HTTP ETag - Strong and Weak Validation

Strong and Weak Validation

The ETag mechanism supports both strong validation and weak validation. They are distinguished by the presence of an initial "W/" in the ETag identifier, as:

"123456789" -- A strong ETag validator W/"123456789" -- A weak ETag validator

A strongly validating ETag match indicates that the content of the two resources is byte-for-byte identical and that all other entity fields (such as Content-Language) are also unchanged. Strong ETags permit the caching and reassembly of partial responses, as with byte-range requests.

A weakly validating ETag match only indicates that the two resources are semantically equivalent, meaning that for practical purposes they are interchangeable and that cached copies can be used. However the resources are not necessarily byte-for-byte identical, and thus weak ETags are not suitable for byte-range requests. Weak ETags may be useful for cases in which strong ETags are impractical for a web server to generate, such as with dynamically-generated content.

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    —Giuseppe Mazzini (1805–1872)

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    “The enemy is like a woman, weak in face of opposition, but correspondingly strong when not opposed. In a quarrel with a man, it is natural for a woman to lose heart and run away when he faces up to her; on the other hand, if the man begins to be afraid and to give ground, her rage, vindictiveness and fury overflow and know no limit.”
    —St. Ignatius Of Loyola (1491–1556)