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347 lines
19 KiB
Markdown
347 lines
19 KiB
Markdown
# Server Parameters
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Mongo database and router servers (i.e., `mongod` and `mongos`) provide a number of configuration
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options through server parameters. These parameters allow users to configure the behavior of the
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server at startup or runtime. For instance, `logLevel` is a server parameter that configures the
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logging verbosity.
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## How to define new parameters
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Parameters are defined by the elements of the `server_parameters` section of an IDL file. The IDL
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machinery will parse these files and generate C++ code, and corresponding header files where
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appropriate. The generated code will self-register server parameters with the runtime.
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Consider `logLevel` parameter from [`parameters.idl`][parameters.idl] for example:
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```yaml
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...
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server_parameters:
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...
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logLevel:
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description: "Specifies the verbosity of logging"
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set_at: ["startup", "runtime"]
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cpp_class:
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name: LogLevelServerParameter
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override_set: true
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...
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```
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This defines a server parameter called `logLevel`, which is settable at startup or at runtime, and
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declares a C++ class for the parameter (i.e., `LogLevelServerParameter`). Refer to the
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[Server Parameters Syntax](#server-parameters-syntax) documentation for the complete IDL syntax.
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## How to change a defined parameter
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Users can set or modify a server parameter at startup and/or runtime, depending on the value
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specified for `set_at`. For instance, `logLevel` may be set at both startup and runtime, as
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indicated by `set_at` (see the above code snippet).
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At startup, server parameters may be set using the `--setParameter` command line option.
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At runtime, the `setParameter` command may be used to modify server parameters.
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See the [`setParameter` documentation][set-parameter] for details.
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## How to get the value provided for a parameter
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Server developers may retrieve the value of a server parameter by:
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* Accessing the C++ expression that corresponds to the parameter of interest. For example, reading
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from [`serverGlobalParams.quiet`][quiet-param] returns the current value for `quiet`.
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* Registering a callback to be notified about changes to the server parameter (e.g.,
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[`onUpdateFTDCFileSize`][ftdc-file-size-param] for `diagnosticDataCollectionFileSizeMB`).
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Database users may use the [`getParameter`][get-parameter] command to query the current value for a
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server parameter.
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## Server Parameters Syntax
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The following shows the IDL syntax for declaring server parameters. Field types are denoted in each
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section. For details regarding `string or expression map`, see that section
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[below](#string-or-expression-map).
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```yaml
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server_parameters:
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"nameOfParameter": # string
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set_at: # string or list of strings
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description: # string
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cpp_vartype: # string
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cpp_varname: # string
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cpp_class: # string (name field) or map
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name: # string
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data: # string
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override_ctor: # bool
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override_set: # bool
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override_validate: # bool
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redact: # bool
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test_only: # bool
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default: # string or expression map
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deprecated_name: # string or list of strings
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on_update: # string
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condition:
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expr: # C++ bool expression, runtime evaled
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constexpr: # C++ bool expression, compile-time eval
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preprocessor: # C preprocessor condition
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min_fcv: # string
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feature_flag: # string
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validator: # Map containing one or more of the below
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lt: # string or expression map
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gt: # string or expression map
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lte: # string or expression map
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gte: # string or expression map
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callback: # string
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```
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Each entry in the `server_parameters` map represents one server parameter. The name of the parameter
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must be unique across the server instance. More information on the specific fields:
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* `set_at` (required): Must contain the value `startup`, `runtime`, [`startup`, `runtime`], or
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`cluster`. If `runtime` is specified along with `cpp_varname`, then `decltype(cpp_varname)` must
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refer to a thread-safe storage type, specifically: `AtomicWord<T>`, `AtomicDouble`, `std::atomic<T>`,
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or `boost::synchronized<T>`. If `cluster` is specified along with `cpp_varname`, then
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`decltype(cpp_varname)` must refer to an IDL-defined type that has `ClusterServerParameter` attached
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as an inline chained struct.
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* `description` (required): Free-form text field currently used only for commenting the generated C++
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code. Future uses may preserve this value for a possible `{listSetParameters:1}` command or other
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programmatic and potentially user-facing purposes.
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* `cpp_vartype`: Declares the full storage type. If `cpp_vartype` is not defined, it may be inferred
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from the C++ variable referenced by `cpp_varname`.
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* `cpp_varname`: Declares the underlying variable or struct member to use when setting or reading the
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server parameter. If defined together with `cpp_vartype`, the storage will be declared as a global
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variable, and externed in the generated header file. If defined alone, a variable of this name will
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assume to have been declared and defined by the implementer, and its type will be automatically
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inferred at compile time. If `cpp_varname` is not defined, then `cpp_class` must be specified.
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* `cpp_class`: Declares a custom `ServerParameter` class in the generated header using the provided
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string, or the name field in the associated map. The declared class will require an implementation
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of `setFromString()`, and optionally `set()`, `append()`, and a constructor.
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See [Specialized Server Parameters](#specialized-server-parameters) below.
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* `default`: String or expression map representation of the initial value.
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* `redact`: Set to `true` to replace values of this setting with placeholders (e.g., for passwords).
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* `test_only`: Set to `true` to disable this set parameter if `enableTestCommands` is not specified.
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* `deprecated_name`: One or more names which can be used with the specified setting and underlying
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storage. Reading or writing a setting using this name will result in a warning in the server log.
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* `on_update`: C++ callback invoked after all validation rules have completed successfully and the
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new value has been stored. Prototype: `Status(const cpp_vartype&);`
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* `condition`: Up to five conditional rules for deciding whether or not to apply this server
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parameter. `preprocessor` will be evaluated first, followed by `constexpr`, then finally `expr`. If
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no provided setting evaluates to `false`, the server parameter will be registered. `feature_flag` and
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`min_fcv` are evaluated after the parameter is registered, and instead affect whether the parameter
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is enabled. `min_fcv` is a string of the form `X.Y`, representing the minimum FCV version for which
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this parameter should be enabled. `feature_flag` is the name of a feature flag variable upon which
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this server parameter depends -- if the feature flag is disabled, this parameter will be disabled.
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`feature_flag` should be removed when all other instances of that feature flag are deleted, which
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typically is done after the next LTS version of the server is branched. `min_fcv` should be removed
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after it is no longer possible to downgrade to a FCV lower than that version - this occurs when the
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next LTS version of the server is branched.
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* `validator`: Zero or many validation rules to impose on the setting. All specified rules must pass
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to consider the new setting valid. `lt`, `gt`, `lte`, `gte` fields provide for simple numeric limits
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or expression maps which evaluate to numeric values. For all other validation cases, specify
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callback as a C++ function or static method. Note that validation rules (including callback) may run
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in any order. To perform an action after all validation rules have completed, `on_update` should be
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preferred instead. Callback prototype: `Status(const cpp_vartype&, const boost::optional<TenantId>&);`
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Any symbols such as global variables or callbacks used by a server parameter must be imported using
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the usual IDL machinery via `globals.cpp_includes`. Similarly, all generated code will be nested
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inside the namespace defined by `globals.cpp_namespace`. Consider the following for example:
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```yaml
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global:
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cpp_namespace: "mongo"
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cpp_includes:
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- "mongo/util/net/ssl_parameters.h"
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server_parameters:
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opensslCipherConfig:
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...
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validator:
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callback: "validateOpensslCipherConfig" # The callback is declared in "ssl_parameters.h"
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```
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### Specialized Server Parameters
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When `cpp_class` is specified on a server parameter, a child class of `ServerParameter` will be
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created in the `gen.h` file named for either the string value of `cpp_class`, or if it is expressed
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as a dictionary, then `cpp_class.name`. A `cpp_class` directive may also contain:
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```yaml
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server_parameters:
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someParameter:
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cpp_class:
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name: string # Name to assign to the class (e.g., SomeParameterImpl)
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data: string # cpp data type to add to the class as a property named "_data"
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override_ctor: bool # True to allow defining a custom constructor, default: false
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override_set: bool # True to allow defining a custom set() method, default: false
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```
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`override_ctor`: If `false`, the inherited constructor from the `ServerParameter` base class will be
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used. If `true`, then the implementer must provide a
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`{name}::{name}(StringData serverParameterName, ServerParameterType type)` constructor. In addition
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to any other work, this custom constructor must invoke its parent's constructor.
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`override_set`: If `true`, the implementer must provide a `set` member function as:
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```cpp
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Status {name}::set(const BSONElement& val, const boost::optional<TenantId>& tenantId);
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```
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Otherwise the base class implementation `ServerParameter::set` is used. It
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invokes `setFromString` using a string representation of `val`, if the `val` is
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holding one of the supported types.
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`override_validate`: If `true`, the implementer must provide a `validate` member function as:
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```cpp
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Status {name}::validate(const BSONElement& newValueElement, const boost::optional<TenantId>& tenantId);
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```
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Otherwise, the base class implementation `ServerParameter::validate` is used. This simply returns
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`Status::OK()` without performing any kind of validation of the new BSON element.
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If `param.redact` was specified as `true`, then a standard append method will be provided which
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injects a placeholder value. If `param.redact` was not specified as `true`, then an implementation
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must be provided with the following signature:
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```cpp
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Status {name}::append(OperationContext*, BSONObjBuilder*, StringData, const boost::optional<TenantId>& tenantId);
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```
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Lastly, a `setFromString` method must always be provided with the following signature:
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```cpp
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Status {name}::setFromString(StringData value, const boost::optional<TenantId>& tenantId);
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```
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Note that by default, server parameters are not tenant aware and thus will always have `boost::none`
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provided as `tenantId`, unless defined as cluster server parameters (discussed
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[below](#cluster-server-parameters)).
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Each server parameter encountered will produce a block of code to run at process startup similar to
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the following:
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```cpp
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/**
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* Iteration count to use when creating new users with
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* SCRAM-SHA-1 credentials
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*/
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MONGO_COMPILER_VARIABLE_UNUSED auto* scp_unique_ident = [] {
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using T = decltype(saslGlobalParams.scramSHA1IterationCount);
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constexpr auto setAt = ServerParameterType::kStartupAndRuntime;
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auto ret = new IDLServerParameterWithStorage<T>(
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"scramIterationCount",
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saslGlobalParams.scramSHA1IterationCount, setAt);
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ret->addBound<predicate::GTE>(5000);
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return ret;
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}();
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```
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Any additional validator and callback would be set on `ret` as determined by the server parameter
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configuration block.
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### Cluster Server Parameters
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As indicated earlier, one of the options for the `set_at` field is `cluster`. If this value is
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selected, then the generated server parameter will be known as a cluster server parameter. These
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server parameters are persisted on a cluster-wide basis via the `setClusterParameter` and
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`getClusterParameter` commands. Cluster server parameters should be used instead of implementing
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custom commands to propagate an option cluster-wide, whenever possible.
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`setClusterParameter` persists the new value of the indicated cluster server parameter onto a
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majority of nodes on non-sharded replica sets. On sharded clusters, it majority-writes the new value
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onto every shard and the config server. This ensures that every mongod in the cluster will be able
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to recover the most recently written value for all cluster server parameters on restart.
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Additionally, `setClusterParameter` blocks until the majority write has succeeded on every replica
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set in the cluster, which guarantees that the parameter value will not be rolled back after being
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set. Mongoses poll the config server for updated cluster server parameter values every `clusterServerParameterRefreshIntervalSecs`.
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`getClusterParameter` returns the cached value of the requested cluster server parameter on the node
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that it is run on. It can accept a single cluster server parameter name, a list of names, or `*` to
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return all cluster server parameter values on the node.
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As indicated in the [Server Parameters Syntax](#server-parameters-syntax) section above, specifying
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`cpp_vartype` for cluster server parameters must result in the usage of an IDL-defined type that has
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`ClusterServerParameter` listed as a chained struct. It is also highly recommended to implement a
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validator callback function and register it to the cluster server parameter via the `validator`
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field. These validators are called before the new value of the cluster server parameter is written
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to disk during `setClusterParameter`.
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See [server_parameter_with_storage_test.idl][cluster-server-param-with-storage-test] and
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[server_parameter_with_storage_test_structs.idl][cluster-server-param-with-storage-test-structs] for
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examples.
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Cluster server parameters can also be specified as specialized server parameters. The chart below
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helps depict how base `ServerParameter` methods can be handled:
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| `ServerParameter` method | Override | Default Behavior |
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| --------------------------- | ------------------- | --------------------------------------------|
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| constructor | `override_ctor` | Instantiates only the name and type. |
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| `set()` | Required | None, won't compile without implementation. |
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| `setFromString()` | Prohibited | Returns `ErrorCodes::BadValue`. |
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| `append()` | Required | None, won't compile without implementation. |
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| `validate()` | `override_validate` | Return `Status::OK()` without any checks. |
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| `reset()` | Required | None, won't compile without implementation. |
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| `getClusterParameterTime()` | Required | Return `LogicalTime::kUninitialized`. |
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* Specifying `override_ctor` to true is optional. If the cluster server parameter needs to have any
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storage initialized at the same time as parameter registration, then an overridden constructor could
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be useful. Otherwise, the default likely suffices provided that all storage modified via `set()` is
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instantiated in another way.
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* It is highly recommended to specify `override_validate` to true and provide a custom implementation
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of the `validate` method. This ensures that cluster parameters do not get set to nonsensical values.
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* `set()` must be implemented in order to update in-memory parameter storage. It will be called from
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an `OpObserver` after observing a change to the cluster parameter document on-disk.
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* `append()` must be implemented in order to serialize the parameter into BSON for use in
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`getClusterParameter`.
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* `setFromString()` must never be implemented as cluster server parameters are only set via BSON
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during runtime.
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* `getClusterParameterTime` must be implemented and should return a `LogicalTime` corresponding to
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the current version of the cluster server parameter.
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* `reset()` must be implemented and should update the cluster server parameter back to its default
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value.
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All cluster server parameters are tenant-aware, meaning that on serverless clusters, each tenant has
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an isolated set of parameters. The `setClusterParameter` and `getClusterParameter` commands will pass
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the `tenantId` on the command request to the `ServerParameter`'s methods. On dedicated
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(non-serverless) clusters, `boost::none` will be passed. IDL-defined cluster server parameters will
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handle the passed-in `tenantId` automatically and store separate parameter values per-tenant.
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Specialized server parameters will have to take care to correctly handle the passed-in `tenantId` and
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to enforce tenant isolation.
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Like normal server parameters, cluster server parameters can be defined to be dependent on a minimum
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FCV version or a specific feature flag using the `condition: min_fcv/feature_flag:` syntax discussed
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above. During FCV downgrade, the cluster parameter's stored on-disk value will be deleted if either:
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(1) The downgraded FCV is lower than the cluster parameter's `min_fcv`, or (2) The cluster
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parameter's `feature_flag` is disabled on the downgraded FCV. While a cluster server parameter is
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disabled due to either of these conditions, `setClusterParameter` on it will always fail, and
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`getClusterParameter` will fail on `mongod`, and return the default value on `mongos` -- this
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difference in behavior is due to `mongos` being unaware of the current FCV.
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See [server_parameter_specialized_test.idl][specialized-cluster-server-param-test-idl] and
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[server_parameter_specialized_test.h][specialized-cluster-server-param-test-data] for examples.
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### String or Expression Map
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The default and implicit fields above, as well as the `gt`, `lt`, `gte`, and `lte` validators accept
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either a simple scalar string which is treated as a literal value, or a yaml map containing an
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attribute called `expr`, which must be a string containing an arbitrary C++ expression to be used
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as-is. Optionally, an expression map may also include the `is_constexpr: false` attribute, which
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will suspend enforcement of the value being a `constexpr`.
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For example, consider:
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```yaml
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server_parameters:
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connPoolMaxInUseConnsPerHost:
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...
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cpp_varname: maxInUseConnsPerHost
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default:
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expr: std::numeric_limits<int>::max()
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...
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```
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Here, the server parameter's default value is the evaluation of the C++ expression
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`std::numeric_limits<int>::max()`. Additionally, since default was not explicitly given the
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`is_constexpr: false` attribute, it will be round-tripped through the following lambda to guarantee
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that it does not rely on runtime information.
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```cpp
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[]{ constexpr auto value = <expr>; return value; }()
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```
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[parameters.idl]: ../src/mongo/db/commands/parameters.idl
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[set-parameter]: https://docs.mongodb.com/manual/reference/parameters/#synopsis
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[get-parameter]: https://docs.mongodb.com/manual/reference/command/getParameter/#getparameter
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[quiet-param]: https://github.com/mongodb/mongo/search?q=serverGlobalParams+quiet+extension:idl&type=code
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[ftdc-file-size-param]: ../src/mongo/db/ftdc/ftdc_server.idl
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[cluster-server-param-with-storage-test]: ../src/mongo/idl/server_parameter_with_storage_test.idl
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[cluster-server-param-with-storage-test-structs]: ../src/mongo/idl/server_parameter_with_storage_test_structs.idl
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[specialized-cluster-server-param-test-idl]: ../src/mongo/idl/server_parameter_specialized_test.idl
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[specialized-cluster-server-param-test-data]: ../src/mongo/idl/server_parameter_specialized_test.h
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