Struct etag::EntityTag

pub struct EntityTag {
    pub weak: bool,
    // some fields omitted
}

An entity tag, defined in RFC7232

The ETag HTTP response header is an identifier for a specific version of a resource. It allows caches to be more efficient, and saves bandwidth, as a web server does not need to send a full response if the content has not changed. On the other side, if the content has changed, etags are useful to help prevent simultaneous updates of a resource from overwriting each other (“mid-air collisions”).

If the resource at a given URL changes, a new Etag value must be generated. Etags are therefore similar to fingerprints and might also be used for tracking purposes by some servers. A comparison of them allows to quickly determine whether two representations of a resource are the same, but they might also be set to persist indefinitely by a tracking server.

Size limit

In order to avoid allocation, ETag size is limited to 62 characters, which should be sufficient for any hashing mechanism.

Format W/"<etag_value>"

• ‘W/’ (case-sensitive) indicates that a weak validator is used. Weak validators are easy to generate but are far less useful for comparisons. Strong validators are ideal for comparisons but can be very difficult to generate efficiently. Weak Etag values of two representations of the same resources might be semantically equivalent, but not byte-for-byte identical.

• “<etag_value>” Entity tags uniquely representing the requested resources. They are a string of ASCII characters placed between double quotes (Like “675af34563dc-tr34”). The method by which ETag values are generated is not specified. Oftentimes, a hash of the content, a hash of the last modification timestamp, or just a revision number is used. For example, MDN uses a hash of hexadecimal digits of the wiki content.

Comparison

To check if two entity tags are equivalent in an application always use the strong_eq or weak_eq methods based on the context of the Tag. Only use == to check if two tags are identical.

The example below shows the results for a set of entity-tag pairs and both the weak and strong comparison function results:

ETag 1	ETag 2	Strong Comparison	Weak Comparison
W/"1"	W/"1"	no match	match
W/"1"	W/"2"	no match	no match
W/"1"	"1"	no match	match
"1"	"1"	match	match

Fields

weak: bool

Weakness indicator for the tag

Implementations

impl EntityTag

pub fn new(weak: bool, tag: &str) -> Self

Constructs a new EntityTag, asserting that it doesn’t overflow and valid ASCII string.

Assertions are performed in debug mode only.

pub fn weak(tag: &str) -> Self

Constructs a new weak EntityTag, using the same checks as new.

pub fn strong(tag: &str) -> Self

Constructs a new strong EntityTag, using the same checks as new.

pub fn checked_new(weak: bool, tag: &str) -> Result<Self, ParseError>

Constructs a new EntityTag, verifying it’s size and whether it includes ASCII.

pub fn checked_weak(tag: &str) -> Result<Self, ParseError>

Constructs a new weak EntityTag, using the same checks as checked_new.

pub fn checked_strong(tag: &str) -> Result<Self, ParseError>

Constructs a new strong EntityTag, using the same checks as checked_new.

pub fn from_file_meta(metadata: &Metadata) -> Self

Creates weak EntityTag from file metadata using modified time and len.

Format:

[modified-]<len>

pub const fn const_from_data(bytes: &[u8]) -> Self

Creates strong EntityTag by hashing provided bytes.

Format:

<len>-<hash>

pub fn from_data(bytes: &[u8]) -> Self

Creates strong EntityTag by hashing provided bytes.

Format:

<len>-<hash>

pub fn tag(&self) -> &str

Get the tag.

pub fn strong_eq(&self, other: &EntityTag) -> bool

For strong comparison two entity-tags are equivalent if both are not weak and their opaque-tags match character-by-character.

pub fn weak_eq(&self, other: &EntityTag) -> bool

For weak comparison two entity-tags are equivalent if their opaque-tags match character-by-character, regardless of either or both being tagged as “weak”.

pub fn strong_ne(&self, other: &EntityTag) -> bool

The inverse of EntityTag.strong_eq().

pub fn weak_ne(&self, other: &EntityTag) -> bool

The inverse of EntityTag.weak_eq().

Trait Implementations

impl Clone for EntityTag

fn clone(&self) -> EntityTag

Returns a copy of the value.

pub fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for EntityTag

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for EntityTag

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for EntityTag

impl FromStr for EntityTag

type Err = ParseError

The associated error which can be returned from parsing.

fn from_str(text: &str) -> Result<EntityTag, ParseError>

Parses a string s to return a value of this type.

impl PartialEq<EntityTag> for EntityTag

fn eq(&self, other: &EntityTag) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &EntityTag) -> bool

This method tests for !=.

impl StructuralEq for EntityTag

impl StructuralPartialEq for EntityTag