Reference: Sorting and Filtering

This section supplements the reference documentation for the projects.alertPolicies and projects.notificationChannels resources in the Stackdriver Monitoring API. It provides syntactic details on the filter and orderBy parameters used by the API in these list methods:

Sorting and Filtering Basics

Support for sorting and filtering in a list operation is indicated by the presence of filter and orderBy string fields in the list request body (see the API reference documentation to see if the request body has these fields). Both fields use a simple language for referring to the fields in the object that is being sorted or filtered.

Sort-order syntax

The orderBy field consists of a comma-separated list of field paths that are optionally prefixed with a minus sign to reverse the order.

For example, user_label.team,display_name orders by team name in ascending order, and then -- for entries with the same team -- orders by the display name, also in ascending order. If you add a minus sign in front of one of the comma-separated fields, then that field is sorted in descending order; for example, if you are trying to cut down on the verbosity of your titles, you could use -display_name.size to sort by the title length, with objects ordered from the most wordy title to the briefest title.

To provide a more realistic example, user_label.team,display_name groups results first by team (in lexicographic order) and then, within each team grouping, organizes results by the title (also in lexicographic order).

Formally, the syntax can be described as:

   ORDER_BY_SPEC :=
      # Comma-separated list of fields.
      ORDERED_FIELD_LIST

   ORDERED_FIELD_LIST :=
      # Single field on which to sort.
      ORDERED_FIELD

      # Sort by the first field and then by the remaining fields.
      | ORDERED_FIELD "," ORDERED_FIELD_LIST

   ORDERED_FIELD :=
      # Sort by the field in ascending order.
      FIELD_REFERENCE

      # Sort by the field in descending order.
      | "-" FIELD_REFERENCE

   FIELD_REFERENCE :=
      # Simple field reference
      FIELD_NAME

      # Map value or list index lookup. For string-valued keys, the
      # supplied key in this notation must be quoted.
      | FIELD_NAME "[" LITERAL "]"

   FIELD_NAME :=
      # Immediate element
      IDENTIFIER

      # Subfield dereference or map element dereference. Note that,
      # in the case of maps, the IDENTIFIER on the left can also
      # be the singular form of the name of the map. That is, it is
      # permitted to use `user_label.mykey` and not just
      # `user_labels.mykey`. This is done for consistency with the
      # metric filters which permit `resource.label.` and `metric.label.`
      # which are in the singular form even though the map is `labels`.
      | IDENTIFIER "." FIELD_NAME

   LITERAL :=
       # Number
       [0-9]+(.[0.9]+)?

       # String literal using single quotes. Note that strings must
       # always be quoted. This is to avoid ambiguity with attempting
       # to refer to the value of a field.
       |  '[^']*'

       # String literal using double quotes. Note that strings must
       # always be quoted. This is to avoid ambiguity with attempting
       # to refer to the value of a field.
       |  "[^"]*"

       # Literal boolean true.
       |  true

       # Literal boolean false.
       |  false

   IDENTIFIER :=
       # Non-digit followed by any number of letters or digits.
       [a-zA-Z_]+[a-zA-Z0-9_]*

Filter syntax

The filter syntax consists of a simple expression language for constructing a predicate from one or more fields of the objects being filtered. Negation, conjunction, and disjunction are written using NOT, AND, and OR keywords. Fields can be compared with literal values by use of : (containment), = (equality), > (greater), < (less than), >= (greater than or equal to), <= (less than or equal to), and != (inequality) operators. Special functions starts_with, ends_with, monitoring.regex.full_match as well as special properties empty and size provide support for more advanced comparisons.

Examples

To return all entries with a non-empty display name or description where the user_labels field has the active key set (with any value):

(NOT display_name.empty OR NOT description.empty) AND user_labels='active'

To return all entries whose description contains 'cloud':

description:'cloud'

To return all entries whose title matches "Temp XXXX":

display_name=monitoring.regex.full_match('Temp \\d{4}')

Formal specification

The filter expression syntax can be summarized as follows:

   FILTER_EXPRESSION :=
         # Negation
         "NOT" FILTER_EXPRESSION

         # Short-circuiting AND
         | FILTER_EXPRESSION "AND" FILTER_EXPRESSION

         # Short-circuiting OR
         | FILTER_EXPRESSION "OR" FILTER_EXPRESSION

         # Implicit short-circuiting AND
         | FILTER_EXPRESSION FILTER_EXPRESSION

         # Parenthesized sub-expression
         | "(" FILTER_EXPRESSION ")"

         # Basic expression
         | SIMPLE_EXPRESSION

   SIMPLE_EXPRESSION :=
         # Field implicitly converted to boolean
         FIELD_REFERENCE

         # Field binary comparison. Note that the right-hand side must
         # be compatible with the type on the left-hand side; one cannot
         # compare a number with a string. Sensible implicit conversions
         # are permitted, however; comparing an integer and double will
         # succeed with appropriate conversion/widening taking place.
         | FIELD_REFERENCE OP LITERAL

         # Function invocation
         | FIELD_REFERENCE "=" FUNCTION_EXPRESSION


   FIELD_REFERENCE :=
         # Simple field reference
         FIELD_NAME

         # Map value or list index lookup. For string-valued keys, the
         # supplied key in this notation must be quoted.
         | FIELD_NAME "[" LITERAL "]"

   FIELD_NAME :=
         # Immediate element
         IDENTIFIER

         # Subfield dereference or map element dereference. Note that,
         # in the case of maps, the IDENTIFIER on the left can also
         # be the singular form of the name of the map. That is, it is
         # permitted to use `user_label.mykey` and not just
         # `user_labels.mykey`. This is done for consistency with the
         # metric filters which permit `resource.label.` and `metric.label.`
         # which are in the singular form even though the map is `labels`.
         | IDENTIFIER "." FIELD_NAME

   OP :=
         # Equality comparison. Should be avoided for double-valued fields.
         "="

         # Less than.
         | "<"

         # Greater than.
         | ">"

         # Less than or equal.
         | "<="

         # Greater than or equal.
         | ">="

         # Containment. This is equivalent to '=' for numeric types.
         | ":"

         # Not equal.
         | "!="

   LITERAL :=
       # Number
       [0-9]+(.[0.9]+)?

       # String literal using single quotes. Note that strings must
       # always be quoted. This is to avoid ambiguity with attempting
       # to refer to the value of a field.
       |  '[^']*'

       # String literal using double quotes. Note that strings must
       # always be quoted. This is to avoid ambiguity with attempting
       # to refer to the value of a field.
       |  "[^"]*"

       # Literal boolean true.
       |  true

       # Literal boolean false.
       |  false

   FUNCTION_EXPRESSION :=
       # Starts with.
       "starts_with" "(" LITERAL ")"

       # Ends with.
       |  "ends_with" "(" LITERAL ")"

       # Regular expression match.
       |  "monitoring.regex.full_match" "(" LITERAL ")"

   IDENTIFIER :=
       # Non-digit followed by any number of letters or digits.
       [a-zA-Z_]+[a-zA-Z0-9_]*

Supported fields

The fields which may be referenced in filter or orderBy depends on the type of object that is being listed.

AlertPolicy

The following fields may be referenced in filter and orderBy when enumerating AlertPolicy objects:

  • name
  • display_name
  • documentation.content
  • documentation.mime_type
  • user_labels
  • conditions.size
  • combiner
  • disabled
  • notification_channels

NotificationChannel

The following fields may be referenced in filter and orderBy when enumerating NotificationChannel objects:

  • name
  • type
  • display_name
  • description
  • labels
  • user_labels

Advanced topics

Field casing

Field names may be expressed in both lower_case_with_underscores and in camelCase forms. That is, both display_name and displayName are supported.

Strings

Special properties

String-valued fields automatically have a size property generated that computes the number of Unicode characters in the string. This enables, for examples, a filter such as display_name.size > 3 AND display_name.size < 10. In addition to size, strings also have a bool empty property.

Sort order

When listing a string in orderBy, the strings are compared using byte-wise lexicographic ordering in the UTF-8 representation of the string; the strings are not sorted according to Unicode collation order.

Implicit bool conversion

It is possible to implicitly convert a string to bool in a filter as in user_label.enabled. Note that this conversion is not identical to testing that the string is non-empty; under this conversion, the contents of the string are parsed to a bool and strings that unambiguously parse to a boolean take on that boolean value; if the string is not obviously boolean-valued, then non-empty strings are interpreted as true and empty strings are interpreted as false.

Strings that case-insensitively match "false", "f", "no", "n", or "0" are considered to be unambiguously false; strings that case-insensitively match "true", "t", "yes", "y", or "1" are considered to be unambiguously true.

Lists

Special properties

List-valued fields automatically have a size property generated that computes the number of elements in that list. For example, you can use notification_channels.size in orderBy to sort alert policies by the number of channels that they notify.

Use in binary comparisons

When comparing a list-valued field to a literal with one of the various binary operators, the comparison is interpreted as applying an element-wise comparison and then computing the OR of the results. For example, the filter notification_channels:"123" will evaluate to true if any of the notification channels has "123" as a substring. For the != operator, specifically, the element-wise comparison is ANDed rather than ORed; in other words, for !=, none of the elements are permitted to match. Or, to restate, x!=y is logically equivalent to the pseudo-code x[0]!=y AND x[1]!=y AND ... AND x[x.size-1]!=y whereas x=y is logically equivalent to the pseudo-code x[0]=y OR x[1]=y OR ... OR x[x.size-1]=y. This inconsistency is designed to address the likely intent of x!=y and prevent confusion with such an expression returning results that contain the forbidden/filtered value.

In addition to restricting on the list as a whole, it is also possible to refer to specific list entries via the indexing ([]) operator. For example, you can use notification_channels[0]:"123" to test just the first element. A default (empty, zero, etc.) is generated in the case of out-of-bounds indexes.

Implicit conversion to bool

When specified in a filter without a binary comparison, a list will be implicitly converted to boolean. This conversion is distinct from testing that the list is non-empty; a_list and NOT a_list.empty return different results for {false, false, false} or any other non-empty list whose values all are or implicitly convert to the boolean value false.

Maps

Special properties

Map-valued fields automatically have a size property generated that computes the number of elements in that map. For example, you can use user_labels.size in orderBy to sort by number of user labels defined. Similarly, a boolean-valued empty property is also auto-generated.

Testing for key existence

Maps may be compared with literal values with the various supported binary operators. The interpretation is logically equivalent to projecting the map into a list by extracting the map's keys and then executing the comparison. To give an example, you can use user_labels="phase" to determine if the user_labels map contains a key whose value is equal to "phase".

Referencing values by key

Map entries can be referenced using one of two notations: dot (.) notation and index ([]) notation. For example, you can use user_labels.team or user_labels["team"] to refer to the value corresponding to the key "team" in the user_labels field. For consistency with the metrics API which uses metric.label. and resource.label. prefixes rather than metric.labels. and resource.labels., map fields may also be referenced using the singular form of the name (i.e. user_label.team is also permitted). Note that, for keys named size or empty, you must use index notation; using dot notation will refer to the special, auto-generated map properties.

Implicit conversion to bool

In implicit conversions to bool, a map is considered to be true if it is non-empty and at least one map value implicitly converts to true. This is not the same as testing that the map is non-empty.

Ordering by compound types

All fields that can be referenced in a filter may be referenced in order_by, as well. This also applies to complex and compound types. The sort order for these types is stable and well-defined, though not necessarily intuitive; therefore, this usage is discouraged but allowed.

Lists

Lists are compared using an element-wise, lexicographic comparison, with the smaller list coming first in the case of their common elements being equal. To give an example, {0, 1} is less than {0, 2} but greater than {0}.

Maps

Maps are compared by performing an element-wise comparison of the map values corresponding to the union of their keys. For keys defined in one map but not the other, a default (empty, zero, etc.) value is used for the comparison. To give an example, {"x":0, "y":0} is less than {"x":1, "y":1} but greater than {"a":-1} and equal to {}.

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