ETag stands for Entity Tag and is an HTTP header that transports a current version identifier of a resource. In REST and OData APIs, it is primarily used for optimistic concurrency control: The clients A resource can only be modified if its ETag still matches the server-side version. This prevents overwrites of parallel changes without the server having to hold server-side locks.
Context
The process in practice: the client calls a resource via GET and receives in the response header a ETag: "123abc". If he changes them, he sends at PATCH or PUT the header If-Match: "123abc" If the current server ETag still matches, the change is executed and a new ETag is returned. If someone else has written in the meantime, the server responds with HTTP 412. Precondition Failed; The client must reload the current state, resolve the conflict, and try again. In the SAP Business One Service Layer, this pattern is relevant when multiple integrations write to the same business partner, item, or open document simultaneously, such as a webshop sync, the ERP front-end, and a migration tool. ETags are automatically generated in OData for entities with configured concurrency properties; on the client-side, a wrapper is recommended that If-Match consistently sets and transfers 412 responses into a reload-and-retry flow.
Demarcation
ETag-based concurrency is optimistic: there is no locking, conflicts are only detected when writing. This distinguishes it from pessimistic locking (e.g. SELECT FOR UPDATE), which in the Database or enforced by a blocking service. ETags are also not a cache-control mechanism in the strictest sense, although they are related to If-None-Match Additionally, support HTTP caching. And they do not replace functional consistency checks: that two write operations follow one another technically cleanly does not mean that the content result is correct – for that, validations and transaction boundaries in the server remain mandatory.
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