Provided by: varnish_5.2.1-1ubuntu0.1_amd64 
NAME
vsl-query - Varnish VSL Query Expressions
OVERVIEW
The Varnish VSL Query Expressions extracts transactions from the Varnish shared memory log, and perform
queries on the transactions before reporting matches.
A transaction is a set of log lines that belongs together, e.g. a client request or a backend request.
The API monitors the log, and collects all log records that make up a transaction before reporting on
that transaction. Transactions can also be grouped, meaning backend transactions are reported together
with the client transaction that initiated it.
A query is run on a group of transactions. A query expression is true if there is a log record within the
group that satisfies the condition. It is false only if none of the log records satisfies the condition.
Query expressions can be combined using boolean functions. In addition to log records, it is possible to
query transaction ids (vxid) in query.
GROUPING
When grouping transactions, there is a hierarchy structure showing which transaction initiated what. The
level increases by one on an 'initiated by' relation, so for example a backend transaction will have one
higher level than the client transaction that initiated it on a cache miss. Request restart transactions
don't get their level increased to make it predictable.
Levels start counting at 1, except when using raw where it will always be 0.
The grouping modes are:
• session
All transactions initiated by a client connection are reported together. Client connections are open
ended when using HTTP keep-alives, so it is undefined when the session will be reported. If the
transaction timeout period is exceeded an incomplete session will be reported. Non-transactional data
(vxid == 0) is not reported.
• request
Transactions are grouped by request, where the set will include the request itself as well as any
backend requests or ESI-subrequests. Session data and non-transactional data (vxid == 0) is not
reported.
• vxid
Transactions are not grouped, so each vxid is reported in its entirety. Sessions, requests,
ESI-requests and backend requests are all reported individually. Non-transactional data is not reported
(vxid == 0). This is the default.
• raw
Every log record will make up a transaction of its own. All data, including non-transactional data will
be reported.
Transaction Hierarchy
Example transaction hierarchy using request grouping mode
Lvl 1: Client request (cache miss)
Lvl 2: Backend request
Lvl 2: ESI subrequest (cache miss)
Lvl 3: Backend request
Lvl 3: Backend request (VCL restart)
Lvl 3: ESI subrequest (cache miss)
Lvl 4: Backend request
Lvl 2: ESI subrequest (cache hit)
MEMORY USAGE
The API will use pointers to shared memory log data as long as possible to keep memory usage at a
minimum. But as the shared memory log is a ring buffer, data will get overwritten eventually, so the API
creates local copies of referenced log data when varnishd comes close to overwriting still unreported
content.
This process avoids loss of log data in many scenarios, but it is not failsafe: Overruns where varnishd
"overtakes" the log reader process in the ring buffer can still happen when API clients cannot keep up
reading and/or copying, for instance due to output blocking.
Though being unrelated to grouping in principle, copying of log data is particularly relevant for session
grouping together with long lasting client connections - for this grouping, the logging API client
process is likely to consume relevant amounts of memory. As the vxid grouping also logs (potentially long
lasting) sessions, it is also likely to require memory for copies of log entries, but far less than
session grouping.
QUERY LANGUAGE
A query expression consists of record selection criteria, and optionally an operator and a value to match
against the selected records.
<record selection criteria> <operator> <operand>
Additionally, a query expression can occur on the transaction itself rather than log records belonging to
the transaction.
vxid <numerical operator> <integer>
A vxid query allows you to directly target a specific transacion, whose id can be obtained from an
X-Varnish HTTP header, the default "guru meditation" error page, or Begin and Link log records.
Record selection criteria
The record selection criteria determines what kind records from the transaction group the expression
applies to. Syntax:
{level}taglist:record-prefix[field]
Taglist is mandatory, the other components are optional.
The level limits the expression to a transaction at that level. If left unspecified, the expression is
applied to transactions at all levels. Level is a positive integer or zero. If level is followed by a '+'
character, it expresses greater than or equal. If level is followed by a '-', it expresses less than or
equal.
The taglist is a comma-separated list of VSL record tags that this expression should be checked against.
Each list element can be a tag name or a tag glob. Globs allow a '*' either in the beginning of the name
or at the end, and will select all tags that match either the prefix or subscript. A single '*' will
select all tags.
The record prefix will further limit the matches to those records that has this prefix as their first
part of the record content followed by a colon. The part of the log record matched against will then be
limited to what follows the prefix and colon. This is useful when matching against specific HTTP headers.
The record prefix matching is done case insensitive.
The field will, if present, treat the log record as a white space separated list of fields, and only the
nth part of the record will be matched against. Fields start counting at 1.
An expression using only a record selection criteria will be true if there is any record in the
transaction group that is selected by the criteria.
Operators
The following matching operators are available:
• == != < <= > >=
Numerical comparison. The record contents will be converted to either an integer or a float before
comparison, depending on the type of the operand.
• eq ne
String comparison. 'eq' tests string equality, 'ne' tests for not equality.
• ~ !~
Regular expression matching. '~' is a positive match, '!~' is a non-match.
Operand
The operand is the value the selected records will be matched against.
An operand can be quoted or unquoted. Quotes can be either single or double quotes, and for quoted
operands a backslash can be used to escape the quotes.
Unquoted operands can only consist of the following characters:
a-z A-Z 0-9 + - _ . *
The following types of operands are available:
• Integer
A number without any fractional part, valid for the numerical comparison operators. The integer type is
used when the operand does not contain any period (.) characters.
• Float
A number with a fractional part, valid for the numerical comparison operators. The float type is used
when the operand does contain a period (.) character.
• String
A sequence of characters, valid for the string equality operators.
• Regular expression
A PCRE regular expression. Valid for the regular expression operators.
Boolean functions
Query expressions can be linked together using boolean functions. The following are available, in
decreasing precedence:
• not <expr>
Inverts the result of <expr>
• <expr1> and <expr2>
True only if both expr1 and expr2 are true
• <expr1> or <expr2>
True if either of expr1 or expr2 is true
Expressions can be grouped using parenthesis.
QUERY EXPRESSION EXAMPLES
• Transaction group contains a request URL that equals to "/foo"
ReqURL eq "/foo"
• Transaction group contains a request cookie header
ReqHeader:cookie
• Transaction group doesn't contain a request cookie header
not ReqHeader:cookie
• Client request where internal handling took more than 800ms.:
Timestamp:Process[2] > 0.8
• Transaction group contains a request user-agent header that contains "iPod" and the request delivery
time exceeds 1 second
ReqHeader:user-agent ~ "iPod" and Timestamp:Resp[2] > 1.
• Transaction group contains a backend response status larger than or equal to 500
BerespStatus >= 500
• Transaction group contains a request response status of 304, but where the request did not contain an
if-modified-since header
ReqStatus == 304 and not ReqHeader:if-modified-since
• Transactions that have had backend failures or long delivery time on their ESI subrequests. (Assumes
request grouping mode).
BerespStatus >= 500 or {2+}Timestamp:Process[2] > 1.
• Log non-transactional errors. (Assumes raw grouping mode).
vxid == 0 and Error
HISTORY
This document was written by Martin Blix Grydeland.
COPYRIGHT
This document is licensed under the same licence as Varnish itself. See LICENCE for details.
• Copyright (c) 2006 Verdens Gang AS
• Copyright (c) 2006-2015 Varnish Software AS
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