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Subsections


4.1 HTCondor's ClassAd Mechanism

ClassAds are a flexible mechanism for representing the characteristics and constraints of machines and jobs in the HTCondor system. ClassAds are used extensively in the HTCondor system to represent jobs, resources, submitters and other HTCondor daemons. An understanding of this mechanism is required to harness the full flexibility of the HTCondor system.

A ClassAd is is a set of uniquely named expressions. Each named expression is called an attribute. Figure 4.1 shows ten attributes, a portion of an example ClassAd.

Figure 4.1: An example ClassAd
\begin{figure}\footnotesize \begin{verbatim}MyType = ''Machine'' TargetType = ... ...\vert LoadAvg<=0.3 && KeyboardIdle>15*60\end{verbatim} \normalsize\end{figure}

ClassAd expressions look very much like expressions in C, and are composed of literals and attribute references composed with operators and functions. The difference between ClassAd expressions and C expressions arise from the fact that ClassAd expressions operate in a much more dynamic environment. For example, an expression from a machine's ClassAd may refer to an attribute in a job's ClassAd, such as TARGET.Owner in the above example. The value and type of the attribute is not known until the expression is evaluated in an environment which pairs a specific job ClassAd with the machine ClassAd.

ClassAd expressions handle these uncertainties by defining all operators to be total operators, which means that they have well defined behavior regardless of supplied operands. This functionality is provided through two distinguished values, UNDEFINED and ERROR, and defining all operators so that they can operate on all possible values in the ClassAd system. For example, the multiplication operator which usually only operates on numbers, has a well defined behavior if supplied with values which are not meaningful to multiply. Thus, the expression 10 * "A string" evaluates to the value ERROR. Most operators are strict with respect to ERROR, which means that they evaluate to ERROR if any of their operands are ERROR. Similarly, most operators are strict with respect to UNDEFINED.


4.1.1 ClassAds: Old and New

ClassAds have existed for quite some time in two forms: Old and New. Old ClassAds were the original form and were used in HTCondor until HTCondor version 7.5.0. They were heavily tied to the HTCondor development libraries. New ClassAds added new features and were designed as a stand-alone library that could be used apart from HTCondor.

In HTCondor version 7.5.1, HTCondor switched the internal usage of ClassAds from Old to New. All user interaction with tools (such as condor_q) as well as output of tools is still done as Old ClassAds. Before HTCondor version 7.5.1, New ClassAds were used in just a few places within HTCondor, for example, in the Job Router. There are some syntax and behavior differences between Old and New ClassAds, all of which will remain invisible to users of HTCondor for this version. A complete description of New ClassAds can be found at http://htcondor.org/classad/classad.html, and in the ClassAd Language Reference Manual found on that web page.

Some of the features of New ClassAds that are not in Old ClassAds are lists, nested ClassAds, time values, and matching groups of ClassAds. HTCondor has avoided using these features, as using them makes it difficult to interact with older versions of HTCondor. But, users can start using them if they do not need to interact with versions of HTCondor older than 7.5.1.

The syntax varies slightly between Old and New ClassAds. Here is an example ClassAd presented in both forms. The Old form: 

Foo = 3
Bar = "ab\"cd\ef"
Moo = Foo =!= Undefined

The New form: 

[
Foo = 3;
Bar = "ab\"cd\\ef";
Moo = Foo isnt Undefined;
]

HTCondor will convert to and from Old ClassAd syntax as needed.

4.1.1.1 New ClassAd Attribute References

Expressions often refer to ClassAd attributes. These attribute references work differently in Old ClassAds as compared with New ClassAds. In New ClassAds, an unscoped reference is looked for only in the local ClassAd. An unscoped reference is an attribute that does not have a MY. or TARGET. prefix. The local ClassAd may be described by an example. Matchmaking uses two ClassAds: the job ClassAd and the machine ClassAd. The job ClassAd is evaluated to see if it is a match for the machine ClassAd. The job ClassAd is the local ClassAd. Therefore, in the Requirements attribute of the job ClassAd, any attribute without the prefix TARGET. is looked up only in the job ClassAd. With New ClassAd evaluation, the use of the prefix MY. is eliminated, as an unscoped reference can only refer to the local ClassAd.

The MY. and TARGET. scoping prefixes only apply when evaluating an expression within the context of two ClassAds. Two examples that exemplify this are matchmaking and machine policy evaluation. When evaluating an expression within the context of a single ClassAd, MY. and TARGET. are not defined. Using them within the context of a single ClassAd will result in a value of Undefined. Two examples that exemplify evaluating an expression within the context of a single ClassAd are during user job policy evaluation, and with the -constraint option to command-line tools.

New ClassAds have no CurrentTime attribute. If needed, use the time() function instead. In order to mimic Old ClassAd semantics in this HTCondor version 7.5.1 release, all ClassAds have an explicit CurrentTime attribute, with a value of time().

In current versions of HTCondor, New ClassAds will mimic the evaluation behavior of Old ClassAds. No configuration variables or submit description file contents should need to be changed. To eliminate this behavior and use only the semantics of New ClassAds, set the configuration variable STRICT_CLASSAD_EVALUATION to True. This permits testing expressions to see if any adjustment is required, before a future version of HTCondor potentially makes New ClassAds evaluation behavior the default or the only option.


4.1.2 Old ClassAd Syntax

ClassAd expressions are formed by composing literals, attribute references and other sub-expressions with operators and functions.


4.1.2.1 Literals

Literals in the ClassAd language may be of integer, real, string, undefined or error types. The syntax of these literals is as follows:
Integer
A sequence of continuous digits (i.e., [0-9]). Additionally, the keywords TRUE and FALSE (case insensitive) are syntactic representations of the integers 1 and 0 respectively.

Real
Two sequences of continuous digits separated by a period (i.e., [0-9]+.[0-9]+).

String
A double quote character, followed by an list of characters terminated by a double quote character. A backslash character inside the string causes the following character to be considered as part of the string, irrespective of what that character is.

Undefined
The keyword UNDEFINED (case insensitive) represents the UNDEFINED value.

Error
The keyword ERROR (case insensitive) represents the ERROR value.


4.1.2.2 Attributes

Every expression in a ClassAd is named by an attribute name. Together, the (name,expression) pair is called an attribute. An attribute may be referred to in other expressions through its attribute name.

Attribute names are sequences of alphabetic characters, digits and underscores, and may not begin with a digit. All characters in the name are significant, but case is not significant. Thus, Memory, memory and MeMoRy all refer to the same attribute.

An attribute reference consists of the name of the attribute being referenced, and an optional scope resolution prefix. The prefixes that may be used are MY. and TARGET.. The case used for these prefixes is not significant. The semantics of supplying a prefix are discussed in Section 4.1.3.


4.1.2.3 Operators

The operators that may be used in ClassAd expressions are similar to those available in C. The available operators and their relative precedence is shown in figure 4.2.
Figure 4.2: Relative precedence of ClassAd expression operators
\begin{figure}\begin{verbatim}- (unary negation) (high precedence) * / + -... ... >= > == != =?= =!= && \vert\vert (low precedence)\end{verbatim} \end{figure}
The operator with the highest precedence is the unary minus operator. The only operators which are unfamiliar are the =?= and =!= operators, which are discussed in Section 4.1.3.


4.1.2.4 Predefined Functions

Any ClassAd expression may utilize predefined functions. Function names are case insensitive. Parameters to functions and a return value from a function may be typed (as given) or not. Nested or recursive function calls are allowed.

Here are descriptions of each of these predefined functions. The possible types are the same as itemized in Section 4.1.2. Where the type may be any of these literal types, it is called out as AnyType. Where the type is Integer, but only returns the value 1 or 0 (implying True or False), it is called out as Boolean. The format of each function is given as 

ReturnType FunctionName(ParameterType parameter1, ParameterType parameter2, ...)
Optional parameters are given within square brackets.

AnyType eval(AnyType Expr)
Evaluates Expr as a string and then returns the result of evaluating the contents of the string as a ClassAd expression. This is useful when referring to an attribute such as slotX_State where X, the desired slot number is an expression, such as SlotID+10. In such a case, if attribute SlotID is 5, the value of the attribute slot15_State can be referenced using the expression eval(strcat("slot", SlotID+10,"_State")). Function strcat() calls function string() on the second parameter, which evaluates the expression, and then converts the integer result 15 to the string "15". The concatenated string returned by strcat() is "slot15_State", and this string is then evaluated.

Note that referring to attributes of a job from within the string passed to eval() in the Requirements or Rank expressions could cause inaccuracies in HTCondor's automatic auto-clustering of jobs into equivalent groups for matchmaking purposes. This is because HTCondor needs to determine which ClassAd attributes are significant for matchmaking purposes, and indirect references from within the string passed to eval() will not be counted.

String unparse(Attribute attr)
This function looks up the value of the provided attribute and returns the unparsed version as a string. The attribute's value is not evaluated. If the attribute's value is x + 3, then the function would return the string "x + 3". If the provided attribute cannot be found, an empty string is returned.

This function returns ERROR if other than exactly 1 argument is given or the argument is not an attribute reference.

AnyType ifThenElse(AnyType IfExpr,AnyType ThenExpr, AnyType ElseExpr)
A conditional expression is described by IfExpr. The following defines return values, when IfExpr evaluates to Where IfExpr evaluates to give a value of type String, the function returns the value ERROR. The implementation uses lazy evaluation, so expressions are only evaluated as defined.

This function returns ERROR if other than exactly 3 arguments are given.

Boolean isUndefined(AnyType Expr)
Returns True, if Expr evaluates to UNDEFINED. Returns False in all other cases.

This function returns ERROR if other than exactly 1 argument is given.

Boolean isError(AnyType Expr)
Returns True, if Expr evaluates to ERROR. Returns False in all other cases.

This function returns ERROR if other than exactly 1 argument is given.

Boolean isString(AnyType Expr)
Returns True, if the evaluation of Expr gives a value of type String. Returns False in all other cases.

This function returns ERROR if other than exactly 1 argument is given.

Boolean isInteger(AnyType Expr)
Returns True, if the evaluation of Expr gives a value of type Integer. Returns False in all other cases.

This function returns ERROR if other than exactly 1 argument is given.

Boolean isReal(AnyType Expr)
Returns True, if the evaluation of Expr gives a value of type Real. Returns False in all other cases.

This function returns ERROR if other than exactly 1 argument is given.

Boolean isBoolean(AnyType Expr)
Returns True, if the evaluation of Expr gives the integer value 0 or 1. Returns False in all other cases.

This function returns ERROR if other than exactly 1 argument is given.

Integer int(AnyType Expr)
Returns the integer value as defined by Expr. Where the type of the evaluated Expr is Real, the value is truncated (round towards zero) to an integer. Where the type of the evaluated Expr is String, the string is converted to an integer using a C-like atoi() function. When this result is not an integer, ERROR is returned. Where the evaluated Expr is ERROR or UNDEFINED, ERROR is returned.

This function returns ERROR if other than exactly 1 argument is given.

Real real(AnyType Expr)
Returns the real value as defined by Expr. Where the type of the evaluated Expr is Integer, the return value is the converted integer. Where the type of the evaluated Expr is String, the string is converted to a real value using a C-like atof() function. When this result is not a real, ERROR is returned. Where the evaluated Expr is ERROR or UNDEFINED, ERROR is returned.

This function returns ERROR if other than exactly 1 argument is given.

String string(AnyType Expr)
Returns the string that results from the evaluation of Expr. Converts a non-string value to a string. Where the evaluated Expr is ERROR or UNDEFINED, ERROR is returned.

This function returns ERROR if other than exactly 1 argument is given.

Integer floor(AnyType Expr)
Returns the integer that results from the evaluation of Expr, where the type of the evaluated Expr is Integer. Where the type of the evaluated Expr is not Integer, function real(Expr) is called. Its return value is then used to return the largest magnitude integer that is not larger than the returned value. Where real(Expr) returns ERROR or UNDEFINED, ERROR is returned.

This function returns ERROR if other than exactly 1 argument is given.

Integer ceiling(AnyType Expr)
Returns the integer that results from the evaluation of Expr, where the type of the evaluated Expr is Integer. Where the type of the evaluated Expr is not Integer, function real(Expr) is called. Its return value is then used to return the smallest magnitude integer that is not less than the returned value. Where real(Expr) returns ERROR or UNDEFINED, ERROR is returned.

This function returns ERROR if other than exactly 1 argument is given.

Integer pow(Integer base, Integer exponent)
OR Real pow(Integer base, Integer exponent)
OR Real pow(Real base, Real exponent)
Calculates base raised to the power of exponent. If exponent is an integer value greater than or equal to 0, and base is an integer, then an integer value is returned. If exponent is an integer value less than 0, or if either base or exponent is a real, then a real value is returned. An invocation with exponent=0 or exponent=0.0, for any value of base, including 0 or 0.0, returns the value 1 or 1.0, type appropriate.

Integer quantize(AnyType a, Integer b)
OR Real quantize(AnyType a, Real b)
OR AnyType quantize(AnyType a, AnyType list b)
quantize() computes the quotient of a/b, in order to further compute ceiling(quotient) * b. This computes and returns an integral multiple of b that is at least as large as a. So, when b >= a, the return value will be b. The return type is the same as that of b, where b is an Integer or Real.

When b is a list, quantize() returns the first value in the list that is greater than or equal to a. When no value in the list is greater than or equal to a, this computes and returns an integral multiple of the last member in the list that is at least as large as a.

This function returns ERROR if a or b, or a member of the list that must be considered is not an Integer or Real.

Here are examples: 

     8     = quantize(3, 8)
     4     = quantize(3, 2)
     0     = quantize(0, 4)
     6.8   = quantize(1.5, 6.8)
     7.2   = quantize(6.8, 1.2)
     10.2  = quantize(10, 5.1)

     4     = quantize(0, {4})
     2     = quantize(2, {1, 2, "A"})
     3.0   = quantize(3, {1, 2, 0.5})
     3.0   = quantize(2.7, {1, 2, 0.5})
     ERROR = quantize(3, {1, 2, "A"})

Integer round(AnyType Expr)
Returns the integer that results from the evaluation of Expr, where the type of the evaluated Expr is Integer. Where the type of the evaluated Expr is not Integer, function real(Expr) is called. Its return value is then used to return the integer that results from a round-to-nearest rounding method. The nearest integer value to the return value is returned, except in the case of the value at the exact midpoint between two integer values. In this case, the even valued integer is returned. Where real(Expr) returns ERROR or UNDEFINED, or the integer value does not fit into 32 bits, ERROR is returned.

This function returns ERROR if other than exactly 1 argument is given.

Integer random([ AnyType Expr ])
Where the optional argument Expr evaluates to type Integer or type Real (and called x), the return value is the integer or real r randomly chosen from the interval 0 <= r < x. With no argument, the return value is chosen with random(1.0). Returns ERROR in all other cases.

This function returns ERROR if greater than 1 argument is given.

String strcat(AnyType Expr1 [ , AnyType Expr2 ... ])
Returns the string which is the concatenation of all arguments, where all arguments are converted to type String by function string(Expr). Returns ERROR if any argument evaluates to UNDEFINED or ERROR.

String substr(String s, Integer offset [ , Integer length ])
Returns the substring of s, from the position indicated by offset, with (optional) length characters. The first character within s is at offset 0. If the optional length argument is not present, the substring extends to the end of the string. If offset is negative, the value (length - offset) is used for the offset. If length is negative, an initial substring is computed, from the offset to the end of the string. Then, the absolute value of length characters are deleted from the right end of the initial substring. Further, where characters of this resulting substring lie outside the original string, the part that lies within the original string is returned. If the substring lies completely outside of the original string, the null string is returned.

This function returns ERROR if greater than 3 or less than 2 arguments are given.

Integer strcmp(AnyType Expr1, AnyType Expr2)
Both arguments are converted to type String by function string(Expr). The return value is an integer that will be Case is significant in the comparison. Where either argument evaluates to ERROR or UNDEFINED, ERROR is returned.

This function returns ERROR if other than 2 arguments are given.

Integer stricmp(AnyType Expr1, AnyType Expr2)
This function is the same as strcmp, except that letter case is not significant.

String toUpper(AnyType Expr)
The single argument is converted to type String by function string(Expr). The return value is this string, with all lower case letters converted to upper case. If the argument evaluates to ERROR or UNDEFINED, ERROR is returned.

This function returns ERROR if other than exactly 1 argument is given.

String toLower(AnyType Expr)
The single argument is converted to type String by function string(Expr). The return value is this string, with all upper case letters converted to lower case. If the argument evaluates to ERROR or UNDEFINED, ERROR is returned.

This function returns ERROR if other than exactly 1 argument is given.

Integer size(AnyType Expr)
Returns the number of characters in the string, after calling function string(Expr). If the argument evaluates to ERROR or UNDEFINED, ERROR is returned.

This function returns ERROR if other than exactly 1 argument is given.

List split(String s [ , String tokens ] )
Returns a list of the substrings of s that have been split up by using any of the characters within string tokens. If tokens is not specified, then all white space characters are used to delimit the string.

List splitUserName(String Name)
Returns a list of two strings. Where Name includes an @ character, the first string in the list will be the substring that comes before the @ character, and the second string in the list will be the substring that comes after. Thus, if Name is "user@domain", then the returned list will be {"user", "domain"}. If there is no @ character in Name, then the first string in the list will be Name, and the second string in the list will be the empty string. Thus, if Name is "username", then the returned list will be {"username", ""}.

List splitSlotName(String Name)
Returns a list of two strings. Where Name includes an @ character, the first string in the list will be the substring that comes before the @ character, and the second string in the list will be the substring that comes after. Thus, if Name is "slot1@machine", then the returned list will be {"slot1", "machine"}. If there is no @ character in Name, then the first string in the list will be the empty string, and the second string in the list will be Name, Thus, if Name is "machinename", then the returned list will be {"", "machinename"}.

Integer time()
Returns the current coordinated universal time, which is the same as the ClassAd attribute CurrentTime. This is the time, in seconds, since midnight of January 1, 1970.

String formatTime([ Integer time ] [ , String format ])

Returns a formatted string that is a representation of time. The argument time is interpreted as coordinated universal time in seconds, since midnight of January 1, 1970. If not specified, time will default to the value of attribute CurrentTime.

The argument format is interpreted similarly to the format argument of the ANSI C strftime function. It consists of arbitrary text plus placeholders for elements of the time. These placeholders are percent signs (%) followed by a single letter. To have a percent sign in the output, use a double percent sign (%%). If format is not specified, it defaults to %c.

Because the implementation uses strftime() to implement this, and some versions implement extra, non-ANSI C options, the exact options available to an implementation may vary. An implementation is only required to implement the ANSI C options, which are:

%a
abbreviated weekday name
%A
full weekday name
%b
abbreviated month name
%B
full month name
%c
local date and time representation
%d
day of the month (01-31)
%H
hour in the 24-hour clock (0-23)
%I
hour in the 12-hour clock (01-12)
%j
day of the year (001-366)
%m
month (01-12)
%M
minute (00-59)
%p
local equivalent of AM or PM
%S
second (00-59)
%U
week number of the year (Sunday as first day of week) (00-53)
%w
weekday (0-6, Sunday is 0)
%W
week number of the year (Monday as first day of week) (00-53)
%x
local date representation
%X
local time representation
%y
year without century (00-99)
%Y
year with century
%Z
time zone name, if any

String interval(Integer seconds)
Uses seconds to return a string of the form days+hh:mm:ss. This represents an interval of time. Leading values that are zero are omitted from the string. For example, seconds of 67 becomes "1:07". A second example, seconds of 1472523 = 17*24*60*60 + 1*60*60 + 2*60 + 3, results in the string "17+1:02:03".

AnyType debug(AnyType expression)
This function evaluates its argument, and it returns the result. Thus, it is a no-operation. However, a side-effect of the function is that information about the evaluation is logged to the evaluating program's log file, at the D_FULLDEBUG debug level. This is useful for determining why a given ClassAd expression is evaluating the way it does. For example, if a condor_startd START expression is unexpectedly evaluating to UNDEFINED, then wrapping the expression in this debug() function will log information about each component of the expression to the log file, making it easier to understand the expression.

String envV1ToV2(String old_env)
This function converts a set of environment variables from the old HTCondor syntax to the new syntax. The single argument should evaluate to a string that represents a set of environment variables using the old HTCondor syntax (usually stored in the job ClassAd attribute Env). The result is the same set of environment variables using the new HTCondor syntax (usually stored in the job ClassAd attribute Environment). If the argument evaluates to UNDEFINED, then the result is also UNDEFINED.

String mergeEnvironment(String env1 [ , String env2, ... ])
This function merges multiple sets of environment variables into a single set. If multiple arguments include the same variable, the one that appears last in the argument list is used. Each argument should evaluate to a string which represents a set of environment variables using the new HTCondor syntax or UNDEFINED, which is treated like an empty string. The result is a string that represents the merged set of environment variables using the new HTCondor syntax (suitable for use as the value of the job ClassAd attribute Environment).

For the following functions, a delimiter is represented by a string. Each character within the delimiter string delimits individual strings within a list of strings that is given by a single string. The default delimiter contains the comma and space characters. A string within the list is ended (delimited) by one or more characters within the delimiter string.

Integer stringListSize(String list [ , String delimiter ])
Returns the number of elements in the string list, as delimited by the optional delimiter string. Returns ERROR if either argument is not a string.

This function returns ERROR if other than 1 or 2 arguments are given.

Integer stringListSum(String list [ , String delimiter ])
OR Real stringListSum(String list [ , String delimiter ])
Sums and returns the sum of all items in the string list, as delimited by the optional delimiter string. If all items in the list are integers, the return value is also an integer. If any item in the list is a real value (noninteger), the return value is a real. If any item does not represent an integer or real value, the return value is ERROR.

Real stringListAvg(String list [ , String delimiter ])
Sums and returns the real-valued average of all items in the string list, as delimited by the optional delimiter string. If any item does not represent an integer or real value, the return value is ERROR. A list with 0 items (the empty list) returns the value 0.0.

Integer stringListMin(String list [ , String delimiter ])
OR Real stringListMin(String list [ , String delimiter ])
Finds and returns the minimum value from all items in the string list, as delimited by the optional delimiter string. If all items in the list are integers, the return value is also an integer. If any item in the list is a real value (noninteger), the return value is a real. If any item does not represent an integer or real value, the return value is ERROR. A list with 0 items (the empty list) returns the value UNDEFINED.

Integer stringListMax(String list [ , String delimiter ])
OR Real stringListMax(String list [ , String delimiter ])
Finds and returns the maximum value from all items in the string list, as delimited by the optional delimiter string. If all items in the list are integers, the return value is also an integer. If any item in the list is a real value (noninteger), the return value is a real. If any item does not represent an integer or real value, the return value is ERROR. A list with 0 items (the empty list) returns the value UNDEFINED.

Boolean stringListMember(String x, String list [ , String delimiter ])
Returns TRUE if item x is in the string list, as delimited by the optional delimiter string. Returns FALSE if item x is not in the string list. Comparison is done with strcmp(). The return value is ERROR, if any of the arguments are not strings.

Boolean stringListIMember(String x, String list [ , String delimiter ])
Same as stringListMember(), but comparison is done with stricmp(), so letter case is not relevant.

Integer stringListsIntersect(String list1, String list2 [ , String delimiter ])
Returns TRUE if the lists contain any matching elements, and returns FALSE if the lists do not contain any matching elements. Returns ERROR if either argument is not a string or if an incorrect number of arguments are given.

The following three functions utilize regular expressions as defined and supported by the PCRE library. See http://www.pcre.org for complete documentation of regular expressions.

The options argument to these functions is a string of special characters that modify the use of the regular expressions. Inclusion of characters other than these as options are ignored.

I or i
Ignore letter case.
M or m
Modifies the interpretation of the caret (^) and dollar sign ($) characters. The caret character matches the start of a string, as well as after each newline character. The dollar sign character matches before a newline character.
S or s
The period matches any character, including the newline character.

Boolean regexp(String pattern, String target [ , String options ])
Uses the description of a regular expression given by string pattern to scan through the string target. Returns TRUE when target is a regular expression as described by pattern. Returns FALSE otherwise. If any argument is not a string, or if pattern does not describe a valid regular expression, returns ERROR.

String regexps
(String pattern, String target, String substitute [ , String options ]) Uses the description of a regular expression given by string pattern to scan through the string target. When target is a regular expression as described by pattern, the string substitute is returned, with backslash expansion performed. If any argument is not a string, returns ERROR.

Boolean stringList_regexpMember
(String pattern, String list [ , String delimiter ] [ , String options ]) Uses the description of a regular expression given by string pattern to scan through the list of strings in list. Returns TRUE when one of the strings in list is a regular expression as described by pattern. The optional delimiter describes how the list is delimited, and string options modifies how the match is performed. Returns FALSE if pattern does not match any entries in list. The return value is ERROR, if any of the arguments are not strings, or if pattern is not a valid regular expression.


4.1.3 Old ClassAd Evaluation Semantics

The ClassAd mechanism's primary purpose is for matching entities that supply constraints on candidate matches. The mechanism is therefore defined to carry out expression evaluations in the context of two ClassAds that are testing each other for a potential match. For example, the condor_negotiator evaluates the Requirements expressions of machine and job ClassAds to test if they can be matched. The semantics of evaluating such constraints is defined below.

4.1.3.1 Literals

Literals are self-evaluating, Thus, integer, string, real, undefined and error values evaluate to themselves.


4.1.3.2 Attribute References

Since the expression evaluation is being carried out in the context of two ClassAds, there is a potential for name space ambiguities. The following rules define the semantics of attribute references made by ClassAd $A$ that is being evaluated in a context with another ClassAd $B$:
  1. If the reference is prefixed by a scope resolution prefix,

  2. If the reference is not prefixed by a scope resolution prefix,

  3. Finally, if the reference refers to an expression that is itself in the process of being evaluated, there is a circular dependency in the evaluation. The value of the reference is ERROR.


4.1.3.3 Operators

All operators in the ClassAd language are total, and thus have well defined behavior regardless of the supplied operands. Furthermore, most operators are strict with respect to ERROR and UNDEFINED, and thus evaluate to ERROR or UNDEFINED if either of their operands have these exceptional values.


4.1.3.4 Expression Examples

The =?= operator is similar to the == operator. It checks if the left hand side operand is identical in both type and value to the the right hand side operand, returning TRUE when they are identical. A key point in understanding is that the =?= operator only produces evaluation results of TRUE and FALSE, where the == operator may produce evaluation results TRUE, FALSE, UNDEFINED, or ERROR. Table 4.1 presents examples that define the outcome of the == operator. Table 4.2 presents examples that define the outcome of the =?= operator.


Table 4.1: Evaluation examples for the == operator
expression evaluated result
(10 == 10) TRUE
(10 == 5) FALSE
(10 == "ABC") ERROR
(10 == UNDEFINED) UNDEFINED
(UNDEFINED == UNDEFINED) UNDEFINED



Table 4.2: Evaluation examples for the =?= operator
expression evaluated result
(10 =?= 10) TRUE
(10 =?= 5) FALSE
(10 =?= "ABC") FALSE
(10 =?= UNDEFINED) FALSE
(UNDEFINED =?= UNDEFINED) TRUE


The =!= operator is similar to the != operator. It checks if the left hand side operand is not identical in both type and value to the the right hand side operand, returning FALSE when they are identical. A key point in understanding is that the =!= operator only produces evaluation results of TRUE and FALSE, where the != operator may produce evaluation results TRUE, FALSE, UNDEFINED, or ERROR. Table 4.3 presents examples that define the outcome of the != operator. Table 4.4 presents examples that define the outcome of the =!= operator.


Table 4.3: Evaluation examples for the != operator
expression evaluated result
(10 != 10) FALSE
(10 != 5) TRUE
(10 != "ABC") ERROR
(10 != UNDEFINED) UNDEFINED
(UNDEFINED != UNDEFINED) UNDEFINED



Table 4.4: Evaluation examples for the =!= operator
expression evaluated result
(10 =!= 10) FALSE
(10 =!= 5) TRUE
(10 =!= "ABC") TRUE
(10 =!= UNDEFINED) TRUE
(UNDEFINED =!= UNDEFINED) FALSE


4.1.4 Old ClassAds in the HTCondor System

The simplicity and flexibility of ClassAds is heavily exploited in the HTCondor system. ClassAds are not only used to represent machines and jobs in the HTCondor pool, but also other entities that exist in the pool such as checkpoint servers, submitters of jobs and master daemons. Since arbitrary expressions may be supplied and evaluated over these ClassAds, users have a uniform and powerful mechanism to specify constraints over these ClassAds. These constraints can take the form of Requirements expressions in resource and job ClassAds, or queries over other ClassAds.


4.1.4.1 Constraints and Preferences

The requirements and rank expressions within the submit description file are the mechanism by which users specify the constraints and preferences of jobs. For machines, the configuration determines both constraints and preferences of the machines.

For both machine and job, the rank expression specifies the desirability of the match (where higher numbers mean better matches). For example, a job ClassAd may contain the following expressions: 

Requirements = (Arch == "INTEL") && (OpSys == "LINUX")
Rank         = TARGET.Memory + TARGET.Mips
In this case, the job requires a 32-bit Intel processor running a Linux operating system. Among all such computers, the customer prefers those with large physical memories and high MIPS ratings. Since the Rank is a user-specified metric, any expression may be used to specify the perceived desirability of the match. The condor_negotiator daemon runs algorithms to deliver the best resource (as defined by the rank expression), while satisfying other required criteria.

Similarly, the machine may place constraints and preferences on the jobs that it will run by setting the machine's configuration. For example, 

    Friend        = Owner == "tannenba" || Owner == "wright"
    ResearchGroup = Owner == "jbasney" || Owner == "raman"
    Trusted       = Owner != "rival" && Owner != "riffraff"
    START         = Trusted && ( ResearchGroup || LoadAvg < 0.3 &&
                         KeyboardIdle > 15*60 )
    RANK          = Friend + ResearchGroup*10

The above policy states that the computer will never run jobs owned by users rival and riffraff, while the computer will always run a job submitted by members of the research group. Furthermore, jobs submitted by friends are preferred to other foreign jobs, and jobs submitted by the research group are preferred to jobs submitted by friends.

Note: Because of the dynamic nature of ClassAd expressions, there is no a priori notion of an integer-valued expression, a real-valued expression, etc. However, it is intuitive to think of the Requirements and Rank expressions as integer-valued and real-valued expressions, respectively. If the actual type of the expression is not of the expected type, the value is assumed to be zero.


4.1.4.2 Querying with ClassAd Expressions

The flexibility of this system may also be used when querying ClassAds through the condor_status and condor_q tools which allow users to supply ClassAd constraint expressions from the command line.

Needed syntax is different on Unix and Windows platforms, due to the interpretation of characters in forming command-line arguments. The expression must be a single command-line argument, and the resulting examples differ for the platforms. For Unix shells, single quote marks are used to delimit a single argument. For a Windows command window, double quote marks are used to delimit a single argument. Within the argument, Unix escapes the double quote mark by prepending a backslash to the double quote mark. Windows escapes the double quote mark by prepending another double quote mark. There may not be spaces in between.

Here are several examples. To find all computers which have had their keyboards idle for more than 60 minutes and have more than 4000 MB of memory, the desired ClassAd expression is 

KeyboardIdle > 60*60 && Memory > 4000
On a Unix platform, the command appears as 
% condor_status -const 'KeyboardIdle > 60*60 && Memory > 4000'

Name               OpSys   Arch   State     Activity LoadAv Mem  ActvtyTime
100
slot1@altair.cs.wi LINUX   X86_64 Owner     Idle     0.000 8018 13+00:31:46
slot2@altair.cs.wi LINUX   X86_64 Owner     Idle     0.000 8018 13+00:31:47
...
...
slot1@athena.stat. LINUX   X86_64 Unclaimed Idle     0.000 7946  0+00:25:04
slot2@athena.stat. LINUX   X86_64 Unclaimed Idle     0.000 7946  0+00:25:05
...
...

The Windows equivalent command is 

>condor_status -const "KeyboardIdle > 60*60 && Memory > 4000"

Here is an example for a Unix platform that utilizes a regular expression ClassAd function to list specific information. A file contains ClassAd information. condor_advertise is used to inject this information, and condor_status constrains the search with an expression that contains a ClassAd function. 

% cat ad
MyType = "Generic"
FauxType = "DBMS"
Name = "random-test"
Machine = "f05.cs.wisc.edu"
MyAddress = "<128.105.149.105:34000>"
DaemonStartTime = 1153192799
UpdateSequenceNumber = 1

% condor_advertise UPDATE_AD_GENERIC ad

% condor_status -any -constraint 'FauxType=="DBMS" && 
  regexp("random.*", Name, "i")'

MyType               TargetType           Name                          

Generic              None                 random-test

The ClassAd expression describing a machine that advertises a Windows operating system: 

OpSys == "WINDOWS"
Here are three equivalent ways on a Unix platform to list all machines advertising a Windows operating system. Spaces appear in these examples to show where they are permitted. 
% condor_status -constraint ' OpSys == "WINDOWS"  '

% condor_status -constraint OpSys==\"WINDOWS\"

% condor_status -constraint "OpSys==\"WINDOWS\""

The equivalent command on a Windows platform to list all machines advertising a Windows operating system must delimit the single argument with double quote marks, and then escape the needed double quote marks that identify the string within the expression. Spaces appear in this example where they are permitted. 

>condor_status -constraint " OpSys == ""WINDOWS"" "


4.1.5 Extending ClassAds with User-written Functions

The ClassAd language provides a rich set of functions. It is possible to add new functions to the ClassAd language without recompiling the HTCondor system or the ClassAd library. This requires implementing the new function in the C++ programming language, compiling the code into a shared library, and telling HTCondor where in the file system the shared library lives.

While the details of the ClassAd implementation are beyond the scope of this document, the ClassAd source distribution ships with an example source file that extends ClassAds by adding two new functions, named todays_date() and double(). This can be used as a model for users to implement their own functions. To deploy this example extension, follow the following steps on Linux:

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