How would you authenticate and authorize service-to-service API calls?

Authenticate service-to-service API calls

Explains API keys, client credentials, managed identity, mTLS, scopes, rotation, and per-client observability.

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I would give each calling service or client its own identity, not one shared secret for everything. Depending on the environment, that might be OAuth client credentials, managed identity, mTLS, or an API key for simpler partner integrations. The API should authorize by client identity and scope, not only by network location.

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API keys, OAuth client credentials, managed identities, mTLS, scopes, rotation, least privilege, and auditability.

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Internal or partner calls need identity too. Treating service traffic as trusted just because it is server-side creates quiet security holes.
I would give each calling service or client its own identity, not one shared secret for everything. Depending on the environment, that might be OAuth client credentials, managed identity, mTLS, or an API key for simpler partner integrations. The API should authorize by client identity and scope, not only by network location.
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I would keep service permissions least-privileged. A billing worker might be allowed to update invoices but not manage users. Secrets or credentials need rotation, expiry where appropriate, and safe storage in a secret manager. For API keys, I would store only hashes, show the raw key once, and support key rollover with overlapping active keys during migration.

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Using one shared secret across many services.
Authorizing only by IP or internal network.
No credential rotation plan.
No per-client logging or rate limits.

How would you authenticate and authorize service-to-service API calls?

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