先看这个实体的字段声明（api/core/entities/provider_configuration.py:60）：

# api/core/entities/provider_configuration.py:60
class ProviderConfiguration(BaseModel):
    """
    This class handles:
    - Provider credentials CRUD and switch     # 供应商级凭据的增删改和切换
    - Custom Model credentials CRUD and switch  # 模型级凭据
    - System vs custom provider switching       # 系统额度 / 自定义 二选一
    - Load balancing configurations             # 负载均衡
    - Model enablement/disablement              # 模型启用/禁用
    """
    tenant_id: str
    provider: ProviderEntity                    # ① 这家供应商的"说明书"(有哪些模型、怎么配)
    preferred_provider_type: ProviderType       # ② 用户更想用哪种(系统/自定义)
    using_provider_type: ProviderType           # ③ 实际正在用哪种
    system_configuration: SystemConfiguration   # ④ 系统额度配置(平台送的试用)
    custom_configuration: CustomConfiguration   # ⑤ 自定义配置(用户自填的 key)
    model_settings: list[ModelSettings]         # ⑥ 每个模型的开关 + 负载均衡

Day05 里 ModelInstance 取凭据，最终落到这个方法（api/core/entities/provider_configuration.py:157）：

# api/core/entities/provider_configuration.py:157
def get_current_credentials(self, model_type, model) -> dict | None:
    if self.model_settings:                                  # ① 先查"这个模型被禁了没"
        for model_setting in self.model_settings:
            if model_setting.model_type == model_type and model_setting.model == model:
                if not model_setting.enabled:
                    raise ValueError(f"Model {model} is disabled.")   # 禁用 → 直接报错

    if self.using_provider_type == ProviderType.SYSTEM:      # ② 走"系统额度"分支
        restrict_models = []
        for quota_configuration in self.system_configuration.quota_configurations:
            if self.system_configuration.current_quota_type != quota_configuration.quota_type:
                continue
            restrict_models = quota_configuration.restrict_models   # 试用额度限定的模型
        copy_credentials = self.system_configuration.credentials.copy() ...
        return copy_credentials                              # 用平台内置凭据
    else:                                                    # ③ 走"自定义"分支
        credentials = None
        if self.custom_configuration.models:                 #    先看模型级有没有单配 key
            for m in self.custom_configuration.models:
                if m.model_type == model_type and m.model == model:
                    credentials = m.credentials; break
        if not credentials and self.custom_configuration.provider:  # 没有 → 退回供应商级 key
            credentials = self.custom_configuration.provider.credentials

Dify 把模型分成六大类型（ModelType 枚举，来自 model_runtime）。它们的"技能对象"基类各不相同，但 Day05 的 ModelInstance 对每一类都提供了一个 invoke_* 门面方法：

Day05 说 model_type_instance 是"真正会调某类模型的技能对象"。它由 ProviderConfiguration.get_model_type_instance（api/core/entities/provider_configuration.py:1538）产出：

# api/core/entities/provider_configuration.py:1538
def get_model_type_instance(self, model_type: ModelType) -> AIModel:
    if self._bound_model_runtime is not None:
        model_runtime = self._bound_model_runtime           # ① 请求已绑好的运行时，直接复用
    else:
        model_runtime, _ = self._get_runtime_and_provider_factory()  # 否则现查

    provider_schema = self._cached_provider_schema or self.provider  # ② 复用已解析的说明书

    return create_model_type_instance(                       # ③ ★按"运行时+说明书+类型"造技能对象
        runtime=model_runtime,
        provider_schema=provider_schema,
        model_type=model_type,
    )

回到 Day05 的 ModelInstance。有了类型体系，你会发现六个 invoke_* 方法长得几乎一样。对比 invoke_text_embedding（api/core/model_manager.py:214）和 invoke_rerank（api/core/model_manager.py:272）：

# api/core/model_manager.py:214
def invoke_text_embedding(self, texts, input_type=EmbeddingInputType.DOCUMENT):
    if not isinstance(self.model_type_instance, TextEmbeddingModel):   # ① 类型防呆
        raise Exception("Model type instance is not TextEmbeddingModel")
    return self._round_robin_invoke(                                    # ② 统一走 Day05 的轮询包装
        self.model_type_instance.invoke,
        model=self.model_name, credentials=self.credentials,
        texts=texts, input_type=input_type,
    )

# api/core/model_manager.py:272
def invoke_rerank(self, query, docs, score_threshold=None, top_n=None):
    if not isinstance(self.model_type_instance, RerankModel):          # ① 换个基类防呆
        raise Exception("Model type instance is not RerankModel")
    return self._round_robin_invoke(                                    # ② 一模一样的包装
        self.model_type_instance.invoke,
        model=self.model_name, credentials=self.credentials,
        query=query, docs=docs, score_threshold=score_threshold, top_n=top_n,
    )