This disclosure relates to providing a high availability computing service in a distributed system. In one aspect, a method includes sending, by a computing unit of multiple computing units that are each executing a respective copy of a computing task, a certificate request to a trusted certificate generator. The request includes authentication information. The authentication information includes a code hash of the computing task. The computing unit receives a certificate report including a public key certificate in a certificate chain generated for the code hash and a private key corresponding to the public key certificate. The public key certificate and the private key form a certificate pair. The certificate chain includes multiple certificates including the public key certificate and a root certificate corresponding to the public key certificate. The computing unit is used as a TLS server. The certificate pair is set as a certificate pair of the TLS server.