@Override
    public String retrieveIdentifier(final WebContext webContext, final Client client) {
        for (val sessionKey : SESSION_KEYS) {
            val clientId = getSessionStore().get(webContext, sessionKey);
            if (clientId.isPresent()) {
                LOGGER.trace("Found client id [{}] for key [{}]", clientId, sessionKey);
                getSessionStore().set(webContext, sessionKey, null);
                return clientId.map(Object::toString).orElse(StringUtils.EMPTY);
            }
        }
        return StringUtils.EMPTY;
    }
This is {@link DelegatedClientOidcSessionManager}.

@author Misagh Moayyed
@since 7.1.0
support/cas-server-support-pac4j-oidc/src/main/java/org/apereo/cas/pac4j/web/DelegatedClientOidcSessionManager.java