
The rewrite function looks up a row that has 'host' and 'url'
matching the incoming request and, if found, returns the value of its
'dest' column. The corresponding query is:
SELECT dest FROM redirects WHERE host='$host' AND url='$url'
The variables 'host' and 'url' are supposed to contain the actual
host and URL parts of the incoming request.
Handling regular expression matches is a bit trickier. Your query
should first return such rows that could possibly match the request.
Then the 'vmod-dbrw' engine will do the rest, by iterating over the
returned set and finding the row that actually matches the request. It
will iterate over the rows in the order they were returned by the
database server, so it might be necessary to sort them by some criterion
beforehand.
The following is an example table structure:
CREATE TABLE rewrite (
id INT AUTO_INCREMENT,
host varchar(255) NOT NULL DEFAULT '',
url varchar(255) NOT NULL DEFAULT '',
dest varchar(255) DEFAULT NULL,
value varchar(255) DEFAULT NULL,
pattern varchar(255) DEFAULT NULL,
flags char(64) DEFAULT NULL,
weight int NOT NULL DEFAULT '0',
KEY source (host,url)
);
The meaning of 'id', 'host', and 'dest' is the same as in the previous
example. The meaning of 'url' is described below. Other columns are
(*note regex matching::):
value
The value to be compared with the pattern.
pattern
Regular expression to use.
flags
Optional flags.
weight
Relative weight of this row in the set. Rows will be sorted by
this column, in ascending order.
The simplest way to select candidate rows is by their 'host' column:
SELECT dest,pattern,value,flags
FROM rewrite
WHERE host='$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 '${NAME}' is replaced by the actual
value of the variable NAME from the table. Expanding an
undefined variable is considered an error.
* Establishing the database connection
Unless the connection has already been established by a prior
call to 'rewrite', the function establishes it using the
parameters supplied earlier in a call to 'config'. If the
connection fails, the function returns NULL immediately.
Database connections are persisting and thread-specific. This
means that each thread keeps its own connection to the
database and attempts to re-establish it if it goes down for
some reason.
* Query execution
The query is sent to the server and the resulting set
collected from it.
* Result interpretation
The resulting set is interpreted as described in *note result
interpretation::. This results in a single value being
returned to the caller.
Assuming the database structure similar to the one discussed in the
previous chapter, the following example illustrates how to use 'rewrite'
to redirect the incoming request.
sub vcl_recv {
dbrw.config("mysql",
"database=rewrite;user=varnish;password=guessme",
{"SELECT dest
FROM redirects
WHERE host='$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 (*note 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
*****************************************
Version 1.2, November 2002
Copyright (C) 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
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The author(s) and publisher(s) of the Document do not by this
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5. COMBINING DOCUMENTS
You may combine the Document with other documents released under
this License, under the terms defined in section 4 above for
modified versions, provided that you include in the combination all
of the Invariant Sections of all of the original documents,
unmodified, and list them all as Invariant Sections of your
combined work in its license notice, and that you preserve all
their Warranty Disclaimers.
The combined work need only contain one copy of this License, and
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copy. If there are multiple Invariant Sections with the same name
but different contents, make the title of each such section unique
by adding at the end of it, in parentheses, the name of the
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unique number. Make the same adjustment to the section titles in
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combined work.
In the combination, you must combine any sections Entitled
"History" in the various original documents, forming one section
Entitled "History"; likewise combine any sections Entitled
"Acknowledgements", and any sections Entitled "Dedications". You
must delete all sections Entitled "Endorsements."
6. COLLECTIONS OF DOCUMENTS
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.
7. AGGREGATION WITH INDEPENDENT WORKS
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
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License does not apply to the other works in the aggregate which
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If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half
of the entire aggregate, the Document's Cover Texts may be placed
on covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic
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the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may
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4. Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
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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.
9. TERMINATION
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/>.
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have the option of following the terms and conditions either of
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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
* Menu:
* append query string: Overview. (line 111)
* backslash interpretation: Configuration. (line 159)
* cacert: Configuration. (line 205)
* case: Overview. (line 105)
* case-insensitive regular expression: Overview. (line 101)
* case-sensitive regular expression: Overview. (line 105)
* certificate authority file, SSL: Configuration. (line 205)
* command expansion: Expansions. (line 293)
* config: Configuration. (line 138)
* config <1>: Configuration. (line 196)
* configuration: Configuration. (line 139)
* database: Configuration. (line 193)
* database engines: Overview. (line 49)
* database name: Configuration. (line 193)
* database password: Configuration. (line 229)
* database query: Configuration. (line 232)
* database server: Configuration. (line 182)
* database server port: Configuration. (line 189)
* database user name: Configuration. (line 226)
* debug: Configuration. (line 179)
* debugging level: Configuration. (line 179)
* discard query string: Overview. (line 116)
* eq: Overview. (line 108)
* escape sequences: Configuration. (line 159)
* exact matching: Overview. (line 108)
* expansion, command: Expansions. (line 293)
* expansion, variable: Expansions. (line 255)
* expansions: Expansions. (line 255)
* FDL, GNU Free Documentation License: Copying This Manual. (line 550)
* flags: Overview. (line 98)
* group: Configuration. (line 200)
* idle timeout, SQL: Configuration. (line 210)
* initialization: Configuration. (line 139)
* MySQL options file: Configuration. (line 196)
* name, database: Configuration. (line 193)
* NC: Overview. (line 101)
* nocase: Overview. (line 101)
* options: Configuration. (line 223)
* options, MySQL: Configuration. (line 196)
* options, PostgreSQL: Configuration. (line 223)
* password: Configuration. (line 229)
* port: Configuration. (line 189)
* QSA: Overview. (line 111)
* qsappend: Overview. (line 111)
* QSD: Overview. (line 116)
* qsdiscard: Overview. (line 116)
* query: Configuration. (line 232)
* query <1>: Query. (line 317)
* query string handling: Overview. (line 111)
* query string handling <1>: Overview. (line 116)
* redirection code: Overview. (line 121)
* regex: Overview. (line 126)
* regex matching: Overview. (line 82)
* regular expression matching: Overview. (line 126)
* regular expression, case-insensitive: Overview. (line 101)
* regular expression, case-sensitive: Overview. (line 105)
* result interpretation: Overview. (line 79)
* rewrite: Rewrite. (line 436)
* rewrite rules: Intro. (line 29)
* rewrite rules <1>: Overview. (line 49)
* server: Configuration. (line 182)
* SSL connection, MySQL: Configuration. (line 205)
* status code: Overview. (line 121)
* strict matching: Overview. (line 79)
* supported databases: Overview. (line 49)
* timeout: Configuration. (line 210)
* timeout, idle, SQL: Configuration. (line 210)
* urlprefixes: Expansions. (line 307)
* user: Configuration. (line 226)
* variable expansion: Expansions. (line 255)
* Varnish: Intro. (line 29)
* vcl_recv: Configuration. (line 139)
* vcl_recv <1>: Rewrite. (line 479)
* vcl_synth: Rewrite. (line 501)
* vmod_std: Rewrite. (line 501)
* X-VMOD-DBRW-Error: Overview. (line 70)
* X-VMOD-DBRW-Status: Overview. (line 121)
* X-VMOD-DBRW-Status <1>: Rewrite. (line 501)