49 lines
2.1 KiB
PHP
49 lines
2.1 KiB
PHP
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.. _generations:
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==========================================
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Resource Provider and Consumer Generations
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==========================================
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Placement handles concurrent requests against the same entity by maintaining a
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**generation** for resource providers and consumers. The generation is an
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opaque value that is updated every time its entity is successfully changed on
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the server.
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At appropriate microversions, the generation is returned in responses involving
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resource providers and/or consumers (allocations), and must be included in
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requests which make changes to those entities. The server checks to make sure
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the generation specified in the request matches the internal value. A mismatch
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indicates that a different request successfully updated that entity in the
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interim, thereby changing its generation. This will result in an HTTP 409
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Conflict response with `error code <error_codes_>`_
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``placement.concurrent_update``.
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Depending on the usage scenario, an appropriate reaction to such an error may
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be to re-``GET`` the entity in question, re-evaluate and update as appropriate,
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and resubmit the request with the new payload.
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The following pseudocode is a simplistic example of how one might ensure that a
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trait is set on a resource provider.
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.. note:: This is not production code. Aside from not being valid syntax for
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any particular programming language, it deliberately glosses over
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details and good programming practices such as error checking, retry
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limits, etc. It is purely for illustrative purposes.
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::
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function _is_concurrent_update(resp) {
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if(resp.status_code != 409) return False
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return(resp.json()["errors"][0]["code"] == "placement.concurrent_update")
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}
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function ensure_trait_on_provider(provider_uuid, trait) {
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do {
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path = "/resource_providers/" + provider_uuid + "/traits"
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get_resp = placement.GET(path)
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payload = get_resp.json()
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if(trait in payload["traits"]) return
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payload["traits"].append(trait)
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put_resp = placement.PUT(path, payload)
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} while _is_concurrent_update(put_resp)
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}
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