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As previously mentioned, Cert-Manager consumes Certificate CRD to issue certificates. But before first certificate is issued, Issuer or ClusterIssuer CRD has to be configured first. For basic use cases that's all. One can use very rich functionality to configure issued certificate and use various types of issuers.
Underneath, out of Certificate CRD, Cert-Manager creates CertificateRequest CRD which is more suitable for M2M processing as it contains Certificate Singing Request (CSR). CertificateRequest CRD is further on consumed by Issuer which processes CSR stored there and in return puts signed certificate and trusted certs in CertificateRequest's status and marks CertificateReqeust as Ready. Cert-Manager notices such state change and from CertificateRequest marked as Ready creates K8s secret originally requested in Certificate CRD. Such K8s secret is ready to be mounted to K8s workload as any other secret.
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